Peridotite xenoliths from the Jagersfontein kimberlite pipe: I. Primary and primary-metasomatic mineralogy

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Geochimrcaet Cosmochimica Acta Vol. 54, pp. 329-341 1990 krgamon Press plc.Printedin U.S.A.

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Peridotite xenoliths from the Jagersfontein kimberlite pipe: I. Primary and primary-metasomatic mineralogy PETER A. WINTERBURN,“~‘*BEN HARTE,’ and JOHN J. GURNEY’ ‘Grant Institute of Geology, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW, Scotland 2Department of Geochemistry, University of Cape Town, Rondebosch 7700, Cape Town, South Africa ‘Isotope Geochemistry Facility, DEMAST, CSIR, PO Box 395, Pretoria 000 1, South Africa (Received February 9, 1989; accepted in

revisedform October 3 1, 1989)

Abstract-The geochemistry and textures of peridotite xenoliths from the Jagersfontein kimberlite pipe are reported. The xenoliths have a primary minCralogy of olivine + orthopyroxene + clinopyroxene k garnet & spinel. They are subdivided into coarse and deformed xenoliths corresponding to high- and low-temperature estimates, respectively. Coarse-grained xenoliths are further subdivided into low- and medium-temperature groups. Mineral chemistry of these two groups is distinct, e.g., clinopyroxene 100 Al/(Al + Cr) 24 to 60 and 60 to 70 in the medium- and low-temperature groups, respectively. Lowtemperature xenoliths have undergone exsolution of pyroxene, spinel, and garnet in their pyroxenes. Primary modal metasomatism has occurred in the coarse xenoliths with the replacement of orthopyroxene by edenitic amphibole in the low-temperature xenoliths and of clinopyroxene by low-Ti phlogopite in the medium-temperature xenoliths. The amphibole stability limit confines it to shallower depths. Metasomatised xenoliths have been enriched in K, Na, Al, and Ca, and trace incompatible elements. Metasomatism is considered to have occurred at around 1 Ga by the infiltration of, and reaction with, ascending HzO-rich fluids with Sr and Nd isotopic characteristics similar to group II kimberlites. The widespread chemical equilibrium seen in metasomatised xenoliths suggests that the particularly distinctive features of the low-temperature Jagersfontein xenoliths, namely exsolution and very low equilibration temperatures, may also be a result of the primary metasomatism. was noted by JOHNSON (1973) and HARTE and GURNEY (1982), but evidence for its metasomatic origin along with phlogopite awaited the work of WINTERBURN(1987). HAGGERTY (1983) described the occurrence of titanate minerals with significant contents of large ion lithophilic elements (LILs) in concentrates from the Jagersfontein kimberlite. At Bultfontein these minerals are associated with the metasomatic events involving the formation of richterite (ERLANK et al., 1987). However despite an extensive search no such

INTRODUCTION THE JAGERSFONTEINKIMBERLITEPIPE, Orange Free State, lies 75 km to the southeast of the Kimberley kimberlite pipe cluster and contains a varied suite of megacrysts, peridotite and eclogite xenoliths, and crustal material: The peridotite xenoliths show some similarity to xenolith suites from other kimberlite pipes in that there are two major groups of xenoliths present: a low-temperature coarse group and a high-temperature deformed group (e.g., NIXON and BOYD, 1973; BOYQ 1974; BOYDand NIXON, 1975; DANCHIN and BOYD, 1976; HARTE and GURNEY, 1980; HARTE, 1983). However, the coarse xenoliths from Jagersfontein can be further subdivided. Little detailed work has been published on the Jagersfontein peridotite xenoliths. JOHNSON(1973) and HARTE and GURNEY(1982) published extended abstracts on xenolithic suites collected from Jagersfontein. The preliminary work of Harte and Gurney has been considerably extended by studies by WINTERBURN(1987) and HOPS (unpubl. data), respectively dealing with the lower-temperature co&e and higher-temperature deformed xenoliths (HOPS et %l., 1986, 1990; WINTERBURNand HARTE, 1987). BOYDand MERTZMAN(1987) presented analyses of several Jagersfontein xenoliths, noting the occurrence of minerals emplaced during the intrusion of the kimberlite. One of the distinctive features of the Jagersfontein lowtemperature xenoliths is the occurrence of amphibole which does not appear to be a product

of late-stage alteration.

titanates have been identified within the xenoliths themselves

(HACGERTY, pers. commun., 1986). This paper is concerned with establishing the evidence for several groups of peridotite xenoliths at Jagersfontein and, in particular, for noting differences between low-temperature xenoliths and their metasomatic equivalents. This paper will also consider the origin of the metasomatised xenoliths. The terminology of metasomatism and, in particular, the use of the term “modal metasomatism” follows HARTE (1983) in designating a metasomatic event leading to changes in modal mineralogy. The designation “primary-metasomatic” essentially follows HARTE et al. (1975), whilst the term “secondarymetasomatic” is largely a substitute for the term “late-secondary” of these authors. Secondary-metasomatic refers to metasomatism associated with entrainment in the kimberlite and eruption, whilst primary-metasomatic refers to metasomatism at depth in the mantle, which clearly precedes secondary-metasotiatism.

This Analytical techniques

Bulkroek major and minor element analyses were performed using standard X-ray fluorescence analyses at the Universities of Cape Town (JJG-timples) and Edinburgh (J-samples). Mineral analyses were

* Present address., Anglo American ResearchLaboratories,PO Box 106, Crown Mines 2025, South Africa. 329

330

P. A. Winterburn. B. Hane. and J. J. Gurney

undertaken at Edinburgh University usinga Cambridge Mk5 Electron probe microanalyser. Where no significant differences were noted analyses comprise the averages of core and rim values on. where possible. three grains of each mineral type. Bulkrock trace element analyses were undertaken by Isotope Dtlution with the assistanceof Peter van Calsteren at the Open University, Milton Keynes England. Isotopic analyses were undertaken at the Open University, Milton Keynes, England, and the Isotope Geochemistry Facility, CSIR. South Africa. Mineral separation, dissolution, and analytical procedures are described in WINTERBURN (1987, 1989). Complete petrographic descriptions and bulkrock and mineral analyses are available upon request from the main author.

From gross textural characteristics.the whole range OfJagersfontein mantle xenoliths can be divided into two groups: one coarse and one deformed. More detailed subdivision can be made concerning chemical and metasomatic features. However. it is important to note that within all groups of xenoliths one can distinguishthree setsof minerals. These comprise the following:

(ieotht~rmoharometr~ melhods

I. A primary mmc~mlgroup (olivine. orthopyroxenc. clinopyroxene. garnet, spinel). consisting of those minerals present prior to any

Pressure and temperatures of equilibration have been calculated using the 2-pyroxene thermometer of BERTRAND and MERCIER (1985) (BM85) and the alumina content of orthopyroxene barometer of PER~UNSet al. (I 98 1) (PE8 I). Calculated values are given in Table I. Although some authors (HOPS et al., 1986; FINNERTY and BOYD, 1987) have noted that the BERTRAND and MERCIER (1985) thermometer produces temperature values similar to the LINDSLEY and DIXON(1976) (LD76) and RNNERTY and BOYD ( 1986) (FB86) single pyroxene thermometers, it was found in this study that such correlation only occurred in xenoliths with calculated temperatures greater than 800°C. At temperatures less than 800°C both the LD76 and FB86 methodsgave anomalously low temperatures up to 300°C lower than the BM85 method. Calculated pressures and temperatures fit

closely to the 40 mW/m’ continental geotherm, with the deepest xenoliths (>I40 km) tending towards higher temperatures (up to

+ 200°C) than predicted by the geotherm. ,411of these latter xcnolrths (high-temperature deformed xenoliths: see below) lie on the diamond side of the diamond-graphite stahilitv curve. PETROGRAPHY

AND

P-l

CLASSIFIC.ATION

evidence of formation of modal metasomatic minerals within the xenoliths. In discussion of these minerals, and particularly their chemistry, we include not only their earliest textural representatives, but also the same minerals where they are the products of exsolution or deformation and recrystallisation. 2. A pri~,zar.~-tnetasomalf~ mineral group (phlogoptte. edemtc.

richtetite), comprising those minerals which appear to post-date the primary mineral group but have apparently achieved a high degree of textural and chemical equilibrium with the primary minerals, Rare replacement textures and the presence of elements (e.g.. Na. K. H,O) normally depleted in peridotites support a metasomatic origin postdating the formation of the primary mineralogy. 3. A sc~c(~ndur)~-mefasoma~~~~ mrneral group (phlogopite. clinopyroxene. orthopyroxene. spinel. pargasite, calcite, perovskitc, serpentine), comprising those minerals exhibiting a poor degree of textural and chemical equilibrium with both the primary and primarymetasomatic mineral groups. These minerals are also chemically

distinct from the primary and primary-metasomatic minerals and are considered to be late-stage phenomena related to kimberlite magmatism. These minerals and the processes involved in their formation are not considered further in this paper (see WINTERBI!RN and HARTE. 1990).

These xenoliths have an average grain size greater or equal to 2 mm and show no evidence of deformation and recrystallisation. They constitute approximately 80% of the peridotite xenoliths from Jagersfontein. They are further subdivided into two groups, both of which have a primary mineralogy of olivine f orthopyroxene f clinopyroxene t garnet ? Cr-spine1 but with the following distinguishing characteristics:

:!I8 162 162 162 165 165

I. Many of these xcnoliths show exsolution textures in their pyroxenes. and have a primary-metasomatic mineralogy ofedenite and rare richterite and Ba-phlogopite. They have low temperature and pressure estimates in the ranges ~950°C and 24-35 Khan (Table 1). 2. Exsolution textures are absent in these xenoliths. and the primary-metasomatic mineralogy comprises phlogopite. They have medium temperature and pressure estimates in the ranges 91% I02 I “C and 37-4 I Kban (Table I ). Xenolilhs wirh deformed ~exmres

These xenoliths show extensive evidence of deformation and recrystallisation with the formation of neoblasts, resulting in porphyroclastic and porphyroelastic-mosaic textures (HARTE, 1977). There is no evidence of exsolution textures or of the formation of new minerals by primary-metasomatic events in these xenoliths. The primary mineralogy comprises olivine ?zorthopyroxene f. clinopyroxene t garnet. They have high temperature and pressure estimates in the ranges I I29- 1346°C and 43-56 Kban (Table 1). The complete classification scheme concerning these xenoliths is shown in Fig. I. However, two exceptional xenoliths fah outside these ranges, a low-temperature. deformed xenolith (WINTERBURN, 1987). which appears similar to two xenoliths described by BORLEY and SUDDABY (1973) and a high-temperature coarse xenolith (Jt 17 in Table I) (WINTERBURN, 1987, Hops et al., 1990).

Metasomatism of peridotite xenoliths in kimberlites

PERIDOTITE

I

I-

1

DEFORMED

COARSE

I

I

I

1

High-T

Medium-T

Low-T

Non-metasomatised Metasomatised Non-me~asoma~ised

Fhlopopiie

Metasomatised Edenite, + rare Ba-phloQOpit6, richterits

FIG. 1.Classification scheme for peridotite xenoliths from the Jagersfontein kimberlite pipe. See text for details.

DETAILED PETROGRAPHY OF COARSE-GRAINED XENOLITHS Primary mineralogy

Coarse-grained xenoliths have an avemge grain size range of 4-7 mm for the primary mineralogy, with exceptional grains up to 20 mm. Garnet, spinel, and clinopyroxene are generally finer grained than the average grain size which is dominated by abundant large grains of olivine and o~hop~oxene (Fig. 2a). The development of texturally equilibrated polygonal-granuloblastic textures (HARTE, 1977) is limited. In the majority of low-temperature coarse xenoliths, garnet and clinopyroxene are minor constituents (O-6%) and o&en appear strung out between other mineral grains, giving an inte~itial appearance, suggesting either late crystallisation or formation by exsolution from other phases. Clinopyroxene in medium-temperature coarse xenoliths frequently has complex, irregularly shaped grain boundaries. Garnets frequently show considerable fracturing and kelyphitisation. Spine1 appears to be a primary phase in numerous coarse-grained xenoliths, including garnet-bearing xenoliths where it is occasionally interfingered with the garnet. Spine1 grains usually form as small, rounded blebs (< 1 mm) or as stringers along grain boundaries. Exsolution features. Many of the coarse, low-temperature xenoliths show evidence of exsolution of spinet, pyroxene, and garnet from orthopyroxene and clinopyroxene, the most common occurrence being that of tiny platelets (co.05 mm) or lamellae (0.05-l mm) of spine1 and/or clinopyroxene along the (010) plane in orthopyroxene (Fig. 2b). No exsolution features are present in any medium- or higbtemperature xenoliths. Sym@jec& ifftergron~f~. In some low-tem~ratu~, coarse-grained xenoliths, symplectic intergrowths of spine1 and clinopyroxene are present, similar to those described by BASUand MACCREWR (1975). In one xenolith (Jl35), the symplectite is concentrated around garnet. BASUand MACGREGOR( 1975) suggested that symplectic intergrowths might form by the breakdown of garnet to spine1 and clinopyroxene. Although this intimate association between garnet and the symplectite seems to support such a subsolidus reaction, it also appears, in this case, that the whole assemblage has exsolved from orthopyroxene. All other symplectite-bearing xenoliths are garnet free. Primary metasomatic mineralogy Primary metasomatic edenitic amphibole. Edenite occurs in 15

coarse, low-temperature xenoliths and one exceptional low-temperature, deformed xenolith. The modal abundance of edenite in amphibole-bearing xenoliths varies considerably, although it isgenerally

331

less than 10%. Usually the edenite forms discrete coarse grains with an average grain size range similar to that of the host peridotite, and edenite is generally in textural equilibrium with the primary mineralogy. Thus, the edenite in many xenoliths appears to be part of the primary mineralogy. However, one xenolith (J 134) offers clear evidence that the edenite formed later than the primary mineralogy (Fig. 2~). In J 134 a grain of edenite is in contact with orthopyroxene containing exsolution lamellae of garnet. These garnet lamellae protrude from the orthopyroxene and extend into the eden&e volume, suggesting that the edenite has replaced the orthopyroxene, whilst leaving the garnet unaffected. This evidence is critical in establishing both that the edenite formation was a metasomatic replacement event and that metasomatism occurred after the exsolution event. All eden&es have a similar mineral chemists, and the ~enite-~ng xenoliths also show common, distinctive geochemical features (see below). Thus, although good textural evidence exists in only one case, we suggest that the edenite is a primary metasomatic mineral formed in a modal metasomatic event. The edenite occurs with a variety of peridot&e mineral assemblages, including harzburgites or lherzolites with or without garnet and/or spinel. Unlike the richterite-bearing xenoliths from Bultfontein (ERLANKet al., 1987), there appear to be no vein-like structures in the Jagersfontein xenoliths, and the edenite is not associated with any particular mineral assemblage. Except for the occurrence of edenite, there are no textural or mineralogical differences between amphibole-bearing and amphibole-free xenoliths. Thus, the amphibole-bearing xenolitbs appear to have been derived from material similar to the coarse, low-temperature, amphi~le-fry xenoliths. Primary metasomatic richterite. Primary-metasomatic richterite is rare at Jagersfontein and has been found in only one xenolith in our collection, a coarse-grained spine1 dunite (J 11); although its presence has been reported by ERLANKet al. (1987). The richterite is poikiloblastic and contains numerous grains of olivine (~2 mm), frequently in optical continuity with each other and other grains outside of the richterite, suggesting the richterite may be replacing olivine. Sulphides are also abundant in this xenolith, both within the primary mineralogy and within the richterite. Primary met~omatie ph[ogopite. urns-me~matic p~~opite occurs in eight coarse, medium-temperature xenoliths and appears to be in textural equilibrium with the primary mineral phases in these xenoliths. The phlogopite forms coarse grains with a similar grain size range to their host rock, and it is typically only weakly pleochroic; ~thou~ some grains are noted to have distinct, highly pleochroic alteration rims. No vein-like structures are noted. Jmportant textural evidence for a metasomatic origin for the phlogopite is seen in two xenoliths. In JJG1795 and JJG1720, phlogopite appears to have partially replaced clinopyroxene. In JJG 1795 the phlogopite contains islands of clinopyroxene in optical continuity with clinopyroxene around the margin of the phlogopite (Fig. 2d). In both cases the phlogopite contains symplectic-like intergrowths with Cr-spinel. This symplectite is considered to be a product of the me&somatic replacement of the ~linopyroxene, the ~maining clinopyroxene not being intergrown with spinel. Primary metasomatic phlogopite occurs with a variety of mineral assemblages, including lherzolites and harzburgites with or without garnet and/or spinel. Phlogopite-bearing xenoliths tend, in general, to be poorer in garnet than amphi~le-bung xenoliths, but there is no evidence that phlogopite is replacing garnet. The difference in garnet contents between low-temperature (? edenite) and mediumtemperature (A phlogopite) xenoliths is more probably a direct result of garnet exsolution in low-temperature xenoliths. BULK CHEMISTRY

When discussing all aspects of the primary bulk major and minor element geochemistry of peridotite xenoliths from kim~rlit~, the effects of ponds-me~omatic mine&isation and enrichment associated with kimberlite magma-

332

P. A. Winterburn. B. Harte, and J. J. Gurney

FIG. 2. a) Coarse xenolith illustrating typical coarse-grained texture. Olivine grains are cross-cut by numerous scrpentinefilled fractures. Orthopyroxenes show some (lighter coloured) alteration along cleavage planes, Grain boundaries are generally smooth, and occasional 120” triple junctions are present. J5, coarse low-femperaturexenolith. planepolansed light, field of view is I2 mm. b) Symplectic intergrowth of clinopyroxene and Cr-spinet Some regular orientation of the Cr-spine1 within the clinopyroxene is apparent. The fine parallel lines in the orthopyroxene to the right are exsolution lamellae of Cr-spine1 and clinopyroxene. R, coarse low-temperature xenolith, plane polarised light, Jieid qf view IS 2 mm. c) Coarse xenolith with garnet forming exsolved blebs in orthopyroxene. The orthopyroxene itself is being replaced by primary-metasomatic amphibole (centre). The garnet blebs, which are not being replaced, have been left projecting into the amphibole. J134, coarse low-temperature, primary-metasomatic amphibole-bearing xenolith, plane polarised light, field ofview is 6 mm. d) Replacement of clinopyroxene by primary-metasomatic phlogopite. Only tiny islands of clinopyroxene now remain (e.g., centre left, bottom left); these are all in optical continuity. Cr-spine1 is being produced in the replacement reaction and forms a symplectic intergrowth with the phlogopite. JJGI 7Y5. coar.se mediumtemperature,

primary-metasomatic

phlogopite-bearing

xenolith plane polarised

tism have to be taken into account (HART& 1983). This is especially true for the Jagersfontein suite, where several xenoliths with high bulk KzO and Rb contents are seen to show considerable secondary-metasomatic mineralisation, with the growth of extensive secondary-metasomatic phlogopite along grain boundaries. In order to examine the bulk chemistry of the xenoliths prior to any secondary-metasomatic interaction, those xenoliths showing as little secondary alteration as possible have to be selected. In the present case xenoliths with Rb > 7.5 ppm and K20 > 0.15 wt% are seen to contain significant amounts of secondary-metasomatic mica and have been eliminated from the following discussion on bulk chemistry. Xenoliths containing primary-metasomatic mica have been

light, field of view ic I 2 mm.

allowed through this screening process since their high-Rb and K20 contents are a function of the greater modal primarymetasomatic mica content rather than the secondary-metasomatic mica content. Selected whole rock analyses are shown in Table 2. Major

element chemistry

Figure 3 illustrates the distribution of xenolith types relative to bulk 100 Mg/(Mg + Fe). There is a strong separation of coarse xenoliths from deformed xenoliths, such that the coarse xenoliths tend towards 100 Mg/(Mg + Fe) > 92 (91.4-93.9) and deformed xenoliths to 100 Mg/(Mg + Fe) < 92 (89.3-92.8).

333

Metasomatism of peridotite xenoliths in kimberlites Table

Type' SO2 TiO A= Sii3 Fe XT&O MgO cao Na20 x20 P 0s LX I Total

2. klected

Rb Zr Nb Be

analyses. 5134 CLA

JJG1716 CLA

JJG1727 Cn

JJG1723 CHJ?

JJG1795 Cm

39.73 0.06 0.7. 5.42 0.09 44.63 0.25 0.02 0.04 0.03 7.63

41.30 cl.05 0.50 6.27 0.06 45.05 0.21 n.d 0.02 0.02 -

43.25 0.04 1.61 6.80 0.11 41.62 1.23 0.25 0.22 0.03

44.46 0.05 1.90 6.13 0.11 42.24 1.07 0.16 0.25 0.05 3.11

42.21 6.06 0.10 5.78 0.10 45.46 0.31 0.07 0.05 0.04 5.25

43.74 O.C9 0.65 3.39 0.10 41.99 0.50 0.12 0.29 0.05 6.63

43.00 0.11 0.49 5.71 0.09 42.64 0.59 0.19 0.25 0.05 6.03

99.57

92.43

95.16

100.36

100.21

100.47

99.95

27 2 6 6

50

16 2 7 4

ce Nd SD EU Gd DY Er Yb

rock

J157 CL

Ni CP Fir

whole

JJG1761 CL

1.45 0.24

2269 1316 29 2 6 4 41

2078 2699 127 7 11 14 284 17.70 7.05 1.05 0.25 0.70 0.21 0.08 0.09

135 7 12 21

9

13 8

56 11 13 6

15.eo 6.37 0.92 0.23 0.67 0.30 0.13 0.12

* CL - Coarse Low-temperature xenolith. CLA - Coarse Law-temperature Amphibole-bearing xenolith. CR - Coarse Medium-tcnpcraturt xemlith. CMP - Coarse Medium-temperature Phlogopite-bearing xsnolith. n.d - not detected. A dash QP blank indicates no data available. All oxides and elaslents excluding the REEs were analysed by XRF. REEs were analysad by isotope dilution.

Medium-temperature coarse xenoliths have 100 Mg/(Mg + Fe) ratios of 93-93.5, whilst the low-temperature ones show a much wider range of 91.5-94. In both groups, given the low number of samples, the 100 M&I(Mg + Fe) range appears similar for xenoliths both with and without primary-metasomatic minerals. The deformed high-temperaturn xenoliths show a rough trend of increasing Al, Ca, and to a lesser degree Na and Ti, with decreasing 100 Mg/(Mg + Fe). Furthermore, although coarse, low-temperature xenoliths within themselves show little evidence of significant trends, it is apparent that, in general, they form the ~ntinuation of the trend defined by the deformed high-temperature xenoliths to more Mg-rich compositions. Coarse xenoliths showing primary metasomatism have higher &O and NarO than non-me~mati~ specimens: the higher K20 correlates well with the abundance of phlogopite; the correlation of NazO with the occurrence of edenite is less precise. At a given 100 Mg/(Mg + Fe) there is a tendency for the pan-me~ma~c xenoliths to show slightly higher CaO and Al203 contents than xenoliths without primary-metasomatic minerals.

erage non-metasomatised garnet peridotite (GP) from Bultfontein. Two porno-me~~matic amphi~le-~a~ng xenoliths (J 134 and JJG 17 16) are notably enriched in all the incompatible elements up to Zr, relative to non-metasomatised xenoliths. They also show a strong similarity to the average Bultfontein potassic-richterite and p~ogopite-bung peridotite (PKP). It is notable, however, that the Pa-xenoliths are enriched in Zr, K, and Rb relative to the Jagersfontein xenoliths. Similarly, medium-temperature, primary-metasomatic, phlogopite-bung xenoliths (JJG 1723 and JJGl795), are enriched in incompatible elements relative to non-metasomatised medium-temperature xenoliths (JJG 1727) and furthermore show some similarity with the average phlogopite peridotite (PP) from B~tfontein. The amount of enrichment in these medium-temperature, metasomatised, coarse xenoliths is less than that in the low-temperature ones, except for Rb and K.

Incompatible eiements

Chemical variations in the primary silicate minerals are shown in Fig. 5. Selected analyses from coarse xenoliths are given in Table 3. Although 100 Mg/(Mg + Fe) variations are largely a reflection of bulk rock 100 Mg/(Mg + Fe) variations, there is some evidence that mineral IO0 Mg/(Mg + Fe) ratios extend to lower values in the modal metasomatised xenoliths than in their non-metasomatised equivalents. AlthouaJl this suggests that modal metasomatised xenoliths may have been enriched in Fe, there is no indication of Ti enrichment of the primary mineralogy, such as that described for the modal metasomatised xenoliths from Matsoku (HARTEetal., 1987).

Selected whole rock analyses from primary-metasomatised and non-metasomatised xenoliths are plotted in Fig. 4a and b following the method of TI-IOMPSON (1982) (data from Table 2). Average xenolith data from Bul~on~in (ERLANK et al., 1987) are also plotted. Two coarse low-temperature xenoliths with no traces of primary-metasomatic minerahsation which contain minimum quantities of secondary-metasomatic min~li~tion (JJG1761 and 5157) both have low contents of incompatible elements and are similar to the av-

MINERAL CHEMISTRY Primary mineralogy

P. A. Winterburn, B. Harte. and J. J. Gurnq

334

from primary-metasomatised. low-tcmperaturc \cnoliths arc‘ notably richer in Fe than those In non-mctasomatiscd xcnoliths. They all lie within H,wx:;~K I 1“s ( 1070) ranges li)r spin& from peridotitc xenoliths in kimbcrlites.

0.3 0.1

It is notable that minerals from primary-metasomatIscd. low-temperature xenoliths have a remarkably uniform mineral chemistry and detinc a very narrow composItIonal ranpc.

0.3

Q) u .-

0.1

:

3.0

8

t-

c Primq~ tne!asotnatrc denrtc The primary-mctasomatic amphibole is classified as edenite and edenitic hornblende following the classification of LEAKE (1978). the majorit) falling within the edenitic hornblende field. All compositions lie within the general formula range of

1.0

+

3

5.0

I 41203

3.0 1.0

0.2

i

0.1

1

c 1 OOMg/CMg+Fe)

FIG. 3. Bulk-rock major elements vs. 100 Mg/(Mg + Fe) for peridotites with only minor amounts of alteration. With the exception of primary-metasomatic, phlogopite-bearing xenoliths, all xenoliths with either greater than 7.5 ppm Rb or 0. I5 wt% K20 have not been plotted. Enclosed field is for high-temperature xenoliths; filled circles are medium-temperature xenoliths; filled squares are primary-metasomatic, phlogopite-bearing, medium-temperature xenoliths: crosses are low-temperature xenoliths; triangles are primary-metasomatic. amphibole-bearing, low-temperature xenoliths.

Clinopyroxene 100 Ca/(Ca + Mg) ratios fall into three distinct groups corresponding to low-. medium-. and hightemperature estimates. NazO content of the pyroxenes is notably higher in the medium-temperature xenoliths relative to the low-temperature xenoliths, with garnet-free, low-temperature xenoliths having the lowest Na20 contents. NazO content of the clinopyroxene varies systematically with total A&O, and Cr203 contents. Clinopyroxene A&O3 and Cr203 contents do not demonstrate a simple exchange trend; however, it is notable that clinopyroxenes in low-temperature xenoliths have lower lo0 AI/(AI + Cr) ratios (60-75) than in medium-temperature xenoliths (24.5-60). High-temperature xenoliths define a general trend of increasing 100 Al/(AI + Cr) with decreasing 100 Mg/(Mg + Fe). This trend continues through the medium-temperature group with increasing 100 Mg/(Mg + Fe). Low-temperature xenoliths, which follow on from mediumtemperature xenoliths in terms of increasing 100 Mg/(Mg + Fe), do not continue the 100 AI/(AI + Cr) trend but are displaced to higher IO0 AI/(Al + Cr) values. This may be related to the fact that it is only in the low-temperature xenoliths that exsolution features involving spine1 and garnet are seen. Cr20, would lower the 100 AI/(AI + Cr) ratio, having been expelled from the pyroxene structure to form spinel. Primary spinels are Cr-Mg-rich (7 to I4 cations Cr ” and 4 to 6 cations Mg2+, 0 = 32) and Ti-poor (< I wt%). Spinels

It is interesting to note that, within the low-temperature xenoliths with widely varying proportions of primary minerals, all the amphiboles fall within a very narrow compositional field. Ranges for the minor elements (TiOz. 0.01 to 0.2; MnO, 0.6 to 0.7; NiO. 0.3 to 0.36 wt%) are extremely

a.

b. kb

Ba K

Nb

La Ce

Sr

Nd

P

Zr Ti Y

FIG. 4. Spidergrams for selected Jagersfontein xenoliths. normalised after SUN (1980). a) JJGl727 is a medium-temperature, non-metasomatised xenolith. JJGl723 and JJGl795 are medium-temperature, primary-metasomatic, phlogopite-bearing xenoliths. b) JJG I76 I and J I57 are low-temperature, non-metasomatised xenoliths. JJG 17 I6 and J I34 are low-temperature, primary-mctasomatic, amphibolebearing xenoliths. GPAVG, PPAVG, and PKPAVG are average garnet peridotite, phlogopite peridotite, and phlogopite K-richterite peridotite from Bultfontein (data from ERUNK et al.. 1987).

Metasomatism of peridotite xenoliths in kimberlites

0.1

0.2

0.3

0.4

05

0.6

FDH I : I-: W-HI+ CLA

0, z”

ICMP ICM

-CL

b. Clinopyroxene

-OH +cMp I

0 (5:

ICM Hm*-HCLA

1

‘CL

FIG. 5. Compositional ranges for peridotite minerals from Jagersfontein. DH-Deformed, high-temperature xenoliths; CM-coarse, medium-temperature xenoliths; CMP-coarse, medium-temperature primary-metasomatic, phlogopite-bearing xenoliths; CL-coarse, lowtemperature xenoliths; CLA-coarse, low-temperature, primarymetasomatic, amphibole-bearing xenoliths. Vertical dashes indicate analysed values.

80

82 c.

84

86

88

Garnet

limited, reflecting the lack of variability in major element chemistry. Figure 6 shows a projection from olivine onto the plane of CS-MS-A in the CMAS tetrahedron, of those xenoliths (4)

containing the five-phase assemblage olivine + orthopyroxene + clinopyroxene + garnet + edenite. Tie-lines between the three phases-garnet, clinopyroxene, and edenite-are similarly orientated, suggesting a good degree of chemical equil-

P. A. Winterburn, B. Harte, and J. J. Gurney

33h Table

3. Selected

JJGl7l6

Mineral

S10* T10

0.06

::*03 a Fe M"0 MgO ce0 *a*0 K2O a1

a3

JJC1723

;zy3 '-'IlO MYlO CaO Na2O

2.85

0.79 4.55 0.17 36.36 0.21 0.10

22.22 6.02 0.56 20.49 4.96 0.07

98.96

100.35

100 .I1

0.06 8.99

Kz" otal

SiCl2 TicI*

0.26

0.11 7.23 0.14 51.23 0.02 0.06

2 1.68 29

10.48 2.30

1 55 0.13 16.09 21.49 2.03 0.02 99.43

2.32 0.13 19.67 IO.14 4.43 1.11 97.23

SP 0.25 0.90 15.60 50.92

16.47 0.79

14.42

Coarse Medium-tcm~crature Phlouopitt-bearins OL OP CP PHL SP 41.08 58.41 55.56 42.07 0.18 0.02 0.00 0.11 0.45 0.72 0.01 0.61 1.73 12.58 3.87

SlO2 T1O Al2 a 3

556866

Analyses

Coarse Low-temperature amphibole-bearing OL OP GT CP Ai 40.04 57.62 41.12 53.99 46.17 0.07 0.09 0.10 0.16 0.48

0.12

0.09

51.40

37.53 0.29 0.12

0.30 3.93

4.97 2.08 0.12 15.54 17.80 3.44

101.68

101.28

101.37

0.90 3.13

65.63 16.87

0.00

0.92

26.37 0.04 0.40 IO.10 96.04

12.98

101.17

Cc,arse Lou-temperature amphibole-, Ba-phlogopifc-bearing CP AM PHL SP OL OP 40.6 57.31 53.33 45.56 38.69 0.34 0.03 0.16 0 21 0.37 1.01 0.12 0.68 2.15 9.99 14.12 30.84

A1263 ',:a"3 p(nO MgO CaO Na20

K20 BaO Total

8.72 0.13 49.50

0.16 5.40 0.11 35.48

0.01

0.15 0.05

99.06

Jll Coarse S10* TX0 A12 d 3 ',:a03 MILO MgO CaO k20 K20 Total

0.00 99.37

'Lou-temperature AM OL 39.96 53.53 0.02 4.23 0.03 6.27

0.91 2.78

0.08

0.08

50.34

21.42 5.80 6.56 1.01 96.34

98.1

2.77 3.14 0.15 14.47

2.88 1.71 0.09 19.91

19.71 2.88

10.35 3.72

0.05 0.00 98.81

1.09 0.00 95.51

0.67 2.90 0.02 24.96

32.61 14.96 0.44 16.79

0.96 8.05 3.8 94.54

96.99

richterite-bearing SP 0.23 0.06 7.47

23.77

56.57 0.64 9.72

MODAL METASOMATIC 98.66

ibration and similar temperatures and pressures of formation. This is further illustrated in Fig. 7 with respect to Fe/Mg ratios in coexisting edenite, garnet and clinopyroxene. Primary metasomatic richterite. The formula of the richterite is T-S17.55

100 Mg/(Mg + Fe) ratios of the primary-metasomatic phlogopite (92.7-95.3) reflect the bulk rock and primary mineral variations, and suggest that chemical equilibrium has been achieved between the primary-metasomatic phlogopite aud the host mineralogy. No significant fluorine was detected in any of these phlogopites above the detection limit of the electron microprobe (co.05 wt%). Barium content is also low (<0.2 wt%). Primary-metasomatic phlogopites are chemically distinct from secondary metasomatic phlogopites in having lower Cr, Ti, and Fe contents and higher 100 Mg/(Mg + Fe) and 100 Al/(Al + Cr) ratios (Fig. 9). A similar relationship was noted by DELANEY et al. ( 1980) for primary- and secondary-metasomatic phlogopites from other xenolithic suites. Primary metasomatic Ba-phlogopite. This phlogopite, in the low-temperature xenolith JJG866 (Table 3). is distinct from all other primary-metasomatic phlogopites in that it contains 3.8 wt% BaO. All other primary-metasomatic phlogopites contain less than 0.2 wt% BaO. In terms of 100 Mg/(Mg + Fe). 100 Al/(Al + Cr), Ti02, and Cr203, this phlogopite is similar to other primary-metasomatic phlogopites. However, AL’” is considerably higher in this mica. whilst Si and K are much lower. The substitution, which can be summarised as BaAlK-~‘Si -‘, was also suggested by GASPER and WYLLIE (1982) for high-BaO phlogopites in carbonatites. The K/Ba ratio of this mica (3.4) is considerably lower than the range of peridotite micas (14-319) from SMITH et al. ( 1979).

Al:;5

C-Al&

Cro.lo

B-Feo.ls

C~O.SSNao.94

A--N&M

I(o.18

MIZGO

Feo.15

REACTIONS

The gross textural and structural features of the primarymetasomatised xenoliths suggest that metasomatism in these xenoliths was dominantly a relatively simple equation of 1 “reactant” = 1 ‘product” + “metasomatic exchange,” and involved no volume change within the xenolith. Mass balance calculations, following the method of GRESENS( 1966), have

been used to establish the chemical components involved in the “metasomatic exchange.” Mass balance calculations have been calculated for the formation of the major primary-metasomatic minerals, phlogopite and edenite. Since chemical equilibration has re-

It differs from the edenites essentially in that it contains more Na in the B-site at the expense of Ca, and more Si in the Tsite in place of Al’“, suggesting the exchange NaSiCa-‘Al-‘. This is illustrated in Fig. 8, where the edenites themselves show this substitution, although to a lesser extent. Primary metasomatic phlogopite. The primary-metasomatic phlogopites correspond to a general formula range of Z-Si5.82-5.98

Ali&4-2.1~

Y-Al&-o.os

Tio.os4-o.c96 Cro.o3o.-o.,z4 Mg5.28-5.62

F%.28a).44

X-k.76-2.00

Mno.m.012

&.036--0.162

MS FIG. 6. Five-phase assemblage (garnetedenite-clinopyroxene-orthopyroxene-olivine) compositions plotted onto the plane of CS-MSA in the CMAS system. Tie-lines join co-existing minerals.

337

Metasomatism of peridotite xenoliths in kimberlites

. E ._ c

: 0

2 ’

90

.

\

16'

_

.

.

Jll .

\

.

.

..

..

l

12-

m\

6.5

14.

z - . l

I)

16

16

Mg/Fe

20

CPX

22

30

Mgl Fe

40

50

garnet

‘.Oi

FIG.7. Mg/Fe in edenite vs. Mg/Fe in garnet and clinopyroxene for all low-temperature xenoliths containing primary-metasomatic, edenitic amphibole.

,

,

,

1.5

2.0

2.5

“;enite

3.0

Ca,+AI,

moved evidence of the compositions prior to metasomatism, the pre-me&somatic orthopyroxene and clinopyroxene compositions are assumed to be the same as the compositions now found across the range of xenoliths. This cannot be demonstrated and is clearly a source of error, but it is an approximation and starting point.

RG. 8. NaB+ Si, vs. CaB+ AlTfor all primary-me&somatic amphiboles from Jagersfontein, illustrating the exchange Na!Xa-‘Al-‘. Analysis J I I is a richterite; all other analyses are edenites or edenitic

The reaction clinopyroxene to phlogopite

100 g OPX + 9.5 g Altos + 9.5 g CaO + 3.5 g Na20

TWO xenoliths,

JJG1723 and JJG1795, show primarymetasomatic phlogopite replacing clinopyroxene with the production of spinel, leading to the formation of a phlogopite and spine1 symplectic intergrowth. The clinopyroxene in these two xenoliths is notably Cr-rich. In the case where clinopyroxene is not Cr-rich, no spine1 is produced, for example, JJG853 (WINTERBURN, 1987). Mass balance reactions have been calculated for both JJG1723 and JJG853. The mass balance reactions are

homblendes. Site allocations after LEAKE (1978).

+ 2.0 g CrzOJ + 1.2 g KzO + x Hz0 = 95.7 g EDENITE + 16.8 g MgO + 13.0 g SiOl t 2.4 g FeO. Ti02 contents were not analysed in this xenolith; however, other edenite-beating xenoliths suggest that around 0.22 g of Ti02 are added. Essentially, the main components added to the rock are Al, Ca, Na, and HzO. The components removed are in the same proportions as typical peridotite olivine. Hence, the products side of the reaction could be reduced to

for JJG853 100 g CPX + 10.7 g KzO + 8.7 g A1203

n

4.0- , m

+ 4.8 g MgO + 0.4 g Ti02 + 0.2 g Fe0 + x Hz0 = 85.7 g PHLOCI + 20.7 g CaO + 18.0 g SiOl + 1.4gNazO+

I.0gCr203+0.1gMn0

for JJG I723 (CrzOx assumed constant) 100 g CPX + 9.0 g AlzOs + 8.2 g KzO + 6.8 g MgO

l

0” F

3.0-

# &',C

8

2.0-

mm .:

r

lo.

m

l

n

trn

. l

l

.

+ 1.7gFeO+0.4gTiOz+0.1gMnO+xH20 = 80.9 g PHLOG + 7.3 g SPINEL

mm

m<<

98-

.

.

’ l*

.

.

.

-1-r.

. .

.

+ 2 I .6 g SiO2 + 17.8 g CaO + 3.1 g Na20. It is evident that, disregarding MnO as a minor component, these two reactions are very similar in terms of the additions and subtractions during primary metasomatism. In both cases the essential components added are Al, K, Mg, and H20with Si, Ca and Na being removed. The reaction orthopyroxene to edenite

J 134 shows clear petrographic evidence of edenite replacing otthopyroxene without the involvement of any other mineral phases. The mass balance calculated for this xenolith is

1 ooMg/(Mg+Fe) FIG. 9. 100 AI/(AI + Cr) and wt% Ti02 vs. 100 Mg/(Mg + Fe) for all metasomatic phloaopites from Jagersfontein. Circles are primarymetasomatic phlogopites;squares are secondary-metasomatic phlogopites.

P. A. Winterbum, B. Harte, and _I.J. Gurney

338

“reactants” = 95.7 g EDENITE + 32.2 g “Ohvine.” However. the mechanics of the mass balance equation require that the olivine component is removed from the xenolith. ISOTOPES Selected isotope analyses from WINTERBURN ( 1987, 1989) are given in Table 4. The three whole-rock samples are listed in order of increasing abundance of primary metasomatic amphibole. “Rb/s6Sr and ‘43Nd/‘44Nd ratios decrease with increasing amphibole content, and ‘47Sm/‘44Nd and 87Sr/86Sr ratios increase. The amphibole itself contains 65 ppm Nd, 11 ppm Sm, 2841 ppm Sr, and 5 ppm of Rb, and has the lowest “Rb/“Sr and ‘43Nd/‘44Nd and highest ‘47Sm/‘44Nd and 87Sr/86Sr of these analyses. In terms of isotopic and concentration ratios, the amphibole lies within the group II kimberlite field Of SMITH ( 1983). WINTERBURN ( 1989) suggested that the trend shown by the whole-rock analyses away from the amphibole towards the group I kimberlite field was a result of contamination of the samples by as little as 0.5% kimberlite. With decreasing amphibole content the xenoliths will be more susceptible to the effects of kimberhte contamination by virtue of their decreased Nd and Sr contents. WINTERBURN (1987) suggested, on the basis of the wholerock analyses of JJG 17 16, that the metasomatic event had a minimum Sm-Nd Tdm age of 0.6 Ga. The new data (Table 4) indicate that the minimum Tdm age of metasomatism is in the region of 1 Ga. WALKER et al. (1989) analysed two coarse-textured, non-modal, metasomatised xenoliths from Jagersfontein and suggested a minimum T,,,, age for isotopic enrichment in their xenoliths of I Ga. These two studies suggest that the modal metasomatism and incompatible element enrichment seen in both non-modal and modal metasomatised xenoliths may be different aspects of the same event and that this event occurred at at least 1 Ga. DISCUSSION

Summary of principal aspects Qfprimary metasomatism

1. The primary-metasomatic mineralogy at Jagersfontein is dominated by the hydrous minerals edenite and phlogopite, and these minerals show clear evidence of chemical equilibration with the primary minerals in the rocks. 2. The edenite is confined to xenoliths showing low-temperature and low-pressure characteristics (~950°C and 2435 Kbars), indicating shallow depths of derivation of
S'Rb/66Sr S'srP6sr 67sr/S6s,,

WI7 0.1550 0.70631,3 0.70612

5134 UR 0.1425 0.70715+1 0.70698

JJG666 UR 0.0122 0.70749~2 0.70748

JJG666 EDENITE 0.0051 0.70773-4 0.70773

147Sm/144Nd 14'Nd/141Nd 143Nd/144Nd1

0.0874 0.51255-3 0.51250

0.0908 0.51239+1 0.51234

0.0965 0.51229+1 0.51224

0.1098 0.51217%1 0.51211

Tdm

612 Ma

820 Ita

985 na

1268 Ma

5561716

Sm-Nd

3. The phlogopite (excluding the one Ba-phlogopite) IS confined to xenoliths showing medium-temperature characteristics (9 13-102 I “C and 37-4 1 Kbars) and whose pressure estimates indicate a deeper depth of derivation in rhc region of 120- I40 km. 4. Bulk-rock major and minor element analyses (when allowance for late-stage alteration and secondary-metasomatism is made). show the general addition of K20 and less regular addition of NazO to the modally metasomatised rocks. An increase in A&O3 and CaO may be indicated in some cases by bulk-rock chemistry. In addition, rocks showing primary metasomatism are relatively enriched in incompatible elements, and this enrichment may have arisen at the same time as the evident modal metasomatism. 5. Considerations of the present mineral chemistry and mass balance calculations performed using mineral analyses indicate the major addition of A&O3 to all primary-metasomatic rocks, as well as KzO to the phlogopite-bearing rocks and NazO and CaO to the edenite-bearing rocks. The mass balance calculations show that CaO and Na20 should be released during the formation of phlogopite. Thus, it appears possible that CaO and NazO have passed into the fluid, with phlogopite formation at greater depths, and been carried to shallower depths, where they have been re-incorporated in the peridotites during edenite formation.

Chemical equilibrium, primar?) metasomatism, and exsolution The data obtained on primary and primary-metasomatic minerals show chemical equilibrium has been achieved throughout the coarse xenolith suite, in spite of the fact that in the low-temperature xenoliths exsolution textures have been preserved and rare replacement textures provide petrographic evidence of metasomatism. Excluding secondary metasomatic effects (WINTERBURN and HARTE, 1990) the attainment of a high degree of chemical equilibrium is demonstrated by the following: I. No core-rim zoning exists within individual mineralsboth primary and primary-metasomatic. 2. Chemical homogeneity in any particular mineral species within the xenoliths is considerable, to the extent that in the low-temperature xenoliths exsolution lamellae of garnet, spinel, or pyroxene within pyroxene have the same composition as other grains elsewhere in the xenolith. 3. Mg/(Mg + Fe), AI/(AI + Cr), and Ca/(Ca -t- Mg) ratios indicate that a high degree of chemical equilibrium has been established between the different minerals within the xenoliths. On the thin section scale it cannot be demonstrated that there has been a change in the primary mineral chemistry on modal metasomatism. Comparisons between different xenoliths show that mineral compositions for both metasomatised and non-metasomatised xenoliths within the two temperature groupings have considerable overlap. Thus, it is uncertain which, if any, of the non-metasomatised xenoliths shows pre-metasomatic compositions.

Metasomatism of peridotite xenoliths in kimherlites In addition to the above observations we have evidence from bulk-rock chemistry (including trace incompatible elements) that all coarse xenoliths, given their depleted major element compositions, may have undergone some metasomatism and enrichment, even where there is no evidence of modal metasomatism. This re-enforces the view that there may well no longer be any material with mineral or bulk compositions representative of the situation prior to primarymetasomatism. Apart from the distinctive aspects of ptimary-metasomatism noted for the Jagersfontein coarse xenoliths, the same rocks show two other distinctive features when compared with other coarse peridotite xenoliths from South Africa: 1. the extensive development of exsolution in the orthopyroxenes and clinopyroxenes of the xenoliths recording the lowest temperatures; and 2. the consistently low temperatures (<8OO”C) recorded by peridotites showing exsolution features. The widespread evidence of chemical equilibrium in exsolved phases, host phases, and other primary minerals noted above makes the conjunction of these two phenomena all the more remarkable. It contrasts with the preservation of chemical gradients in xenoliths showing exsolution from clinopyroxene at higher temperatures (HARTE and GURNEY, 1975; SAUTTER and HARTE, 1988) and the occurrence of chemical inhomogeneities in some xenoliths showing metasomatism associated with melts at higher temperatures (SMITH and EHRENBERG,1984; HARTE et al., 1987). One way of explaining the low temperatures of equilibration in the Jagersfontein peridotites, given the evidence of equilibration at the time of primary metasomatism, is that they are products of the primary metasomatism. Thus, the fluids (hydrous, as argued below) which cause the metasomatism may also promote chemical transport and equilibration between the minerals. The effect of HZ0 promoting diffusion is well known and has recently been linked with the availability of protons (ELPHICKand GRAHAM, 1988). Nature of metasomatic fluids In this section we shall endeavor to ascertain the composition of the primary metasomatic fluid involved in the production of edenite and phlogopite in low- and medium-temperature xenoliths. First we may consider the proportions of volatile species H20, COZ, and CH., . Several lines of evidence suggest that the fluid is dominantly HzO-rich rather than COz-rich. These are 1. EGGLER (1987) noted that the movement of COz-rich fluids would be restricted by buffering to depths of less than 60 km (18 Kbars), whereas the movement of H20-rich fluids would likewise be restricted to depths of greater than 70 km (22 Kbars). Furthermore, if the fluid was CO*-rich we would expect to see the presence of carbonate minerals in the metasomatised xenoliths. The only carbonate mineral identified in these xenoliths is, on the basis of texture, considered to be secondary-metasomatic.

339

2. EGGLER (1987) suggested that COsxx (HzO-rich) fluids would be able to carry a far greater inventory of elements compared to COTrich fluids and additionally that COT poor fluids would have a lower capacity for carrying Ti than C&rich fluids. The primary metasomatism at Jagersfontein shows very little evidence of Ti02 modification. Flouride is indicated to be present in the fluid by its presence in the primary-metasomatic minerals. However, whether it was a major component is uncertain. Its presence would enhance the solubility of many of the elements in the fluid (for example, the alkalis, alkaline earths, Fe, and the REE), while at the same time depressing the solubility of others such as Al and Si (EGGLER, 1987). There is no significant evidence bearing on the CH4 content or oxygen fugacity of the fluid. Thus it seems probable that the agent responsible for the primary metasomatism at Jagetsfontein was an HZO-rich fluid containing minor amounts of F. Such a fluid would be capable of containing all the elements required in the new metasomatic minerals, as well as being capable of carrying those removed. The actual composition of the metasomatic fluid cannot be calculated. However, the metasomatism is dominantly potassic at depth (phlogopite) and sodic/calcic at shallower depths (edenite). RYABCHIKOVet al. (1982) suggested that metasomatism in the mantle would be essentially potassic or sodic and that the two metasomatic zones would be spatially separated, with sodic metasomatism (leading to the formation of amphibole) at shallower depths and potassic metasomatism (leading to the formation of phlogopite) at greater depths. Although SCHNEIDERand EGGLER (1984) could not substantiate the results of RYABCHIKOVet al. (1982) the suggestion of RYABCHIKOVet al. ( 1982) appears to be supported by the above results. Primary metasomatic model The conditions of formation indicated by the P-T estimates for Jagersfontein xenoliths suggest mantle zones 75- 140 km deep for low- and medium-temperature coarse xenoliths, and 140- 190 km deep for high-temperature deformed xenoliths. Xenoliths with primary-metasomatic amphibole have depth signatures of approximately 75-120 km, whilst those with primary-metasomatic phlogopite give depths of 120- 140 km. Only the Ba-phlogopite-beating xenolith has both primarymetasomatic phlogopite and primary-metasomatic amphibole, and for this rock no pressure and temperature data can be calculated. Thus, it is reasonable to assume that the phlogopite-amphibole boundary is around 120 km. HARIYA et al. (1974) placed the maximum depth of pargasitic amphibole at approximately 120 km, which (although pargasitic rather than edenitic) shows remarkable agreement. Phlogopite stability extends to considerably greater depths (MENGELand GREEN, 1990) than those indicated in this suite by the primary-metasomatic, phlogopite-bearing xenoliths. The only xenoliths representing material from depths greater than 140 km are the high-temperature deformed xenoliths. These show no evidence of having undergone a modal metasomatic event involving primary-metasomatic phlogopite.

P. A. Winterburn, B. Harte, and J. J. Gurney

340

Therefore, we envisage an event, probably around 1 Ga, in which a HzO-rich fluid percolated through the mantle section represented at Jagersfontein and, at depths of at least 120-140 km, the fluid reacted with the peridotite, precipitating phlogopite at the expense of clinopyroxene in an esscntially potassic me&somatic style. The residual fluid, now enriched in Ca and Na, migrated upwards to shallower depths at less than 120 km to within the amphibole stability field. Here it precipitated edenite in an essentially sodic and calcic metasomatic style at the expense of orthopyroxene. The source of the H@-rich fluid can only be a matter of speculation given the available info~ation. A devolatili~tion process giving rise to the fluid at depths beiow those of the Jagersfontein xenoliths may or may not involve magmatic activity. Although there is no evidence of clearly igneous phenomena involving melts in the coarse xenoliths, magmatic activity beneath the portion of liihospheric section represented by the coarse xenoliths is not precluded. WYLLIE( 1987, 1990) and EGCLER (1987) have suggested that magmas arising from the asthenosphere beneath undisturbed cratonic areas would evolve H&-rich fluids at depths of around 185 km. This would occur where the local geotherm intersects with the lherzolite-C-O-H solidus and leads to the crystallisation of the magma within the lithosphere and hence the loss of volatiles contained within the melt. Such lhrids would be enriched in dissolved minerals from the magma body and have a capacity to metasomatise the mantle. It is also unclear what the source region of the fluid, or magma giving rise to the fluid, may have been, although isotopic evidence suggests that the source of the metasomatic fluid may be related to group II kimberlite-type magmas or be a source of these magmas. CONCLUSIONS Peridotite xenoliths from Jagersfontein consist of a primary mineral assemblage of olivine +- o~hopyroxene i garnet + spine1 r clinopyroxene. On the basis of gross texture the xenoliths can be divided into essentially two groups: a coarsegrain&

group and a group showing

deformation

textures.

These two groups are further subdivided on the basis of minera1 textures and chemical equilibria into high-, medium-. and low-temperature groups. The high-temperature group comprises those xenoliths which show deformation textures. Pressure and temperature estimates for these xenoliths are 43-56 Kbars and 11291346°C. The medium-temperature and low-temperature groups both comprise coarse-textured xenohths. Pressure and temperature ranges for the low-temperature xenoliths are 24-35 Kbars and <950°C, and for the medium-temperature xenoliths are 37-4 1 Kbars and 9 13- 102 1‘C. Exsolution of clinopyroxene, orthopyroxene, spinel, and garnet from orthopyroxene and clinopy~xene has occurred in the low-tem~rature xenoliths. No exsolution is evident in either the medium- or high-temperature xenoliths. The coarse, medium- and low-temperature xenoliths show evidence of mcdai me~matism which has occurred at depth in the mantle, as shown by the chemical and textural equi-

hbrium achieved, and this is referred to as primary metasomatism. New minerals of the primary-metasomatic suite are dominantly phlogopite and edenite, although minor richterite is also present. The phlogopite is generally restricted in its occurrence to medium-temperature xenoliths and the edenite to low-temperature xenoliths. A single occurrence of Baphlogopite has been noted in an edenite-bearing, low-temperature xenohth. Rare replacement textures show that in the low-tempcrature xenoliths edenite is replacing orthopyroxene, and in the medium-temperature xenoliths phlogopite is replacing clinopyroxene. There is no evidence of melt injection in the Jagersfontein coarse xenoliths, either on an intimate scale or in the form of vein- or dyke-like bodies. Bulk-rock chemistry reflects the presence of the metasomatic minerals with increased K20 and A&O3 in the phiogopite-bearing xenoliths, and increased KrO, NazO, Al~0~, and CaO in the edenite-bearing xenoliths. Rb is notably enriched in phlogopite-bearing xenoliths and Sr in edenite bearing-xenoliths. A general, incompatible-element enrichment is also suggested in all coarse xenoliths regardtess of the presence or absence of primary-metasomatic minerals. The edenite and phlogopite are considered to have been emplaced during a single episode of metasomatism by the infiltration of a HzO-rich fluid at around l Ga. with Sr and Nd isotopic signatures similar to group II kimberlites. Metasomatism is dominantly potassic at depth, where the replacement of clinopyroxene by phlogopite enriches the fluid in Na and Ca, and more sodic at shallower depths. where edenite is formed. The growth of edenite at shallow depths, less than 120 km, probably reflects the limited stability field of amphibole within the upper mantle. We suggest that for the lowest tem~rature xenoliths with exsolution microstructures, the hydrous metasomatic event was also important in giving rise to the widespread chemical equilibrium. ~c~~(~~~e~g~e~~~-Ba~ Hawthorne and the De Beers Kimberlite Petrology Unit are thanked for assistance with fieldwork and sample provision. Doug Smith and Joe Boyd are thanked for reviews that considerably improved and shortened this manuscript. This research was undertaken with the aid of a NERC research studentship to PAW. Editorial handling: F. A. Frey

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