ELSEVIER Journal of Volcanology and Geothermal Research 74 (1996) 89-99

JOurnalOf~OlOgy andgeothermalmeamh

In situ chemical fractionation in thin basaltic lava flows: examples from the Auckland volcanic field, New Zealand, and a

general physical model

William Rogan a’ *, Stephen Blake b, Ian Smith a

a Department of Geology, The Uniuersiry of Auckland, Private Bag 92019, Auckland, New Zealand b Department of Earth Sciences, The Open Uniuersity, Walton Hall, Milton Keynes, MK7 6AA, UK

Received 3 March 1995; accepted 30 July 1996

Abstract

Basaltic lava flows of the Auckland volcanic field, northern New Zealand, commonly contain near-vertical cylinders and sub-horizontal sheets of a rock more coarse grained and vesicular than the host. The cylinders and sheets, referred to collectively as pegmatoid autoliths, are l-8 cm in diameter/thickness. The flows in which they occur are all less than 10 m thick and of pahoehoe type. Pegmatoid autoliths are enriched in most elements except Cr, Ni, Mg and Ca which are depleted, and Si, Al, and Sr which remain unchanged with respect to the host flow. Pegmatoid autoliths represent in situ chemical fractionation of a basaltic lava flow. Mass-balance calculations show that pegmatoid autoliths have the same composition as the interstitial liquid after 36-50% crystallisation of the host. The segregation process requires a rigid permeable crystal framework through which the interstitial liquid moves by gas filter-pressing. A physical model of gas filter-pressing in a cooling lava is developed and predicts that a combination of high permeability, low melt viscosity and thick lava should favour the segregation of interstitial melt. A review of geological observations shows that the occurrence of pegmatoid autoliths conforms with the model’s predictions.

Keywords: segregation: segregation vesicle; segregation vein; vesicle cylinder; Auckland volcanic field

1. Introduction

Lava flows are generally regarded as crystallised sheets of magma which show variation in physical characteristics such as jointing (e.g., DeGraff and Aydin, 1987), surface texture (e.g., pahoehoe, aa, blocky), crystallinity and vesicle distribution (e.g.,

* Corresponding author. Present address: Department of Earth

Sciences, The Open University, Walton Hall, Milton Keynes,

MK7 6AA, UK. Fax: +44 1908 655151. E-mail:

w.j.rogan@open.ac.uk.

Philpotts and Lewis, 1987; Aubele et al., 1988; Sahagian et al., 1989; Walker, 1989). Many lava flows are compositionally homogeneous, but some are compositionally variable as a result of tapping a heterogeneous chamber (e.g., Bacon, 1986; Camp et al., 1987; Smith, 1992) or in situ differentiation by crystal settling (e.g., Mathews et al., 1964), liquid immiscibility (e.g., Philpotts, 1982) or liquid segre- gation (e.g., Kuno, 1965; Smith, 1967; Wright and Okamura, 1977; Anderson et al., 1984; Helz, 1987; Puffer and Horter, 1993, and references therein). In the latter category are lava flows which contain

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90 W. Rogan et al. / Journal of Volcanology and Geothemal Research 74 (1996) 89-99

“inclusions’ ’ of conspicuously vesicular and

coarser-grained material which, although texturally distinct from the host flow, have mineralogical simi-

larities and systematic compositional contrasts indi-

cating a consanguineous relationship. The term

“ vesicle cylinder” has been applied to near-vertical

cylindrical forms of these inclusions (Goff, 1977)

and these are commonly associated with and inter-

sect sheet forms known as vesicle sheets or segrega-

tion veins. Individual vesicles which are wholly or

partly filled with such material are known as segre- gation vesicles (Smith, 1967). Vesicle cylinders and

sheets are not to be confused with other vesicular

features of lava flows, for instance pipe vesicles

(Philpotts and Lewis, 1987). The features discussed

in this study are distinguished by the fact that they

are composed of vesicular rock, rather than being vesicles themselves. The term “pegmatoid autolith”

is used in this study to collectively refer to vesicle

cylinders, vesicle sheets, and segregation veins. They

are described as “pegmatoid” because in all in-

stances in this study they are coarser grained than the

host flow, and “autoliths” because they are derived from within the lava flows in which they occur.

Pegmatoid veins found in the Whin Sill in the

north of England (Tomkeieff, 1929) and the Pal-

isades Sill in New Jersey (Steiner et al., 1992) are

similar to vesicle sheets but have no associated

cylinders. Cylinders with no associated sheets have been observed in the Rattlesnake Mountain Sill,

western Texas (Carman, 1994). Studies of pegmatoid

autoliths show that their composition is chemically differentiated from that of the host flow. Pegmatoid

autoliths are the result of in situ fractionation and

auto-intrusion of a lava flow, and as such they offer insights into the processes of crystallisation and dif-

ferentiation of magma. In Sections 2, 3, 4 we de-

scribe the distribution and petrology of pegmatoid autoliths that occur within lava flows of the Auck-

land volcanic field in northern New Zealand. In Section 5 we derive a general model for segregation by gas filter-pressing which is shown to compare favourably with observations from Auckland and

elsewhere.

2. Field observations and petrography

The Auckland volcanic field is a young (Late Pleistocene-Recent) intraplate basaltic field contain-

ing 49 monogenetic eruption centres (Smith, 1989). Early activity from each centre is typically explosive

(phreatomagmatic or Hawaiian to Strombolian) and

later stages are characterised by effusion of lava

flows. The magmas erupted by the Auckland volca-

noes are most commonly alkali basalt and basanite,

with subordinate nephelinite and less commonly

transitional to tholeiitic basalt. Within many centres

there are clear differentiation patterns which trend from alkali basalt to basanite (Smith, 1992).

Vesicle cylinders and sheets are found in some of

the lava flows of the Auckland volcanic field. For

this study we examined nine well exposed flows. Three flows contained vesicle cylinders, and two

contained vesicle cyliners and vesicle sheets (Table

1). The flows containing pegmatoid autoliths range

in thickness from 3 to 10 m and are of pahoehoe

type. Other pahoehoe flows examined did not con- tain autoliths, and no occurrence in aa flows was

observed. Apart from surface morphology, no other

field parameter, e.g., thickness, substrate, flow length,

correlated with the presence of pegmatoid autoliths.

Data from four flows are presented here.

Vesicle cylinders are sub-circular in cross section

(Fig. 1), and sheets are roughly planar, and usually,

but not always, sub-horizontal. The boundaries of pegmatoid autoliths with the host lava are sharp and

well defined. The cylinders are for the most part

vertical, and occur between about 15 cm above the base of a flow to 75 cm below the flow surface,

regardless of the thickness of the flow. When a

Table 1

Localities visited during the course of this study, and the features

found at each

Volcano

Pupuke

Rangitoto

Three Kings

Mt Wellington

Wiri

Locality

Thome Bay

Smale’s Quarry

Islington Bay Quarry

Navy Quarry

Western Springs

Penrose

Winstone’s Quarry

Railway Quarry

Rosconnnon Quarry

features

vc, vs

vc, vs

vc, vvs

“C ‘I

vc

vc = vesicle cylinder; vs = vesicle sheet; vvs = vertical vesicle

sheet. a Only one vesicle cylinder found in 100 m of lateral exposure at

the Railway Quarry. Difficulty of access precluded sampling.

W. Rogan et al./ Journal of Volcanology and Geothermal Research 74 (1996) 89-99 91

Fig. 1. Photograph of a horizontal section through a vesicle cylinder from the Islington Bay Quarry on Rangitoto Volcano. Scale in

centimetres.

vesicle cylinder is traced upward it commonly devi- ates by up to 20” from the vertical. Near the top of a flow, vesicle cylinders may intersect vesicle sheets (Fig. 21, and in flows without sheets, vesicle cylin- ders are commonly capped by a cavity of the same diameter as the cylinder. Near-vertical vesicle sheets are also observed at the same level as the lowest vesicle cylinders in some flows. These sheets have no obvious systematic arrangement.

Vesicle cylinders range in diameter from 2 to 8 cm with an average of approximately 3 cm and are spaced at intervals between 15 cm and 2 m or more.

Flow surface

Vesicle cylinder

Fig. 2. Idealised cross section through a lava flow containing

pegmatoid autoliths.

The distance between vesicle cylinders within a given flow is more or less constant, and there is a crude positive correlation between vesicle cylinder diame- ter and spacing. Vesicle sheets are approximately 3 cm thick. Calculation of the total volume of pegma- toid autoliths shows that only 1 to 4% of a flow is segregated magma.

Basalts of the Auckland volcanic field are charac- teristically porphyritic and contain phenocrysts of olivine and less common clinopyroxene in a ground- mass of plagioclase, clinopyroxene, subordinate olivine and minor iron-titanium oxides; nepheline occurs in some of the more undersaturated rocks. All flows that contain pegmatoid autoliths have a dikty- taxitic texture, i.e., they have intercrystalline voids. This texture indicates that interstitial liquid was drained from a network of interlocking crystals. Peg- matoid autoliths essentially contain the same mineral phases as their hosts, but in different proportions and usually of more evolved compositions. The abun- dance of olivine decreases markedly from host to pegmatoid autolith and pyroxene is more abundant than in the host. Phenocrysts of olivine are rare in the pegmatoid autoliths. These have the same com- position and size ( = 0.4 mm across> as phenocrysts in the host flow, and are interpreted as having grown in the host flow, and then been entrained into the

92 W. Rogan et al. / Journal of Volcanology and Geothermal Research 74 (1996) 89-99

autolith. Plagioclase and pyroxene crystals in the pegmatoid autoliths are 0.2-0.5 mm in length, which is approximately 1.5 times their size in the host flow. Plagioclase crystals in the pegmatoid autoliths are commonly aligned parallel to the host/autolith boundary, a feature also noted by Goff (1977). Iron- titanium oxide phases are titanomagnetite and il- menite (determined by microprobe), and are more abundant in the pegmatoid autoliths where they are commonly acicular, but also occur as skeletal rhombs. Pegmatoid autoliths have a vesicular&y of approxi- mately 30%; calculated using densities:

autolith vesicularity = 1 -

autolith( bubble - free)

=l- 1840kgmp3

2660kg me3 = 0.3

Densities were calculated by weighing samples in air and immersed in water. Bubble-free density was calculated with water-saturated samples, and the den- sity of the rock as a whole was calculated by weigh- ing samples covered in plastic film when in water. The vesicularity is therefore an estimate of the total void space, and pegmatoid autoliths have values two to five times those of their hosts. The bubble-free

density of pegmatoid autoliths is slightly higher than that of the host flow (2660 kgm-” compared to 2600 kgmp3 for the host flow).

3. Geochemistry

The host lava flows in this study have the chemi- cal composition of alkali basalt or olivine tholeiite. Samples from the top, middle and bottom of each flow did not reveal any significant compositional variation. This contrasts with the vertical variation found in certain much thicker flows of flood basalts which contain vesicle sheets (Philpotts et al., 1995). The major- and trace-element geochemistry of peg- matoid autoliths shows small but significant differ- ences from their host flows (Table 2). With respect to their host flows, pegmatoid autoliths are consis- tently depleted in Mg, Ni, Cr and Ca, and enriched in most other components (Fig. 3). The variable nature of the enrichment patterns of Nb, Ti and V can be attributed to different amounts of opaque minerals in the host and pegmatoid autolith pairs. These elements are partitioned into opaque phases. The host flows have chondrite-normalised REE pat-

4

3

5 X E .‘: 2 a

z

1

0

Fig. 3. Plot of autolith compositions normalised to that of their host flow, illustrating their compositional relationship. The x-axis is arranged

in order of decreasing autolith compatibility, calculated using average values for autolith/host. The Rb value for one of the Rangitoto

autoliths is suspiciously low (see Table 2), and has therefore been omitted from this diagram. The horizontal line separates enrichment

(above the line) from depletion. Circles represent Rangitoto, squares Pupuke, and diamonds are Wiri.

W. Rogan et al. /Journal of Volcanology and Geothermal Research 74 (19%) 89-99 93

Table 2

Whole-rock major- (XRF) and trace-element (REE by ICP-AES, others by XRF) compositions of pegmatoid autoliths and their host flows

Pupuke Volcano wiri Volcano Rangitoto Volcano

Locality: Thome Bay Smale’s Quarry Roscommon Quarry Islington Bay

AU cat #: 44521 44522 44524 44525 44536 44538 44528 44530 44531

wt. % SiO,

TiO,

Al,% FeO’Otal

MnO

MgO CaO

Na,O

J&G

p205

H,O-

LO1

Total

ppm V

Cr

Ni

Cu

Zn

Rb

Sr

Y

Zr

Nb

Ba

La

Ce

Nd

Sm

EU

Gd

DY Yb

Lu

Pb

Th

Minerals Olivine

Plagioclase

Pyroxene

48.23 49.26 45.89 47.14 44.36 45.45 49.18 50.56 53.92

1.77 3.48 2.18 2.73 2.59 2.67 1.88 4.02 2.46

13.71 12.93 13.38 14.1 12.79 15.4 14.19 12.56 13.06

13.58 15.95 13.59 14.31 14.13 13.36 12.64 15.91 14.69

0.18 0.23 0.2 0.23 0.16 0.2 0.17 0.24 0.18

10.04 4.38 10.71 7.38 11.27 5.98 9.45 3.34 2.03

9.38 9.17 9.54 9.14 10.19 8.85 9.15 7.65 5.99

2.45 3.33 2.97 3.74 2.62 3.71 2.54 3.86 4.49

0.54 1.05 0.86 1.31 1.08 1.88 0.68 1.48 2.23

0.22 0.44 0.4 0.64 0.44 0.76 0.24 0.59 0.94

0.12 0.12 0.39 0.34 0.21 0.57 0.15 0.25 0.3

-0.7 - 0.55 0.09 - 0.26 0.06 0.71 - 0.66 - 0.74 - 0.55

99.52 99.79 100.2 100.8 99.9 99.54 99.61 99.72 99.74

190 302 187 165 233 149 188 326 123

317 22 317 182 412 162 294 nd 12

256 40 258 155 247 93 204 20 12

86 144 84 129 80 98 79 132 144

116 150 111 131 126 132 106 164 178

5 10 8 17 15 29 14 29 15

300 298 448 466 544 622 336 283 318

19 37 22 30 22 31 22 46 60

107 217 149 223 170 248 136 306 450

16 31 31 50 44 74 15 33 44

100 199 180 245 251 410 121 237 317 10.0 20.1 24.1 36.9 15.1 35.6 13.0 29.6 44.5 22.0 42.6 45.9 72.9 50.1 68.1 28.6 64.7 99.5 13.5 25.0 23.3 35.8 26.0 33.6 16.9 36.3 56.3 3.4 6.7 4.9 7.7 5.5 6.8 4.3 9.3 13.8 1.3 2.1 1.8 2.6 1.9 2.3 1.4 2.8 3.8 3.7 6.7 4.9 7.0 4.9 5.9 4.3 8.9 12.4 3.3 6.1 3.8 5.7 4.0 4.9 3.9 8.3 11.6 1.4 2.5 1.5 1.8 1.3 1.7 1.6 3.2 4.6 0.2 0.4 0.2 0.2 0.2 0.2 0.2 0.4 0.6

nd nd nd nd nd nd nd 9 5 nd 5 nd 6 nd 8 nd 4 4

Fo 80-66 An 60-52

Fo 78-21 An 50-27

Fo 84-76 Fosz-72 Fo 84-81 An 64-58

Fo 84-79 An 71-58

Wo47%2

Wo5+n39

Fo 79-75 An 66-35

Wo4,En44

Wo5,En3,

Fo 40-25 An 43-37

Wo4,En4,

Fo 40-25 An 43-37

Wo37En43

Wo45En36 Wo&n43 Y&n39 Wo43En38 Wo40En32 Wo&n39

Host flows are the left hand column in each set of analyses. nd = not detected. End-member mineral compositions give the range of

compositions. Full microprobe analyses available on request.

94 W. Rogan et al. / Journal of Volcanology and Geothermal Research 74 (19961 89-99

Table 3

Mass-balance modelling of fractionation using the Thome Bay host and autolith compositions

Fraction Host 01 Plag CPX UlVii Liquid Autolith %diff

0.18 0.21 0.11 0.00 0.50

SiOz 48.23 31.66 54.60 50.00 49.26 49.26 0.00 TiO, 1 .I1 I .42 19.00 3.24 3.48 0.07 Al&‘, 13.71 28.63 I .07 4.30 15.10 12.93 0.17 FeO’ 13.58 24.20 14.44 73.08 15.51 15.95 0.03 MnO 0.18 0.40 0.20 0.22 0.23 0.05 MgO 10.04 37.41 11.37 4.38 4.38 0.00 CaO 9.38 0.35 II.56 19.41 9.54 9.17 0.04 Na,O 2.45 4.53 0.42 2.90 3.33 0.13 K,O 0.54 0.19 1 .oo I .05 0.04 P,% 0.22 0.44 0.44 0.00 Sum 100.10 99.62 99.50 98.12 96.58 101.41 100.22 0.53

K 0.54 0 0.2 I 0.01 0 I .05 I .05 0.00 Sr 300 0 2.52 0.13 0 41 I 298 0.38 Ba 100 0 0.38 0.00 0 190 199 0.05 Y 19 0 0.02 0.37 0 31 31 0.00

Mineral compositions are averages of the observed range as determined by microprobe analysis. The trace-element mineral values given are

D values; plagioclase values come from Blundy and Wood (1991) (Sr and Ba) and an unpublished parameterisation of experimental results

on K partitioning (Rogan, work in progress), and pyroxene values from Hart and Dunn ( 1993) (K, Sr. Ba) and unpublished paramerisation

of experimental results on Y partitioning (Rogan, work in progress).

terns showing variable LREE enrichment

[(La,/Yb,) 4.8-131 and insignificant Eu anomalies.

In contrast, although the pegmatoid autoliths are

considerably REE enriched with respect to their host

flows, the chondrite-normalised patterns of the peg-

matoid autoliths and hosts are sub-parallel, with

La,/Yb, ratios only slightly higher than their hosts.

The pegmatoid autoliths have Eu anomalies which

are slightly lower than those of their hosts.

The composition of minerals in pegmatoid au-

toliths overlaps those in the host flow, but the most primitive compositions in the pegmatoid autoliths are

more evolved than the most primitive in the host

flow (Table 21. The more evolved mineral and chemical composi-

tions of the pegmatoid autoliths with respect to the

host flows suggest that they are related to the host magma by fractional crystallisation. We propose that the magma which was originally erupted, fraction- ated and separated into pegmatoid autoliths and host. Since only 1 to 4% of a flow is segregated material the host composition should be a good approxima- tion of the initial, unfractionated composition. Mass-balance modelling using average major-ele- ment phenocryst compositions from the host indi-

cates that 36 to 50% crystallisation is required to yield the pegmatoid autoliths’ compositions (e.g.,

Table 3). These results are corroborated by enrich- ments in highly incompatible elements (Fig. 3) and

fractional crystallisation modelling of Sr and Ba

(Table 3). This modelling shows that pegmatoid

autoliths are the host flow minus the crystals that formed first, i.e., they represent an interstitial liquid.

4. Discussion

Our observations of the physical and chemical

properties of pegmatoid autoliths and their host flows place the following constraints on any model that

attempts to explain their existence: After 25 to 54%

crystallisation of the lava, a small amount of intersti- tial liquid was able to collect in vertical cylinders, and in some cases horizontal and or vertical sheets, which are entirely confined to the interior of the flow. The material that fills these bodies is two to five times more vesicular than the host flow, and is more coarsely crystalline than the host. The forma- tion of these bodies leaves no vertical chemical zonation in the host flow, and the only distinctive

W. Rogan et al./ Journal of Volcanology and Geothermal Research 74 (1996) 89-99 95

physical characteristics of these flows are that they solidification, we proceed by comparing the rates of are pahoehoe type and have a diktytaxitic texture. these competing effects.

The mass-balance modelling shows that the segre- gated liquid is actually the interstitial liquid of the host lava, and the diktytaxitic texture tells us that the liquid was somehow removed from between the crystals. If this was achieved by compaction (Phil- Potts et al., 1995), crystal settling (Gibb and Hender- son, 1992), flow differentiation (Komar, 19721, or compositional convection (Tait and Jaupart, 1992; Car-man, 1994), the segregation process would result in compositional zonation in the host body, and there would be no diktytaxitic texture. Vertical sampling of autolith bearing flows failed to highlight any zonation of the flow, meaning these processes can be ruled out. Gas filter-pressing (Anderson et al., 1984; Sanders, 1986) offers a means by which liquid can be moved from between crystals, and explains how the diktytaxitic texture is produced.

A liquid of viscosity p flows through a porous medium of porosity 4 and permeability k at a speed, U, given by a modified version of Darcy’s Law (e.g., Sleep, 1974):

kP’ UC-

u4 (1)

where P’ is the pressure gradient driving the flow. Models of permeability-porosity relationships for porous media composed of mono-sized spherical or cylindrical grains are such that k/4 + 0 for 4 -+ 0 (Dullien, 1979; Higdon and Ford, 1996); other ge- ometries should behave similarly such that Eq. (1) predicts decreasing speeds with decreasing porosity. If the pressure gradient arises from a pressure differ- ence AP across a distance L, then P’ = AP/L and the time for flow through the porous medium is:

5. Gas filter-pressing

Anderson et al. (1984) have argued that the pro- cess of gas filter-pressing best accounts for the pres- ence of segregation vesicles within diktytaxitic host lavas. In this process, a crystallising vesicular lava reaches a stage where the crystallinity is sufficient for a rigid permeable network of crystals to have formed. This is most likely to be achieved in re- sponse to the undercooling which results from erup- tive degassing (Sparks and Pinkerton, 1978; Sato, 1995). Continued cooling and growth of anhydrous minerals causes further vesiculation of the residual melt. This effervescence forces vesicular residual melt to flow through the permeable matrix into low pressure regions such as cracks (forming segregation veins) and large vesicles (forming segregation vesi- cles) (Anderson et al., 1984; Sanders, 1986). Ander- son et al. (1984) calculated that this would produce pressure differences of about 0.1-0.5 MPa and that these are sufficient to drive residual melt through inter-crystalline capillaries before the lava solidified. We now seek a more general physical model to define the conditions under which segregation can occur. Recognising that segregation of differentiated melt is driven by porous flow and inhibited by

L L2Ud 4lcxv N ; - -

kAP (2)

This time can be compared with the timescale for lava at a distance D from a cooling surface to solidify:

D2 ‘solidify _ -

K (3)

where K is thermal diffusivity. The solidification time which is relevant to the problem of liquid segregation is the time taken for the lava to cool from the temperature at which it acquires a perme- able matrix to a lower temperature at which the permeability is effectively negligible. In the-case-~of Makaopuhi lava lake, Hawaii, Wright and Okamura (1977) estimated that differentiated liquid was able to segregate between 1070 and 1030°C. Their tem- perature-depth-time data for the lava crust show that the time taken to cool between these tempera- tures depended on position according to:

t= 1.14 x 105D2

or:

0.034D2 t=

K

96 W. Rogan et al. /Journal of Volcanology and Georhemal Research 74 (1996) 89-99

using a typical value for K from Keszthelyi (1994) of 3 X lo-’ m* s-‘. This is in good accord with the

parameterised solidification timescale given by Eq.

(3). It follows that segregation is favoured if tflow <

‘solidify 1 i.e., when:

kAPD* p>l ‘&‘L2

(4)

Inequality (4) expresses the intuitive result that porous flow, and hence segregation, occurs faster

than solidification in thick, highly permeable lavas

containing residual melt of low viscosity.

To test inequality (4) requires each physical quan-

tity to be evaluated. The most problematic quantity is

the permeability, k. For mono-sized particles, perme- ability can range over several orders of magnitude

and increases with grain size and porosity. For exam-

ple, when 0.35 < 4 < 0.70, k = 0.18d245,5 for

spheres of diameter d (Dullien, 1979) and k = 0.32d2+5.0 for lattices of long cylinders of diameter

d (calculated from fig. 4b of Higdon and Ford, 1996). However, the groundmass of a lava comprises

a wide range of crystal sizes and shapes, and a

general formula for k for such mixtures is not avail-

able (Dullien, 1979). This precludes a fully quantita-

tive test of inequality (4) although qualitatively a

large grain size, high porosity and a narrow range of

crystal sizes will produce the most permeable crystal

network (Pettijohn, 1975; Dullien, 1979). The amount

of crystallisation required to achieve a rigid frame-

work will also depend on these textural factors, but

studies of lava lake crusts lead geologists to consider

50% crystallisation as a reasonable amount (e.g., Wright and Okamura, 1977; Wright and Peck, 1978)

so C$ _ 0.5. Lava thickness defines the maximum

possible value for D and is readily measured in the

field; values of less than lm to several hundred

metres are possible. The melt viscosity, p, can vary from about 10 Pas for hot basaltic liquid to about 10’ Pas for cool rhyolitic residuum. Of the other variables in inequality (4) K is not expected to vary by more than an order of magnitude between differ- ent lavas (Delaney, 1987; Keszthelyi, 1994) the spacing between sinks for residual melt is con- strained by observations of the spacing of cylinders as L _ 0.1 to 1 m, and AP N lo5 Pa (Anderson et

al., 1984). Thus, the three most variable physical

parameters which play a role in determining whether

segregation will occur are predicted to be lava thick-

ness, the crystallisation dynamics which influence

the texture of the groundmass (Lofgren, 1980) and

hence its rigidity and permeability, and melt viscos- ity.

In the Auckland lavas, the role of flow depth is

expressed by the observation that the vesicular up-

permost and lowermost parts of autolith-bearing lavas

are devoid of segregation features. The outermost 0.5 m of these flows are interpreted to have cooled

sufficiently fast to prevent porous flow. Similar ob-

servations were made by Anderson et al. (1984) in

high-alumina basalt lavas. Puffer and Hotter (1993)

noted that the thickest and most abundant segrega- tion veins are found in some of the thickest lava

lakes, lava flows and sills. This can be accounted for by the fact that the interiors of thick magma bodies

take the longest time to cool, giving more opportu-

nity for segregation to occur.

Our model predicts that the most permeable lavas

should favour the segregation of interstitial melts. In

the Servilleta Basalts of New Mexico, Dungan et al.

(1989) report that the flows with the coarsest grain

size tend to be the ones in which segregation vesicles are best developed. Because permeability increases

with grain size, all else being equal, the simplest

interpretation is that these are the most permeable

flows.

The permeability of the groundmass matrix is

determined by its texture, but the texture also influ- ences the bulk rheology of the lava and hence the

pahoehoe-aa transition (e.g., Kilbum, 1990; Sato,

1995). Field and laboratory evidence indicates that aa flows are favoured by high undercoolings which

cause high crystal nucleation densities and an entan-

gled network of small crystals producing high appar- ent bulk viscosities. The large number of small

crystals will produce a relatively impermeable crys- tal network which will not be conducive to rapid segregation of interstitial melt. In contrast, pahoehoe flows can have a more open groundmass structure produced by lower nucleation densities at low under- cooling. This will be a relatively permeable structure and, therefore, more likely to allow melt to segregate rapidly. The notable correlation between the occur- rence of pegmatoid autoliths and pahoehoe host lava

W. Rogan et al./.loumal of Volcanology and Geothermal Research 74 (1996) 89-99 91

in the Auckland flows may be best accounted for by

this argument that the textures which favour pahoe-

hoe also favour segregation. Qualitatively, thin sec- tions of the Auckland lavas sampled for this study do

not reveal any obvious differences in the crystal

density in the groundmasses of flows with and with-

out segregation features. A systematic quantitative

study of samples from throughout different lavas are

required to test this hypothesis further, although it

must be remembered that permeability is not the

only variable which influences segregation rates. The viscosity of interstitial melt is a function of

the bulk composition of the lava and the amount of

crystallisation, which in turn determines the porosity

and permeability. In a lava of given composition, the viscosity of the residual melt will increase with

increasing crystallisation and hence decreasing per-

meability such that the left hand side of inequality

(4) will become smaller with time. The conditions in

a lava flow may, therefore, change from being suit-

able for segregation to being unsuitable. Anderson and Gottfried (1971) provide a possible example of

this where they describe high-alumina basalt lavas

containing segregations of basaltic and basaltic an-

desite composition formed by 30 to 70% crystallisa-

tion. The same lavas contain a rhyolitic residual

glass (formed by more than 90% crystallisation) but

this had not segregated, presumably due to its very high viscosity and the low permeability of the nearly

totally crystallised lava. Similar relationships are dis-

played in Hawaiian lava lakes where basaltic liquids have been squeezed into fractures but rhyolitic liq-

uids have remained trapped (e.g., Wright and Peck,

1978). The viscosity of the segregated liquids from

the Auckland lavas have been calculated, by the

method of Shaw (19721, at a temperature calculated using the model of Nielsen and Dungan (1983) (this

model was found to predict liquidus mineral compo-

sitions which are in good agreement with the ob- served compositions in the segregates). The viscosity

of the most silicic liquid is 520 Pas, which is in the

range of viscosities of segregated liquids calculated for Hawaiian lava lakes (50-1000 Pas). All other

Auckland segregated liquids have much lower vis- cosities (lo-60 Pas) as a result of their basic and alkaline composition. This will have assisted these

liquids to segregate quickly from the thin lava flows. A summary illustration of the competing thermal

and fluid dynamic controls embodied in inequality

(4) is given by observations of certain pillow basalts made by Bideau and Hekinian (1984). Traversing

from rim to core the crystallinity became larger and

coarser because of decreasing cooling rates. Segrega-

tion vesicles had formed where the crystallinity was

greater than about 50% and was best developed in

the centre of the pillow, where the coarse permeable

texture had persisted the longest. The conditions under which gas filter-pressing is

predicted to be most effective are present in flows

which contain segregation features.

6. Conclusions

Vesicle cylinders and sheets, segregation veins

and segregation vesicles are evidence of the fact that

lava flows can and do fractionate after eruption. We

propose the following model of the segregation pro-

cess:

Basaltic magma is erupted and feeds a vesicular

lava flow. The crust of the flow cools rapidly, pre-

venting the escape of gas from the flow interior. The

lava crystallises to the point where it forms a rigid

crystal framework, and volatiles exsolve from the

residual liquid. The growth of small bubbles forces interstitial liquid into zones of lower pressure (gas

filter-pressing, Anderson et al., 19841, such as large

vesicles and cracks. In the completely frozen flow,

these are recognised as segregation vesicles and seg- regation veins, respectively. Large cavities which

cap some vesicle cylinders have been interpreted by

Philpotts and Lewis (1987) as the accumulation of

bubbles at the top of a molten vesicle cylinder.

Vertically elongate vesicles are observed in some lava flows, and the vesicle cylinders capped by a

cavity may represent partially filled vesicles of this

type. Gas filter-pressing can operate in any lava with a

rigid permeable matrix and vesicular interstitial melt, but the efficiency of liquid segregation depends on the competing effects of porous flow and solidifica- tion. A model of these effects indicates [inequality (411 that an optimal combination of high permeabil- ity, low melt viscosity and thick lava are most

98 W. Rogan et al. /Journal of Volcanology and Geothermal Research 74 (1996) 89-99

favourable for segregation to take place. This is compatible with our observations.

Acknowledgements

Many thanks to Ritchie Sims and John Wilmshurst

(University of Auckland) for technical support dur-

ing the analytical part of this study, and to the quarry

owners and managers for allowing access to their

sites. We also thank Bruce Marsh, Kathleen

Schwindinger and three other referees for their help- ful comments.

Gaff, F.E., 1977. Vesicle cylinders in vapour differentiated basalt

flows. Ph.D. Thesis, Univ. California, Santa Cruz, CA.

Hart, S.R. and Dunn, T., 1993. Experimental clinopyroxene/melt

partitioning of 24 trace elements. Contrib. Mineral. Petrol.,

113: l-8.

Helz, R.T., 1987. Differentiation behavior of Kilauea Iki lava

lake, Kilauea volcano, Hawaii: an overview of past and cur-

rent work. Geochem. Sot. Spec. Pub]., I: 241-258.

Higdon, J.J.L. and Ford, G.D., 1996. Permeability of three-dimen-

sional models of fibrous porous media, J. Fluid Mech., 308:

341-361.

Keszthelyi, L., 1994. Calculated effect of vesicles on the thermal

properties of cooling basaltic lava flows. J. Volcanol.

Geotherm. Res., 63: 257-266.

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