Chromite spinels from ultramafic xenoliths

Geochimlca et Cosmochimica Acta,1975, Vol.39.pp.937to945.Pergamon Press.Printed inNorthern Ireland

Chromite spinels from ultramafic xenoliths ASISH R. BASU Department (Received

of Geology, 13 February

and IAN D. MACGREGOR

University 1974;

of California, U.S.A.

accepted

Davis,

in revised form

California

5 April

95616,

1974)

Abstract-The spin& in ultramafic xenoliths from kimberlites and alkali olivine basalts show a wide range of compositional variation, far in excess of the ranges shown by their coexisting silicate phases. The chemical variation of the spinels is a function of the host magma, texture and depth of origin. The spinels occur in five textural types: (a) euhedral spinels restricted only to the kimberlites; (b) spinels intergrown with silicates commonly found in kimberlites; (c) exsolved spine1 from orthopyroxene in xenoliths from both alkali olivine basalt and kimberlite; (d) interstitial spine1 texture generally restricted to xenoliths from alkali olvine basalts; and (e) spin& in kelyphitic rims around garnet in xenoliths from kimberlites. The chemistry of the spinels varies systematically with texture. The highest Cr/(Cr + Al) ratios and lowest Mg/(Mg + Fez+) ratios occur in the euhedral spinels, whereas the spinels showing intergrowth and exsolution textures are compositionally intermediate. Spinels from kimberlite xenoliths have a higher and wider range of the ratio Fe3+/(Cr + Al + Fe3+) th an the spinels from alkali olivine basalt xenoliths. The ratio Cr/(Cr + Al + Fe3+) increases with pressure or depth of origin of the xenoliths. The Also, content of the spinels varies sympathetically with the Al,O, content of the coexisting orthopyroxenes, the value being lowest for euhedral spinel-orthopyroxene pairs in kimberlite xenolith and highest for interstitial spinol-orthopyroxene pairs in alkali basalt xenolith. The wide range of chemistry of the spinels and their correspondence with geologic environment suggest that the chromite spinels from ultramafic xenoliths are particularly sensitive minerals in examining the environmental conditions of the Earth’s upper mantle.

INTRODUCTION CISROMITE spinels

are a common accessory phase in ultramafic rocks of mantle origin. The stability field of spinel-bearing peridotites lies between that of plagioclase (KUSHIRO and YODER, 1966) and garnet-bearing (MACGREGOR, 1964, 1965) peridotites at lower and higher pressures, respectively. The boundary between the spine1 and garnet-bearing varieties is critically dependent on the bulk ratio of the trivalent oxides Cr203, Fe,O, and Al,O,, with the spine1 phase assemblage extending to higher and probably lower pressures with increasing Cr,O, and Fe,O, content (MACGREGOR, 1970). The spinels in ultramafic xenoliths from alkali olivine basalts and kimberlites show a wide range of compositional variation, far in excess of the ranges shown by the coexisting silicate phases, and therefore represent potentially sensitive indicators of the intensive and extensive parameters that control the chemistry of the co-existing phases. This paper examines the chemical variation of chromite spine1 chemistry as a function of the host magma, texture and depth of origin. The data come from xenoliths in the alkali olivine basalt field near San Quintin in Baja California and from kimberlite pipes in the Kimberley region, South Africa. TEXTURAL TYPES

Chromite spinels in ultramafic xenoliths can be classified into five distinctive textural types. Their textures can generally be related to the nature of the host 937

938

A. R. BASU and I. D. MACGREUOR

basalt and appear to reflect significant differences in their chemistry and depth of origin. The five textural types are as follows: 1. Euhedral spinels that occur only in harzburgites from kimberlite pipes where the spinels are found as small octahedra varying in size from about 0.1 to 1 mm in size. The octahedra are found within the olivine and enstatite of the host rock. 2. Spinels intergrown with silicates are commonly found in harzburgites and occasionally in lherzolites from kimberlite xenoliths (BOYD, 1971; DAWSON and SNITH, 1973). The characteristic texture (Fig. 1) consists of a wormy or eutectoid intergrowth of spine1 with clinopyroxene, although comparable intergrowths with orthopyroxene, garnet and amphibole (BOYD, 1971) are also found. The intergrowths always occur interstitially between large olivine and orthopyroxene grains, and are usually marginal to orthopyroxene. In one critical texture the wormy intergrowth is seen to merge with spine1 lamellae exsolved from orthopyroxene. Similarly, except for amphibole, the combination of intergrowth phases are all seen as exsolved lamellae from orthopyroxene, and in the harzburgites commonly occur as intergranular grains marginal to large orthopyroxene crystals. The observations lead to the interpretation that the intergrowth textures result from the contemporaneous exsolution of silicate and spine1 from orthopyroxenes into intergranular areas. 3. Exsolution of spine1 from orthopyroxene is observed in xenoliths from alkali olivine basalts and kimberlites (Figs. 2 and 3). In the kimberlite xenoliths the spinels occur as small rods or platelets in the (010) plane of the host. In a few lherzolite xenoliths from San Quintin, the orthopyroxene porphyroclasts show exsolution of spinels. These orthopyroxenes are deformed by kinking in the system (100) [OOl]. The exsolved spinels are restricted to the kink band boundary (Fig. 3). &O, amounts to 4.65 wt. % in the host orthopyroxene at 2 mm distance from this boundary, and diminishes gradually to 3.50 wt. % at the boundary. This subsolidus exsolution of spine1 along the kink band boundary of the host orthopyroxene is probably due to stress-induced deformation. However, it is interesting to note that the spinels in the groundmass of the same rock, outside the orthopyroxene porphyroclasts, have almost identical composition with the exsolved spinels. DAWSON and SMITH (1973) reported a strikingly similar textural feature in a deformed garnet lherzolite from the kimberlite of the Monastery Mine, O.F.S., South Africa. In this case, garnet appears to have exsolved along the kink band boundaries of the host orthopyroxene. Dawson and Smith also noted the similarities in the composition of the exsolved garnet along the kink band boundary of the host orthopyroxene to the composition of the host-free garnet in the surrounding finer grained matrix. 4. Interstitial textures are represented by the spine&lherzolit’e xenoliths in basanites from San Quintin, Baja California. The typical texture is porphyroelastic, with large porphyroclasts of olivine and aluminous enstatite in a finergrained recrystallized groundmass of olivine, orthopyroxene, diopside and spinel. Occasionally, the diopsides also occur as large porphyroclasts with deformation twin lamellae. The flattened orthopyroxenes and olivines define the foliation. The spinels are usually flattened parallel to the porphyroclasts and the elongation of the spinels on the foliation plane defines a lineation in the rock. The spinels occupy intergranular locations between the silicate grain boundaries (Fig. 4). 5. Chromite spinels also occur as small (0.01-0.1 mm) grains in the kelyphitic

Fig.

1. Wormy

intergrowth or symplect& of spine1 ad clinopyroxene in a lrimberlito xenolith (longer dimension = 2 mm). Fig. 2. Exsolution of spine1 along (010) plane of host enstiztite crystal in a kimberlite xenolith (longer dimension = 1 mm).

Fig. 3. Kink bands in a porphyroclastic enstatite in a lllereolit~c xenolith in basalt from San Quintin, Baja, C’alifornia. Not,e the droplets of spinels concentrabed along the kink band boundary (longer dimension = 3 mm). Fig. 4. Interstitial large grains of spine1 in bet,\wen grain boundaries of the associated silicates in a basaltic lhcrzolite xenolith (longer dimension = 3 mm).

939

Chromite spinels from ultramafic xenoliths Table 1. Chemistry of some representative spinels and their associated silicate phases Sample no. 23 (Spine1 hamburg&e symplectite xenolith in kimbedite,

Clinopyroxene

Spine1 (in symplectitc) TiO, ALO, Cr,Os Fe0 (total Fe)

0.04 21.72 4673 14.92

MnO MgO

0.18 14.18

Total

98.41

Mn Mg

55.60 trace 1.39 1.07 1.17

-41203 cr203

Fe0 (total Fe) MnO

0.10 16.27 24.07 0.84 0.06 10056

MgO CaO N&,0 K2O

Total

Atomic proportions on the basis of 6 oxygen atoms

O*OOl 0.791 1.142 0411 0.067 0.005 0.683

Fe3+

(in symplectite)

SiO TiO:

Atomic proportions on the basis of 4 oxygen atoms Ti AI Cr Fes+

from Jaegersfontein)

Si Ti Al Cr Fe Mn Mg ca, Na K

1.999 0.000 0.059 0.030 0.035 0.003 0.873 0,927 0,059 0*003

Sample no. R-72 (Spine1 harzburgite xenolith in kimberlite from the Roberts Victor Mine) spine1 TiO, Af203 Cr203

Fe0 (total Fe) M.IlO

Orthopyroxene 0.02 10.26 58.99 1531 0.38

MgC’

12.98

Total

97.94

SiO TiO: M2o3 Cr203

Fe0 (total Fe) AdnO MgO C&O N&,0 Total

57.79 0.01 0.76 0.27 4.04 0.09 3628 0.27 0.06 99.57

Olivine SiO, TiO, -42% Cr203

Fe0 (total Fe) MnO MgO X0 NE%,0 TotaI

42.00 o-03 trace trace 6.38 0.06 52.23 0.41 0.01 101.12 .___-

A. R. BASU and I. D. MACGREGOR

940

Table 1. (Cont.) Atomic proportions on the basis of 4 oxygen atoms Ti Al Cr Fez+ Fe3+ M.n Mg

Atomic proportions on the basis of 6 oxygen atoms Si Ti Al Cr Ff3 Mn

0.000 0.401 1.547 0.360 0.065 0.011 0.642

Atomic proportions on the basis of 4 oxygen atoms Si Ti Al Cr Fe Mn Mg Ni Na

1.978

0.000 0.030 0.010 0.115 0.002 1+351 0.009 0.004

Mg Cal Na

1.002 0.001 0.000 0.000 0.127 0.001 1.858 0.008 0.001

Sample no. SQ l-6-9 (Spine1 lherzolite xenolith in alkali basalt from San Quintin, Baja California) Orthopyroxene Clinopyroxene Olivine spine1 TiO, Al203 Cr2Os Fe0 (total Fe) MnO

MgO

Total

0.02 66.16 11.35 10.50

SiO, TiO, Al,% Cr2%

55.07 0.16 5.03 0.55

0.12

Fe0 (total Fe) MnO MgO C&O

5.96

21.16

99.29

Atomic proportions on the basis of 4 oxygen atoms Ti Al Cr Fez+ Fes+ Mn Mg

0.000 1.728 0.234 0.182 0.047 0.003 0.824

Total

0.12 32.95 0.85

100.69

Atomic proportions on the basis of 6 oxygen atoms Si Ti Al Cr Fe Mn Mg CE&

1.888 0.004 0.203 0,015 0.171 0.003 1.684 0.031

SiO, TiO, A12o3 cr203

Fe0 (total Fe) MnO MgO CaO Na,O K2O NiO Total

0.43 6.74 0.93

SiO, Fe0 MIlO MgO

2.75

NiO

0.37

0.09

cao

0.07

52.29

15.79 21.02 1.81 0.01 0.08 101.94

Atomic proportions on the basis of 6 oxygen atoms Si 1.862 Ti 0.012 Al 0.283 Cr 0.026 Fe 0.082 Mn 0.003 Mg 0.838 CL% 0.802 NE 0.125 K 0.001 Ni 0.002

Total

40.91 9.97 0.13 49.22

100.67

Atomic proportions on the basis of 4 oxyg0n atoms Si Fe Mn Mg Ni ceo

0.998 0.203 0.003 1.790 0.007 0.002

Chromite spinels from ultrcimafic xenoliths

941

that characteristically surround the garnets in xenoliths from kimberlites. As a rule, the kelyphitic rims are composed of a fine-grained intergrowth of spine1 and phlogopite. NIXON and BOYD (1973) have shown that the spinels in the kelyphitic rims have a wide range of chemistry. rims

CHEMISTRY OF THE CHROMITE SPINELS

The spinels from the various ultramafic xenoliths were analyzed using an electron microprobe. The chemistry of representative spinels and their coexisting silicates are shown in Table 1. Iron was determined as total iron and the amount of Fez+ and Fe3f was distributed by assuming an R2+RZ3+0, formula for the spinels. The chemical variations of the spinels are shown in Figs. 5-9. Figure 5 shows the Cr-Al-Fe3+ plot for the various groups of spinels. It is evident that the spinels from basaltic xenoliths are Al-rich, whereas those of kimberlites are Cr-rich. Both groups merge compositionally with spinels of symplectite-texture in kimberlites. The kimberlite xenolith spinels also show higher Fe3+ content. IRVINE (1965) showed that a series of ‘equipotential surfaces’ can be calculated within the spine1 compositional prism. These surfaces represent possible olivinespine1 equilibrium assemblages for given temperatures and pressures. These surfaces can also be represented with respect to small differences in the olivine composition. Plots of Cr x lOO/(Cr + Al) against Mg x lOO/(Mg + Fez+) are shown in Fig. 6. It is seen that the Cr/(Cr + Al) ratios vary greatly. The Mg/(Mg + Fez+) ratios of the spinels and their coexisting olivines decrease with the decrease of the Cr/(Cr + Al) ratio in the spinel. The forsterite content of the coexisting olivines defines surfaces

Fig. 5. Variation of Fe3+: Cr : Al in spinels from various xenoliths.

A. R. BASU and I. D. MACGREGOR

942

90 -

70-

j 50JHJURFS q t-uhdrul + LXSOIU/~OR&

i L 0 3@-

sqnplecfife

o hfefsfifiaf

-90.5

1

Kimberlite lenoliths Alkali divine

basotj I xenolltk

Mol. % forsterite

IO-

I

I

90

I

70 h@XIOO/Mg+Fe’+

I

50

Fig. 6. Variation of Cr x lOO/(Cr + Al) vs Mg X lOO/(Mg + Fez+) in spine1with mole/o for&rite of coexisting olivines.

comparable to Irvine’s theoretical surfaces. Temperature estimates using the diopside solvus (DAVIS and BOYD, 1966) indicate that xenoliths from alkali-olivine basalts fall in the temperature range from 850’ to lOOO”C, and those from kimberlites formed at temperatures between 940“ and 1050°C. Figure 7 shows that the cation ratio Fe3+/(Cr + Al + Fe3+) is low for the spinels in lherzolite xenoliths in basalts, whereas for the kimberlite xenolith spinels, the same ratio shows a wide range of variation. This difference would suggest that the spinels in the kimberlite xenoliths formed at a relatively higher oxygen fugacity than the spinels in basaltic xenoliths. It is interesting to note that the compositional range of the ratio Fe3+/(Cr + Al + Fe3+) in kimberlite-xenolith spinels is strikingly comparable to that in spinels from the three major stratiform igneous complexes, the Bushveld, the Great Dyke and the Stillwater (IRVINE, 1967, Fig. 9). The pressure and temperature conditions at which the various xenoliths last equilibrated can be estimated by using available experiment data. Equilibration temperatures are estimated first by the diopside solvus (DAVIS and BOYD, 1966) and then the equilibration pressures by the Al,O, contents of the coexisting enstatites (MACGREGOR, 1974). Figure 8 shows a plot of the pressures of equilibration of the xenoliths against the Cr x lOO/(Cr + Al + Fe3+) ratio in the spinels. Two main groups of spinels appear, suggesting that spinels equilibrated at higher pressures have higher ratios of Cr/(Cr + Al + Fe3+). However, this apparent relationship may well be the result of bulk chemical differences between the different xenolith suites. Figure 9 shows that the Al,O, contents in spinels correlate with their coexisting orthopyroxenes, supporting the importance of bulk chemistry. It shows that the orthopyroxenes and spinels in basaltic xenoliths are much more

Chromite spine18from ultramafic xenoliths

943

Kimberlite xenoliths

Fig.

7.

Vaziation of Fe3+

x

lOO/(Cr + Al + Fe3+) vs Mg in spinels.

x

lOO/(Mg + Fez+)

aluminous than those of kimberlite xenoliths. The G-Al ratios in spinels of various ultramafic xenoliths, therefore, must reflect bulk compositional effects. At present, it is not possible to distinguish clearly between the effects of composition and pressure on the distribution of Cr,03 and Al,O, between coexisting pyroxenes and spinels. DISCUSSION As a general rule, the different textures of the spinels correspond to their geoThus euhedral, intergrowth and reaction rim textures are logic environments. characteristic of spinels in xenoliths from kimberlite pipes, whereas interstitial textures are generally restricted to xenoliths from alkali olivine basalts. Only exsolution textures are found in both occurrences. In a similar way the chemistry of the spinels vary systematically. The highest Cr/(Cr + Al) ratios and lowest Mg/(Mg + Fe2f) ratios occur in the euhedral spinels, whereas the spinels showing intergrowth and exsolution textures are compositionally intermediate, and the interstitial spinels lowest and highest, respectively. Corresponding differences are

A.

944

R.

BASU and I. D. MACGREUOR

IO

70

50

30 PRESSURE.(kb)

Fig. 8. Plot of Cr x

lOO/(Cr + Al + Fe3+) in spine1 against their estimated bretion pressures.

equili

-

0 6!!

50

40; :

o+

r; 30-

0

A

Y 0

] Klmberlitf

i 20

xenoi;ths

Alkali olivine : basalt ) xenoliths IO

c ‘I

Fig. 9. Variation

of wt.%

Al,O,

in orthopyroxenes in xenoliths.

and their

coexisting

spinels

Chromite spinels from ultramafic xenoliths

945

shown by the Fex+/(Cr $- Al + Fe3+) ratio where the xenoliths from kimberlites have spine18 which have a higher and wider range of values than spinels from alkali olivine basalt xenoliths. There is also an apparent relationship of the Cr/Cr + Al + Fe”+) ratio with pressure (Fig. 8), which seemingly indicates that spinels formed at higher pressures have higher Crf(Cr + Al + Fe3+) ratios. However, the close correspondence of spine1 and orthopyroxene chemistry (Fig. 9) would suggest that bulk composition is the critical variable, and the restriction of Cr-rich compositions to deeper subcontinental mantle rocks may indicate a potential for using the euhedral and interstitial textured spinels to establish the relative ‘refractoriness’ of the mantle. Preliminary estimates suggest that the suboceanic mantle, represented by the lherzolite xenoliths from San Quintin, is significantly less ‘refractory’ than t,he subA further difference in bulk composition is illustrated by continental counterpart. the more oxidized nature of the kimberlite spinel, suggesting higher partial presThe wide range of ehromite spine1 chemistry sures of oxygen in this environment. and the correspondence with geologic environment points to the chromite spinels as a particularly sensitive mineral in examining environmental conditions. This paper illustrates some interesting systematics that encourage more detailed examination of the mineral group. Acknowledge~~ent+-We are gmteful to JUDY JOHNSTON for use of her analyses of the intorgrowth spinels and Jagersfontein samples. F. R. BOYD, T. N. IRVINE,and L. G. MEDARIS,JR. reviewed the manuscript and made useful criticisms which have been incorporated into the manuscript. Financial support was provided by a Penrose grant from the Geological Society of America to A. R. BASU, and a D-grant from the University of California, Davis to I. D.

MACGREGOR. REFERENCES BOYI) F. R. (1971) Pargasite-spine1 peridotite xenolith from the Wesselton Mine. Camegke I’nst. $~a&. Yearb. 70, 138-142. DAVIS B. T. C. and BOYD F. R. (1966) The join ~g~Si~O~-CaM~Si~O~at 30 kb pressure and its application to pyroxenes from kimberlites. J. Geophys. Res. ‘91, 3567-3576. DAWSONJ. B. and SMITHJ. V. (1973a) Garnet exsolution from stressedorthopyroxene in garnet lherzolitefrom the Monastery mine. International Conference on Kimberlites, Abstracts, Univ. of Cape Town, 8 1. DAWSONJ. B. and S~%IITH J. V. (1973b) Chemistry of opaque minerals from peridotite and eologite xenoliths. Inte~~t~~l Co~~e~e~e 0% K~~be~~~tes~Abstracts, Univ. of Cape Town, 83. IRVINET. N. (1965) Chromian spine1as a petrogeneticindicator. Part 1. Theory. Can. J. Earth Sci. 2, 648. IRVINET. N. (1967) Chromian spine1as petrogoneticindicator. Part 3. Petrologic applications. Can. J. Earth. Sci. 4, 71. KUSHIROI. and YODERH. S. JR. (1966) Anorthite-forsterite and anorthite-enstatite reactiona and their bearing on the bas&--eclogite transformation. J. Petrol. $‘, 337-362. MACGREOORI. 13. (1964) The reaction 4 enstatite + spine12 forst,erit~e + pyrope. Cuezegie Inst. Wash. Yearb. 63, 157. MACGREGOR I. D. (1965) Stability fields of spine1and garnet peridotites in the synthetic system MgO-CaO-Al,O,-SiO,. Camzegie Inst. Wash. Pearb. 64, 126-134. MACGREGOR I. D. (1970) The affect of CaO, CrzOs, Fe,O, and Al,O, on the stability of spine1 and garnet peridotites. Phv.9. Earth Planet. Interiors 3, 372-377. MACGREGOR I. D. (1974) The system Mg~Al~Os-SiO~-solubility of A&O, in enstatite for spine1 and garnet-peridotite compositions. Amer. Milaerd. 59, 11 G-1 19. NIXON P. H. and BOYD F. R. (1973) Petrogenesis of the granular and sheared ultra basic nodule suite in kimberlites. In Lesotho KimberZites, (editor P. H. Nixon), pp. 48-56. Lesotho National

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