a study of macro-rhythmic layering and cumulate processes

A Study of Macro-Rhythmic Layering andCumulate Processes in the Jimberlana Intrusion,

Western Australia. Part I: The Upper Layered Series

byl. H. CAMPBELL

School of Geology, University of Melbourne, Parkville, Victoria 3052, Australia1

{Received 14 August 1975; in revised form 30 March 1976)

ABSTRACT

The upper layered series of the Jimberlana Intrusion rests unconformably on the lowerlayered series and has resulted from a major new pulse of magma which entered the magmachamber during the final stages of crystallization of the lower series. All parameters whichvary systematically with fractionation are sharply reversed at the contact between the twolayered series.

The lower portion of the ultramafic zone of the upper layered series is composed of a re-peated macro-rhythmic succession of olivine cumulates, bronzite-olivine cumulates, andbronzite cumulates. A detailed investigation was made of this sequence so that the contactsbetween the units could be compared with the contact between the upper and lower layeredseries. Cu, Ni, Cr, P, and U were measured on a whole rock basis; the Mg/Mg+Fe ratiosof the pyroxenes and olivines, the Cr content of pyroxenes and the Ni content of olivineswere determined by the electron microprobe. Each of the macro-rhythmic units is associatedwith a reversal in mineral variation and in the Ni-Cr trends and has a zone of high sulphidevalues at its base. These features are also found at the contact between the upper and lowerlayered series and, if the upper series is due to a new influx of magma, it follows that themacro-units are also due to multiple injection. This conclusion is strongly supported by thesize of the reversals which are too large to be explained by the alternative hypotheses tomultiple injection suggested by Jackson (1961) and Wager (1959).

The anomalous sulphide-rich zones at the contacts between the upper and lower layeredseries and between the macro-rhythmic units are thought to have formed from a narrowsupercooled zone which developed for a brief period at the bottom of the magma chamberfollowing each new magma pulse. The olivines from these sulphide-rich zones are depletedin Ni, suggesting that Ni has been scavenged by the sulphides. This is only possible if theolivine grains crystallized with the sulphides in the restricted supercooled zone at the bottomof the magma chamber.

INTRODUCTION

MACRO-CYCLES of crystallization or macro-rhythmic layering are a prominentfeature of many layered intrusions including the Stillwater Complex of Montana(Jackson, 1961), the Great Dyke of Rhodesia (Worst, 1960), the Bushveld Com-plex of South Africa, and the Rhum Intrusion of Northwestern Britain (Wager& Brown, 1968). A typical macro-rhythmic unit is 30-300 m thick and beginswith the crystallization of olivine cumulates at its base passing upwards grada-tionally or abruptly into bronzite (e.g. Stillwater) or plagioclase (e.g. Rhum)

1 Present address: Department of Geological Sciences, Queen's University, Kingston, OntarioK7L 3N6, Canada.IJouraal of Petrology, VoL 18, P«rt 2, pp. 83-215, 1977|

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184 I. H. CAMPBELL

cumulates at the top. Brown (1956) showed that this type of layering contrastedin scale with the fine-scale rhythmic layering of Duluth and Skaergaard andhence required a different mechanism from that of crystal sorting by convectioncurrents. Although macro-rhythmic layering has been widely recognized anddocumented, its origin remains uncertain and has been explained by three con-trasting hypotheses.

The first, suggested by Cooper (1936) and developed by Brown (1956) andIrvine & Smith (1967), is that the units are due to periodic influxes of undifferenti-

Zone of crystallisation '

Pile of accumulated crystals

vto

ZoneNof crystallisation^

Pile of accumulated

)

— - ^

crystals

B

1100 1105 1110 1115 1120TEMPERATURE(°C)

1125

FIG. 1. Diagrammatic illustration showing the relationship between the adiabatic temperaturegradient and the melting-point gradient with increasing pressure (thickness). According to Jackson(1961), crystallization occurs from a stagnant zone of magma below the intersection of the twocurves (Fig. 1 A) and continues until the released latent heat of crystallization raises the temperatureof the stagnant layer to the melting-point temperature gradient (Fig. 1B). The zone of crystallizationis now at a super-adiabatic temperature and is in a favourable condition to join the convection of the

main body of magma (Jackson, 1961, Fig. 91, modified).

ated magma. Each influx mixes with the fractionated magma in the chambermaking it more ultrabasic and raising its temperature, resulting in a repetitionof the mineral assemblage.

The second hypothesis was proposed by Jackson (1961, p. 93-9) for the form-ation of the macro-rhythmic units of the Stillwater Complex. Jackson noted thatthe mineral melting-point gradient in a thick sill is greater than the adiabatictemperature gradient. He suggested that crystallization occurs near the bottomof the magma chamber from a layer of supercooled magma below the point ofintersection of the two curves (Fig. 1). The crystals which form within this layer


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MACRO-RHYTHMIC LAYERING AND CUMULATE PROCESSES 185

raise its density and prevent it convecting. The remainder of the magma is freeof crystals and convects readily. The magma chamber therefore divides into two,a convecting upper part and a stagnant lower part. Continued crystallization ofolivine from the lower stagnant layer of magma raises its SiO2 and lowers itsMgO content until, eventually, olivine is replaced by bronzite as the stableliquidus phase. The cycle ends when the released latent heat of crystallizationraises the temperature of the stagnant layer to that of the melting-point curve,making further crystallization impossible. The stagnant layer is now at a super-adiabatic temperature and, when crystal settling is complete, is in a favourablecondition to join the convection of the main body of magma. The cycle is re-started when a major convectional overturn sweeps away the stagnant layer andreplaces it with magma, higher in MgO and lower in SiO2, from the convectingupper layer.

A third hypothesis has been suggested by Wager (1959) and Wager & Brown(1968, p. 294-5) as a viable alternative to multiple injection for explaining themacro-rhythmic layering of Rhum and Bushveld. They believe that the type ofmacro-layering found in these intrusions may result from the order of nucleationof the cumulus phases from a supercooled magma. Their hypothesis applies tothe particular type of macro-rhythmic layer found in these intrusions and is notintended to have general application.

The problem in deciding between these hypotheses has been the lack of aclear-cut example for which one hypothesis could be established beyond ques-tion, and then used as a basis for comparison in more difficult cases. The con-tact between the upper and lower layered series of the Jimberlana Intrusion,Western Australia, is a positive example of multiple injection (Campbell et al.,1970). The purpose of this project was to study that contact and to identify theparameters which are indicative of this process. These parameters will then beused to decide whether or not the macro-rhythmic layers found in the upperlayered series are also due to multiple injection.

ANALYTICAL TECHNIQUES

The minerals were analysed on a 'Geoscan' electron-probe microanalyser andthe intensity data were corrected by the computer program of Mason et al.(1969). Most of the analyses were partial, using the methods 'pyroxene','olivine', and 'feldspar' (Mason et al., 1969) in which only Fe, Mg, Ca (±Cr)in pyroxene; Fe, Mg (±Ni) in olivine; and Ca, Na, K in plagioclase were meas-ured. The program calculates the remaining elements for correction purposes,making the assumption that the minerals are stoichiometric. Analyses werecarried out at 15 kV using a beam focussed to a diameter of approximately 2 p.The presence of fine exsolution lamellae in the pyroxenes presented an analyticalsampling problem which was overcome by analysing 7 points 10 fj. apart on eachof 3 grains in the case of a Ca-poor pyroxene and 10 points on each of 3 grainsin the case of a Ca-rich pyroxene. Each analysis is therefore considered to be a


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186 I. H. CAMPBELL

close approximation to the average composition of the grain including thethe exsolution lamellae.

Cu and Ni were analysed by atomic absorption spectroscopy at the Kal-goorlie laboratory of Western Mining Corp. Ltd. Inter-laboratory checks madeby Dr. J. Ross indicate that the results are accurate to within ±15 per cent. Crwas determined by X-ray fluorescence.

The U determinations were carried out at Aldermaston (U.K.) using themethod of Gale (1967). Henderson et al. (1971) found that the precision of thismethod was ±10 per cent in orthocumulates and ±20 per cent where the Ucontent was very low.

P was analysed colorimetrically by a modification of the 'heteropoly blue'

LAKE COWAN

/ / / • NORSE MACowan t-// \

LAKE JOHNSON

L EGEND

Upper layered series

Lower layered series

Marginal series

S.W. AUSTRALIA

FIG. 2. Location diagram and plan of the Jimberlana Intrusion showing the distribution of the com-plexes and the arrangement of the layered series.

method (Boltz, 1938, p. 32). Samples were run in batches of twenty-seven whichincluded two standards provided by Henderson (1968). The results agree withHenderson's and have a precision of ±10 per cent at 100 ppm P.

GENERAL GEOLOGY OF THE JIMBERLANA INTRUSION

The Jimberlana Intrusion is a layered, horizontal, pipe-like body (McClay& Campbell, 1976), similar in many respects to the Great Dyke of Rhodesia(Campbell et al, 1970). It is 180 km in length, 5-5 km thick, up to 2-5 km wide,and is located near Norseman, Western Australia.


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At several points along its length the intrusion widens into canoe-shapedcomplexes which are joined by a narrower connecting pipe (Fig. 2). The intru-sion is subdivided into three layered series; the lower layered series, the upperlayered series, and the marginal series. The lower layered series is made up of analternating sequence of olivine cumulate and bronzite cumulate layers overlainby a thick plagioclase-augite-hypersthene cumulate layer. Layering at thecentres of the complexes is horizontal but it steepens towards the edge of theintrusion, finally approaching vertical at the margin. The marginal series occupiesthe lower part of the intrusion, surrounding the lower layered series at this leveland filling the connecting pipe. It shows reversed fractionation in that the olivineand bronzite cumulate layers overlie the plagioclase-augite-hypersthene cumulatelayer and, in this respect, is similar to the marginal series in the Muskox Intru-sion (Bhattacharji & Smith, 1964). The upper layered series, which is found nearthe top of the intrusion, is like the lowerHayered series and consists of a repetitive

UPPER LAYERED SERIES

LOWER LAYEREDSERIES

Granophyrlc layar

|- j Plagloclaae-auglt*

-hyperathene cumulate

Bronzite cumulate

flTTI Olivine cumulate

FIG. 3. An idealized cross-section ,of Jimberlana showing the distribution of the major rock typesand the relationship between the layered series. The diagram has been simplified by omitting themacro-rhythmic layering in the ultramafic zone of the upper layered series and by showing only two

macro-rhythmic units in the lower layered scries.

sequence of olivine cumulate and bronzite cumulate layers overlain by a plagio-clase-augite-hypersthene cumulate layer and a granophyric layer. The upperlayered series is considerably smaller in volume than the lower layered seriesand its layering is nearer horizontal. The relationship between the three layeredseries and the geometry of their phase layering is shown in Fig. 3.

THE CONTACT BETWEEN THE UPPER AND LOWER LAYERED SERIES

Geochemical trends have been established for the lower layered series (Camp-bell & Borley, 1974). The Mg/Mg+Fe ratio and Cr content of the pyroxenes all


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188 I. H. CAMPBELL

decrease with fractionation. These trends, however, are sharply reversed at thelower contact of the upper layered series but rapidly become re-establishedhigher in the series (Fig. 4). The magnitude of these reversals and the change inthe cumulus mineral assemblage at the contact are too great to be explained by

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FIG. 4. Mineral and trace element variation curves for the upper and lower layered series. The layeringhas been simplified as in Fig. 3. A full line in the mineral variation curve indicates that the phase iscumulus and a dotted line indicates that it is intercumulus. The dotted line in the trace element curves

Mg(Ni and Cr) gives the composition of the liquid. En = in Ca-poor pyroxene. The layers areFe+Mgmarked at the top of the diagram with letters indicating the cumulus phase. Thus OCL = olivinecumulate layer, BCL = bronzite cumulate layer, and PAHCL = plagioclase-augite-hypersthenecumulate layer. UZ = ultramafic zone, ULS = upper layered series, and LLS = lower layered series.


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convectional overturn or ordered nucleation from a supercooled melt. The onlyhypothesis which can account for a reversal of this magnitude is that the magmachamber has been modified by a major new influx of magma. If this is correct,any anomalous feature associated with the contact between the upper andlower layered series may be indicative of multiple injection. Of particular im-portance in this regard is the zone of anomalously high sulphides (indicated byhigh Cu-Ni contents, see Fig. 4) found at the contact.

Detailed mapping of the contact between the upper and lower layered serieshas revealed many minor irregularities and has shown that the layering of theupper layered series is oblique to that of the lower layered series. These featuresindicate that the two series are unconformable, a fact confirmed by drill holeND1 (Fig. 5).

One other feature of the contact between the upper and lower layered series isnoteworthy. Within 30 m of the contact, there is an abundance of gabbro-andesite dykes. These dykes have unchilled margins and are thought to haveresulted from partial melting of the lower layered series but, because they cut the

Rhythmiclayering

UPPER LAYERED

LOWER LAYERED . . ^ = ^ = SERIESSERIES

Scale (metres)

FIG. 5. A cross-section of the Norcott Complex through drill hole ND1 showing the relationshipbetween the upper and lower layered series. Symbols as for Fig. 3. The plagioclase-bronzite cumulate

layer is indicated by *.

upper layered series, they cannot have formed until an appreciable amount ofthat series had crystallized. At the time of formation of the gabbro-andesitedykes, both layered series had recently crystallized and were just below theirmelting points. The upper series, because it formed at a higher temperature, washotter and therefore lost some of its heat through the lower layered series. Thisheat was sufficient to melt some of the lower temperature components of thelower layered series, resulting in the partial melting required for the productionof these dykes.


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190 I. H. CAMPBELL

PETROLOGY OF THE ULTRAMAFIC ZONE OF THE UPPER LAYERED

SERIES

Petrography

The ultramafic zone has been subdivided into four macro-rhythmic units asshown in Table 1. Due to the unconformity between the layered series, the bot-tom of unit 4 is not exposed at the surface or in drill hole ND1 (Fig. 5).

TABLE 1

Stratigraphic succession of the upper layered series

Layers

1. Granophyric layer2. Plagioclase-augite-hypersthene cumulate layer

3. No. 1 bronzite cumulate layer

4. No. 2 Bronzite-plagioclase cumulate layer5. No. 2 bronzite cumulate layer6. No. 2 olivine cumulate layer

7. No. 3 bronzite cumulate layer8. No. 3 olivine cumulate layer

9. No. 4 bronzite cumulate layer

LOWER LAYERED

Symbols

PAHCL

BCL-1

BPCL-2BCL-2OCL-2

BCL-3OCL-3

BCL-4

SERIES

Units

Unitl

Unit 2

Unit 3

Unit 4

Zones

Mafic zone

Ultramaficzone

Bronzite is a cumulus mineral in all layers except the olivine cumulate layersin which it forms poikilitic intercumulus grains up to 5 mm in diameter. Thenormal grain size of cumulus bronzite is 1-2 mm but there are two exceptions.First, near the contact with the lower layered series, slight chilling has reducedits size to 0-5-10 mm. Secondly, in the vicinity of the bronzite-plagioclasecumulate layer, bronzite occurs in two distinct grain sizes, 0-5-10 mm and2-3 mm (Plate IB). The larger grains have a higher En content than the smallerones (Fig. 6) and when plotted on the pyroxene quadrilateral, the direction oftheir tie-lines (Fig. 7) indicates that they are not in equilibrium with augite fromthe same sample.

Partially serpentinized olivine and small amounts of chromite occur togetheras cumulus minerals in all rocks below the bronzite-plagioclase cumulate layer.Chromite occurs as separate grains up to 005 mm in diameter or as inclusionsin the other mineral phases. It is most abundant in rocks with a high olivinecontent and shows evidence of resorption at all levels. Many of the olivinegrains have also undergone resorption which partly explains their wide rangeof size (0-2-2-0 mm).

Plagioclase and augite are intercumulus phases except in the bronzite-plagioclase cumulate layer. In this layer, plagioclase is cumulus but the relation-ship of augite to the other minerals is uncertain.


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WO SO 0 -50

HEIGHT IN SEQUENCE (METRES)

FIG. 6. Variation in mineral composition, the P and U contents of the rocks, and textural type,plotted against height in the sequence. Note the P and U values are expressed in jigm/cc to indicatethe amount of liquid trapped between the cumulus grains (see Henderson, 1970). The large bronzitegrains referred to in the text are indicated by large dots. The letters indicating the texture are: C = cu-mulate, D = dual grain-size, P = poikilitic, and T = transitional. Vertical lines indicate the range in

compositions of four grains and the horizontal bars indicate the mean. En = •=—, \ . in both Ca-Fe+Mg

rich and Ca-poor pyroxenes. The marking of the layers follows the convention adopted in Fig. 4except that the units are numbered. Thus BCL-2 = No. 2 bronzite cumulate layer.

Many rocks are strongly orthocumulate and contain up to 2-3 per cent late-stage minerals of which biotite, apatite, titaniferous magnetite, and quartz arethe most important.


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192 I. H. CAMPBELL

Textural types

Two main textural types are recognized in the ultramafic zone. The first ischaracterized by poikilitic intercumulus plagioclase, bronzite, or augite grainsup to 7 mm in diameter (Plate 1 A). Augite and bronzite develop as reaction rimsaround several cumulus olivine grains which show strong evidence of re-sorption. However, resorption is rare when the cumulus grains are enclosed inplagioclase. In the second textural type, plagioclase and augite form small inter-cumulus grains 0-2-0-5 mm in diameter (Plate IB). Bronzites associated withthis second textural type have potkilitic margins which enclose small grains of

Hed

MOLE %

Fio. 7. The upper layered series pyroxenes of Jimberlana plotted on the pyroxene quadrilateral. Partof the Ca-poor trend does not pass through the analysed points. The grains involved are invettedpigeonites and differential polishing of the host and lamellae has made it difficult to obtain the Cacontent of the original grains. In this case only the Ca-poor host was analysed and the Ca content of theoriginal grain was estimated. In two instances it was possible to measure the Ca content of theoriginal grain and these points are indicated by X. The large Ca-poor pyroxene grains referred to inthe text are indicated by large dots and they are joined to coexisting Ca-rich pyroxenes by thick

dashed lines.

plagioclase and augite. Plagioclase grains, including those enclosed by bronzite,show normal continuous zoning without unzoned centres. This is importantbecause it is the only textural feature which allows these small intercumulus lathsto be distinguished from otherwise similar cumulus plagioclases of the bronzite-plagioclase cumulate layer (Plate 1C). Another important feature of this texturaltype, which will be called 'dual grain-size texture', is that augite does not formreaction rims around olivine and bronzite.

A transitional texture is also found (Plate ID). It consists of small, stronglyzoned poikilitic plagioclases up to 3 mm in diameter, and laths 1 mm in lengthshowing significantly less zoning. The distribution of these textural types isshown in Fig. 6.

MINERAL VARIATION CURVES

Olivine, orthopyroxene, and clinopyroxene

The mineral variation curves for olivine, orthopyroxene, and clinopyroxeneare similar. They all show a reversal to more Mg-rich compositions at the bottom


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194 I. H. (CAMPBELL

of each rhythmic unit, which corresponds closely to a zone in which the inter-cumulus texture is poikilitic. This contrasts with the upper half of the unitswhere the mineral variation curves show a normal fractionation trend and theintercumulus textures are of the dual grain-size or transitional type (Fig. 6). Theratio of the amount of reversed to normal mineral variation decreases in thehigher rhythmic units.

2OOO

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1000

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CHROMIUM

CHROMIUM

COPPER

CHROMIUM IN Ca-RICH PYROXENE

IN Ca-POOR PYROXENE

LLS

50 0 -SOHEIGHT IN SEQUENCE (METRES)

FIG. 8. Variation in Ni, Cr, and Cu contents of the rocks and the Ni and Cr contents of selectedminerals plotted against height in the sequence. Vertical lines indicate the range in compositions offour grains and the horizontal bars indicate the mean. The layers are marked as in Figs. 4 and 6.


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Plagioclase

In the previous section the small plagioclase grains in bronzite cumulates withdual grain-size texture were interpreted as being intercumulus. Their composition(Fig. 6) is consistent with this as they have a lower An content than the firstcumulus plagioclase at the bottom of the bronzite-plagioclase cumulate layer.In this respect they are similar to the intercumulus plagioclase from the No. 1bronzite cumulate layer of the lower layered series (Fig. 4). Variation in thecomposition of the intercumulus plagioclase is irregular (Figs. 4 and 6) in boththe upper and lower layered series.

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B.ron l̂te

£ Fe as FeO%

o o c

Cumulate Layer

Cumulate Layer

Cumulate Layer

Ti02%Mg0% Mn0%° 9 9

—•

1 1 1

FIG. 9. Chemical variation in chromites from the upper layered series. The vertical line gives thevariation within each specimen and the horizontal bar is the mean of four analyses.

Chromite

The geochemistry of chromites from macro-unit 2 is summarized in Fig. 9.The variation in the composition of different grains from the same specimen isgreater than the variation within the unit and the results are therefore of littlevalue as an indicator of fractionation trends.

TRACE ELEMENTS

Phosphorus and uranium

P and U enter the cumulus phases in very small amounts and their per-centage in a cumulate is directly proportional to the amount of pore materialtrapped, that is, the extent to which its texture is orthocumulate (Wager, 1963;Henderson, 1968; Henderson et al., 1971). The P and U contents of the rockshave been measured (Fig. 6) to determine the distribution of adcumulate and


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196 I. H. CAMPBELL

orthocumulate textural types within the macro-rhythmic units. Mesocumulatespredominate especially in the dual grain-size textural type, but cumulates withpoikilitic textures show a wide range in P and U values and, in some cases, arestrongly orthocumulate. The only adcumulates are found in the bronzite-plagioclase cumulate layer. Note that the bottoms of the Nos. 1, 2, and 3bronzite cumulate layers are more adcumulate in character than the remainderof these layers.

Copper

Chalcopyrite is present in samples with greater than 10 ppm Cu, indicating thatthe average Cu content of the cumulus phases is less than 10 ppm. Cu valuesabove this level indicate the presence of sulphides. Four zones with high Cuvalues were found, one near the bottom of each of the macro-rhythmic units(Fig. 8). These zones correspond closely to the zones which have reversed min-eral variation and poikilitic or transitional textures. Note that they extend a littlebeneath the bottom of each unit into the top of the unit below.

Investigations of lateral variations in the sulphide content of the lowerlayered series of Jimberlana (Campbell, 1973) and of the Muskox Intrusion,Canada (Chamberlain, 1967) have shown that sulphides are concentrated at themargins by a factor of about 10. No detailed investigations of this type werecarried out for this study but preliminary results indicate a similar pattern inthe upper layered series.

Nickel and chromium

Ni and Cr partition strongly into the early cumulus phases and are there-fore sensitive indicators of fractionation (Irvine & Smith, 1967; Irvine, 1975).Ni values are highest at, or a little below, the base of each unit and fall towardsthe top (Fig. 8). Cr values increase sharply at the base of the units and, like Nidecrease towards the top.

The Ni content of olivines and the Cr content of pyroxenes have also beendetermined (Fig. 8). The Ni variation in the olivines does not follow the Nivariation in the rocks. With the exception of one sample at the base of unit 3,Ni values for olivines are highest at the top of the units and lowest at the bot-tom. The Cr content of the pyroxenes varies sympathetically with the Cr contentof the rocks. The results, however, are difficult to interpret due to the widerange of Cr values for different pyroxene grains within the same sample.

DISCUSSION

Variable-depth convection model

Jackson (1961, p. 96) justifies the introduction of his variable-depth hypothesisby arguing that the hypothesis of multiple injection has the following difficultieswhen applied to Stillwater.

(i) The initial magma injection of Stillwater was accompanied by the em-


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placement of considerable amounts of Fe-Cu-Ni sulphides along the base of theintrusion, but no sulphides have been observed at the base of the macro-rhythmic units.

(ii) Feeder dykes to the higher rhythmic units should cut the lower units, butnone have been found.

(iii) The total olivine: bronzite ratio in the units falls gradually but continu-ously from bottom to top. Jackson believes that multiple injection can explainthis only if the successive injections of magma form part of a fractionating se-quence. This, as Jackson points out, would require the existence of a secondmagma chamber below the main chamber which would be an unlikely coinci-dence.

TABLE 2

Comparison between the composition of samples from possible feeder dykes to theupper layered series and the calculated bulk composition of the upper layered series

SiOjTiO2

A1,O3

FejO,FcOMnOMgOCaONa2OK2OH2O

TOTAL

1

52-800-40

12-901-207-90017

11-608-402100-67200

2

53-060-46

13011088-49017

11-999-262100-550-54

100-71

3

50040-31

15-352136-900179-48

13181-540-210-56

99-87

4

50-730-52

16091-588190178-06

11-522-240-200-55

99-85

5

48-670-59

13110-80

11110-21

11-3710-512120-24204

100-77

Key1. Preliminary estimate of the bulk composition of the upper layered series based on a weighted

average of fifty samples.2. N144. Western dyke, analysts M. Haukka and P. Hannaker.3. N14. Eastern dyke, analyst J. A. Hallberg.4. N15. Eastern dyke, analyst J. A. Hallberg.5. N140. Eastern dyke, analysts M. Haukka and P. Hannaker.

Jackson's first objection is specific to Stillwater and does not apply to Jim-berlana as the bottoms of the macro-rhythmic units of the latter have an anoma-lously high sulphide content.

His second objection is the strongest argument supporting his hypothesis.To the best of the writer's knowledge, the feeder dykes essential to the multipleinjection hypothesis have not been recorded as cutting the lower rhythmic unitsof any layered intrusion. Two bronzite-rich gabbro dykes, 1000 m long and 20 mwide, have been found cutting the lower layered series of Jimberlana. Althoughthese dykes are not linked directly with the base of the upper layered series, theirchemical compositions are similar to a preliminary estimate of the bulk compo-sition of the upper layered series (Table 2) and it seems likely that they are feed-


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198 I. H. CAMPBELL

ers to this series. A similar dyke, 5 m wide and parallel to one of the maindykes, may be the feeder to one of the later pulses. No feeder dykes were posi-tively identified cutting the lower rhythmic units, although hand specimens ofpossible feeder material were found in rubbly outcrop. The bronzite-rich gabbrodykes have a similar appearance in the field to plagioclase-rich bronzite cumu-lates, and dykes of this composition would be extremely difficult to identify inthe ultramafic zone of the upper layered series, the outcrop of which is usuallypoor. It is also worth remembering that the bottom of the upper layeredseries is only exposed at the edges of the complex. The most likely location offeeders is the unexposed centre. This applies to other intrusions, such as theGreat Dyke, Rhum, Bushveld, and Stillwater, where the most likely locationsfor the feeders are not exposed.

The third objection is of a more general nature and, although the upperlayered series of Jimberlana is not a particularly good example, it is true formost layered intrusions. Jackson, in arguing that this is evidence against mul-tiple injection, appears to have overlooked the fact that the later magma pulsesmust mix with the magma in the chamber which has undergone a degree offractionation. Although each new injection of magma will raise the MgO contentof the melt above the level present at the end of the preceding cycle, it cannotreturn to the concentration present at the start of that cycle unless the newpulse is much larger than the preceding pulses. Furthermore, it cannot returnto the MgO content of the initial injection unless the new pulse has a higherMgO content than the original magma. The observed general reduction in theolivine: bronzite ratio of the successive macro-rhythmic units is therefore con-sistent with a multiple injection model if, as seems likely, each successive magmapulse mixes with a more highly fractionated residue in the magma chamber.

The weakness of Jackson's variable-depth convection model is, as Wager &Brown (1968, p. 340) argue, the difficulty in isolating a stagnant magma from theoverlying magma so that the SiO2 enrichment and MgO depletion necessary tobring about bronzite crystallization at the end of the cycle can occur. Thenorites above the ultramafic zone of Stillwater are finely layered and each layeris attributed, by both Hess (1960) and Jackson (1961), to a convection currentsweeping across the floor of the magma chamber. Thus, at the contact betweenthe ultramafic zone and the norites, Jackson requires the convection style ofStillwater to change suddenly from variable-depth convection (with the magmachamber divided into a lower stagnant portion and a rapidly convecting upperportion) to a single, rapidly convecting system. This is unlikely.

There is a second difficulty in applying Jackson's hypothesis to the upperlayered series of Jimberlana. Undercooling, essential to the variable-depthmodel, can be derived in one of two ways. First, by heat loss through the floorof the magma chamber and secondly, as the result of the difference in the adia-batic temperature gradient and the melting-point gradient at the start of arhythmic unit (Fig. 1). If Jackson's model is applied to the upper layered series,


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the mineral-variation reversals of unit 4, and possibly unit 3, would be due tosupercooling produced by heat loss through the lower layered series. However,the extensive zone of normal variation at the top of unit 3 indicates that heat lossthrough the lower layered series has lost its capacity to induce supercooling.For Jackson's model to apply, the mineral-variation reversal of unit 2 musttherefore be due to the difference between the melting-point and adiabatictemperature gradients. It is probable that the amount of supercooling requiredto produce a compositional reversal of En5 in pyroxenes is at least 6 °C and ifthe melting-point temperature gradient is 3 °C per km greater than the adiabaticgradient, the point of intersection of the two curves must be 2 km above thefloor of the magma chamber. Jackson's variable-depth convection model re-quires this distance to be small compared with the total thickness of the magmachamber and a minimum thickness of 6 km would therefore seem reasonable.Since the total thickness of the upper layered series is less than 0-5 km, Jackson'smodel is unable to account for the compositional reversal of the pyroxenes inunit 2.

MULTIPLE INJECTION

Each of the macro-units is associated with a reversal in the mineral variationand in the Ni-Cr trends, and has a zone of high sulphide contents at its base.These features are also found at the contact between the upper and lower layeredseries and if the upper layered series is due to a new influx of magma, it followsthat the macro-units are also due to multiple injection. This conclusion isstrongly supported by the size of the reversals which, as argued above, are toolarge to be explained by the hypothesis of Jackson (1961).

An essential part of the multiple injection hypothesis, as proposed by Brown(1956) and Irvine & Smith (1967), is that the process involved the flushing ofsome residual magma from the chamber, possibly through a volcano. Brown(1956) postulated this to explain the lack of variation in mineral compositionbetween the lowest and highest macro-units of Rhum, and Irvine & Smith (1967)to account for the disproportional amount of ultramafic rocks in Muskox com-pared with the basaltic composition of the chilled margin. An attraction of theflushing concept is that it requires no increase in the volume of the magmachamber if the volume of magma removed is equal to the volume added bythe new pulse.1

There is no clear evidence to indicate whether or not magma was removedfrom the upper layered series magma chamber during the injection process.One of the possible feeder dykes (the western dyke, Table 2) has a similar com-position to the bulk composition of the upper layered series magma, indicatingthat magma of the correct composition was available for the formation of the

1 e.g. Multiple injection can produce a mixture of two old magma to one new if a third of theresidual magma is removed at the time of injection. The same mixture of old and new magma,without flushing, implies a 50 per cent increase in the volume of the magma chamber.


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200 I. H. CAMPBELL

upper layered series without requiring the removal of material from the magmachamber. The other dyke has a lower MgO content. If its composition is repre-sentative of the magma pulses which formed the upper layered series, some frac-tionated magma must have been flushed from the magma chamber duringmultiple injection to give the MgO content of the upper layered series bulkcomposition (Irvine & Smith, 1967). The flushing concept, although interesting,has no bearing on the discussion which follows.

The diagnostic feature in the intercumulus textural types is their differentgrain sizes. Since grain size is chiefly a function of the rate of cooling, it islikely that the dual grain-size type of texture resulted from a faster rate of cool-ing than the coarser poikilitic type. The systematic distribution of these tex-tural types within the macro-units is probably due to different rates of inter-cumulus heat loss during the early and late stages of formation of the rhythmicunits.

The poikilitic textural type is found only at the bottom of units and is closelyassociated with zones which have reversed mineral variation and anomalouslyhigh Cu values (Figs. 6 and 8). The close association between these three featuresand the bottoms of the units suggests that they are genetically related to themultiple injection process and the period of adjustment that followed.

There are two possible models for multiple injection which need to be con-sidered.

Model 1

Injection took the form of a sudden pulse which modified the composition ofthe magma already in the magma chamber by making it more ultrabasic andraising its temperature above that of its environment (by At, Fig. 10A and B).The temperature difference between the top of the crystal pile and the main bodyof the magma gave rise to a temperature gradient in the magma above the crystalpile. Within this zone the magma was supercooled (Fig. 10B) and crystallizedlower temperature silicates than those which would have equilibrated withthe contemporary magma (Fig. 10B and C). A combination of convection,conduction, and the release of latent heat of crystallization soon lowered At andreduced the temperature gradient at the bottom of the magma chamber.Eventually At became very small and 'normal' cooling conditions were re-established. During the period of adjustment, although the temperature of themelt was falling at a faster rate than usual, the temperature of the top of thecrystal pile was rising (A/,, Fig. 10A-D). It was the reduction in the amountof supercooling at the top of the pile which produced the reversed mineralvariation. The transition from reversed to normal mineral variation marks thedisappearance of the supercooled zone at the bottom of the magma chamber andthe olivines and pyroxenes with the highest Mg/Mg+Fe ratios formed at thispoint in the cycle (ZZ in Fig. 10D).

The effect of raising the temperature gradient within the crystal pile during


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EIG

HT

f1

z

Magma

chambs

A

/ ^ U p p e r layered/series crystal pllt

-» - TEMPERATURE

' xjSlurt or new .

j c

j^~ Invnr

! Y

; E

New layer j^f ;

. - A t -

Supercooled

New layer

•At,-a; 0

fMagmachamber

New layer T '

Fio. 10. Diagrammatic representation of multiple injection by a sudden pulse of magma (model 1,see text).

A represents conditions before a new magma pulse. The upper layered series crystal pile has a highthermal gradient due to heat loss to the cooler crystal pile of the lower layered series.

B gives conditions immediately after injection. The temperature of the magma chamber is raised A/which produces a strong temperature gradient in the magma above the crystal pile.

C depicts the period of adjustment following the injection. The temperature gradient above the crystalpile has been reduced.

D. When normal heat-loss conditions are re-established, the temperature gradient in the magma issmall and it is under these circumstances that the minerals with the highest Mg/Mg + Fe ratio form.Note that the temperature in the crystal pile at point X in C has fallen only by the distance between thetwo dots in D. These are the conditions which produce the poikilitic type of crystallization. E and Fshow the effect of heat loss through the crystal pile under normal heat-loss conditions. Cooling of theintercumulus liquid is fast so that the point Y in E is cooled by the distance between the two dots inF. It is this fast cooling which produces the dual grain-size type of texture. G gives the conditions

immediately after another magma pulse.The dashed line in C-G gives the temperature conditions in B as a basis for comparison. The

dotted line (ZZ) in E-G is the level at which the highest Mg/Mg + Fe minerals form in the newlayer.


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202 I. H. CAMPBELL

the period of adjustment was to reduce the rate at which the intercumulusliquid lost heat (Fig. IOC and D). This influenced the nature of intercumuluscrystallization in two ways. First, it slowed the rate of intercumulus crystal-lization which resulted in the formation of a poikilitic texture. Secondly, theslow rate of crystallization permitted equilibrium to be maintained betweenthe cumulus grains and the intercumulus liquid, resulting in resorption of theformer. This resorption of cumulus grains in bronzite and augite oikocrysts,which is such a prominent textural feature of the upper layered series, is onlypossible if the poikilitic mineral formed at a lower temperature than the cumulusgrains (Wager & Brown, 1968, p. 315). Thus the strong resorption found at thebottom of the macro-units is compatible with the cumulate textural types beingpoikilitic mesocumulates or orthocumulates as indicated by the high P and Ucontents of these rocks. Resorption is not possible in heteradcumulates (poikiliticadcumulates) as denned by Wager et al. (1960).

The distinctive dual grain-size texture is unusual and its occurrence in theupper layered series must be due to some feature rarely found in layered intru-sions. One feature of the upper layered series crystallization which distinguishesit from the crystallization of other cumulates is that the new magma has beeninjected on to the relatively cool crystal pile of the lower layered series. Thelower layered series, during periods of normal crystallization, became a heat sinkand a significant temperature gradient would have developed in the crystal pileof the upper layered series (Fig. 10E and F). Heat loss through the floor of themagma chamber, under these circumstances, would be an important considera-tion. Since it was lost through the crystal pile, it would have led to the high rateof intercumulus heat loss that resulted in the unusual dual grain-size texture,characteristic of the upper half of the macro-units.

During the formation of the lower macro-units of the upper layered seriesthe temperature of the temporary floor of the magma chamber was raised byeach successive magma pulse (by Af3 in Fig. 10F). If each magma pulse raisedthe temperature of the magma chamber to approximately the same level,there would be a steady reduction in the value of A/ (Fig. 10G). Since thegreater the value of A;, the greater would be the width of the zone of reversedmineral variation, it follows that there would have been a steady reduction in thereversed: normal mineral variation in the higher units. This would have con-tinued until a macro-rhythmic unit formed (unit 2, Fig. 6) in which the normalmineral variation was greater than the reversed mineral variation. This phe-nomena was therefore confined to the lower macro-units (units 2, 3, and 4). Thesize of subsequent reversals depended on the composition, size, and timing ofthe injection pulses.

Model 2

Injection was not 'instant' but extended over a period of time correspondingto the width of the mineral variation reversals. The reversals, in this case, were


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due to the crystallization of the cumulus phases from a magma which wasgradually becoming more ultrabasic and steadily increasing in temperature.The explanation for the distribution of the poikilitic and dual grain-size typesof texture given for model 1 applies equally to model 2. The difficulty with model2 is that it requires continuous injection of magma while over 15 m of cumulateswere accumulating on the floor of the magma chamber.

TIMING OF THE INJECTION OF THEUPPER LAYERED SERIES MAGMA

The contact between the upper and lower layered series is subparallel to thelayering in the lower layered series (Fig. 5) and the lower contact of the upperlayered series is only slightly chilled. These factors suggest that the magmawhich formed the upper series was injected into the magma chamber beforecrystallization of the lower series was complete. If the new magma mixed witha small amount of residual magma in the chamber, this would explain both theabsence of late-stage differentiates at the top of the lower layered series, and thelower Mg: Fe ratio of pyroxenes and olivines from the upper layered series com-pared with those from the lower layered series (Fig. 4). There is no structuralevidence to indicate that the volume of the magma chamber increased duringthe injection of the upper layered series magma, suggesting that injection mayhave been accompanied by the flushing of an equal volume of lower layeredseries residual magma.

Chromium

The Cr content of the cumulus phases is chromite > Ca-rich pyroxene >Ca-poor pyroxene > olivine (Campbell & Borley, 1974). During the formationof the olivine cumulate layers, chromite crystallized as a minor but significantcumulus phase. The percentage of cumulus chromite decreased sharply with thereplacement of cumulus olivine by cumulus Ca-poor pyroxene, but this changehad little effect on the total Cr content of the rocks due to incongruent meltingrelationship between Cr-rich pyroxene and chromite (Dickey et al., 1971).

The chemistry of the chromites from the upper layered series is variable andthey are of little value as indicators of fractionation trends. Chromites arehighly reactive (Henderson & Suddaby, 1971) and react with the intercumulusliquid during the period of orthocumulate crystallization. The released Cr isabsorbed by the crystallization of intercumulus pyroxenes.

Cr3+ substitution for Mg2+ or Fe2+ in pyroxenes requires the simultaneousreplacement of Si4+ by Al3+ (or the entry of Na+ into an M site) so that chargebalance can be maintained. This is well illustrated by Ca-rich and Ca-poorpyroxenes from the bottom of the upper layered series near the contact withthe lower layered series. The Cr contents of individual grains within the samespecimen vary from 0-48 to 1 -05 per cent in Ca-rich pyroxenes and from 0-29 to0-58 per cent in Ca-poor pyroxenes (Table 3). Pyroxenes with low Cr contents


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204 I. H. CAMPBELL

also have low Al, indicating that the geochemistry of Cr3"1" in pyroxenes ispartly dependent upon the availability of trivalent octahedral sites.

The Cr3"*" content of a pyroxene is dependent on the Cr3"1" content of the meltfrom which it crystallized and the Al3+ content of its tetrahedral site which, inturn, depends upon the aslOl of the magma. Kushiro (1960) and Brown (1968)have shown that the Al3+ content of a pyroxene is reduced by raising theflsioi °f the magma from which it crystallized. The effect of fractionation wouldbe to lower the Cr content of the magma but to raise its aSioj- The Cr content ofthe pyroxenes should therefore fall with fractionation.

The wide range in the Cr3+-Al3+ contents of individual pyroxene grainswithin the same rock indicate a degree of disequilibrium for these elements

TABLE 3

Analyses of individual pyroxene grains from a single sample (N18) near thecontact between the upper and lower layered series

SiOjTiO2

AJ2O3FeOMnOMgOCaONa2ONiOCr2O3

51-200-272-498-140-23

16-8219-410-340 0 8101

99-99

52-280-601-858-850-20

161318-820-310 0 70-48

99-59

52-370-272-597-85017

16-5419160-350-071-05

100-42

51180-272-567-930-25

15-6420010-370-060-98

99-25

54020 1 41-31

15090-34

26-782-320 0 30 0 80-29

100-40

53-560 1 61-70

15-400-28

26-252-560 0 20-090-58

100-60

54010 1 61-44

14-810-26

26-652-300 0 20 0 60-39

10010

53-960-201-23

15150-31

26-632-250020070-35

10017

54170-221-26

15010-31

26-991-860 0 10 0 60-36

100-25

which makes a detailed interpretation of their distribution difficult. The higherCr3"1" of pyroxenes from the base of the units compared with those from the topis consistent with the multiple injection hypothesis, but the difference is less thanis predicted by Jackson's variable-depth convection hypothesis. The distribu-tion coefficient (D) for Cr between Ca-poor pyroxenes and the magma is about 10(from data of Campbell & Borley, 1974). If crystallization of the rhythmic unitswas from a small, isolated, stagnant layer of magma, the Cr content of thatlayer would be rapidly depleted and the Cr content of the pyroxenes crystallizingat the end of a crystallization cycle might be expected to be lower than for thosefound at the top of the upper layered series rhythmic units.

The total Cr content of the rocks provides a way to distinguish between thetwo models for multiple injection. Model 1 requires a sharp increase in the Crcontent of the magma at the time of injection. Since Cr partitions strongly intothe early cumulus phases, the Cr content of rocks should be greatest at the bot-tom of the units and fall with fractionation. Model 2 requires the Cr content ofthe magma to increase gradually, reaching a maximum at the end of the periodof injection. According to this model, the rocks with the highest Cr content


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will also contain pyroxenes and olivines with the highest Mg:Fe ratios. Thedistribution of Cr in units 2, 3, and 4 (neglecting the chilled zone at the bottomof unit 4) fit model 1, but model 2 offers a better explanation for unit 1.

Phosphorus and uranium

As already shown, the P and U contents of rocks are indicators of the degreeto which they are orthocumulate. Continuous reaction between the cumulusgrains and the cooling intercumulus liquid dilutes the liquidus composition ofthe cumulus minerals in rocks which have an orthocumulate texture, loweringthe Mg:Fe ratio of the pyroxenes and oh"vines. One of the objectives of measur-ing the P and U contents of the rocks (Fig. 6) was to monitor the effect of thisdilution on the olivine and pyroxene mineral-variation curves. The effect onthese phases is small, probably about 2-3 mole per cent (Campbell & Borley,1974), and no changes in these curves can be correlated directly with variationsin the P or U contents of the rocks. Nevertheless, it is probable that the Mg:Feratios of the olivines and pyroxenes have been depressed where the P-U con-tents of the rocks are high, as at the top of the No. 2 and No. 3 olivine cumulatelayers. Of particular importance is the systematic change from an adcumulateto a mesocumulate texture at the bottom of the No. 1 bronzite cumulate layer(between +40 and +50 m in Fig. 6), which may have suppressed the mineral-variation reversal at the bottom of this layer.

The diluting effect of post-depositional reactions on the liquidus compositionof the cumulus grains will be more effective on small grains with a large surfacearea : volume ratio than it is on large grains. The cumulus bronzite grains be-tween +30 and +50 m in units 1 and 2, which occur in two distinct sizes, sup-port this hypothesis. The larger grains have a consistently higher Mg : Fe ratiothan the smaller grains and the more orthocumulate the sample, the greater thedifference in the compositions (Fig. 6). Note that the large grains at the bottomof unit 1 which, if this argument is correct, are more representative of the liquiduscomposition than the smaller grains, show a more pronounced mineral reversal.This supports the argument that the magnitude of this reversal has been sup-pressed by the change from adcumulate to mesocumulate texture.

A second example of the relationship between grain size and the extent ofpost-depositional reaction is provided by the chromites. The effectiveness ofreactions during the orthocumulate stage of crystallization in producing thewide range in the chemical compositions of the chromites (Fig. 9) has beengreatly enhanced by the very small grain size of this mineral.

The rapid changes from mesocumulate to orthocumulate in rocks which havea poikilitic texture are associated with irregular variations in the compositionsof the intercumulus plagioclase. Apparently nucleation of the intercumulusplagioclase took place at irregular centres within the crystal pile. These centreshave high-An plagioclase and a low P-U content. The liquid not required for thecrystallization of plagioclase diffused away from these early growth centres


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206 I. H. CAMPBELL

and crystallized later, at a lower temperature. The resulting rocks have anorthocumulate texture, a high P-U content and low-An plagioclase. Note therough correlation between high An and low P-U in Fig. 6. The correlation is notperfect because the variations occur on a scale of centimetres and the samplesselected for P-U analyses were not always representative of the thin section.The poikilitic top of the No. 1 bronzite cumulate layer, which is on a coarserscale, shows the relationship more clearly (Campbell, 1973).

Copper

The presence of anomalous Cu values at the base of each of the macro-units is, perhaps, the best evidence of multiple injection. It is difficult to recon-cile these sulphide horizons with the variable-depth convection hypothesissuggested by Jackson (1961). There are three mechanisms by which the sul-phides may have been emplaced.

Mechanism (i). The sulphides may have been emplaced as fine, immiscible drop-lets which settled slowly from the magma over an extended period of time. Thedifficulty with this mechanism is to explain why the periods of settling shouldcorrespond closely with the periods of reversed mineral variation. The correla-tion for units 2 and 3 is too good to be coincidental.

Mechanism (ii). The sulphide entered the intrusion as somewhat coarser,immiscible globules in a turbid magma which settled quickly to the floor of themagma chamber. If injection took place over an extended period of time, asrequired by model 2, the period of settling of the sulphides would correspondto the period of reversed mineral variation.

The difficulty with mechanisms (i) and (//) is that they fail to account for theconcentration of sulphides at the margin of the intrusion. The case against thesemechanisms is further strengthened by the distribution of sulphides in the lowerlayered series. The bottoms of the macro-rhythmic units of this series are alsostrongly anomalous in sulphides, but only at the margin of the intrusion. Thiscannot be reconciled with mechanisms (/) or (ii). The macro-rhythmic units ofthe lower layered series, unlike those of the upper layered series, are stronglyU-shaped (Fig. 3) and if the sulphides entered the magma chamber as immis-cible droplets, they should have gravitated to the bottom of the chamber andconcentrated at the base of the 'U', that is, at the centre of the intrusion.

Mechanism (Hi). The sulphides separated from narrow zones of supercooled meltwhich existed at the margin of the magma chamber during periods of normalcrystallization, and at the bottom of the magma chamber early in the injectioncycle of model 1 (Fig. 10B). The solubility of S in a basaltic magma is dependenton its composition, temperature, fOl and/Sj. Haughton et al. (1974) have shownthat the equation of Fincham & Richardson (1954)

(~i JOim — »"« 7~~i

JSi


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for predicting the solubility of sulphur (Sm) in silicate melts can also be appliedto basalts and that the relationship between the constant (C,) and temperature islinear. They have stressed the role of temperature and their calculations showthat, at a constant fOl to fSl ratio, the solubility of S in a basaltic melt can beincreased by a factor of 5 if the temperature of the magma is raised from1100 °C to 1200 °C. Although their work cannot be used to make an accuratecalculation of the solubility of S in the Jimberlana magma, it can be used tocalculate changes in the S solubility for small changes in temperature, providedthe effect on the S solubility of changes in the/O2 to/S 2 ratio of the Jimberlanamelt is small compared with the effect of changing temperature. If a tempera-ture and S solubility are assumed for the parent magma, the S solubility can becalculated at any other temperature. The temperature and S solubility that havebeen assumed are 1230 °C and 1200 ppm respectively. It has also been necessaryto assume that the parent magma was saturated or near-saturated with S(Haughton et al, 1974; Moore & Fabbi, 1971) and that its chemical compositionwas kept homogeneous by regular convection.

Consider the conditions likely to exist in the magma chamber some time afterthe initial injection of magma when the temperature of the melt has fallen to,say, 1200 °C. It is apparent from the shape of Jimberlana that heat loss throughthe top and sides of the intrusion would have been greater than heat loss throughthe floor (the crystal pile). The magma adjacent to the margin would thereforebe cooler than the magma above the crystal pile at the centre by, say, 5 °C (Fig.11, Example 1). The calculated S solubility at the centre is 830 ppm (at 1200 °C)but at the margin it is only 770 ppm (at 1196 °C). The S content of the melt willclearly be controlled by the lower of these two solubilities, that is, by the solu-bility at the margin of the intrusion.

It is envisaged that the Jimberlana magma convected frequently and that theseconvection cycles acted as a conveyor system, continually moving magma fromthe warm centre of the intrusion to the cooler margin. If the S content of themagma exceeded its solubility at the margin, S would be precipitated as immis-cible globules of sulphides. The efficiency with which S was removed from themagma as it passed through the marginal zone would depend on the rate ofdiffusion of S to the growing sulphide globules. If the rate of diffusion was slow,the amount of S removed from the melt would be small and the S supersatura-tion would increase as the temperature of the magma fell. This would lead toan increase in the chemical gradient which, in turn, would result in a faster rateof diffusion. A balance or steady state would be achieved when the rate ofdiffusion was sufficiently fast to prevent a build-up in the S supersaturation of themagma. If the chemical gradient1 necessary to produce this balance was lessthan the difference in the solubility of S between the margin and centre of theintrusion {i.e. less than 60 ppm in the case cited above), sulphides would not

1 The figures presented in Fig. 11 assume that the S content of the magma was 20 ppm above thesolubility of S at the margin.


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208 I. H. CAMPBELL

Example 1 dtc - 30°C dtd - 5°C

Before injection After injection

Marginalzone

t - 1195V 7 7 0

t - 1200S m - 8 3 0

Example 2 d t c - 10°C dt d -10°C

Before Injection After Injection

FIG. 11. Two hypothetical models of multiple injection with different temperature contrasts betweenthe parent and residual magma (dtc) and between the centre and margin of the intrusion (dtd). It hasbeen assumed that injection involves the addition of new magma in the ratio 1 new:2 old and that theadded magma comes from the parent source which has a temperature of 1230 °C and an S content(Sc) of 1200 ppm. t = temperature °C; Sm = sulphur solubility; dashed arrows represent the sul-phides precipitating from the roof of the magma chamber which become redissolved while passingthrough the undersaturated main body of magma; full arrows indicate convection currents: stippledareas show zones of S-supersaturation and the ringed figures indicate the approximate degree ofsupersaturation in ppm. The zone of S-supersaturation at the margin of the intrusion in Example 2has been extended towards the centre of the intrusion to indicate that the conditions at the margin

and the centre are gradational after a new magma pulse.


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separate at the centre. The sulphides which formed at the top must gravitatethrough the magma chamber to reach the floor but would be redissolved by theundersaturated magma before they did so. Sulphide deposition would thereforebe confined to the side of the intrusion. These are the conditions postulated tohave prevailed throughout most of the crystallization of the Jimberlana Intru-sion as they explain why the concentration of sulphides at the margin is at leastan order of magnitude higher than at the centre.1

If the magma chamber now received a new pulse of primary magma (tempera-ture 1230 °C, S = 1200 ppm) equal in volume to half the volume of magma al-ready in the chamber, the resulting mixture would have a temperature of 1210°C, a S content of 927 ppm and a S solubility of approximately 931 ppm(Fig. 11, Example 1). The temperature at the bottom of the supercooled zoneat the base of the magma chamber would be the temperature at the top of thecrystal pile immediately before injection of the new magma pulse, that is, 1195°C at the margin and 1200 °C at the centre, and the S solubilities would be 770ppm and 830 ppm respectively. Sulphides would be unable to precipitate fromthe main body of magma, but they would be able to separate from the narrowzones of supercooled magma at the margin and bottom of the intrusion (Figs.11 and 10B). Some of these sulphides gravitated a small distance through thecrystal pile into the top of the unit below (Fig. 8F). S would continue to separatefrom the zone at the bottom of the intrusion until its temperature rose to a levelat which its S solubility was greater than the S content of the melt. This wouldhave occurred towards the end of the period of reversed mineral variation (Fig.10D), which explains the close correlation between these zones and the zonesof high sulphide values. At the end of the period of reversed mineral variation,the magma chamber returned to the conditions described earlier where thesulphides could only precipitate at the margin.

THE DIFFERENCE IN THE DISTRIBUTION OF SULPHIDESIN THE UPPER AND LOWER LAYERED SERIES

It is evident from the above argument that even during periods of high sul-phide precipitation, the rate of precipitation at the margin would be greaterthan at the centre. To further illustrate this point and to explain the differencein the distribution of sulphides in the upper and lower layered series, severaladditional hypothetical cases of multiple injection were considered. The resultsshowed that the degree of oversaturation, and therefore the ratio of the rate ofdeposition at the margin to the rate of deposition at the centre, is increased byraising the temperature difference between the centre and the margin or bylowering the temperature contrast between the injected magma and the residual

1 A similar explanation is offered by Chamberlain (1967) to explain the distribution of sulphides inthe Muskox Intrusion. He, however, relies on diffusion to move the sulphides from the centre of theintrusion to the margin. The rate of diffusion in silicate melts is of the order of 1 m/year and it seemsunlikely that diffusion alone can effect the necessary mass transfer.


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210 I. H. CAMPBELL

magma. If the temperature contrast between the magmas was sufficiently smalland/or the temperature difference between the centre and margin was sufficientlygreat, the precipitation of sulphides at the centre of the intrusion may have beenprevented after a new influx of magma (Fig. 11, Example 2).

There are two important differences between the conditions of crystallizationof the upper and lower layered series. First, heat loss through the margin andfloor of the upper layered series magma chamber was, in both cases, through thewarm crystal pile of the lower layered series whereas, for the lower layeredseries, it would be dominantly through the margin to cold country rock. It istherefore likely that temperature differences between the centre and marginwould be greater for the lower layered series than for the upper layered series.Secondly, the Mg:Fe ratio in the pyroxenes and olivines from the lower layeredseries is higher than in those from the upper layered series and the changes in thisratio at the contact between units are less pronounced. This suggests that theresidual magma in the upper layered series magma chamber was more highlyfractionated than that of the lower layered series and that the contrast intemperature between the fractionated magma and the injected pulses of parentmagma was greater. These differences between the upper and lower layered serieshave apparently been sufficiently great to prevent the precipitation of sulphidesat the centre of the lower layered series after new influxes of magma.

Nickel

Ni enters both sulphide and silicate phases and, at equilibrium, partitionsbetween olivine, sulphides, and the melt according to the following equations:

„ _ XN1 in sulphides , nD s u l ~ ~Km in liquid U )

„ XN, in olivine n ,D°l ~ XN1 in liquid ( 2 )

where DS u I ;>D o ,>l .The ability of Ni to enter both sulphides and silicates limits the importance

of the total Ni content of the rocks (Fig. 8A) as a petrological indicator. Thewhole-rock Ni curve is made up two components, a sulphide component similarto the whole-rock Cu curve (Fig. 8F) and a silicate component similar to thewhole-rock Cr curve (Fig. 8C). The whole-rock Ni curve is, in effect, the sum ofthese two curves.

The geochemistries of Cr and Ni are similar in that they both receive a highcrystal field stabilization energy in octahedral sites and partition strongly intothe early pyroxenes and olivines, respectively. The low Ni content of theolivines (Fig. 8B) from the bottoms of the macro-rhythmic units, which is thereverse to the Cr content of the pyroxenes (Fig. 8D and E), is therefore anomal-ous and requires explanation. Ni partitions between the coexisting phases in


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accordance with equations (1) and (2), both of which must be satisfied at equi-librium. Variation in the Ni content of the olivines must therefore reflect eitherchanges in the Ni content of the melt or the effect of temperature on DOi.

Consider first the effect of temperature on DO1. Both models for multipleinjection require the temperature at the bottom of the magma chamber toincrease slightly during the early stages of formation of the macro-rhythmicunits. The data of Nagamori (1974) shows that the likely effect of raising thetemperature of a magma is to lower the oNIO in the melt and so reduce the valueof Do l . This conclusion is supported by Irvine (1975, Fig. 56). Changes in DOiwith temperature, which in any case are probably small for the temperaturechanges involved, cannot therefore explain the variation in the Ni content ofthe olivines shown in Fig. 8B.

An important difference between the geochemical behaviour of Ni and Cr isthat Ni partitions strongly into sulphides, whereas Cr does not. The low-Ni oli-vines from the bottom of each of the units are closely associated with zones ofhigh Fe-Cu-Ni sulphides, and a simple mass-balance calculation shows that thetotal amount of Ni held in the sulphides is approximately equal to the Nideficiency of the olivines. Ni partitions preferentially into the sulphides (i.e.DSul<^Dol) and it appears that the low Ni content of these olivines is due toscavenging by the sulphides.

If the olivines and sulphides crystallized in equilibrium, equation (2) requiresthe Ni content of the magma from which the olivine grains have crystallized torise during the formation of the macro-rhythmic units. Since the total Nicontents of the phases separating from the magma is always greater than the Nicontent of the melt this is not possible for a magma chamber when taken as awhole. If, however, crystallization of the sulphides and the cumulus silicatephases occurred within a restricted volume of magma, and if the rate of removalof Ni by the combined crystallization of sulphides and silicates was greater thanthe rate of entry of Ni into the zone of crystallization, the melt would becomelocally depleted in Ni and would crystallize low-Ni olivines in accordance withequation (2). As soon as sulphide crystallization ceased the rate of removal ofNi from this zone would have fallen substantially and the Ni content of the zoneand of the olivines crystallizing from it would have begun to rise until, even-tually, normal crystallization conditions were re-established.

An essential pre-requisite of this argument is that the sulphides and cumulussilicates crystallize together from a restricted volume of magma within themagma chamber, thus supporting the argument previously advanced concerningthe location of the sulphide crystallization. It was argued that the most likelyplace for the sulphides to crystallize is from the narrow zone of supercooledmagma that existed at the bottom of the magma chamber immediately follow-ing the injection of a new magma pulse (Mechanism (Hi)). If the Ni depletionin the olivines is due to sulphide scavenging, the cumulus olivines must alsocrystallize from this zone.


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212 I. H. CAMPBELL

Thermodynamically, the most favourable location for the formation ofcumulus grains is at the floor of a magma chamber. Although the temperatureat the top of a magma chamber will be lower than at the bottom, the verticaltemperature gradient within a large magma chamber will be small and will bemore than offset by the rise in the melting point of the minerals with increasedpressure (Fig. 1). These factors led Jackson (1961) to suggest that the cumulusgrains in the Stillwater Complex nucleated and crystallized near the floor of themagma chamber and gravitated to their final resting position. Jackson arguesthat each macro-rhythmic unit formed from a stagnant layer of magma whichwas isolated from the rest of the magma chamber. It is clear that the thicknessof the stagnant layer must be several times greater than the macro-unit whichcrystallizes from it, for otherwise the units would show stronger evidence offractional crystallization.

The amount of Ni held in the Jimberlana sulphide horizons is small comparedwith the total Ni content of the upper layered series magma chamber, and thevolume of magma which could be effectively scavenged of Ni by this volume ofsulphides is considerably less than the wide zone of crystallization envisaged byJackson (1961, p. 96). A more appropriate model for the crystallization of theupper layered series, immediately following the injection of a magma pulse, isthat it occurred from a narrow zone of supercooled magma at the bottom of themagma chamber which, because of its low temperature, higher viscosity and thecrystals suspended in it, was unable to join the convection cycle of the main bodyof magma. The rate of mass transfer by diffusion is slow compared with the rateof transfer by convection and this layer has, to a degree, acted as a barrier whichplaced an upper limit on the rate of mass transfer between the converting mainbody of magma and the growing crystals. Ni, during the period that the sulphideswere crystallizing, partitioned strongly into the crystallizing phases and exceededthe upper limit of mass transfer. Its partitioning into the silicate phases wascorrespondingly affected. The partitioning of elements such as Fe, Mg, and Sihowever, which do not concentrate so strongly into the cumulus phases, does notappear to have been greatly influenced by the diffusion barrier.

Once the zone of supercooling had been released, the layer of magma im-mediately above the floor of the magma chamber would no longer have beencooler than the rest of the chamber and would have been able to join the con-vection of the main body of magma. There is no evidence in the upper layeredseries to suggest that the stagnant layer persisted once the zone of supercoolinghad gone, and it seems likely that normal convection, as envisaged by Wager& Deer (1939) and Hess (1960), developed during the formation of the upperpart of the macro-rhythmic units.


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THE POSITION OF THE OLIVINE CUMULATE LAYERS

The relationship between Mg-rich olivine and Ca-poor pyroxene is controlledby the following equations:

Mg2Si206 = SiO2+Mg2SiO4 (3)

Mg2Si2O6+2MgO = 2Mg2SiO4 (4)

Campbell & Nolan (1974) have shown that raising the temperature of crystalli-zation expands the stability field of olivine through the effect of temperature onequation (3). The Mg:Fe ratios of the olivines and pyroxenes indicate that thetemperature at the bottom of the magma chamber reached its maximum wellafter the start of the injection cycle. If temperature was the dominant parametercontrolling the distribution of olivine within the macro-units, the olivine layersshould not be found at the bottom of the units but should be centred near thepeak in the Mg:Fe ratios.

The stability of olivine and pyroxene is also influenced by the aSio2 ar*d #MgO

of the melt from which they crystallize. The formation of Mg-rich olivine isfavoured by lowering the aslO2 of the melt (equation (3)) or by raising its aMgO

(equation (4)). During the early stages of formation of units 2 and 3, a combina-tion of low aslOj and high aMtO resulted in the formation of Mg-rich olivine.The continued crystallization of olivine, with its low SiO2 and high MgO con-tents, raised the aSiOl of the magma and lowered its aMgO until, eventually,olivine was replaced by Ca-poor pyroxene in accordance with equations (3) and(4), despite the increase in the temperature of crystallization. Apparently therole of temperature was overshadowed by the influence of aSiOi and aMgO.

THE ORIGIN OF MACRO-RHYTHMIC LAYERING IN THE ULTRAMAFICZONES OF OTHER LAYERED INTRUSIONS

The magnitude of the mineral variation reversals associated with the bottomsof the macro-rhythmic units in Jimberlana are greater than those recordedfrom the Great Dyke, Muskox, and Stillwater Intrusions. Two factors havecontributed to this:

(i) The composition and temperature of the later pulses contrasted sharplywith the magma in the chamber, which had been substantially modified by mix-ing with the highly fractionated residue of the lower layered series.

(ii) The small size of the upper layered series and the presence of the lowerlayered series heat sink produced a high rate of heat loss, preventing the upperlayered series magma approaching the temperature of the later magma pulses.This has resulted in a further increase in the temperature difference between theold and new magmas {i.e. Af in Fig. 10B is always large).

If later magma pulses have modified the composition of the other intrusionsduring the formation of their ultramafic zones, the additions have occurred be-


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214 I. H. CAMPBELL

fore sufficient fractionation had taken place to produce a strong contrast betweenthe old and new magmas. The effects of the new additions have therefore beensubtle compared with the changes produced in the upper layered series of Jim-berlana.

The general similarity of macro-rhythmic layering in the ultramafic zones ofthe Great Dyke, Muskox, and Stillwater Intrusions leads to the inescapableconclusion that this type of layering has the same origin in each of these intru-sions. The macro-rhythmic units of the upper layered series in Jimberlana arealso similar but for the presence of:

(i) Dual grain-size texture at the top of the units,(ii) A zone of anomalous sulphides at the base of each unit,

(iii) The magnitude of the mineral variation reversals at the bottoms of theunits.

It has been argued that the first of these differences was caused by the highrate of heat loss through the floor of the magma chamber due to the presenceof the lower layered series heat sink, and that the other differences were due tothe strong contrast in temperature and composition between the residualmagma and the new magma pulses. Thus the very factors which strengthen thecase for multiple injection in Jimberlana have produced conspicuous differencesbetween the macro-rhythmic units of Jimberlana and those of other intrusions.These differences give rise to some doubt as to whether the conclusions arrivedat in this paper, regarding the origin of macro-rhythmic layering, can be ex-tended to other intrusions. Nevertheless, it is hoped that the data presentedwill be of value to workers studying intrusions where the variation in the para-meters associated with macro-rhythmic layering are less marked.

ACKNOWLEDGMENTS

This work was carried out at Imperial College, London and at MelbourneUniversity, Australia. The author wishes to thank Dr. G. D. Borley, Dr. R.LeMaitre, Professor J. F. Lovering, Dr. G. J. H. McCall, Mr. P. Suddaby, andProfessor J. Sutton, F.R.S. for their advice and encouragement, and ProfessorG. M. Brown, Dr. T. N. Irvine, Dr. H. W. Nesbitt, and Dr. R. N. Thompson forcritically reviewing the manuscript. Newmont Pty. Ltd. are thanked for makingavailable the material from diamond drill hole ND1 and their map (by D. S.Tyrwhitt) of the Norcott Complex. Western Mining Corp. Ltd. financed thetrace element analyses. The author was supported by an Overseas Scholarshipfrom the Royal Commission for the Exhibition of 1851 and by a ResearchFellowship from the University of Melbourne.

REFERENCES

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Lond. Ser. B, 240, 1-53.


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1968. Experimental studies on inversion relations in natural pigeonitic pyroxenes. Yb. CarnegieInstn. Wash. 66, 347-53.

CAMPBELL, I. H., 1973. Aspects of the petrology of the Jimberlana Layered Intrusion of WesternAustralia. Unpublished Ph.D. thesis, University of London {Imperial College).& BORLEY, G. D., 1974. The geochemistry of pyroxenes from the lower layered series of the

Jimberlana Intrusion, Western Australia. Contr. Miner. Petrol. 47, 281-97.& NOLAN, J., 1974. Factors effecting the stability field of Ca-poor pyroxene and the origin

of the Ca-poor minimum in Ca-rich pyroxenes from tholeiitic intrusions. Ibid. 48, 205-19.MCCALL, G. J. H., & TYRWHTTT, D. S., 1970. The Jimberlana Norite, Western Australia—a

smaller analogue of the Great Dyke of Rhodesia. Geol. Mag. 107, 1-12.CHAMBERLAIN, J. A., 1967. Sulphides in the Muskox Intrusion. Can. J. Earth Sci. 4, 105-53.COOPER, J. R., 1936. Geology of the southern half of the Bay of Islands igneous complex. Nfld. Dept.

Nat. Res. Geol. Sec. Bull. 4, 1-62.DICKEY, J. S., JR., YODER, H. S., JR., & SCHAIRER, J. F., 1971. Chromium in silicate-oxide systems.

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melts. Phil. Trans. R. Soc. Lond. 223, 40-62.GALE, N. H., 1967. Development of the delayed neutron technique as a rapid and precise method for

the determination of uranium and thorium at trace levels in rocks and minerals with applicationsto isotope geochronology. In Radioactive dating and methods of low level counting. Paper Sm-87/38, 431-52. International Atomic Energy Agency, Vienna.

HAUGHTON, D. R., ROEDER, P. L., & SKINNER, B. J., 1974. Solubility of sulphur in mafic magmas.Econ. Geol. 69, 461-67.

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trusion. Contr. Miner. Petrol. 33, 21-31.MACKINNON, A., & GALE, N. H., 1971. The distribution of uranium in some basic igneous

cumulates and its petrological significance. Geochim. cosmochim. Ada, 35, 917-25.HESS, H. H., I960. Stillwater igneous complex, Montana: a quantitative mineralogical study. Mem.

geol. Soc. Am. 80.IRVINE, T. N., 1975. Chromitite layers in stratiform intrusions. Yb. Carnegie Instn. Wash. 74, 300-16.

& SMITH, C. H., 1967. The ultramafic rocks of the Muskox Intrusion, Northwest Territories,Canada. In WYLLIE, P. J. (Ed.), Ultramafic and Related Rocks, New York, John Wiley and Sons.

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KUSHIRO, I., 1960. Si-Al relations in clinopyroxenes from igneous rocks. Am. J. Sci. 258, 548-54.MCCLAY, K. R., & CAMPBELL, I. H., 1976. The structure and shape of the Jimberlana Intrusion,

Western Australia as indicated by a combined geological and geophysical investigation of theBronzite Complex. Geol. Mag. 113, 129-139.

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WAGER, L. R., 1959. Differing powers of crystal nucleation as a factor producing diversity in layeredigneous intrusions. Geol. Mag. 96, 75-80.1963. The mechanism of adcumulus growth in the layered series of the Skaergaard intrusion.

Min. Soc. Amer. Spec. Pap. 1,1-9.& BROWN, G. M., 1968. Layered igneous rocks. Edinburgh: Oliver and Boyd.& DEER, W. A., 1939 (re-issued in 1962). Geological investigations in East Greenland. Pt. III.

The petrology of the Skaergaard Intrusion, Kangerdlugssuaq, East Greenland. Meddr. Grenland,105, No. 4, 1-352.BROWN, G. M., & WADSWORTH, W. J., 1960. Types of igneous cumulates. / . Petrology, 1, 73-85.

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a study of macro-rhythmic layering and cumulate processes

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