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Department of Chemistry, Geoscience, and Environmental Science, Tarleton State University, Stephenville, Texas, U.S.A.
Correspondence: * E-mail: field{at}tarleton.edu
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ABSTRACT |
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 Top Abstract IntroductionExperimental methodsPeridotite petrographyMineral chemistryDiscussion: Origin of...AcknowledgmentsReferences cited |
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Key Words: Symplectite • discontinuous precipitation • garnet • harzburgite • mantle
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INTRODUCTION |
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TopAbstractIntroductionExperimental methodsPeridotite petrographyMineral chemistryDiscussion: Origin of...AcknowledgmentsReferences cited |
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Symplectite-forming processes have been proposed in several papers. Williams (1932) suggested that spinel-enstatite-olivine symplectites were eutectic intergrowths. Dawson and Smith (1975) proposed breakdown of an AB2O4 phase as a mechanism of symplectite growth. Basu and MacGregor (1975) suggested formation of symplectites by contemporaneous exsolution of spinel and silicates from orthopyroxene into intergranular areas. Neumann (1991) favored crystallizing melt inclusions as a way of forming spinel + clinopyroxene symplectites in upper mantle xenoliths found in the Canary Islands. Green and Burnley (1988) proposed eutectoid-like decomposition of chromium garnet as a symplectite-forming process. Takahashi and Arai (1989) also argued that garnet is a precursor mineral of symplectites in spinel lherzolites from the Horman peridotite.
Symplectites are abundant in upper mantle peridotite xenoliths included in South African kimberlites. Symplectites are described from the DeBeers, Newlands, Monastary, Bultfontein, and Wesselton kimberlites (Dawson and Smith 1975), the Frank Smith Mine (Exley et al. 1982), and the Jagersfontein kimberlite (Dawson and Smith 1975; Harte and Gurney 1983; Winterburn et al. 1990; Field and Haggerty 1994). Symplectites in the South African peridotites vary in mineralogy and mineral abundance. Minerals commonly found in the symplectites are clinopyroxene (diopside), orthopyroxene (enstatite), Cr-rich spinel, pyrope garnet, Mg-rich amphibole, and phlogopite.
This study examines symplectites in peridotites recovered from South African kimberlites to characterize the suite of symplectites found in upper mantle peridotites, to characterize symplectite-bearing peridotites, and to determine the mechanism of formation of the symplectites. The peridotites come predominantly from the mine dumps scattered around Kimberley, South Africa. These samples will be referred to in this paper as the Kimberley peridotites. The Kimberley samples were collected from waste dumps by S.W. Field in 1989 and 1995, and by S.E. Haggerty in 1984. Peridotites from the Premier kimberlite and Newlands kimberlite were also studied. The Newlands and Premier kimberlite samples are in the peridotite collection at the University of Cape Town in South Africa. The symplectite mineral assemblages studied are clinopyroxene + spinel (cpx + spl), orthopyroxene + spinel (opx + spl), clinopyroxene + orthopyroxene + spinel (cpx + opx + spl), garnet + clinopyroxene + spinel (grt + cpx + spl), garnet + spinel (grt + spl), amphibole + spinel (amph + spl ± cpx), and phlogopite + spinel (phl + spl). [All mineral abbreviations used in this paper are from Kretz (1983)]. Chemical data from peridotites from the Jagersfontein kimberlite are used as a reference to compare the Kimberley, Premier, and Newlands data. The Jagersfontein samples were collected from waste dumps by S.E. Haggerty in 1984 and 1990. Most of the Jagersfontein data are from Field and Haggerty (1994); however, previously unpublished Jagersfontein data are also used. This study does not include the mineral intergrowth known as kelyphite, generally assumed to be a decompression breakdown product of garnet.
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EXPERIMENTAL METHODS |
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TopAbstractIntroductionExperimental methodsPeridotite petrographyMineral chemistryDiscussion: Origin of...AcknowledgmentsReferences cited |
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The mineral chemistry of 55 Kimberley symplectite-bearing peridotites were analyzed in detail on CAMECA microprobes in the Geology Department at the University of Cape Town in South Africa, and in the Geosciences Department at the University of Massachusetts. Two Premier peridotites and two Newlands peridotites were also analyzed. Bence and Albee (1968) and Albee and Ray (1970) and the PAP and ZAF correction procedures were utilized in microprobe analyses. A variety of natural and synthetic standards were used. Bulk major- and trace-element data from 21 Kimberley symplectite-bearing peridotites were obtained at the XrF facilities at the University of Massachusetts, and at the University of Cape Town. Five symplectite-free peridotites were analyzed with the microprobe and XrF for comparison to symplectite-bearing peridotites.
Symplectite structures were examined in transmitted and reflected light, and SEM micrographs were taken of numerous symplectites. Symplectite modes were determined by scanning electron micrographs into a computer. The intergrowth volume percents were analyzed with an image analysis program (ImageJ) obtained from the National Institute of Health. Symplectite bulk compositions were determined by combining the volume percentage data, mineral chemistry data, and mineral specific gravity data (Deer et al. 1996).
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PERIDOTITE PETROGRAPHY |
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TopAbstractIntroductionExperimental methodsPeridotite petrographyMineral chemistryDiscussion: Origin of...AcknowledgmentsReferences cited |
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Fine-grained harzburgites
Fine-grained harzburgites are dark green, with a granoblastic texture (Harte 1977), and have grain sizes of less than 2 mm. The rocks are composed of ~60% olivine and 40% enstatite. The olivine shows slight to moderate serpentinization, but no metasomatic minerals are present. These rocks contain between 36 and 84 symplectites per standard petrographic thin section.
Coarse-grained harzburgites
Coarse-grained harzburgites are a lighter green color with an average grain size from 4 to 7 mm. The rocks average about 60% olivine and 40% enstatite. A few small, discrete spinel grains are scattered throughout some of these peridotites. The olivines show moderate to extensive serpentinization. No metasomatic minerals are present in these rocks. Coarse-grained harzburgites contain between 12 and 32 symplectites per thin section.
Garnet harzburgites
Garnet harzburgites are composed predominantly of granular olivine and enstatite, have grain sizes that mostly fall between 4 and 7 mm, and have a granoblastic texture. There is a moderate amount of serpentinization in these peridotites. Some samples also contain coarse granular Cr-rich diopside. Garnet grains are fine grained, and occur as interstitial grains between olivine and enstatite. Kimberley garnet does not occur, as is often the case with Jagersfontein garnet, as lamellae in enstatites. Kimberley garnet harzburgites contain only a single identifiable symplectite each. There are, however, several garnet grains in each sample that contain spinel inclusions, which may be remnant symplectites. The Premier sample contains five symplectites.
Metasomatized harzburgites
Metasomatized harzburgites are similar in texture and grain size to the coarse-grained harzburgite but are defined as having abundant modal phlogopite and/or amphibole. The premetasomatic mineralogy appears to have been similar to the coarse-grained harzburgites. However, the enstatite grains have a distinct golden brown color, caused by the presence of hundreds of small exsolution lamellae of spinel. The phlogopite is found predominantly rimming the edges of, and as inclusions within the enstatite. Phlogopite also occurs as veins cutting through the harzburgites. Associated with phlogopite is fine-grained pale-green diopside, and occasional clusters of red to opaque spinel. Amphibole is found in some metasomatized peridotites, rimming enstatite, or as inclusions within enstatite. Amphibole-bearing peridotites are less common than peridotites dominated by phlogopite. Metasomatized peridotites contain between 6 and 17 symplectites per thin section, and average 11 per thin section.
Bulk-rock chemistry
Kimberley symplectite-bearing peridotites are all depleted in Al2O3 and CaO, and enriched in MgO, compared to lherzolites and garnet lherzolites (Nixon et al. 1981; Dawson 1980; see Table 11). They also tend to have higher Cr and Ni contents than more fertile peridotites.
There are no systematic major oxide chemical differences between symplectite-bearing peridotites and their non-symplectite equivalents. There are also, for most major oxides, no apparent systematic differences between symplectite-bearing harzburgites and symplectite-bearing garnet harzburgites, except that garnet harzburgites tend to have lower Al2O3 contents (Fig.1
). There are no major chemical differences between the fine-grained symplectite-bearing harzburgites and the coarse-grained symplectite- bearing harzburgites. However, the range of TiO2 and K2O contents is greater in the coarse-grained harzburgites than in the fine-grained harzburgites.
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) and Nb and Zr compared to the fine-grained harzburgites. This is probably a result of metasomatic fluids that have affected the coarser-grained peridotites to a greater extent.
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). These peridotites contain large amounts of phlogopite and amphibole, which are clearly of metasomatic origin, and the elevated TiO2, K2O, Zr, Nb, Sr, and Ba contents are a result of metasomatizing events.
Temperature and pressure
The temperature of the garnet harzburgites from Jagersfontein was estimated to be 800–1200 °C at 15–35 kbar pressure (Field and Haggerty 1994). Temperatures for the Kimberley harzburgites and garnet harzburgites were determined by utilizing the Brey and Köhler (1990) geothermometer after the method of Witt-Eickschen and Seck (1991) for spinel peridotites. It is unclear what these temperatures mean. The T-P conditions were determined for assemblages in equilibrium with garnet; however there is no evidence of garnet in any of the fine- or coarse-harzburgites. In addition, many of the cpx compositions used to determine temperature are from symplectites, which may not represent equilibrium conditions. Regions of some host opx show apparent depletion of Al, Cr, and Ca adjacent to symplectites. Smith and Barron (1991) argued that diffusion gradients in opx adjacent to garnet indicate disequilibrium conditions. With these cautions in mind, temperatures calculated for the Kimberley peridotites ranged from 740 to 903 °C. Temperatures between 642 and 829 °C were calculated for the metasomatized peridotites. Temperature estimates for fine-grained harzburgites were 861–901 °C; estimates for coarse-grained harzburgites were 867–903 °C; and estimates for garnet harzburgites were 740–829 °C.
Symplectite structure and petrography
Symplectites in the Kimberley peridotites are composed of Cr-rich spinel plus one or more of the silicates diopside, enstatite, amphibole, phlogopite, and garnet. Symplectites having different mineral assemblages are associated with the four symplectite-bearing harzburgites. The fine-grained harzburgites contain mostly cpx + spl, opx + spl, or cpx + opx + spl symplectites. The medium-grained harzburgites are dominated by cpx + opx + spl symplectites, with lesser numbers of cpx + spl symplectites. Kimberley garnet harzburgites contain only grt + spl, but grt + cpx + spl symplectites are found in the Premier Mine samples. This assemblage contrasts with Jagersfontein where grt + cpx + spl symplectites are common. It is unclear whether this is a real distinction or a result of sampling. Amphibole + cpx + spl and phl + spl symplectites characterize the metasomatized symplectites. Some cpx + spl and opx + spl symplectites are also found in metasomatized peridotites.
All symplectites consist of numerous spinel grains embedded in one, or a few, coarser-grained silicates. This type of structure is common in manufactured alloys and metals and is called a cellular structure (Gupta 2001). In the metasomatized peridotites, the silicates may be replaced by fine-grained phlogopite and/or amphibole. Symplectites are substantially smaller than the granular olivine and enstatite that dominate the peridotites. Symplectite sizes in the fine-grained harzburgites are on the order of tenths of millimeters, and the symplectites in the coarser-grained peridotites may be up to 2 mm across.
The symplectites are mostly at grain boundaries between primary enstatite and olivine, but a few symplectites are found within enstatite grains. All symplectites are located directly adjacent to, and are genetically related to a primary enstatite grain, which herein is called the host enstatite. The symplectites have a flat edge that abuts the host enstatite. The symplectites generally have a hemispherical or triangular cross-section, and jut into adjacent olivines. The symplectites disrupt the normally straight olivine-enstatite grain boundary. The spinel grains within the cellular structure of the symplectite are rod- or lath-shaped and are crystallographically oriented with respect to the host enstatite-olivine boundary (Fig. 4). Sections cut through symplectites parallel to the enstatite boundary show that individual spinel grains have an oval- or rounded-lath shaped cross-section. Symplectites occur only sporadically along host enstatite boundaries. A single host enstatite can have multiple symplectites adjacent to it, but not all enstatite grains within a thin section have visible associated symplectites.
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). Both host and symplectite orthopyroxenes display simultaneous extinction, identical interference colors, identical optical figures, and identical cleavage patterns.
Symplectites containing cpx + opx + spl are composed of two distinct mineralogical regions separated by a sharp boundary (Fig. 5). Adjacent to the host enstatite is a region of opx + spl. The opx is in optical and structural continuity with the host enstatite. On the outer edge of the cellular structure, adjacent to olivine, is a region, rim, or partial rim of cpx + spl. The cpx in an individual symplectite is a single crystal, or a few crystals. The spinel lamellae in the two regions are oriented approximately in the same direction, however, the spinel in the cpx + spl region are smaller, more numerous, and more closely spaced than the spinel in the opx + spl region of the symplectite. The cpx in an individual symplectite is uniform in optical properties. The volume proportions of the two regions varies considerably among symplectites.
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). The core region contains spinel lamellae embedded within a single cpx grain. The structure is identical to the cpx + spl symplectites without garnet. A region of spinel embedded within garnet forms a rim or partial rim around the core structure. The spinel grains are not as abundant as in the core, are small, and generally oval or rounded rectangles. The long axes of small spinel grains are parallel to the orientation of the spinel in the core region. The boundary between the two regions is sharp, and spinel lamellae in the core region commonly are terminated abruptly at the core-rim boundary. The garnet rim is most commonly adjacent to olivine, but in some symplectites, the core is completely surrounded by the garnet and cut off from the host enstatite. The thickness of the grt + spl rim varies from peridotite to peridotite.
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). The spinel grains within the garnet are generally short, rounded laths or oval in shape. The spinel lamellae are less abundant than the spinel in pyroxene symplectites; however, the radial pattern of the laths is still present and discernable in some symplectites (Fig. 7a). These symplectite structures are associated with an adjacent enstatite, and protrude away from the pyroxene into olivine.
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). The spinel grains display a similar orientation and pattern shown by spinel in the pyroxene-dominated symplectites. The amphibole appears to be a single grain throughout the entire symplectite. Clinopyroxene in amphibole symplectites exists as small patches within volumetrically larger amphibole. The cpx regions contain embedded oriented spinel lamellae, and the features appear to be a relict cpx + spl symplectite. Phlogopite + spl symplectites are structurally different from other symplectites. These intergrowths are composed of spinel with the typical cellular pattern. However, between the spinel grains are hundreds of small phlogopite grains. Small slivers of cpx or opx are present in some symplectites adjacent to spinel laths. The spinel grains are irregular, distorted, less numerous, and coarser-grained than spinel in pyroxene symplectites. The relationship of these symplectites to a host enstatite and olivine are similar to those of the other symplectite types.
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MINERAL CHEMISTRY |
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TopAbstractIntroductionExperimental methodsPeridotite petrographyMineral chemistryDiscussion: Origin of...AcknowledgmentsReferences cited |
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Spinel in Kimberley peridotites occurs primarily as a symplectite component, and more rarely as discrete grains. Spinel shows a general trend of increasing Al/(Al+Cr) with increasing Mg/(Mg+Fe). There is no systematic chemical difference between spinels in the three non-metasomatized peridotite types. Spinel in highly metasomatized peridotite tends to contain less Al and Mg, and more Fe and Cr, than spinel in non-metasomatized peridotites. Aluminum contents (Table 41) range from 3.99 to 29.94 wt%, however, spinel with a low Al content is present in highly metasomatized peridotites (Fig. 8). The range of Al contents in non-metasomatic peridotites ranges from 10.93 to 29.94 wt%. Spinel Cr contents vary from 40.25 to 59.68 wt%, and Mg contents from 13.00 to 14.61 wt%.
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), Cr contents range from 0.82 to 0.08 wt% (avg. 0.31 wt%), and Ca contents run from 1.09 to 0.08 wt% (avg. 0.37 wt%). Garnet harzburgite opx tends to contain more Al (2.56–0.61 wt%, avg. 1.32 wt%), approximately the same range of Cr (0.42–0.23 wt%, avg. 0.39 wt%); and lower Ca (0.34–0.24 wt%, avg. 0.31 wt%) than opx in other peridotites.
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) from 3.27 to 1.43 wt% (avg. 2.59 wt%), Cr contents range from 0.87 to 0.51 wt% (avg. 0.76 wt%), and Ca contents between 1.04 and 0.38 wt% (avg. 0.77 wt%). Enstatite in symplectites have Al contents in the range from 2.61 to 1.85 wt% (avg. 1.85 wt%), Cr contents from 0.88 to 0.28 wt% (avg. 0.66 wt%), and Ca contents from 0.52 to 0.27 wt% (avg. 0.39 wt%). Symplectite opx (Table 6) always shows lower Al contents than co-existing discrete opx (Fig. 10). Symplectite enstatite typically has higher Si and Mg contents (Fig. 11), and lower Cr and Ca content than coexisting discrete enstatite. These same chemical relationships exist in Jagersfontein peridotites containing symplectite-discrete opx pairs.
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). A Premier Mine garnet has a Cr2O3 content of 2.19 wt%. Garnet grains are homogeneous. Jagersfontein symplectite garnet grains are richer in Al and contain less Cr (Field and Haggerty 1994).
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DISCUSSION: ORIGIN OF SYMPLECTITES |
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TopAbstractIntroductionExperimental methodsPeridotite petrographyMineral chemistryDiscussion: Origin of...AcknowledgmentsReferences cited |
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Evidence for this process is both textural and chemical. Clinopyroxene + spl symplectites are most common in harzburgites, and are very common in fine-grained harzburgites. The precursor host rock to a symplectite-bearing rock would therefore be a fine-grained harzburgite with no symplectites. These harzburgites exist at Kimberley, and several were analyzed chemically and mineralogically to compare them to symplectite-bearing harzburgites. The symplectite-free harzburgites are chemically and mineralogically identical to symplectite-bearing harzburgites (Fig 1). Clinopyroxene + spl symplectites are regions of high concentrations of Ca-Al-Cr. There are no phases in the fine-grained symplectite-free harzburgites with the exception of olivine and opx. Chemically, the rocks show little evidence of metasomatism (Fig. 2) indicating that the symplectite material is not metasomatic in origin. If symplectite-free harzburgites are precursors to symplectite-bearing harzburgites, there are no other phases other than Ca-Al-Cr discrete opx that could be sources of symplectite material.
Clinopyroxene + spl symplectites bulge into adjacent olivine grains, and the veniform spinel lamellae are commonly oriented approximately at right angles to the adjacent opx (Fig. 4). These textures are similar to those developed in experiments on alloys of Cu-Cd (Guo et al. 1993), Fe-Zn (Gupta 2001), Mg-Al (Kashyap et al. 2000), and Al-Li-Zr (Pragnell et al. 1994). The symplectites in these alloys develop at grain boundaries, grow into adjacent regions, and develop cell structures approximately adjacent to grain boundaries. The symplectites in these alloys developed from symplectite-free starting materials. The formation of the symplectite is often attributed to discontinuous precipitation, that is, precipitation at grain boundaries followed by grain boundary movement. Boland and Roermund (1982) indicated that discontinuous precipitation is the mechanism that forms clinopyroxene + plagioclase symplectites in eclogites from Scandinavia. Wirth and Voll (1987) suggested discontinuous precipitation as the mechanism that formed quartz and Na-plagioclase symplectites (myrmekite) between alkali feldspar grains in Italian gneisses, and make an analogy to similar features in alloys. It should be noted, however, that cellular structures in alloys and in the Italian gneiss developed between grains of the same material. Symplectites in peridotites, on the other hand, form between chemically similar, but mineralogically different forsterite and enstatite.
The alternative hypothesis is that the symplectites form by garnet breakdown. The bulk chemistry (Table 10) of Kimberley symplectites have been calculated from mineral compositions, volume percents, and densities from Deer et al. (1996). A series of garnet grains with a range of Cr and Al contents from Kimberley and Jagersfontein were analyzed to compare to the bulk symplectite chemistry (Table 11). The Kimberley and Jagersfontein cpx + spl symplectites are chemically distinct from garnet grains in garnet harzburgites (Fig 12). The garnet grains are enriched, for example, in Al with respect to the symplectites, and depleted in Cr. If the symplectites formed by garnet breakdown, the intergrowths would have needed to lose substantial Al. The only non-symplectite phase containing Al is discrete opx. The excess Al would have to diffuse into the opx, and distribute uniformly to produce a chemically homogenous grain. Symplectite chemistry does resemble the chemistry of some high-Cr green garnet grains from Jagersfontein, but these garnet grains are found only in highly sheared, high-temperature peridotites.
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Cpx + opx + spl and opx + spl symplectites
Clinopyroxene + opx + spl symplectites are divided into two mineralogically distinct regions, a fine-grained cpx + spl domain and a coarser grained opx + spl domain (Fig. 5). The natural symplectites are similar to dual-region symplectites produced experimentally in some alloys. Fine-grained symplectites develop at grain boundaries with increasing temperature. The alloy symplectites can form a coarser-grained symplectite by a process called discontinuous coarsening (Gupta 2001). The secondary symplectite forms at the original grain boundary and, through diffusion and crystal growth, gradually replaces the finer-grained symplectite. Incomplete replacement leaves a coarse-grained symplectite with a fine-grained symplectite rim.
The dual structure of symplectites in peridotites most likely develops by a similar process. The initial stage results in the growth of a fine-grained cpx + spl domain. The opx + spl symplectite forms secondarily, and at the expense of the initial symplectite accompanied by diffusion of Ca, Fe, and Al to the outer edge of the symplectite. This idea is supported by the relatively Ca-Al-Cr depleted nature of the symplectite opx. Boland and Roermund (1982) found two symplectite domains in eclogites, a coarse-grained types (symplectite a) and a finer-grained type (symplectite b). They suggested that the original symplectite b transformed by secondary processes into coarse symplectite a.
Orthopyroxene + spl symplectites are not common, and always occur in peridotites that also contain cpx + opx + spl symplectites. The opx + spl symplectites are identical in appearance to the opx-rich regions of cpx + opx + spl symplectites. Orthopyroxene + spl symplectites in the Kimberley peridotites are interpreted to be accidental cuts through cpx + opx + spl symplectites, which have avoided cpx + spl regions.
These common opx-symplectites further complicate the formation of symplectites by garnet breakdown. A garnet breakdown process would not only have to produce a symplectite, but would also have to produce single opx grains in the symplectite, which are crystallographically oriented exactly the same as the adjacent discrete enstatite. The process also needs to produce a region composed of, in many cases, a single crystal of cpx with finer-grained spinel lamellae at the boundary between the opx + spl domain and adjacent olivine. Although garnet breakdown and some subsequent recrystallization event is not beyond the realm of possibility, discontinuous precipitation and subsequent discontinuous coarsening seem a more reasonable explanation. The discontinuous precipitation hypothesis is also supported by the abundance of studies on the formation of symplectites in alloys.
Amphibole + spinel symplectites
Amphibole + spl symplectites formed by a metasomatic replacement of pre-existing cpx + spl, or cpx + opx + spl symplectites. The amphibole grains in the symplectites are typical of metasomatic amphiboles reported from upper mantle peridotites (Haggerty 1995; Dawson and Smith 1982). Arai (1986) described amphiboles containing partially digested pyroxene-spinel symplectites from Ichinomegata, northeastern Japan. Replacement textures in Kimberley metasomatic peridotites, including remnant fragments of cpx, are common in some amphibole grains. Some amphibole-spinel symplectites have a rim of spinel-free amphibole surrounding a core region (Fig. 7) of vermicular spinel and amphibole. This texture suggests the amphibole is not simply replacing the symplectite, but that another phase was also involved in amphibole growth. Bulk amph + spl symplectite chemistry tends to be more Mg-rich than cpx + spl or cpx + opx + spl symplectites (Fig. 13). Symplectite amphibole growth may involve discrete opx or phlogopite. Experiments (Tronnes 2002) indicate that potassic richterite replaces phlogopite-diopside assemblages at pressures above 8–9 Gpa and temperatures of 1000–1300 °C.
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Garnet-spinel symplectites
Garnet symplectites are not as common in the Kimberley peridotites as in samples from Jagersfontein. Only three garnet symplectites were found in the Kimberley collection. An additional garnet symplectite was found in the Premier Mine collection. Kimberley garnet symplectites consist of only spinel lamellae and blebs in garnet. Typical garnet rims surrounding cpx + spl cores, common in Jagersfontein samples, were not found in the Kimberley samples. It has been suggested that pyroxenespinel symplectites form by eutectoid-like decomposition as garnet breaks down with decreasing temperature and or pressure (Morishita and Arai 2003; Green and Burnley 1988).
The presence of garnet in symplectites may be used to support a symplectite origin by either garnet decomposition or symplectite replacement by garnet growth from pre-existing pyroxene-spinel symplectite via a reaction of the form:
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Bulatov et al. (1991) suggested that garnet could form by metamorphic re-equilibration of Cr-spinel harzburgite. Schmädicke and Evans (1997) indicated that garnet and olivine in the Bohemian Massif formed from opx + spl, based in part on relict spinel inclusions in garnet. Clarke and Carswell (1977) attributed the growth of Cr-rich garnets from the Newlands kimberlite to subsolidous reaction of opx and Cr-rich spinel. Evidence from Kimberley, like the evidence from Jagersfontein, supports the garnet growth hypothesis. However, the textural evidence of garnet-growth at Kimberley is incomplete. Symplectites (especially in Kimberley sample AJE 1049) show a well-defined symplectite pattern completely contained within garnet. If garnet growth has occurred, the pyroxene in the pre-existing symplectite has been completely consumed by the reaction. However, the textural sequence of garnet replacement of symplectite can be seen by combining the Kimberley and Jagersfontein SEM images. Figures 6a, 6b, and 7a are SEM micrographs of a possible sequence of garnet growth. Figure 6a shows a thin rim of garnet around a core of cpx + spl symplectite. Spinel grains in the symplectite are terminated by the garnet rim, implying garnet formation subsequent to symplectite formation. Figure 6b shows remnant cpx + spl symplectite surrounded by a garnet. Included within the garnet is a region of grt + spl. Figure 7a shows a typical spinel fingerprint pattern contained within a garnet. A spinel-free zone surrounds the garnet-spinel region. The garnet texture in sample AJE 1049 shows a remarkable resemblance to the amphibole-spinel symplectites found in some metasomatized Kimberley peridotites. Ionov et al. (1993) argued that the poikilitic appearance of pyrope in garnet peridotites from the Vitim Highlands of the lake Baikal region of Siberia suggested garnet growth at the expense of spinel and pyroxene. Das et al. (2006) described garnet in high-grade metamorphic terrains with a garnet + spinel core surrounded by a spinel-free garnet rim. One of the possible causes suggested by the authors was that the garnet formed from a precursor symplectite composed of orthopyroxene and sillimanite.
Garnet growth by symplectite replacement is complicated by the same chemical data that argue against symplectites being breakdown products of garnets, specifically that symplectite bulk compositions to do not match garnet compositions. Garnets are deficient in Cr (Fig.14) and Ca, and enriched in Al with respect to symplectites. Even the most Cr-rich green garnet grains are still Cr-poor and Al-rich compared to most pyroxene-spinel symplectites. The main difficulty addressing garnet growth in harzburgites is the supply of Al, and to a lesser extent Ca. The only major source of excess Al and Ca in most of the Kimberley garnet harzburgites are the large discrete grains of orthopyroxene. A more accurate chemical equation for the growth of garnet is
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Garnet in the Kimberley symplectite-bearing garnet harzburgites is volumetrically low and the volume of discrete orthopyroxene is large. The opx could be the source of the extra Al. Discrete opx in garnet harzburgite is deficient in Al and Ca (Fig. 9) with respect to opx in symplectite-bearing harzburgites. This difference suggests that opx in garnet harzburgite may have experienced more extensive Al diffusion than opx in harzburgites.
Possible causes of symplectite formation and garnet growth
Symplectite growth (cpx ± opx + spl) in the Kimberley peridotites is probably caused by an increase in mantle temperatures followed by cooling. Symplectites in experiments on initially symplectite-free alloys develop after heating. Symplectites form in Fe-Zn alloys after heating to 650 °C (Gupta 2001), Al-Li-Zr alloys after heating to 580 °C (Pragnell et al.1994), and in Cu- Cd alloys after heating to 400 °C (Guo et al. 1993). Piccardo et al. (2006) attributed vermicular spinel at the borders of opx to exsolution from a higher-temperature, Al-rich pyroxene, and Garrison and Taylor (1981) suggested high-temperature exsolution from a high-temperature, non-stoichiometric pyroxene as an origin for spl + mag + opx + cpx symplectites in a spinel websterite from Kentucky. Smith (2000) suggested that all cpx in some xenoliths formed by exsolution from opx. There is some metallographic evidence (Guo et al. 1993) indicating that stress may have an effect on morphology and location of cellular structures in alloys. There is little or no evidence in fine-grained Kimberley harzburgites, however, of structures caused by stress. Metasomatism is unlikely to be a triggering event, as fine-grained harzburgites with the most numerous symplectites show the least effects of metasomatism. Harzburgites and garnet harzburgites are depleted peridotites that may have formed as a result of partial melting of fertile pre-existing peridotite. This depletion event may have set up the necessary chemical conditions for subsequent symplectite formation. The lack of symplectites in lherzolites may be caused by the Ca enriched nature of lherzolites with respect to harzburgites. Symplectites are volumetrically small, and the symplectite Al, Cr, and Ca in harzburgites, may be accommodated in the abundant Cr-rich diopside, which is characteristic of the more fertile rock.
Garnet grains in the Kimberley peridotites are interpreted to form by a prograde metamorphic process. It is generally accepted that spinel peridotites represent lower-temperature and lower-pressure regimes in the mantle. Smith and Barron (1991) proposed that garnet formation results from temperature dependent Al-solubility in pyroxene, as did Das et al. (2006). Schmädicke and Evans (1997) suggested that garnet growth from opx + spl is due mostly to pressure increase alone. Clarke and Carswell (1977) also indicated that increasing pressure was the dominant cause of the growth of high-Cr garnets. Eclogites are suspected of being metamorphosed subducted oceanic basalt and some harzburgites may be subducted depleted lithospheric material (McCulloch 1993). Subduction of depleted spinel-bearing harzburgites may have allowed garnet growth from spinel + pyroxene to occur as the subducting slab crossed the spinelgarnet transition zone.
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1 Deposit item AM-08-015, Tables 1–5, 8, and 9 and Figure 3. Deposit items are available two ways: For a paper copy contact the Business Office of the Mineralogical Society of America (see inside front cover of recent issue) for price information. For an electronic copy visit the MSA web site at http://www.minsocam.org, go to the American Mineralogist Contents, find the table of contents for the specific volume/issue wanted, and then click on the deposit link there. 
MANUSCRIPT RECEIVED February 28, 2007; MANUSCRIPT ACCEPTED October 23, 2007
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