chapter 8. palaeomagnetic study of tertiary lava …mimi/chapter8.pdf177 chapter 8. palaeomagnetic...
TRANSCRIPT
177
Chapter 8. Palaeomagnetic Study of Tertiary Lava from Barrington Tops, NSW, Australia
The previous two chapters have dealt with historic lava; the logical next
step is to extend investigations to ancient lava. Tertiary lava from Barrington
volcano, New South Wales, Australia was the chosen study area. The chapter
begins with the reasons for choosing the study area and then describes the
geology and dating of the volcano. Previous palaeomagnetic studies of the
volcano and the construction of Cenozoic Australian APWPs are described. The
background sections are followed by a description of the sampling undertaken for
this study and then the experimental results are discussed. An intensive rock
magnetic investigation, thermal demagnetisation and conventional Thellier
intensity studies have been carried out. These are described and discussed.
Microwave demagnetisation using one sample per flow and a preliminary
microwave intensity study using eight samples from different flows have been
carried out; these results are discussed. The chapter ends with an overall
discussion and summary.
8.1. Introduction
A more comprehensive coverage of palaeomagnetic data in both time and
space is required to gain a more complete understanding of the geomagnetic field.
Australia is one region where palaeomagnetic data, especially field intensity data,
is particularly sparse. This is illustrated in Fig. 8.1, which shows the complete set
of intensity data contained in the 1998 version of the IAGA palaeointensity
database, pint97 (Perrin et al., 1998) for the Australian continent, along with the
recent results obtained from Jurassic intrusions, NSW (Thomas et al., 2000).
There are a total of only 19 field intensity evaluations from six published studies
(Barbetti & McElhinny, 1976; Briden, 1966a; b; McElhinny & Evans, 1976;
Sennayake et al., 1982; Thomas et al., 2000). Directional data are not abundant
either and there are a number of conflicting versions of the Cenozoic Australian
apparent polar wander path (APWP) which are discussed in Section 8.4.2.
178
0
2
4
6
8
10
12
14
0 500 1000 1500 2000
Age Ma
VD
M x
10
22 A
m2
Figure 8.1 All field intensity data for the Australian continent contained in the pint97 database and from Thomas et al. (2000).
0
5
10
15
20
25
30
35
45 50 55 60
Age Ma
VD
M x
10
22 A
m2
Figure 8.2 All field intensity values contained in the pint97 database for the time interval 60 - 45 Ma.
Palaeointensity data are distinctly scarce during the early Tertiary as
illustrated in Fig. 8.2, which contains all the data in the pint97 database for the
time interval from 45-60 Ma, none of which is from Australia. There are ten
palaeointensity evaluations; from Mexico (Urrutia Fucugauchi, 1980), a DSDP
leg at the mid Pacific Ridge (Kono & Tosha, 1980) and from the British Tertiary
Igneous Province (BTIP) (Smith, 1967). The Barrington Volcano, NSW,
Australia is contemporaneous in age with the BTIP and antipodal, thus potentially
allowing the dipolar nature of the magnetic field to be investigated. In addition,
179
this is a key age for investigating the correspondence between field intensity and
the frequency of field reversals as it is an interval in which the frequency of field
reversals is beginning to increase following the Cretaceous Normal Superchron
(c.f. Merrill et al., 1996).
8.2. Geological Setting
Throughout the Cenozoic widespread volcanism took place at sites spread
along the entire length of Eastern Australia (e.g. Johnson, 1989; Wellman &
McDougall, 1974a) with the Barrington province being just one of many lava
fields (Fig. 8.3). The Mount Royal Ranges, situated in northern New South
Wales, 220 km from Sydney, consists of a deeply dissected basalt volcano
(Barrington). The volcanic rocks may have originally formed a low angle shield,
which has subsequently been extensively modified by erosion. The remnants of
the Barrington volcano crop out mainly on the Barrington Tops plateau and its
escarpment. The original total volume of the lava field may have been about 700
km3, but now the volume is about 120 km3 (Johnson, 1989). There is a well
exposed unconformity between the basalt and the underlying deformed
Carboniferous and Devonian sedimentary and volcanogenic rocks. Permian
granites of the Barrington Tops granodiorite are also present in the region (e.g.
Johnson, 1989). Fig. 8.4 illustrates the geology of the Barrington Tops region.
The volcanic pile consists of about 700 m of flat lying flows (Wellman &
McDougall, 1974b) with flow thicknesses of 2-10 m dominating the lava pile.
The majority of the volcanic pile contains alkaline rock types. This includes
transitional basalt, alkali basalt, basanite and nephelinite. Many are porphyritic in
olivine, or clinopyroxene, or both (Johnson, 1989).
180
Figure 8.3 Map of Eastern Australia showing the Cenozoic volcanics (from Wellman et al., 1969).
181
Fig 8.4 Geologic Map of the Barrington Tops Region (from Mason & Kavalieris, 1984).
182
A stratigraphical study has been carried out in the western part of the
Barrington Tops region (Mason, 1982) the main results of which are described in
the following paragraphs. The study concentrated on two areas, around Prospero
Trig and at Semphills Creek. A sketch map of the area is shown in Fig. 8.5 and
the stratigraphic sections are shown in Fig. 8.6. The dominant rock type in the
two sections is alkaline intergranular olivine basalt. Tholeiitic basalt lava is only
present at the Semphills Creek section at the base of the sequence. It was erupted
first and accumulated in topographic lows.
Figure 8.5 Sketch map of Barrington Tops region (from Mason, 1982), stippled area denotes Barrington volcano and the thick black lines denote the location of the two sections studied
by Mason (1982).
183
a l k a l i b a s a l t i c r o c k sol. bas. (intergranular)
ol. bas. (aphitic/subaphitic)
px.-ol. basalts Ankaramite
undifferentiated basaltstholeiitic basalts
lack of exposureCarboniferous sediments
Figure 8.6 Volcanic stratigraphy of Prospero Trig and Semphills Creek sections (from Mason, 1982). Flow thicknesses are approximate.
The volcanic succession in the Prospero Trig area is well exposed in road
cuttings on the Gloucester – Scone road where it traverses the western escarpment
of the Barrington Tops plateau from Moonan Outlook to Moonan Brook. Mason
(1982) states that at least 33 flows are exposed along a ~6 km section of road,
through ~430 m of altitude. The angular unconformity between the folded
Carboniferous sediments and the overlying Tertiary deposits is also well exposed.
The top few tens of metres of Carboniferous sediments are extensively weathered
and the top metre or so comprises conglomeratic material of possible pediment
gravel origin. Over this unconformably lies approximately 2 m of subhorizontal
Tertiary stream bed sediments. On top of this is the oldest basaltic lava flow,
which has an irregular base.
The whole of the volcanic sequence appears to be of lava flow origin with
an average flow thickness of ~10 m (2 – 15 m range). Most of the flows contain a
massive basal section overlain by a vesicular portion. A number of flows, mainly
the top ones, have well developed columnar jointing. White secondary minerals,
including zeolites and calcite cement the clinkery flow tops. These white
secondary minerals also commonly fill vesicles wherever they occur. Some flow
184
tops are very red signifying that they are strongly ferruginised and indicating that
there were significant periods of time between eruptions. Other flows show no
signs of weathering between them suggesting that they were extruded in rapid
succession.
Hand specimens, show that some flows do not contain phenocrysts
whereas others contain large glassy green olivine phenocrysts in a dark greenish
grey groundmass. Some flows contain pyroxene phenocrysts up to 40% in
volume. These rocks are very obvious in the field and are termed ankaramite
(Mason, 1982; 1985). Intergranular olivine basalt dominates the upper half of the
section whereas pyroxene - phyric basalt (including ankaramite) is restricted to
the base of the section.
The second section of the Mason (1982) study is at Semphills Creek. The
section is not as well exposed as that at Prospero Trig but is still good. 27 flows
in 350 m were sampled leaving ~100 m unexposed and unsampled. Rock types
similar to those found at Prospero Trig are also found at Semphills Creek. The
major difference between the sections is the presence of four massive flows
(average 15 m thick) of tholeiitic basalt at the bottom of the section.
For more detailed petrographic results the reader is referred to Mason
(1982; 1985) and Johnson (1989).
8.3. Age of Barrington Volcano
Until the advent of radiometric dating, ages given to the Cenozoic
volcanic rocks of Eastern Australia were mainly based on palaeontological and
physiographic data (Wellman & McDougall, 1974b). Using potassium – argon
(K-Ar) dating the volcanics are now known to range in age from late Palaeocene
to middle Miocene, a larger range than thought from the original studies. The
bulk of the K-Ar dating for Barrington volcano was published in the late 60’s and
early 70’s giving an age for the Barrington volcano of around 52 Ma. When this
age is corrected for currently accepted values of the decay constants, the age
increases to 53 Ma. However, four new determinations have recently been carried
out (Dr. F. L. Sutherland, pers. comm. to N. Thomas) giving the volcano a
slightly wider range of ages.
185
Table 8.1. Whole Rock K-Ar Ages for Barrington Volcano
Sample Calculated age (Ma )
Mean Age (Ma) Location
GA2924
GA2925
GA2949
GA2929
GA2948
51.53 ± 0.7
50.69 ± 0.6
44.76 ± 0.6 44.53 ± 0.6 50.81 ± 0.7
52.41 ± 0.7 52.09 ± 0.9
51.5 ± 0.7
50.7 ± 0.6
44.6 ± 0.6
50.8 ± 0.7
52.2 ± 0.7
Stewarts Brook
GA3465
GA2903
GA2923
GA2902
GA2901
GA2900
GA1961*
47.25 ± 0.6
51.25 ± 0.9
42.57 ± 0.7 43.48 ± 0.6 41.20 ± 0.7 45.60 ± 0.8 52.88 ± 0.9 51.79 ± 0.7 50.02 ± 4.7
48.7 ± 0.5
47.3 ± 0.6
51.3 ± 0.9
43.0 ± 0.7
43.4 ± 0.8
52.3 ± 0.8
50.0 ± 4.7
48.7 ± 0.5
Semphills Creek
GA2946
GA2947
53.3 ± 1.4 53.2 ± 1.4 43.9 ± 1.2 42.3 ± 0.7
53.3 ± 1.4
43.1 ± 1.0
20 km south of Nundle, north of other locations
Stewarts Brook and Semphill Creek determinations from Wellman et al. (1969) apart from * from McDougall & Wilkinson (1967); south of Nundle determinations from Wellman &
McDougall (1974b). Errors for analytical uncertainty are at one standard deviation apart from those from south of Nundle where the uncertainty is given at two standard deviation.
At present a total of 14 published whole rock K-Ar determinations have
been made on samples from Barrington Volcano. McDougall & Wilkinson (1967)
were the first, followed by eleven determinations from Wellman et al. (1969) and
two determinations from Wellman & McDougall (1974b). The results are
summarised in Table 8.1 showing the range in ages from 43 Ma to 53 Ma. An
intruding dyke gave a slightly older plagioclase age of 54.6 ± 2 Ma (Wellman &
McDougall, 1974b) increasing the range of ages to 43 Ma to 55 Ma. The younger
ages have been interpreted as reflecting variable amounts of argon loss by
diffusion, whereas the older ages are more tightly clustered and are thought to
indicate the age of extrusion of the flows. From these results it has been
suggested that the whole of the Barrington Volcano is about 52 Ma (Wellman &
186
McDougall, 1974b). Idnurm (1985a) adjusted the K-Ar ages to conform to the
currently accepted values of the decay constants (Steiger & Jager, 1977)
increasing the age of Barrington volcano to 53 Ma.
From the small dispersion in accepted ages this suggests the flows will
have erupted over a relatively short period of time. Wellman et al. (1969)
estimated the period probably did not exceed 1.4 Ma by taking two standard
deviations of the average age.
Four recent K-Ar whole rock dates have been evaluated (Dr. F. L.
Sutherland of the Australian Museum, Sydney, 2000) and are listed in Table 8.2.
The ages range from 51 to 59 Ma with a mean (and standard deviation around the
mean) of 54 ± 3 Ma. The range of 8 Ma is greater than the range of ages found
previously, however these new samples are all from new locations. This implies
that the lava flows were extruded over a longer period of time than 1.4 Ma. The
mean age using the recent eva luations is consistent with the previous mean age
placing the Barrington volcano as early Eocene.
Table 8.2. New K-Ar whole rock ages for Barrington Volcano
Sample Rock type
Age Ma ± 1 SD Location
DR12662
DR14701
DR16503
DR16517
Basanite, top flow
Basanite flow
Olivine
nephelinite
Alkali basalt flow
58.7 ± 0.5
54.5 ± 0.6
53.3 ± 0.4
50.9 ± 1.5
Thunderbolts lookout, Barrington Tops forest Road, Upper Manning Callemondah Road, Barrington Tops, Ellerston Moppy Lookout Summit, Upper Manning Barrington Tops forest Road, west side Polblue Swamp, Ellerston
Analyses DR12662, 14701, 16503: A. Webb AMDEL Laboratories, Adelaide, South Australia, analysis DR16517: H. Zwingmann, CSIRO Laboratories, North Ryde,
NSW.
187
8.4. Previous Palaeomagnetic Studies
8.4.1. Barrington Tops
The only published palaeomagnetic study of the Barrington volcano was
undertaken by Wellman et al. (1969) in conjunction with their K-Ar dating. In
addition to Barrington they also studied Liverpool and Nandewar volcanoes (Fig.
8.3). The study was purely directional; no rock magnetic analyses were
performed.
At Barrington volcano two sections were studied, 21 km apart, Semphills
Creek and Stewarts Brook (Figs. 8.4 and 8.5). It was due to this study that the
Mason (1982) stratigraphic study chose Semphill Creek as one of their sections
(the Gloucester – Scone road had not been built at the time of the Wellman et al.
(1969) study). Hand samples oriented with a sun compass and level were taken
from a total of 36 flows (13 from Semphill Creek and 23 from Stewarts Brook).
From each hand sample between 3 and 9 cores were taken. An astatic
magnetometer was used to determine the direction and intensity of magnetisation
of the samples after AF cleaning. A test sample from each flow was progressively
AF demagnetised up to a maximum of 60 mT to determine the stable end point.
Thereafter, the other samples were subjected to one AF demagnetisation step at a
field slightly higher than the evaluated stable end point. All flows apart from one
near the base of the Stewarts Brook section were stable to AF demagnetisation.
The flow mean directional results are shown in Fig. 8.7. The Semphill
Creek section is predominantly reversely magnetised with just one flow near the
base of the section being normally magnetised. The magnetisation directions of
the Stewarts Brook section are more complex, with two periods of reversed
polarity separated by flows of normal polarity magnetisation. There are also two
flows that exhibit anomalous directions, which are not included in the overall
directional mean shown in Table 8.3.
188
Figure 8.7 Variation of declination and inclination through the two sections (from Wellman et al., 1969). Dashed line shows overall mean direction.
Table 8.3 Mean directional results of Wellman et al. (1969) for Barrington Volcano
N D I R k α95 s Plat. S Plong. E Α95
33 193.0 +65.5 32.33 48.47 3.6 11.4 70.5 125.6 5.3
Where N is the number of flows, D declination, I inclination, R resultant of N unit vectors, k, precision parameter, α95 radius of circle of 95% confidence about mean direction, s angular
standard deviation, Plat and Plong the palaeomagnetic south pole with associated Α95.
8.4.2. Australian APWP
There are currently differing versions of the Australian Cenozoic APWP,
each with their own uncertainties (e.g. Acton & Kettles, 1996). A key issue is
whether or not the poles derived from the Australian Cenozoic volcanoes, in
particular Barrington and Liverpool, are anomalous.
McElhinny et al. (1974) synthesised all the palaeomagnetic and age data
from igneous bodies that was currently available (including the Barrington results
from Wellman et al. (1969)) to produce an Australian APWP from 60 Ma to
189
recent. Six poles were determined with the Barrington data included in the 60-40
Ma pole (Table 8.4). The derived APWP (Fig. 8.8) “shows a distinct zig-zag with
a westerly excursion around 30 Ma and a knee around 18 Ma” (McElhinny et al.,
1974).
Table 8.4 Different Palaeo South Poles for the early Tertiary
Age Ma Lat Long Α95 ReferenceBasalt pole 60-40 68.5 130.9 5.2 McElhinny et al. , 197450 Ma average 50 65.7 127.6 9.4 Embleton & McElhinny, 1982North Rankin 1 58 61.7 118.4 8.0/3.3/87.0* Idnurm, 1985b
*95% confidence ellipse described by its major axis/minor axis/azimuth of major axis.
Figure 8.8 The McElhinny et al. (1974) Australian APWP (from McElhinny et al., 1974).
Embleton & McElhinny (1982) constructed an alternative APWP using
poorly dated laterite and weathered profile data. The motivation for producing
this new APWP was due to inconsistencies between the basalt derived APWP and
sea floor spreading results. Klootwijk & Pierce (1979) had previously noted that
the Australian APWP did not match the Indian APWP when this was rotated on
to the Australian plate. The accuracy of the pole positions were questioned, in
particular from the Barrington and Liverpool volcanoes (Klootwijk & Pierce,
190
1979; Embleton & McElhinny, 1982) leading to the desirability of determining
the APWP from non igneous bodies. The Embleton & McElhinny (1982) path is
calibrated from estimates based on broad averages of all the available basalt data
(Table 8.4) since no closely constrained ages could be assigned to the laterite and
weathered profile poles. This APWP path fits in better with sea floor spreading
results and exhibits a smoother progression through time. The zig zags of the
McEhinny et al. (1974) path are no longer present and back to about 50 Ma the
path lies near the 120 °E meridian.
Idnurm (1985a; 1994) defined another APWP similar to that of Embleton
& McElhinny (1982) but with a different rate of polar wander. The Idnurm
APWP is derived from dated sedimentary sequences combined with the poorly
dated laterite data. The data from the Barrington, Liverpool, Nandewar and
Tweed volcanoes was all rejected as being anomalous. An azimuthally unoriented
drill core taken from off the north west coast of Australia, North Rankin 1,
provides the pole nearest in age to the Barrington volcano (Idnurm, 1985a) (Table
8.4). The drill core is given a mean depositional age of 58 Ma estimated from
planktonic foraminifera. Remanence of the geomagnetic field at the time of
drilling has been used to azimuthally orient the drill core. All the ancient
directions are of normal polarity indicating that the remanence was acquired
relatively rapidly (thought to be at the time of deposition).
Two reasons have been given for possible anomalous basalt poles, first,
that secular variation has not been averaged out due to episodic extrusion
(Idnurm, 1985a) and second that the magnetisation of the rocks is more
complicated than previously thought (Hoffman, 1984; Embleton & McElhinny,
1982). Musgrave (1989) however, believes that the igneous data may be more
reliable than previously thought. He constructed an APWP based on a polynomial
curve fitted through palaeomagnetic data from the igneous rocks and weathered
profiles. Idnurm believes however that there are problems with the construction
of the Musgrave APWP, which have been outlined in Idnurm (1990).
Fig 8.9 illustrates the Embleton, Idnurm and Musgrave APWPs. In
addition, three poles from averages of global palaeomagnetic data excluding
Australia are shown. The poles were obtained by reconstructing and averaging
poles in to the Australian reference frame (Acton & Gordon, 1994). The
Musgrave APWP is more in line with hot spot derived data than the Idnurm and
191
Embleton APWPs but this discrepancy is believed to be due to true polar wander
(Idnurm, 1985b).
Figure 8.9. Comparison of different Australian Cenozoic APWPs (from Acton & Kettles, 1996). Embleton APWP (region with v pattern), Musgrave APWP (region with dashed line)
and Idnurm APWP (very thick black line). The three triangles are averages from palaeomagnetic data from the rest of the world (excluding Australia) that have been
transposed to the Australian reference frame (Acton & Gordon, 1994). The star, white circle and associated ellipses are the computed pole positions from Acton & Kettles (1996) and is
not of importance here.
8.5. 1997 Field Trip / Sampling
The area chosen for the present palaeomagnetic study was the road section
in the Mason (1982) stratigraphic study near Prospero Trig (Fig. 8.5). This was an
ideal location due to the excellent exposure and the information from the
stratigraphical study. The section is located between the two sections (Semphill
Creek and Stewarts Brook) of the Wellman et al. (1969) study. The start of the
section, EBT (Eocene Barrington Tops) was at the dingo gate at Moonan
Outlook. The section continued west, following the Barrington Tops Forest Road
(Scone – Gloucester Road) down towards Moonan Brook as the road traverses the
western escarpment of the Barrington Tops plateau. The geology and stratigraphy
were as described by Mason (1982) (Section 8.2). Over a distance of about six
192
kilometres we (myself, John Shaw and Tim Rolph) identified 27 distinct lava
flows of which 22 were sampled. Brief field notes are shown in Table 8.5 and
selected field photos in Fig. 8.10.
Table 8.5 Field notes
FLOW (top to bottom)
SITE NOTES
1 2 3 4 5 6 - 8 9 10 11 12 13 14 15 16 17 18 19 20 21
EBT01 EBT02 EBT03 EBT04 EBT05 EBT06 - EBT07 EBT08 EBT09 EBT10 EBT11 EBT12 EBT13 EBT14 EBT15 EBT16 EBT17 EBT19 EBT18 -
Can’t see top of flow. Massive blocks towards bottom of flow. Visible baked contact and clinker of next flow Next flow Small amount exposed Approx 100m down road. Thin, fractured flow. Flow edge exposed. Lots of microfractures in flow. Off road down slope Thin flow missed Down field below EBT06, thin ~3m flow. Back on road. Thin flow, no red contact, think is next flow down but is possibly same as EBT07. More difficult to see contacts between flows now. Next flow down the road Next flow down road. Discrete blocks of lava in flow top rubble. V. thin, badly exposed flow not sampled. By cattle grid. Ankaramite (nodules visible further up road) Discrete blocks in top of flow Same characteristics as EBT13 but flow is horizontal and the road goes down in elevation, so deduce is the next flow. Corner of road. Good exposure of fresh lava, samples taken horizontally along flow. Flow coming towards us – tongues of lava exposed in amongst scoria. Further along road than site EBT18 (road goes up) 1 flow possibly missed
193
22 23 24,25,26 27
EBT21 EBT20 - EBT22
Road goes up from site EBT20. Very thin flow. In soily section, not much exposure. From site EBT19 to site EBT22 minimum of 3 flows missed Along road decrease in altitude ~30m. Blocks in grassy slope, below this are sediments.
An average of seven 2.5 cm diameter cores were taken from each flow
using a petrol powered rock drill. Samples were oriented with a sun compass, or
when there was no sun sited on to a distant point. In the laboratory each core was
cut into, on average, three small cores. This provided one core for thermal
demagnetisation, one for Thellier intensity analysis and one for microwave
studies. Core bottoms were used for rock magnetic analyses.
In addition to the drilled cores, hand samples were also collected from
each flow.
194
Figure 8.10 Field photographs, a) site 2 and 3; b) Tim Rolph drilling site 5, note red top of 6 at base.
a)
b)
195
8.6. Rock Magnetism Two samples per flow (in total 42 samples) were subjected to rock
magnetic analyses.
Table 8.6 Rock magnetic parameters. See text for details of sample type.
sample χlf χlt type RS NRM NRM300°C/NRM Ms Mrs Mrs/Ms Hc Curie Type jh/jc tc1 tc2 TYPE10
-8m
3kg
-110
-5Am
2/kg % Am
2kg
-1Am
2kg
-1mT °C °C
01-02a 1277 1 0.37 1757 7 1.36 0.16 0.12 6.77 4a 0.79 185 525 B01-08a 1141 1 0.17 970 4 1.38 0.15 0.11 4.90 4b 1.07 210 500 B02-04a 1414 1 0.20 61 39 1.46 0.22 0.15 6.91 1a 0.85 278 A02-08a 259 1 0.22 53 43 0.91 0.24 0.26 10.05 1b 0.56 285 A03-01a 937 1/2 0.70 104 91 1.44 0.35 0.24 13.95 4c 1.04 285 565 B03-02a 766 1/2 0.48 14 151 0.87 0.18 0.21 9.68 4c 0.82 278 585 B04-03a 1430 1 0.15 212 4 1.22 0.16 0.13 5.15 1a 0.96 240 A04-07a 1191 1 0.23 2931 3 1.41 0.18 0.13 5.40 4a 1.00 192 495 B05-03a 1682 2 0.74 156 78 2.12 0.47 0.22 12.82 4c 0.94 290 550 B05-04a 1388 1 0.20 53 72 1.96 0.27 0.14 7.54 4a 0.88 290 500 B06-01a 1376 1 0.22 15 260 1.12 0.17 0.15 6.03 1b 0.59 290 A06-06a 826 2 0.81 134 84 1.18 0.19 0.16 19.35 2a 1.05 580 C07-03a 779 2 0.74 101 96 1.33 0.27 0.20 19.73 3 0.97 230 585 C07-06a 1139 2 0.94 107 90 1.46 0.29 0.20 19.60 2b 1.08 558 C08-05a 888 2 0.91 247 90 1.49 0.28 0.19 18.60 2b 1.12 575 C08-07a 894 2 0.94 183 92 1.40 0.29 0.21 21.49 2b 1.10 570 C09-02a 931 1/2 0.42 95 61 1.00 0.19 0.19 9.42 4c 0.93 190 580 B09-03a 841 1 0.17 145 73 0.61 0.10 0.16 5.53 1c 0.66 180 A10-02a 702 3 0.17 395 15 0.07 0.02 0.26 7.79 1c 0.29 190 A10-05a 575 1 0.18 70 20 0.40 0.06 0.16 5.03 1c 0.65 190 A12-06a 417 1 0.24 22 17 0.51 0.07 0.13 4.65 1c 0.82 190 A12-08a 514 2 0.79 63 98 0.99 0.25 0.25 21.49 2b 1.06 555 C13-02a 621 2 1.00 125 75 1.32 0.33 0.25 30.41 2b 1.07 590 C13-07a 448 1 0.25 39 35 0.47 0.08 0.16 5.78 1c 0.54 190 A14-01a 301 1 0.30 51 15 0.21 0.05 0.22 7.26 1c 0.29 185 A14-06a 400 1 0.26 57 20 0.30 0.06 0.19 5.53 1c 0.50 170 A15-01a 412 1 0.19 622 9 0.32 0.07 0.23 6.81 1c 0.31 160 A15-06a 238 1 0.17 405 17 0.22 0.05 0.23 6.79 1c 0.28 150 A16-04a 1174 1 0.09 111 4 1.17 0.08 0.07 3.46 1d 1.09 190 A16-06a 940 1 0.10 95 6 0.88 0.09 0.10 3.64 1d 1.23 170 A17-03a 875 1 0.08 184 4 0.29 0.06 0.22 7.29 1c 0.28 160 A17-04a 828 1 0.12 212 6 0.37 0.08 0.22 7.54 1c 0.24 135 A18-01a 226 3 0.36 116 5 0.14 0.03 0.21 7.54 1c 0.73 240 A18-02a 171 3 0.50 47 9 0.12 0.03 0.21 8.29 1c 0.57 235 A19-01a 956 2 1.03 128 80 1.86 0.52 0.28 28.27 3 0.97 250 580 C19-04a 804 1 0.18 67 34 0.66 0.09 0.13 4.86 4a 0.62 165 520 B20-01a 111 3 0.86 21 34 0.08 0.02 0.26 8.76 1c 0.33 180 A20-05a 102 3 0.87 15 43 0.09 0.02 0.19 9.05 1c 0.32 178 A21-04a 154 3 0.49 28 47 0.09 0.03 0.30 12.19 1c 0.26 190 A21-05a 112 3 0.61 16 54 0.08 0.01 0.15 6.15 1c 0.53 140 A22-04a 1063 1 0.21 235 9 1.16 0.15 0.13 5.65 1a 0.92 270 A22-06a 857 1 0.22 255 3 0.65 0.12 0.19 8.68 1b 0.53 280 A
Table 8.6 lists the main rock magnetic parameters and an assigned sample
classification; type A, B, and C, representing different levels of high temperature
deuteric oxidation. Type A are the least oxidised and type C are the most
oxidised. The classification was initially based on thermomagnetic behaviour
where type A samples contain a single low temperature Curie point; type B
samples contain two discernible Curie points and type C samples are dominated
196
by a single high temperature Curie point. Over half the samples (25) are of type
A, 9 are type B and 8 are type C.
The results from the different rock magnetic experiments are described in
the following sub-sections before a summary of the main results.
8.6.1. Thermomagnetic Experiments Thermomagnetic measurements were carried out using the Curie balance.
The Curie points Tc determined for each sample are listed in Table 8.6 along with
the ratio of the magnetisation on heating and cooling at 100 °C, jh/jc. The curves
have been classified in to different types based on the classification of Mankinen
et al. (1985).
0
100
200
300
400
500
600
700
800
900
1000
0 100 200 300 400 500 600
Temperature °C
Mag
net
isat
ion
AU
Type 1b
0
100
200
300
400
500
600
0 100 200 300 400 500 600
Temerpature °C
Mag
net
isat
ion
AU
Type 1c
0
100
200
300
400
500
600
0 100 200 300 400 500 600
Temperature °C
Mag
net
isat
ion
AU
Type 1d
0
200
400
600
800
1000
1200
0 100 200 300 400 500 600
Temperature °C
Mag
net
isat
ion
AU
Type 1a
Figure 8.11. Type 1 Curie curves. Heating curve is the dark line and cooling curve the lighter line.
Over half (25) of the samples are dominated by a single magnetic phase
with a low Curie temperature ranging from 140 – 290 °C. These low temperature
Curie points are indicative of high Ti titanomagnetite that cooled rapidly so that
no high temperature deuteric oxidation occurred. The curves have been classified
as type 1 Curie curves and are illustrated in Fig. 8.11.
197
There is evidence (type 1a and 1b Curie curves) that low temperature
oxidation has occurred in some flows producing cation deficient titanomagnetite,
titanomaghaemite. On heating, titanomaghaemite inverts to a multiphase
intergrowth of magnetite, ilmenite and other minerals. This is shown in the Curie
curves by the disproportionation peak (type 1b curve) and is diagnostic of highly
maghaemitised samples. The non-reproducibility of the heating curve with an
increase in magnetisation on heating is also an indication of the inversion to a
more highly magnetised magnetite phase. Type 1a curves do not exhibit
disproportionation peaks indicating that the oxidation may be less extreme. Three
samples exhibit type 1a curves, which are similar to the type 1 curve of Mankinen
et al. (1985) and three samples exhibit type 1b behaviour.
The majority (17) of samples exhibiting type 1 Curie curves are of type
1c. A dramatic increase in magnetisation is seen after heating, with magnetisation
values on cooling up to 70% greater at 100 °C. The cooling curve is somewhat
linear with the rise in magnetisation starting at around 520 °C. Disproportionation
peaks are not seen in type 1c curves indicating that low temperature oxidation if
present, is not extreme. The average Curie temperature for type 1c samples is 180
°C, which is lower than the average Curie temperature of 273 °C for types 1a and
1b. As low temperature oxidation is associated with an increase in Curie
temperature, this is further evidence that type 1c samples have not undergone low
temperature oxidation. Type 1c curves are thus indicative of a primary high Ti
titanomagnetite that alters on heating to Ti poor titanomagnetite. The alteration
product contains a range of Curie temperatures indicated by the linear cooling
curves. This is also the case for types 1a and 1b, which also exhibit linear cooling
curves.
The fourth type of type 1 Curie curves; type 1d, are exhibited only by the
two samples from flow 16. These samples exhibit greater reproducibility between
the heating and cooling curves than the other type 1 curves. The cooling curve
crosses the heating curve so at 100 °C the cooling curve is below the heating
curve. These Curie curves are interpreted as being indicative of a primary high Ti
titanomagnetite that has not undergone low temperature oxidation and does not
alter significantly on heating.
198
0
100
200
300
400
500
600
700
800
0 100 200 300 400 500 600
Temperature °C
Mag
net
isat
ion
AU
Type 2a
0
100
200
300
400
500
600
700
800
900
0 100 200 300 400 500 600
Temeprature °C
Mag
net
isat
ion
AU
Type 3
0
200
400
600
800
1000
1200
1400
1600
0 100 200 300 400 500 600
Temperature °C
Mag
net
isat
ion
AU
Type 2b
Figure 8.12. Type 2 and 3 Curie curves. Heating curve is the dark line and cooling curve the lighter line.
Six samples exhibit type 2 Curie curves which are illustrated in Fig. 8.12.
These curves are similar to the type 2 Curie curves of Mankinen et al. (1985). A
single ferrimagnetic phase is present with a Curie temperature between 555 °C
and 590 °C and a decrease in magnetisation after heating. Type 2a curves have
heating and cooling curves that show little or no change in shape and the decrease
in the magnetisation at 100 °C is 10% or less. Only one sample, 06-06, showed
this type of behaviour. The heating and cooling curves show a marked difference
in shape, particularly in the high temperature region in type 2b curves. The
decrease in magnetisation at 100 °C is 12% or less. Five samples exhibited this
behaviour. The magnetic phase producing type 2 curves could be either a primary
low Ti titanomagnetite or may be the result of high temperature deuteric
oxidation of a primary Ti rich titanomagnetite to a Ti poor titanomagnetite
containing ilmenite lamellae. The same magnetic phases are responsible for type
3 curves but with minor variations in magnetic grains or the bulk rock. Type 3
curves differ from type 2 in that they contain a perceptible low Curie temperature
component and the cooling curve crosses the heating curve so that the
magnetisation at 100 °C is higher than before heating. Type 3 behaviour is
199
between type 2 and type 4 behaviour but closer to type 2 than type 4. Two
samples exhibited this behaviour.
0 1 0 0 2 0 03 0 04 0 0
5 0 0 6 0 0
7 0 0 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 T e m p e r a t u r e - 8 C
M a g n e t i s a t i o n A UT y p e 4 a0
1 0 0
2 0 0
3 0 0
4 0 0
5 0 0
6 0 0
7 0 0
0
1 0 0
2 0 0
3 0 0
4 0 0
5 0 0
6 0 0
T e m p e r a t u r e ° C
M
a
g
n
e
t
i
s
a
t
i
o
n
A
U
T y p e 4 b
0
2 0 0 4 0 0 6 0 0 8 0 0 10001200
14000 1 0 02 0 0300 400
5 0 06 0 0 T e m p e r a t u r e ° C
M a g n e t i s a t i o n A U
T y p e 4 c F i g u r e 8 1 3 T y p e 4 C u r i e c u r v e s . H e a t i n g c u r v e i s t h e d a r k l i n e a n d c o o l i n g c u r v e t h e l i g h t e r line. Nine samples
e x h i b i t t y p e 4 b e h a v i o u r i n w h i c h t h e r e a r e t w o d i s t i n c t
ferrimagnetic phases (Fig. 8 13). One phase has Curie temperatures in the range
1 6 5 2 6 2 9 0 °
C a n d t h e o t h e r p h a s e i n t h e r a n g e 4 2 0 2 6 5 8 5
6 0
C. Four samples exhibit
t y p e 4 a b e h a v i o u r w h i c h i s s i m i l a r t o type 4 of Mankinen
et al.
(1985). The curve
i s r e p r o d u c i b l e i n t h e h i g h t e m p e r a t u r e r e g i o n b u t e x h i b i t s a n i n c r e a s e i n
magnetisation at lower temperatures. Only sample 01
- 0 8 e x h i b i t s t y p e 4 b
b e h a v i o u r i n w h i c h t h e c o o l i n g a n d h e a t i n g c u r v e s a r e v e r y s i m i l ar. Four samples e x h i b i t t y p e 4 c c u r v e s w h e r e t h e h e a t i n g c u r v e i s t o t a l l y i r r e v e r s i b l e . T h e h i g h
t e m p e r a t u r e C u r i e p o i n t r e d u c e s a f t e r h e a t i n g a n d t h e c o o l i n g c u r v e i s s i m i l a r t o t h e c o o l i n g c u r v e o f t y p e 2 b c u r v e s .
Type 4 curves are commonly seen where h i g h t e m p e r a t u r e o x i d a t i o n h a s c o n v e r t e d o n l y p a r t o f t h e h i g h T i t i t a n o m a g n e t i t e t o a T i p o o r t i t a n o m a g n e t i t e c o n t a i n i n g i l m e n i t e l a m e l l a e . T h e l o w t e m p e r a t u r e p a r t m a y b e e i t h e r a T i r i c h
t i t a n o m a g n e t i t e ( t y p e 4 b ) o r i f i t h a s u n d e r g o n e l o w t e m p e r a t u r e ox i d a t i o n ,
t i t a n o m a g h a e m i t e ( t y p e 4 a ) . I t i s n o t a l w a y s e a s y t o d i s t i n g u i s h b e t w e e n t h e t w o
200
but maghaematisation is associated with an increase of magnetisation on heating.
The low Curie temperature phase of type 4c could be due to either a Ti rich
titanomagnetite or titanomaghaemite.
The thermomagnetic results indicate that type 1 Curie curves have
experienced the least high temperature oxidation and type 2 and 3 potentially the
most. Hence, the samples are classed as being type A if they contain type 1 Curie
curves, type B if they contain type 4 Curie curves and type C if they contain type
2 or 3 curves.
8.6.2. Hysteresis Experiments The hysteresis parameters Ms, Mrs, and Hc were determined using the
VSM and are listed in Table 8.6. The ratio Mrs/Ms can be used to estimate the
bulk magnetic grain size. The only samples to have Mrs/Ms < 0.1 and hence of
bulk MD grain size are from flow 16. This is the only flow to exhibit type 1d
Curie curves. All other samples have PSD bulk magnetic grain size with 0.11 <
Mrs/Ms < 0.30. The values of Hc also indicate bulk grain size, the values
decreasing as bulk grain size increases. Hc varies from 0.07 mT for one sample
from flow 16 up to a maximum of 30.41 mT for a sample exhibiting type 2 Curie
curve behaviour. The hysteresis results are consistent with a reduction in effective
grain size as a response to high temperature oxidation. This is in agreement with
the thermomagnetic results and sample type classification.
8.6.3. Low Temperature Susceptibility Representative low temperature susceptibility, χLT , results are shown in
Fig. 8.14. The results are generally consistent with the thermomagnetic
interpretations.
Type 1 χLT curves have RS values that range from 0.09 to 0.37. These
curves are similar to the group 1 χLT curve of Senanayake & McElhinny (1982)
which they interpret as indicative of mainly MD Ti rich tianomagnetites. The
isotropic point at –150 °C is suppressed for these samples indicating that the
grains have oxidised (Section 2.1.1.3, Özdemir et al., 1993). Type 1 χLT curves
correspond to type 1 and 4 Curie curve which agrees with the interpretation that
Ti rich titanomagnetite and titanomaghaemite is present.
201
Type 2 χLT curves exhibit the isotropic peak at around –150 °C indicative
of magnetite and MD grains (as they are less prone to oxidation). RS values are
between 0.74 and 1.03. These curves are similar to group 3 of Senanayake &
McElhinny (1982) which they interpret as representing MD Ti poor
titanomagnetite. However, Senanayake & McElhinny (1982) did not consider
mixed grain sizes so a preferred interpretation is that type 2 curves contain SD,
CD (or both) grains as well as MD Ti poor titanomagnetite (Radhakrishnamurty
et al., 1977; Radhakrishnamurty, 1990). Type 2 χLT curves correspond to type 2
Curie curves corroborating the interpretation that Ti poor titanomagnetite is
present.
Type 1/2 χLT curves are a combination of type 1 and type 2 curves. They
exhibit an isotropic peak at around –150 °C and have RS values between those of
type 1 and type 2. The four samples that exhibit type 1/2 χLT curves exhibit type 4
Curie curves.
The increase in RS value from type 1, through type 1/2 to type 2 is
consistent with a reduction in effective grain size as a response to high
temperature oxidation.
A third type of χLT curve; type 3 (Fig. 8.14) was exhibited by 7 samples
from flows 18, 20 21 and one sample from flow 10. A Hopkinson peak is present
at temperatures ranging from –33 to –7 °C which is most likely due to the
presence of haemoilmenite. Haemoilmenites with Curie temperatures in this
region have Ti content 0.7 < y < 0.8 (Dunlop & Özdemir, 1997). All these
samples have type 1c Curie curves and are characterised by low values of
magnetisation and susceptibility. These samples also have significantly lower
values of Ms (~0.1 Am2/kg) and Mrs (~0.02 Am2/kg) than the rest of the samples
indicating a lower concentration of ferrimagnetic material at room temperature.
202
0
0.2
0.4
0.6
0.8
1
1.2
-200 -150 -100 -50 0
Temperature °C
/30
Type 1
Type 2
Type 1/2
0
0.5
1
1.5
2
2.5
3
3.5
-200 -150 -100 -50 0
Temperature °C
/30
Type 3
Figure 8.14. Low temperature susceptibi lity curves exhibiting type 1, 1/2, 2 and 3 behaviour.
8.6.4. Room Temperature Susceptibility Room temperature low frequency susceptibility values are listed in Table
8.5. The frequency dependent susceptibility has been determined from the low
and high frequency room temperature susceptibility measurements (Section
2.1.1.2). The frequencies used look at a small window of grain sizes at the SP /
SD boundary so the influence of smaller SP grains will not be detected. The
frequency dependence was found to be less than 1% in all cases showing that the
contribution of SP grains with grain sizes at the SP / SD boundary is negligible.
203
0
0.5
1
1.5
2
2.5
0 100 200 300 400 500 600
Temperature °C
No
rmal
ised
ro
om
tem
per
atu
re s
usc
epti
bili
ty
Type A 14-01
Type B 05-03
Type C 06-06
Figure 8.15 Typical variation in room temperature susceptibility after heating for sample types A, B and C.
Room temperature susceptibility was measured after heating the samples
during thermal demagnetisation experiments. Fig. 8.15 illustrates representative
results for each of the three sample types. The majority of samples are highly
susceptible to thermal alteration as already seen from the thermomagnetic
experiments. Creation of new magnetic material is indicated by the large increase
in susceptibility. Type A and B samples exhibit an increase in susceptibility
starting at 250 or 300 °C, as expected for inversion of titanomaghaemite and the
alteration of titanomagnetite. Type C samples are more stable with an increase in
susceptibility occurring at higher temperatures, in the region 500 °C and above.
This is as expected for samples having undergone more high temperature deuteric
oxidation.
8.6.5. Stability of NRM
NRM values (listed in Table 8.6) range from 14 to 2931 x 10-5 Am2/kg.
Only three samples have NRM values greater than 900 x 10-5 Am2/kg (the next
highest is 622 x 10-5 Am2/kg). The anomalously high NRM values could be
indicative of an IRM induced by lightning strike. However, it is not usually
possible for an IRM to be removed by 300 °C and sample 04-07, which has the
highest NRM value, loses 97% of its NRM by 300 °C. The average NRM
204
(excluding the three anomalously high values) is 130 x 10-5 Am2/kg. The range of
NRM values found in this study are comparable to the range found by Wellman et
al. (1969).
The stability of NRM to heat, defined as the percentage of NRM
remaining at 300 °C, is listed in Table 8.6. This, in general, corresponds with the
thermomagnetic results indicating that the dominant magnetic minerals seen in
the rock magnetic tests are the dominant remanence carriers. As expected type C
samples are the most stable, exhibiting a maximum of 20% reduction in NRM at
300 °C. Type B samples exhibit a range of stability indicating that the remanence
is either dominantly held in the low Curie temperature phase, the high
temperature phase or else is held in both phases. The majority of type A samples
have lost the bulk of their remanence by 300 °C. Some type A samples however,
show more complicated behaviour. This is most likely due to the remanence
being held in finer magnetic grains not visible from the rock magnetic
experiments.
8.6.6. Summary The 42 samples studied have been divided in to three types A, B and C
relating to their level of high temperature deuteric oxidation, with type A the least
oxidised and C the most oxidised. The majority of samples are of type A and
contain a single low temperature Curie point, have type 1 χLT curves and exhibit
the largest bulk grain sizes. The samples are very susceptible to alteration with
the creation of new magnetic material between 250 and 300 °C. There is evidence
that the Ti rich titanomagnetite has undergone varying degrees of low
temperature oxidation. Type C samples contain a single high Curie temperature
ferrimagnetic phase of a low Ti titanomagnetite and experience less dramatic
alteration on heating. Type C samples exhibit type 2 χLT curves and have the
smallest bulk grain size. Type B samples have oxidation states in between A and
C and contain two ferrimagnetic phases.
Samples that have undergone low temperature oxidation will contain a
CRM and not solely a TRM and hence are unsuitable for palaeointensity analysis.
The tendency for the samples to alter on heating is an additional feature that
renders them unsuitable for conventional palaeointensity analysis. Type C
205
samples are the most promising candidates for any palaeointensity experiments as
they have undergone the greatest high temperature deuteric oxidation (assuming it
was at temperatures greater than the final Curie temperature). The evidence for
low temperature oxidation in these samples is less obvious but is still a
possibility. The results of NRM300°C/NRM indicate that the remanence is
generally held in the magnetic phases dominant in the rock magnetic results.
Reflected light microscopy would undoubtedly aid in the identification of
the magnetic minerals present in these samples, and the extent of both low and
high temperature oxidation. As haemoilmenites are visible under the microscope
the interpretation of the Hopkinson peak observed in type 3 χLT curves could be
verified and it would be interesting to see if discrete haemoilmenite is present in
any of the other samples.
Another complementary rock magnetic experiment that would be
beneficial to this study is IRM acquisition with backfield measurements. With the
determination of Hcr, a Day plot could be produced to confirm the general PSD
bulk grain size of the magnetic minerals. IRM acquisition would allow another
means of discriminating between magnetic phases.
8.7. Directional Analysis Using Thermal Demagnetisation Four samples per flow underwent thermal demagnetisation. Two of the
four samples were sister cores to the samples that underwent rock magnetic
analyses. Four samples per flow is the minimum acceptable for directional
analysis (c.f. Butler, 1992). The temperature of demagnetisation was increased in
50° steps up to 450 °C after which 20 or 25 °C steps were used up to a maximum
of 600 °C.
8.7.1. Demagnetisation Behaviour
Stable characteristic remanence directions (listed in Appendix C) have
been obtained for all samples apart from one sample from site 22. Representative
Zijderveld plots (OVPs) are shown in Fig. 8.16.
Secondary low temperature components of magnetisation, where present,
are generally removed by 200 °C. The low temperature component in most cases
is of random orientation but for a few samples is in the direction of the present
Australian field (e.g. Fig 8.16a). For the majority of samples, the secondary and
206
primary components are clearly distinct, but for a few samples there is a region of
overlapping blocking spectra where the secondary component is seen to swing
round to the converging characteristic remanence (Fig. 8.16b). Some samples
consist solely of a single component of magnetisation (Fig. 8.16c; d), albeit noisy
in the low temperature interval in some cases (Fig. 8.16c).
As expected, demagnetisation behaviour is closely linked to rock
magnetic behaviour. The convergent component on the OVP for some samples
remained up to 350 °C after which the signal became too noisy to yield a
direction (Fig 8.16e). This behaviour is seen for rock magnetic type A samples
that contain a low temperature Curie point. Other samples (including those of
type C) yield convergent components up to 550 °C.
The creation of new magnetic material due to alteration on heating (shown
in the non-reversibility of Curie curves and the increase in room temperature
susceptibility after heating) is also evident from thermal demagnetisation. Many
samples exhibit an increase in intensity at around 400 °C, corresponding to the
additional magnetisation of the newly created material. The direction of
magnetisation at this heating step is anomalous indicating that the newly formed
remanence direction, influenced by the ambient field, is not in the same direction
as the original remanence. However, at the next temperature step the newly
formed remanence is removed and the direction returns to that of the original
remanence. This behaviour is seen as a ‘blip’ on the OVP, seen in Figs. 8.16a; c.
Directions of reversed polarity have been obtained for all sites apart from
sites 1, 4 and 10. Sites 1 and 4 have high NRM values and no viscous component
so it is probable these samples have been affected by lightning strikes. Site 10
does not have anomalously high NRM values but does yield OVP plots with no
viscous component and a very smooth trajectory (Fig. 8.16d). It is therefore
concluded that this site has also been affected by a later overprint.
207
N S
W DOWN
E U
Horizontal
Vertical
SAMPLE EBT106-01A
F I E L D S2 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 4 8 0
5 0 0 5 2 0 5 4 0 5 6 0
0
0.5
1
1.5
2
2.5
3
0 100 200 300 400 500 600Temperature °C
Nor
mal
ised
Inte
nsity
N S
W DOWN
E U
Horizontal
Vertical
SAMPLE EBT107-03A
F I E L D S2 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0
5 5 0 5 7 0
0
0.2
0.4
0.6
0.8
1
1.2
0 100 200 300 400 500 600
Temperature °C
No
rmal
ised
In
ten
sity
N S
W DOWN
E U
Horizontal
Vertical
SAMPLE EBT106-06A
208
F I E L D S2 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 4 8 0
5 0 0 5 2 0 5 4 0 5 6 0
0
0.2
0.4
0.6
0.8
1
1.2
0 100 200 300 400 500 600
Temperature °C
Nor
mal
ised
Inte
nsity
0
0.2
0.4
0.6
0.8
1
1.2
0 100 200 300 400 500 600
Temperature °C
Nor
mal
ised
Inte
nsity
N S
W DOWN
E U
Horizontal
Vertical
SAMPLE EBT110-02A
F I E L D S2 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 4 8 0
N S
W DOWN
E U
Horizontal
Vertical
SAMPLE EBT116-06A
Figure 8.16 Representative Zijderfeld (OVP) plots.
8.7.2. Directional Results
The characteristic remanence directions were deduced from the OVP and
also from stereo projections. The results and associated errors for individual
samples are listed in Appendix C and flow mean directions are listed in Table 8.7.
Some minor variations in the evaluated mean flow directions are seen between
the two approaches. No flow mean could be evaluated for sites 1, 4 and 10 as
after they had been identified as being affected by later overprints they were
discarded, so only two samples per site underwent thermal demagnetisation.
Horizontal
Vertical
d)
e)
100°C
209
Table 8.7 Flow mean directions evaluated using OVP and stereoplot.
OVP stereoplotFlow Site n dec inc α95 k dec inc α95 k
1 12 2 4 243.2 82.6 25.7 13.8 196.2 79.6 17.7 27.83 3 4 206.3 67.7 2.9 1039.5 203.8 67.7 4.1 510.34 45 5 4 163.2 67.6 9.2 101.8 164.2 67.2 10.2 82.96 6 4 186.4 72.1 6.9 179.6 186.0 70.5 6.8 183.278 7 4 200.9 74.5 12.9 51.7 202.2 72.6 11.7 62.99 8 4 174.9 63.5 6.5 200.9 175.7 64.0 5.1 320.910 9 4 199.6 57.7 9.8 89.3 202.8 57.1 8.2 127.411 101213 12 4 167.6 55.4 9.7 90.0 170.2 58.6 6.1 228.914 13 4 182.4 51.7 5.5 279.9 181.5 50.7 5.5 277.015 14 4 188.6 64.5 7.4 157.2 183.4 59.3 4.9 350.816 15 4 190.9 52.6 6.5 202.2 190.7 55.9 5.8 247.817 16 4 187.4 68.4 3.8 571.1 183.7 66.8 6.7 190.718 17 4 189.6 66.0 10.4 79.6 183.0 63.9 9.2 101.519 19 4 189.7 60.8 18.3 26.3 185.9 60.6 16.9 30.420 18 4 205.2 65.3 5.3 305.3 194.3 66.9 9.8 88.12122 21 4 214.4 54.7 9.2 101.1 208.7 60.8 11.0 70.823 20 4 185.9 64.7 19.6 22.9 182.1 65.6 18.7 25.124252627 22 3 226.9 0.1 13.9 81.2 234.4 1.0 15.8 61.7
210
02468
10121416182022242628
0 50 100 150 200 250 300
Declination
Flow
num
ber
02468
10121416182022
242628
-20 0 20 40 60 80 100 120
Inclination
Figure 8.17 Flow mean directions (from OVP). A) declination and inclination through section in stratigraphic order. Solid black line is the overall mean direction (excluding flow 2 and 27), with the dashed line the associated error around the mean. B) Stereo projection of
flow mean directions (circles), triangle is the section mean (excluding flow 2 and 27).
A)
B)
211
Fig. 8.17 plots the flow mean declination and inclination in stratigraphic
order (see Table 8.5) and on a stereo projection. It can be seen that the flow mean
directions are clustered together except for site 22 which has a shallow
inclination. This site direction could be anomalous due to the incomplete removal
of a secondary component, as on the stereo plots the data points tended to lie
along a great circle. This site mean was not included in the evaluation of the
overall section mean. The site 2 mean was not included either in the evaluated
section mean due to the large within flow variation. The mean directional results
for the whole section are given in Table 8.8, with the results of Wellman et al.
(1969) for comparison. The mean direction and palaeo south pole has been
evaluated using the flow means derived from the OVP and from the stereoplots.
Table 8.8 Section mean direction and palaeo south pole determination.
Study N Dec Inc α95 k S Plat Plong Α95 Wellman et al. (1969) 33 193.0 +65.5 3.6 48.47 11.4 -70.5 125.6 5.3
This study (OVP) 16 189.5 +63.6 4.3 76.2 9.3 -74.2 126.4 5.9
This study (stereo projection)
16 187.2 +63.5 3.7 102.4 8.0 -75.2 131.2 5.1
Where k is the precision parameter and S is the angular dispersion.
It can be seen that the three directional evaluations are, within error,
equivalent.
8.7.3 Discussion This study confirms the previous results of Wellman et al. (1969).
However, the two studies differ in the fact that no flows of normal polarity were
found in this study. Only 16 flows were studied compared to the 33 of Wellman
et al. (1969) yet the mean direction and errors are comparable indicating the small
variability between flows.
As this study corroborates the palaeo pole obtained by Wellman et al.
(1969), the question of its validity remains. It has been suggested (Embleton &
McElhinny, 1982) that the direction could be anomalous due to complex
magnetisation of the samples. Hoffman (1984) made a study of the Liverpool
212
volcano, which also gives a questionable palaeo pole. A flow with large internal
variations was investigated using electron microprobe analyses and reflected light
microscopy. It was found that nearly unaltered titanomagnetite extremely rich in
titanium (x > 0.75) was present. Titanomagnetite with x > 0.75 is SP or
paramagnetic at room temperature, so these grains will not obtain a remanence as
the flow cools after extrusion. However, over time, low temperature oxidation of
the extremely high Ti titanomagnetite will produce titanomaghaemite with a
higher Curie temperature. This CRM will be in the direction of the ambient field
at that time, i.e. at an unspecified time after extrusion. This process is the
explanation for samples from one flow that contain reversed, intermediate and
normal polarity directions. The extent of low temperature oxidation is expected to
proceed at varying rates within the flow, resulting in a stable CRM being formed
at different times. In this study of the Barrington volcano there is no comparable
large intra flow variation but there is evidence of low temperature oxidation in
some flows. The majority of flows exhibit type 1c Curie curves which appear to
contain primary Ti rich (x ~ 0.6) titanomagnetite. With a low Curie temperature,
it is feasible that the remanence has been altered due to viscous processes since
Tertiary times. However, there is no significant difference in remanence direction
between flows with low Curie temperatures and those dominated by higher Curie
temperature, Ti poor titanomagnetite. It is therefore not thought likely that the late
acquisition of remanence is likely to have occurred in these Barrington Tops
lavas. However, the small variation between flow directions found at Barrington
Tops could indicate that there was some form of wide spread remagnetisation
event at some point after extrusion. There is no other evidence for this.
Another suggestion for the anomalous pole, advocated by Idnurm (1985a),
is that there is incomplete cancellation of palaeosecular (PSV) variation.
Barrington, Liverpool and Nandewar volcanoes all show successive flows with
similar remanence directions. This directional grouping could be caused by rapid
extrusion of the lavas. If there is episodic extrusion of lava then to eliminate
secular variation SV, unit weights should be assigned to the group means as
opposed to individual flows. At Barrington Tops it was noted that there were
ferruginised layers between some flows, indicating there was a significant length
of time between some flows and not others. However, all the flow means are very
similar with no clear grouping of directions. If the flow means are grouped in
213
some way whilst this will not change the pole position significantly it will
increase the α95 making the pole less anomalous. The total time for the extrusion
of the lava pile at Barrington Tops is not definitively known. It has been
suggested that the spread of K-Ar ages (Section 8.3) could be erroneous (N.
Thomas, pers. comm.) and the lava pile was extruded rapidly. However, the
Wellman et al. (1969) study which included a greater number of flows than the
present study found both normal and reversed directions indicating a longer time
interval than the present study, yet the deduced palaeopoles are the same.
An alternative explanation for the anomalous pole is that significant non
dipole components of the field are present. Global data covering the Cenozoic (0-
65 Ma) indicate that the GAD hypothesis is valid over this time period (Kent &
Smethurst, 1998). However, it has been suggested (e.g. Wilson, 1970; Hailwood,
1977; Livermore et al., 1984; Chauvin et al., 1996) that there was a significant
non dipole component of the geomagnetic field in the Tertiary. Schneider & Kent
(1990) (and previously Coupland & Van der Voo (1980)) examined selected
Tertiary palaeomagnetic data to assess the non dipole component, and concluded
that the non dipolar field was essential quadropolar. The evaluated non dipole
field is not sufficient however, to explain the Barrington pole (Idnurm, 1985a)
(nor for example, does it explain the Tertiary pole for Central Asia obtained by
Chauvin et al. (1996)).
If the dipole field is weak then the non dipole field will have a greater
influence. McFadden et al. (1991) compiled global PSV data through time and
evaluated the relative contributions of primary (dipole) and secondary
(quadropolar) dynamo families. The angular dispersion of PSV for 45-80 Ma
varies between 11° and 24° depending on latitude. The total angular dispersion
(S) from the present study is 9°, so that the between site dispersion is 8° when the
within site dispersion is removed (as described in McFadden et al., 1991). It can
be seen that the 8° dispersion is less than that from the global compilation of
PSV. However, the contribution from the secondary family (which is latitude
independent) is evaluated as 9.7 ± 1.5° which fits more closely to the 8°
dispersion found at Barrington Tops. Therefore the directional grouping could be
a result of extrusion occurring at a time when the amplitude of secular variation
214
(SV) was very small. The strength of the dipole field can be determined from
palaeointensity analysis.
8.8 Conventional Thellier Experiments The Coe (1967a) version of the Thellier technique with pTRM checks
(Section 2.3) has been carried out on sister cores of all the samples used in the
rock magnetic analyses (i.e. two per site). There was no sample pre selection
despite the evidence for low temperature oxidation in some flows. Heating steps
were carried out from 100°C in 50° steps up to 500°C and then 25° steps to
600°C. The laboratory field used was 50 µT.
8.8.1. Results Representative Arai plots with pTRM checks are shown in Fig. 8.18 and
Fig. 8.19. It can be seen that non- ideal behaviour is exhibited by all samples.
Three criteria had to be met for a palaeointensity estimate to be made. Firstly,
there must be a straight-line segment on the Arai plot in the temperature region
that defines the characteristic remanence (taken from the directiona l study).
Secondly, in the selected interval, the direction of magnetisation during the
Thellier experiment should be in the characteristic remanence direction and not
swing towards the direction of the magnetising field. Thirdly, the pTRM checks
should indicate the previous pTRM determination. Only 4 palaeointensity
estimates could be made out of the 42 Thellier experiments that were performed.
The Arai plots are shown in Fig. 8.19 and the results with associated Coe
statistics are in Table 8.9.
Table 8.9 Palaeointensity estimates
Sample N T interval f g σb/b q Ha µT uncertainty05-03 3 100-300 0.32 0.41 0.047 2.79 15.00 5.3414-06 4 250-450 0.20 0.54 0.311 0.35 6.18 17.6020-01 4 200-350 0.49 0.65 0.083 3.84 7.15 1.8621-05 3 200-300 0.12 0.49 0.299 0.20 12.38 63.40
215
01-02
0
0.2
0.4
0.6
0.8
1
1.2
0 0.05 0.1 0.15 0.2 0.25
trm gained
nrm
rem
aini
ng
02-04
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4 5 6 7 8
trm gained
nrm
rem
aini
ng
03-02
0
0.2
0.4
0.6
0.8
1
1.2
0 2 4 6 8 10
trm gained
nrm
rem
aini
ng
04-03
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2 2.5 3
trm gained
nrm
rem
aini
ng06-01
0
0.2
0.4
0.6
0.8
1
1.2
0 2 4 6 8 10 12 14 16
trm gained
nrm
rem
aini
ng
07-03
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2 2.5 3 3.5
trm gained
nrm
rem
aini
ng
08-05
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2 2.5 3
trm gained
nrm
rem
aini
ng05-04
0
0.2
0.4
0.6
0.8
1
1.2
0 2 4 6 8 10 12 14
trm gained
nrm
rem
aini
ng
Figure 8.18 Representative Arai plots (one per site), with pTRM checks (lines).
216
09-03
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2 2.5 3 3.5
trm gained
nrm
rem
aini
ng
10-02
0
0.2
0.4
0.6
0.8
1
1.2
0 0.1 0.2 0.3 0.4 0.5 0.6
trm gained
nrm
rem
aini
ng
12-06
0
0.2
0.4
0.6
0.8
1
1.2
0 2 4 6 8 10
trm gained
nrm
rem
aini
ng
13-07
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4 5 6 7
trm gained
nrm
rem
aini
ng14-01
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4 5 6 7trm gained
nrm
rem
aini
ng
15-06
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2trm gained
nrm
rem
aini
ng
16-06
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4 5trm gained
nrm
rem
aini
ng
17-03
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4 5trm gained
nrm
rem
aini
ng
Figure 8.18 Contd. Representative Arai plots (one per site), with pTRM checks (lines).
217
18-01
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2 2.5 3trm gained
nrm
rem
aini
ng
19-04
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4 5 6 7 8trm gained
nrm
rem
aini
ng
20-05
0
0.2
0.4
0.6
0.8
1
1.2
0 2 4 6 8 10trm gained
nrm
rem
aini
ng
22-04
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2trm gained
nrm
rem
aini
ng
Figure 8.18 Contd. Representative Arai plots (one per site), with pTRM checks (lines).
218
05-03
y = -0.300x + 1.043R
2 = 0.998
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4 5 6trm gained
nrm
rem
aini
ng
21-05
y = -0.236x + 0.868R
2 = 0.913
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2trm gained
nrm
rem
aini
ng
20-01
y = -0.142x + 1.874R2 = 0.986
0
0.2
0.4
0.6
0.8
1
1.2
0 2 4 6 8 10
trm gained
nrm
rem
aini
ng
14-06
y = -0.112x + 0.484R2 = 0.816
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2 2.5 3 3.5 4
trm gained
nrm
rem
aini
ng
Figure 8.19 Samples which produced palaeointensity estimates. Triangles are the accepted points and the dashed line the line of best fit.
219
8.8.2. Discussion
Only 4 out of the 42 determinations gave palaeointensity estimates. The
samples were from flows that did not show evidence of low temperature
oxidation. Two of the palaeointensity estimates use only 3 points and the other
two estimates, 4 points. The estimates are not of high quality as shown by the q
factor and the high uncertainty. However, the dipole appears weak at Barrington
Tops in the Tertiary as the results indicate that the field intensity was probably
around 10 µT, which corresponds to a VDM of 1.3 x 1022 Am2.
The low success (10 %) of these Thellier experiments is consistent with
the rock magnetic results that indicated the high susceptibility to thermal
alteration of the majority of the samples. The non- linear Arai plots and the
extreme failure of pTRM checks are indicators of alteration. The creation of new
magnetic material is also indicated by the formation of CRM in the direction of
the magnetising field used in the Thellier experiment, seen as a swing in the
direction of magnetisation from the characteristic remanence direction towards
the magnetising field direction. The creation of higher blocking temperature
material indicates that the correction methods of McClelland & Briden (1996)
and Valet et al. (1996) would not be suitable.
The success of the Thellier experiments could, perhaps, be improved with
a better choice of temperature steps. A greater number of low temperature heating
steps would be advisable for samples that exhibited the characteristic remanence
direction at low temperatures. Over the temperature interval that produced the
palaeointensity estimates (Table 8.9) better estimates could be made if there were
a greater number of data points. It would also be better to carry out the Thellier
experiments in a laboratory field that is nearer to the estimated palaeointensity.
220
8.9. Microwave Experiments
It is important for the development of the microwave technique that it can
be extended for use with older rocks, in particular ones that contain multi
directional components. Microwave demagnetisation and a preliminary intensity
investigation using microwave demagnetisation / remagnetisation has been
carried out with the Tertiary Barrington Tops basalt.
8.9.1. Microwave Demagnetisation
One standard core per site was selected, and two oriented mini cores from
each standard core underwent microwave demagnetisation. The mini cores were
oriented as described in Section 4.2.1. Selected, representative OVP and intensity
plots of microwave and thermal demagnetisation are shown in Fig. 8.20 and the
evaluated directions listed in Table 8.10. Similar behaviour is shown by thermal
and microwave demagnetisation (e.g Fig 8.20a and f) however, for most samples
the transition between different components of magnetisation is generally more
gradual with microwave demagnetisation (as was also found for some Hawaiian
samples (Section 7.6)). For some samples (Table 8.10 and Fig. 18.20c, d) this
smearing of components means that it is not possible to isolate the primary
component of magnetisation. Only one sample, 04-03 (Fig. 8.20e) exhibited
markedly different behaviour, with the thermal and two microwave experiments
all showing different behaviour.
Table 8.10 Directional results from thermal and microwave demagnetisation.
Thermal Microwave Microwave MeanSample T range D I MAD P range D I MAD P range D I MAD D I α95 k01-08 250-480 126.9 10.3 3.4 8-75 127.2 4.2 1.6 17-50 102.4 -16.9 1.2 119.1 -0.8 31.4 16.502-06 200-300 276.7 74.0 4.0 42-60 230.3 54.6 4.6 29-55 235.3 51.0 5.6 240.6 60.9 25.0 25.503-01 250-540 203.2 67.4 2.5 25-75 249.9 54.6 5.5 17-118 216.3 64.5 2.2 226.6 63.6 20.3 37.904-03 350-450 160.0 38.5 5.905-05 300-575 160.9 74.5 3.006-06 300-560 172.5 68.0 6.3 0-110 147.3 74.3 2.5 25-112 195.6 76.2 2.7 171.0 73.7 11.9 108.307-06 350-550 160.5 72.4 2.7 42-55 158.9 52.9 3.408-07 450-570 164.6 67.6 0.8 55-105 178.4 74.0 1.3 17-138 180.6 72.3 1.2 173.6 71.4 6.7 340.309-02 150-550 212.3 60.2 4.7 42-95 215.7 52.0 3.0 55-110 229.3 50.9 3.5 219.6 54.6 11.2 122.910-0512-08 350-570 180.1 56.0 3.2 37-125 180.9 55.0 1.9 17-110 180.3 56.5 2.8 180.4 55.8 1.2 10280.213-07 300-540 177.1 47.5 2.914-06 250-540 192.6 63.1 2.5 42-105 181.1 79.4 15.215-06 200-500 191.4 48.2 1.5 25-88 177.3 53.5 3.1 25-95 156.0 49.5 1.9 175.0 51.3 18.1 47.516-04 150-480 184.1 68.7 1.1 25-46 213.2 76.6 2.5 17-46 211.6 58.4 4.9 203.1 68.4 16.7 55.317-03 150-500 195.5 72.8 1.5 8-46 158.9 81.1 1.5 8-50 112.8 63.0 1.9 146.9 75.7 24.4 26.718-02 200-400 210.1 64.8 2.0 21-58 196.9 71.9 3.6 17-65 133.2 84.9 5.4 198.2 75.0 19.3 41.819-01 150-540 203.1 59.7 3.0 55-112 261.9 81.9 7.620-05 200-450 137.1 71.0 4.021-04 200-550 212.3 54.8 5.1 125-150 205.9 57.0 11.122-04 200-500 228.8 -4.9 2.2 8-62 224.0 5.8 1.7 8-50 248.5 5.1 4.6 233.7 2.0 22.0 32.4
221
W E
S DOWN
N UP
Hor izonta l
V e r t i c a l
S A M P L E E B T 1 0 1 - 0 8 A
FIELDS20 100 150 200 250 300 350 400 450 480
500 520 540
W E
S DOWN
N U P
H o r i z o n t a l
V e r t i c a l
S A M P L E E B T 1 B T 0 1 0 8 D
FIELDS0 8 12 17 21 25 29 33 37 42
46 50 58 66 75
W E
S DOWN
N U P
H o r i z o n t a l
V e r t i c a l
S A M P L E E B T 1 B T 0 1 0 8 B
FIELDS0 8 17 25 29 33 42 50 55
0 20 40 60
Power Watts
0 20 40 60 80
Power Watts
0 100 200 300 400 500 600
Temperature °C
222
N S
W D O W N
E U
Horizontal
Vert ical
SAMPLE EBT107-06A
F I E L D S2 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0
5 5 0 5 7 0
N S
W DOWN
E U
Horizontal
Vert ical
SAMPLE EBT1BT0706D
F I E L D S0 8 1 7 2 5 3 3 4 2 5 0 5 5 6 0 6 8
7 5 8 8 1 0 0 1 1 2
N S
W DOWN
E U
Horizontal
Vert ical
SAMPLE EBT1BT0706B
F I E L D S0 8 1 7 2 5 3 3 4 2 5 0 5 5 6 2 7 0
8 0 9 2 1 0 5 1 1 8 1 3 0 1 4 2
0 100 200 300 400 500 600
Temperature °C
0 20 40 60 80 100 120
Power Watts
0 50 100 150
Power Watts
N S
W D O W N
E U
Horizontal
Vert ical
SAMPLE EBT114-06A
F I E L D S2 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 4 8 0
5 0 0 5 2 0 5 4 0
N S
W D O W N
E U
Horizontal
Vert ical
SAMPLE EBT1BT1406A
N S
W D O W N
E U
Horizontal
Ver t ica l
SAMPLE EBT1BT1406B
0 100 200 300 400 500 600
Temperature °C
0 50 100 150
Power Watts
0 50 100 150
Power Watts
Figure 8.20c and d.
07-06
14-06
Thermal
Thermal
Microwave
Microwave Microwave
Microwave
Vertical Horizontal
c)
d)
25W
25W
33W
33W
350°C
350°C
150°C
150°C
33W 42W
42W
223
F I E L D S0 8 1 7 2 5 3 3 4 2 5 0 5 8 6 8 7 8
8 8 1 0 0 1 1 2 1 2 5 1 3 8
F I E L D S0 8 1 7 2 5 3 3 4 2 5 0 5 8 6 8 8 0
9 2 1 0 5 1 1 8 1 3 0 1 4 2
W E
S D O W N
N UP
Horizontal
Ver t ica l
SAMPLE EBT104-03A
F I E L D S2 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0
W E
S D O W N
N UP
Hor izonta l
V e r t i c a l
S A M P L E E B T 1 B T 0 4 0 3 D
FIELDS0 8 1 7 2 5 3 3 3 7 4 2 4 6 5 0 5 4
5 8 6 2 7 0 8 0 9 0 1 0 0
W E
S D O W N
N UP
Hor izonta l
Ver t ica l
SAMPLE EBT1BT0403B
FIELDS0 8 1 7 2 5 3 3 4 2 5 0 5 5 6 2 7 0
8 0
0 20 40 60 80
Power Watts0 20 40 60 80 100
Power Watts
0 100 200 300 400 500
Temperature °C
N S
W D O W N
E U
Horizonta l
Ver t ica l
SAMPLE EBT112-08A
F I E L D S2 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0
5 5 0 5 7 0
N S
W D O W N
E U
Horizontal
Ver t ica l
SAMPLE EBT1BT1208A
N S
W D O W N
E U
Horizontal
Ver t ica l
SAMPLE EBT1BT1208B
0 100 200 300 400 500 600
Temperature °C
0 50 100 150
Power Watts
0 50 100 150
Power Watts
Figure 8.20 Representative OVP and intensity plots using thermal and microwave demagnetisation; a) 01-08, b) 18-02, c) 07-06, d) 14-06 e) 04-03 and f) 12-08.
Vertical Horizontal
e)
04-03
12-08
Thermal
Thermal
Microwave Microwave
Microwave Microwave
f)
37W
37W
37W
37W
350°C
350°C
62W
62W
33W
33W
70W
70W
224
Where primary directions were obtainable from both microwave demagnetisation
determinations the mean direction of the three determinations (two microwave
and one thermal) has been evaluated and listed in Table 8.10. The α95 values have
a large range, from 1.2° to 31°. The error is a combination of errors associated
with sample orientation and the uncertainty in the microwave determined
directions. For some samples, only three power steps isolate what is interpreted as
the primary direction.
Table 8.11 Comparison of thermal and microwave directions with no field data included.
Sample Thermal Microwave Difference Microwave Difference D I D I DT-DM IT-IM D I DT-DM IT-IM
01-08 106.6 -9.1 101.3 -12.4 5.3 3.3 101.6 -15.0 5.0 5.902-06 200.0 48.5 219.3 26.5 -19.3 22.0 224.6 27.0 -24.6 21.503-01 178.2 6.0 202.8 0.6 -24.6 5.4 183.6 3.3 -5.4 2.706-06 157.1 16.0 164.6 23.2 -7.5 -7.2 167.0 11.3 -9.9 4.707-06 210.3 61.4 249.5 58.6 -39.2 2.808-07 157.5 -6.0 164.5 -9.5 -7.0 3.5 163.1 -10.6 -5.6 4.609-02 247.0 60.0 215.7 52.0 31.3 8.0 274.0 60.7 -27.0 -0.712-08 128.4 48.8 128.1 47.8 0.3 1.0 128.4 49.3 0.0 -0.514-06 146.6 42.4 168.9 41.0 -22.3 1.415-06 153.5 77.1 192.6 77.3 -39.1 -0.2 228.7 66.9 -75.2 10.216-04 147.8 62.0 158.8 52.3 -11.0 9.7 132.1 49.4 15.7 12.617-03 168.4 61.9 184.7 54.3 -16.3 7.6 220.6 51.1 -52.2 10.818-02 144.7 51.8 158.3 54.3 -13.6 -2.5 182.6 47.8 -37.9 4.019-01 238.2 57.8 273.7 83.2 -35.5 -25.421-04 143.6 26.7 147.0 30.0 -3.4 -3.322-04 25.7 0.5 21.3 11.4 4.4 -10.9 46.0 9.3 -20.3 -8.8
To investigate the sample orientation error, field data has been removed
from the thermal and microwave directions so that the declination and inclination
values as measured by the SQUID and spinner magnetometers can be compared
(Table 8.11). The difference between thermal and microwave declination values
starts at –75°, then –39° up to +31° and the difference between inclination values
range from -25° to +22°. The range of differences in declination is greater than
for the inclination values. This is as expected from sample orientation error, as
the dominant error is in the horizontal plane (see Section 7.6.2). The differences
can be compared to those obtained in the study of Hawaiian lava (Section 7.6.2),
excluding the anomalous –75° declination difference. The differences are listed in
Table 8.12. The range of declination differences is less for the Barrington Tops
study indicating that using the wafering saw, to orient the samples to the
microwave system, is more accurate than the pen line used in the Hawaii study.
The mean declination difference is still significantly negative indicating that the
225
stick and mark on the wall used to orient the samples to the microwave system is
not correct. The range of inclination differences for the Barrington Tops samples
is greater than the Hawaiian study despite extreme care being taken in the drilling
of samples. It is therefore likely that this increased range (and non zero mean) is
indicative of error in the isolation of the primary component and not core
orientation.
Table 8.12 Comparison between the differences in thermal and microwave directions with no field data for the Hawaiian and Barrington Tops studies.
DT-DM IT-IM
N min max mean min max meanHawaii 26 -51 42 -16 -14 18 -0.6Barrinton Tops 28 -39 31 -13 -25 22 3
To summarise, for the Barrington Tops samples that underwent
microwave demagnetisation the presence of secondary components can cause
difficulties in the isolation of the primary component of magnetisation. The
difficulty in removing the secondary components indicates that the microwave
unblocking spectra differ from the thermal unblocking spectra. The orientation of
the samples needs to be improved, in particular the orientation of the sample to
the microwave system, before directions determined with the microwave system
are sufficiently accurate.
8.9.2. Microwave Palaeointensity Study Due to time constraints it was only possible to carry out a preliminary
microwave palaeointensity study. Samples from 8 sites were investigated. Six
samples (two from each rock magnetic group) were chosen that exhibited a stable
characteristic remanence in the expected direction during microwave
demagnetisation experiments. In addition to these samples, a sample (01-08) from
site 01 interpreted as containing an IRM from a lightning strike was investigated.
A sample from core 20-01 also underwent microwave palaeointensity analysis as
this core gave the best estimate of the palaeofield using the Thellier technique.
The ceramic method was used initially so that the directional stability
could be monitored. If the ceramic method produced a successful result the
perpendicular field method was then performed to check for consistency. No
226
alteration checks were carried out. The laboratory field was 20 µT for all
experiments. Unoriented samples were used throughout.
The sample that yielded the best estimate of the palaeofield using the
Thellier method (20-01) was not part of the microwave demagnetisation study so
its behaviour to microwave exposure was not previously known. Unfortunately,
as can be seen in Fig. 8.21 during microwave demagnetisation a stable
characteristic direction of magnetisation was not obtained. It was, therefore, not
possible to obtain a palaeointensity estimate using the microwave technique for
this sample.
N S
W DOWN
E U
Horizontal
Vertical
SAMPLE EBT1BT2001I
FIELDS0 8 17 21 25 29 33 37 42 46
52
Figure 8.21 OVP for sample 20-01 showing that no stable direction is obtained from microwave demagnetisation.
Site 01 is interpreted as having been affected by lightning strike resulting
in a strong stable remanence with no viscous component. A sample (01-08)
underwent palaeointensity analysis using the ceramic method. The results are
shown in Fig. 8.22 along with the Thellier result for comparison. The direction of
the NRM during the experiment, plotted on the OVP, illustrates the stability of
remanence. As the remanence is an IRM, as opposed to a TRM, ideal behaviour
would not be expected for palaeointensity analysis and this was indeed the case. It
52W
52W
8W
8W
227
is interesting to note that during the microwave experiment as the power is
increased the NRM reduces as expected, but the amount of pTRM gained also
reduces. This continues until the last three power steps when the pTRM gained
increases as expected. This behaviour is not seen in the Thellier analysis.
W E
S DOWN
N UP
Horizontal
Ver t i ca l
SAMPLE EBT1BT0108 I
F I E L D S0 8 1 7 2 1 2 5 2 9 3 3 3 7 4 2 4 6
01-08 Ceramic Method
0
500
1000
1500
2000
2500
3000
3500
4000
4500
0 200 400 600 800
TMRM gained
NR
M r
emai
nin
g
01-08 Thellier Method
0
0.2
0.4
0.6
0.8
1
1.2
0 0.1 0.2 0.3 0.4 0.5
trm gained
nrm
rem
aini
ng
Figure 8.22 Comparison of microwave ceramic method and Thellier palaeointensity results for sample 01-08 containing an IRM from lightning strike. The OVP is from NRM
directions during the microwave experiment (unoriented sample).
The two rock magnetic type A samples (16-04 and 18-02) exhibited
anomalous behaviour during the microwave palaeointensity experiment using the
ceramic method. The results are shown in Fig 8.23 with the Thellier results for
comparison and the OVP of the NRM directions during the microwave
experiment. For sample 16-04, during the microwave experiment the NRM
intensity increased for two increasing power steps. This can be seen in both the
Arai and OVP plots in Fig. 8.23a. It is possible that this is due to the incomplete
removal of the TMRM formed in the previous power step. Without the experiment
being repeated with a sister sample it is not possible to say more about this result.
Sample 18-02 produces a good straight- line on the Arai plot however it has a
228
positive slope as opposed to the expected negative slope. For each increasing
power step the NRM decreases as expected, but the amount of TMRM gained for
increasing power decreases. This is similar to the behaviour seen by the sample
01-08 affected by lightning strike and would lead to the conclusion that this
sample could contain an IRM as opposed to a TRM.
16-04 Ceramic Method
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 0.5 1 1.5 2 2.5 3
TMRM gained
NR
M r
emai
nin
g
16-04 Thellier Method
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4 5 6
trm gained
nrm
rem
ain
ing
N S
W DOWN
E U
Horizontal
Vert ical
SAMPLE EBT1BT1604I
F I E L D S0 8 1 2 1 7 2 1 2 5 2 7 3 2 3 5 3 9
4 3
Figure 8.23a
a)
8W
8W
0W
0W
229
18-02 Ceramic Method
y = 0.2831x + 0.04R2 = 0.902
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4
TMRM gained
NR
M r
emai
nin
g
18-02 Thellier Method
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4 5
trm gained
nrm
rem
aini
ng
W E
S DOWN
N UP
Horizontal
Vert ical
SAMPLE EBT1BT1802I
F I E L D S0 8 1 7 2 1 2 5 2 7 3 1 3 5 3 9 4 5
5 2 5 8 6 5
Fig. 8.23 Comparison of microwave ceramic method and Thellier palaeointensity results for type A rock magnetic samples a) 16-04 and b) 18-02. OVPs are of NRM directions during
the microwave experiment (unoriented samples).
The results from the two samples from rock magnetic group B are shown
in Fig. 8.24. Sample 09-02 failed to give a palaeointensity estimate however an
estimate could be made from sample 03-01.
Sample 09-02 exhibited behaviour similar to 01-08 which contains an
IRM (Fig. 8.24a). For the first four power steps the TMRM gained decreased with
increasing power, but for the final three power steps TMRM gained increased.
However, from the OVP it can be seen that the smoothest trajectory, converging
to the origin is obtained for the final four power steps. The previous three power
steps (used in the palaeointensity experiment) do not extend the trajectory and
thus could be due to a secondary component of magnetisation. This would
explain the anomalous Arai plot, but even if the first three points on the Arai plot
are discarded the remaining points do not produce a good straight line. It is
therefore not possible to obtain a palaeointensity estimate from this sample.
27 W
65 W
b)
8W
8W
230
Sample 03-01 behaved well during microwave palaeointensity analysis
using the ceramic method and also using the perpendicular field method (Fig
8.24b). The palaeointensity estimates and associated quality factors are listed in
Table 8.13. The perpendicular method gave a higher estimate, 18.5 ± 2.4 µT, than
the ceramic method estimate, 10.7 ± 1.2 µT. The discrepancy between the two
estimates could be due to a number of factors including sample anisotropy. No
palaeointensity estimate could be made from the Thellier experiment result;
indeed there is no straight line segment on the Arai plot.
N S
W DOWN
E U
Horizontal
Ver t i ca l
SAMPLE EBT1BT0902I
F I E L D S0 4 0 4 2 4 5 5 0 5 8 6 5 7 2 8 2 9 0
09-02 Ceramic Method
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2 2.5 3 3.5
TMRM gained
NR
M r
emai
nin
g
09-02 Thellier Method
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2 2.5 3 3.5
trm gained
nrm
rem
ain
ing
Figure 8.24a
45 W
65 W
72 W
82 W
90 W
a)
65W
65W
231
03-01 Ceramic Method
y = -0.536x + 1.427R2 = 0.998
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2 2.5
TMRM gained
NR
M r
emai
nin
g
03-01 Perpendicular field method
y = -0.922x + 2083R
2 = 0.989
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2 2.5
TMRM gained
NR
M r
emai
nin
g
03-01 Thellier method
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4 5
trm gained
nrm
rem
ain
ing
W E
S DOWN
N UP
Horizontal
Vert ical
SAMPLE EBT1BT0301I
F I E L D S0 8 2 5 3 3 4 5
5 5 6 2 7 0 8 0
8 5 9 2 9 8 1 0 2 1 0 8
Fig. 8.24 Comparison of microwave and Thellier palaeointensity results for type B rock magnetic samples a) 09-02 and b) 03-01. OVPs are of NRM directions during the microwave
ceramic method experiment (unoriented samples).
The two samples with group C rock magnetic characteristics exhibited
similar behaviour to microwave palaeointensity analysis. Both samples produced
palaeointensity estimates as shown in Fig. 8.25 and Table 8.13. The
palaeointensity estimates evaluated using the ceramic, and the perpendicular field
methods produced consistent results. It is interesting to note that the Arai plot
produced using the Thellier method exhib its a straight- line segment with a
gradient that would indicate a palaeointensity similar to that derived from the
microwave analysis for both samples. The segment is composed of only 3 points
for sample 08-07 and 4 points for sample 06-06. The pTRM checks over the
temperature intervals in question fail completely. Without alteration checks being
carried out for the microwave analysis it is not possible to say conclusively that
b)
Fe = 10.7 ± 1.2 µT
Fe = 18.5 ± 2.4 µT
80W
80W
232
no alteration has occurred during experimentation. However the consistency
between the two microwave results using different methods, and the quality of the
estimates lead me to believe that they do give a good indication of the
palaeointensity of the samples.
Table 8.13 Palaeointensity estimates using microwave palaeointensity analysis
Sample Method N P interval f g σb/b q Ha µT Uncertainty03-01 Ceramic 9 55-108 0.58 0.69 0.018 22.0 10.74 1.18
perp field 7 60-92 0.76 0.78 0.047 12.6 18.54 2.3708-07 Ceramic 8 62-108 0.76 0.79 0.033 18.5 14.33 1.63
perp field 6 70-92 0.71 0.77 0.029 18.6 14.02 1.5906-06 Ceramic 5 50-98 0.34 0.39 0.043 3.1 17.02 5.74
perp field 11 50-115 0.73 0.75 0.018 29.7 17.58 1.86
The success of the microwave palaeointensity analysis in this preliminary
study is related to the rock magnetic properties of the samples, with the samples
exhibiting the greatest high temperature deuteric oxidation (type C) yielding the
greatest success. No palaeointensity estimate was obtained from the two type A,
low temperature Curie point samples. One of the type B samples produced an
estimate whilst the other failed. The palaeointensity estimates obtained using the
microwave technique are similar to the ones obtained using the Thellier technique
providing further indication of a low dipole moment in the early Tertiary.
It is desirable to extend the microwave palaeointensity study to include
many more samples and to implement alteration tests. If this is carried out then a
much better idea could be obtained of the strength of the magnetic field recorded
by the Barrington Tops lava.
233
08-07 Perpendicular Field method
y = -0.700x + 1.206R2 = 0.997
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2
TMRM gainedN
RM
lost
08-07 Ceramic Method
y = -0.714x + 1.077R2 = 0.994
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1 1.2 1.4
TMTM gained
NR
M r
emai
nin
g
W E
S DOWN
N UP
Horizontal
Vert ical
SAMPLE EBT1BT0807I
F I E L D S0 1 7 3 3 5 0 5 5 6 2 7 0 7 5 8 0 8 8
9 5 1 0 0 1 0 8
08-07 Thellier method
y = -0.260x + 0.903
R2 = 0.993
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4
trm gained
nrm
rem
ain
ing
06-06 Ceramic Method
y = -0.848x + 1.067
R2 = 0.994
0
0.2
0.4
0.6
0.8
1
1.2
0 0.1 0.2 0.3 0.4 0.5 0.6
TMRM gained
NR
M r
emai
nin
g
Perpendicular field method
y = -0.878x + 0.998
R2 = 0.997
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1
TMRM gained
NR
M r
emai
nin
g
N S
W DOWN
E U
Horizontal
Vert ical
SAMPLE EBT1BT0606I
F I E L D S0 8 1 7 3 3 4 8 5 8 7 0 8 0 8 2 9 0
9 2 9 8 1 0 2 1 1 0
06-06 Thellier method
y = -0.322x + 0.932R2 = 1.000
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4
trm gained
nrm
rem
aini
ng
Fig. 8.25 Comparison of microwave and Thellier palaeointensity results for type C rock magnetic samples a) 08-07 and b) 06-06. OVPs are of NRM directions during the microwave
ceramic method experiment (unoriented samples).
Fe = 14.3 ± 1.6 µT
Fe = 14.0 ± 1.6 µT 0.26 x 50 =13
Fe = 17.0 ± 5.7 µT
Fe = 17.6 ± 1.9 µT 0.32 x 50 = 16
a)
b)
234
8.10. Summary The study area, Barrington Tops, was chosen to extend Australian and
early Tertiary palaeomagnetic data. The samples proved to be very interesting
rock magnetically, but not as suitable for conventional palaeointensity analysis as
had been hoped.
The majority of the flows studied are dominated by what appears to be a
primary high Ti titanomagnetite that converts to a nearer magnetite phase on
heating. Some flows show evidence of having undergone varying degrees of low
temperature oxidation. A few flows contain a single Ti poor titanomagnetite
thought to result from high temperature deuteric oxidation. The mineral magnetic
study could be extended to include reflected light microscopy.
The directional study using thermal demagnetisation confirmed the results
of the previous palaeomagnetic study by Wellman et al. (1969). Little variation is
seen between flow means, giving a section mean declination of 190°, inclination
of 64° and α95 of 6°. The corresponding palaeopole has been questioned as
anomalous when compared to data from sediment sequences and the Indian
APWP. Additional palaeomagnetic studies from different locations are required
to gain a better understanding of the geomagnetic field in Australia in the early
Tertiary.
The rock magnetic analyses indicate that the majority of samples are
highly susceptible to thermal alteration, thus rendering them problematic for
conventional Thellier palaeointensity analysis. The success was extremely low (as
predicted) however, a crude estimate from 4 samples suggested that the intensity
of the field was weak. More success was had with microwave intensity analysis
using samples in which the primary component of magnetisation could be
isolated with microwave demagnetisation, but only a preliminary study has been
carried out. Hence, no firm conclusions regarding the field intensity can be made
until the microwave study is extended. Importantly though, the dipole field
appears weak. This may explain the anomalous palaeopole as the non dipole field
will have greater influence when the dipole field is weak. This is in agreement
with previous studies (e.g. Wilson, 1970; Hailwood, 1977; Livermore et al.,
1984; Chauvin et al., 1996) that suggested the presence of a significant non
dipole component of the geomagnetic field in the Tertiary.