Bimineralic Eclogites Research Articles

A Model for Eclogite-Peridotite Interactions: Activity-Driven Metasomatism, with Evidence from the Zero Eclogite Suite, South Africa

It is well known that the dehydration and melting of basaltic oceanic crust in subduction zones is of crucial importance for the geochemical signature of magmatic-arc. At the same time, deep-seated kimberIite-borne eclogite xenolith suites both of South Africa and Siberia are being interpreted as remnants of such subducted slabs (e.g. Jacob et al., 1994; Snyder et al., 1997), and much speculation is concerned with the possible role of slab-mantle or, more generally, eclogite-peridotite interactions for the compositions of subcratonic lithosphere or in the genesis of some basalts (e.g. Yaxley and Green, 1998). Here, we want to systematize eclogiteperidotite relationships using an analysis of component activities imposed by their differing bulk compositions and arrive at a reaction sequence which must be satisfied by any of the geochemically based models. Whereas it is generally appreciated that 4-phase garnet lherzolite in the upper mantle constitutes a buffer for the silica activity through the equilibrium Ol + SiO2 = Opx (reaction [Gt] in Fig. 1), it is less known that in the system CMAS, the reaction Opx + Cpx + A1203 = Garnet ([Ol] in Fig. 1) also fixes the activity of A1203 at a given P, T, and bulk composition. Thus, mantle peridotite has to reside at an invariant point in a(Al203)-a(SiO2) space as shown in Fig. 1, the exact numerical value of which varies with P, T along a geotherm for any bulk composition. Eclogite, in contrast, generally lacks olivine and orthopyroxene and therefore must be associated with a(A1203)-a(SiO2) to the right of the lherzolite invariant point in Fig. 1. The activity of AI203 in a bimineralic Garnet + Cpx eclogite is defined by the reaction Cpx + A1203 = Garnet and depends on the aluminum (Ca-tschermakite) content of Cpx and the Xca (grossular content) of garnet. Thus a field for bimineralic eclogites is generated in activity space which nonetheless cannot be in equilibrium with peridotite activities. More specifically, kyanite-bearing eclogites must be located at high a(SiO2) to stabilize kyanite, and quartz/coesitebearing kyanite eclogites at still higher a(SiO2), both grossly out of equilibrium with peridotitic compositions at ambient P and T. This is also shown in Fig. 1. The consequence of these relationships is that eclogites e. g. in subducted slabs constitute a disequilibrium couple with surrounding peridotite at mantle depths, and the resulting activity gradients across the slab-mantle interface must drive metasomatic exchange. In other words, the eclogites must lose silica and aluminum to the surrounding mantle peridotites. The resulting metasomatic sequence is first, elimination of any quarz/kyanite in the eclogite, rendering it bimineralic; then, at progressed metasomatism, production of orthopyroxene. We suggest that this is the ultimate reason for the comparative scarcity of coesite/kyanite in kimberlite-borne eclogites vs. orogenic eclogites within the continental crust (both originating from similar oceanic crust). Conversely, orthopyroxene must be produced in peridotites as well below as above a subducted slab, by metasomatic exchange with its eclogites. This may help to explain the high-Si, Opx-rich character of old Archaean lithospheric roots, given that more frequent subduction and a hotter geotherm in the Archaean promote the proposed metasomatic exchange. Both direct slab melts and hydrous fluids from slab dehydration must reflect a(A1203)-a(SiO2) of their source and react with the peridotite they encounter. Still, with both metasomatic agents the nature of the metasomatic reactions is essentially determined by the activity gradients between eclogite and peridotite. We are presently investigating a suite of eclogite xenoliths from the Zero kimbertite, South Africa, which apparently shows evidence for the proposed metasomatic exchange. This suite consists of kyanite

Proton probe quantitative trace elemental microscopy of kyanite bearing eclogytic samples

Bi-mineralic eclogites (clinopyroxene and garnet) commonly occur as xenoliths in kimberlites and are generally thought to be the product of subduction of oceanic crust. Less common and less well understood are the kyanite-bearing eclogites and in particular grospydites. Grospydites were first observed in Siberian kimberlite pipes from the Daldy-Akalkit kimberlite field and specifically the Zagadochnaya Pipe. The name “grospydite” is a descriptive term for Ca- and Al-rich eclogites made up of the rock-forming mineral's gro ssular, py roxene and kyanite ( di sthen) at which the grossular component of garnet is at least > 50 mol%. One of the outstanding questions regarding these rocks, concerns the distribution of trace element concentrations such as Rb and Sr in gropydite forming minerals. In particular the concentrations of these elements in kyanite are virtually unknown, although the concentrations should be in the sub-ppm range. A knowledge of their concentrations is important for future RbSr isotopic studies. A feature of the samples that make them geologically unique but difficult to analyse is that they exhibit many polymineralic phases separated by alteration material on the micron scale. The proton induced X-ray emission (PIXE) technique was used for in-situ, non-destructive quantitative mapping of Rb and Sr trace elemental distributions in the different minerals. The focused beam (15 μm spot) could be scanned over the sample covering an area of up to 2.5 × 2.5 mm 2 to generate elemental distributions as maps. In addition, the scan area could be reduced to an arbitrary line, an arbitrary shape, or simply a spot to improve statistical accuracy while following the complicated shapes of the various phases present in the mineral. The data were also collected in list-mode allowing off-line replaying of the measurement using different scan masks to generate X-ray spectra unique to a given phase. The X-ray spectra were analysed in detail by the GUPIX computer programme to deliver trace elemental concentrations appropriate to each phase. The trace elemental concentration determinations were complemented by major and minor elemental analyses using the electron probe, and this provided an internal calibration via the Fe content of the minerals.

Oxygen isotope systematics of eclogite mineral phases from South Africa

Eleven eclogite xenoliths from the Bobbejaan and Roberts Victor mines, South Africa have been characterized by a variety of geochemical techniques. The samples include nine bimineralic eclogites and two grospydites that span the range of mantle eclogite major element compositions. Primary phase end-member compositions range from Py 75Al 16Gr 09, Jd 18Di 82CaEs 0 to Py 28Al 21Gr 51, Jd 49Di 36CaEs 15. Oxygen isotope analyses of mineral separates were performed for clinopyroxene and garnet. Isotopic values ( δ 18O) range from +4.0 to +7.8 for clinopyroxene and +3.6 to +7.2 for garnet. Differences in isotopic values between the primary phases ( Δ = clinopyroxene minus garnet) range in value from −0.6 to 0.6. Although oxygen isotopes are not expected to fractionate at mantle temperatures, the anion electrostatic site potential values determined by Smyth (1989) are consistent with small degrees of fractionation in the mantle. Oxygen isotope fractionation between minerals may be calculated by the equation of Smyth and Clayton ( F= A×10 6 T 2 , where A=0.7911 eV). Fractionation values at 1300°K range from 0.6 (Jd-Pyr) to 0.03 (CaTs-Grs), while at 1600°K these values are between 0.39 and 0.02, respectively. The re-equilibration temperatures of the studied eclogite suite are 1050 to 1250°C at 30 kbar, therefore the obtained differences between the mineral phases are consistent with the calculated isotope fractionation values. The data obtained for eclogite oxygen isotopes suggest that 18O may fractionate to a small extent at mantle PT-conditions.

Diffusion gradients in an eclogite xenolith from the Roberts Victor kimberlite pipe: (2) kinetics and implications for petrogenesis

In a bimineralic eclogite xenolith (sample JJG41) from the Roberts Victor kimberlite, compositional gradients in clinopyroxene are related to garnet exsolution. Two principal reactions involving clinopyroxene and garnet occur: (i) The net-transfer Al2Si-1Mg-1 which is responsible for garnet growth according to the equation 2Di+Al2Si-1Mg-1=Grossular+MgCa-1 (reaction 1). This has created substantial compositional gradients in Al, Si and Mg within clinopyroxene. (ii) The exchange of Fe−Mg between garnet and clinopyroxene (reaction 2). During the stage of garnet growth (reaction 1) the lamellae crystallized sequentially as a result of a temperature decrease from around 1400 to 1200° C. This exsolution growth-stage was under the control of Al diffusion in clinopyroxene and at around 1200° C further growth of garnet lamellae became impeded by the sluggishness of Al diffusion in the clinopyroxene host. However, reaction 2 continued during further cooling down to about 1000° C; this temperature being inferred from the constant Fe−Mg partitioning at clinopyroxene-garnet interfaces for the whole set of lamellae. The initial clinopyroxene in JJG41 was probably formed by crystallization from a melt in Archaean time. The cessation of Fe−Mg exchange between garnet and clinopyroxene at about 1000° C may well predate the eruption of the eclogite in kimberlite at around 100 Ma. Kinetic models of reaction are examined for both reactions. Modelling of reaction 1, involving both diffusion and interface migration, allows several means of estimating the diffusion coefficient of Al in clinopyroxene; the estimates are in the range 10-16-10-20 cm2/s at 1200° C. These estimates bracket the experimentally determined data for Al diffusion in clinopyroxene, and from these experimental data a preferred cooling rate of about 300° C/Ma is obtained for the period of growth of garnet exsolution lamellae. A ‘geospeedometry’ approach (Lasaga 1983) suitable for a pure-exchange process (reaction 2) is used to estimate the cooling rate in the later stages of the thermal history (after garnet growth); values 4–40° C/Ma are consistent with the shape of the Fe-diffusion gradients in the clinopyroxene. The extensive thermal history recorded by JJG41, including probable melt involvement at ca. 1400° C, demonstrates the complex evolution of rocks within the mantle. Whilst the notion of formation of mantle eclogites from subducted oceanic crust has become fashionable, it is clear that tracing eclogite geochemical and P-T characteristics backwards from their nature at the time of xenolith eruption, through high-temperature mantle events to the characteristics of the original subducted oceanic crust, will be very complex.

Trace element crystal chemistry of mantle eclogites

Rare earth element (REE) concentrations have been measured using instrumental neutron activation analysis on clean separates of primary minerals from 11 eclogite samples from the Bobbejaan and Roberts Victor kimberlites, South Africa. Samples were selected to reflect minimal secondary alteration and represent a broad range of eclogite compositions from coesiteand corundum-grospydites through magnesian bimineralic eclogites. Correlations between REE concentrations and major-element compositions suggest that garnet and clinopyroxene crystal chemistry are the dominant control on REE distribution and that these approach solidstate equilibrium distributions. Reconstructed wholerock REE concentration variation with whole-rock major-element compositions are consistent with an origin by high-pressure igneous fractionation followed by reequilibration to lower temperatures at pressures in excess of three GPa.

Mantle eclogites: evidence of igneous fractionation in the mantle

Despite recent geochemical studies advocating an origin of eclogite xenoliths in kimberlite by subduction of altered sea-floor basalts and crustal cumulates, there are numerous examples showing abundant exsolution of garnet and kyanite from clinopyroxene that could only have occurred on cooling from near-solidus temperatures at pressures in excess of 3 GPa. Samples showing extensive exsolution tend to have more calcic garnets and sodic pyroxenes than the average eclogite, but do not form a group that is geochemically distinct in major, minor, or trace elements. Also, composite samples containing both kyanite and bimineralic eclogites demonstrate the co-genetic relation of extremes of composition of the eclogite suite. Accordingly, we present an igneous fractionation model to account for the major and trace element variations observed in these rocks: grospydites and kyanite eclogites are early pyroxene, or pyroxeneplus-corundum, cumulates that have undergone extensive subsolidus exsolution of kyanite and garnet. These were then followed by more Fe-rich eclogites which had pyroxene plus garnet as solidus phases and fractionated towards more magnesian bimineralic eclogites.

Coronitic reactions and ductile shear zones in eclogitised ophiolite metagabbro, Western Alps, North Italy

The objective of this work is to study the most significant static and syn-tectonic transformations which accompanied the eclogite metamorphism (450 ± 50°C, 12–15 kbar) of an ophiolite metagabbro from the Western Alps. In partly undeformed volumes abundant igneous mineral relics occur and the eclogite metamorphism produced coronitic or (incomplete) pseudomorphic reactions. The disequilibrium or local equilibrium conditions implied by these features suggest that slow diffusion and interface kinetics were the rate-limiting factors which prevented a large-scale eclogite equilibration. The undeformed volumes are bounded by syn-eclogite ductile shear zones where the magmatic minerals and textures are obliterated and a new fabric/foliation develops. The enlargement of equilibrium domains observed in the shear zones indicates that a close approach to eclogite equilibration of these metagabbros was not attained without the catalytic effect of pervasive deformation. The bulk-rock chemistry also played an important role in determining the kind of eclogite assemblage; in particular only sheared gabbros rich in Fe can produce bimineralic eclogites.

Trace-element abundances of iron-rich eclogites, with implications on the geodynamical evolution of the Voltri Group (Pennidic Belt)

Fe (136,900 ppm)- and Ti (28,540 ppm)-rich eclogites contained in the Voltri serpentinites show contents of metallic trace elements different from those of other eclogites described so far, both those of ophiolitic type and those included in ultramafic rocks. The Voltri eclogites are enriched in Mn (1,894 ppm), V (610 ppm), and Co (75 ppm) and depleted in Cr (110 ppm), Ni (56 ppm), and Zr (46 ppm) in comparison with other occurrences. The analyzed samples are: twelve flaser-eclogites, four fine-grained bimineralic eclogites and three eclogites showing an advanced degree of retrometamorphism toward greenschist facies. The given average values (in ppm) are based on nineteen analyzed samples. This trace-element content is believed to reflect the highly fractionated tholeiitic character of the eclogites and suggests that the Voltri rocks originated from the isochemical metamorphism of ferrogabbros which were probably intruded into the serpentinized lherzolites of a mid-oceanic ridge at the time of the opening of the Pennidic trough.

Evolution of clinopyroxene and garnet in an eclogite nodule from the roberts victor kimberlite pipe, South Africa

A bimineralic eclogite (JJG 41) shows clinopyroxene crystals of widely varying grain size (up to 6.0 cm) from which extensive lamellar and granular exsolution of garnet has occurred. The eclogite is rich in CaO and Al 2O 3, but poor in Na 2O; the initial clinopyroxene probably had 14.0 to 14.5 wt.% Al 2O 3 with tschermakite and jadeite contents of approximately 30% and 7% respectively. Considerable variation in chemical composition occurs within single clinopyroxene crystals and between different garnets (both granular and lamellar). Garnets show increasing grossular contents with progressive exsolution, and this is also associated with an increase of K' = Fe Mg Garnet/Fe/Mg clinopyroxene from approximately 1.8 to 5.5. These changes place much importance on declining temperature during exsolution. High pressure experimental work shows a wedge-shaped stability field for clinopyroxene (approximating the initial composition) below the solidus, and, assuming a magmatic derivation, gives an estimate of formation of an initial clinopyroxene + garnet assemblage of approximately 1400°C and 34–38 kb. It is suggested that the subsequent exsolution is largely related to cooling from these conditions towards the geotherm at initially constant depth.

Crystallization of some natural eclogites and garnetiferous ultrabasic rocks at high pressure and temperature

The solidi of two dry bimineralic eclogites (probable high-pressure cumulates) are slightly lower but their liquidi are substantially higher in temperature than the solidus of garnet-lherzolite. Olivine does not appear at the liquidus of either rock and enstatite appears only transiently in the solidus-liquidus interval of one rock at 20 kbar. The two dry eclogites cannot, therefore, represent primary liquids produced from a peridotite source nor can they be partially melted to yield peridotite residua. One eclogite shows total solution of garnet in clinopyroxene at the solidus close to 20 kbar, otherwise garnet is retained as a solidus phase in both rocks between 20 and 30 kbar. A garnet-olivine websterite, thought to have formed originally as an olivine + two aluminous pyroxenes cumulate, develops this mineralogy only close to the dry solidus at 30 kbar, 1575°C. Spinel appears at lower pressures, garnet at higher pressures. An iron- and aluminium-rich garnet-lherzolite, possibly representing a frozen high-pressure primary magma in equilibrium with harzburgite, does not show the required replacement of olivine by orthopyroxene as liquidus phase until pressures in excess of 45 kbar, implying a depth of origin greater than 140 km.

Rb/K ratios and Sr-isotopic compositions of minerals in eclogitic and peridotitic rocks

Rb, K and Sr concentrations and Sr isotopic compositions are reported for omphacite, diopside, garnet and mica concentrates, and whole-rocks, from mafic and ultramafic inclusions of South African kimberlite pipes and two Bavarian metamorphic eclogites. Concentrations were measured by isotope dilution, and details are given of a technique suited to the determination of low K- and Rb-concentrations. Low Rb/K and low 87 Sr/ 86 Sr ratios characterise the pipe eclogite omphacites and higher Rb/K and 87 Sr/ 86 Sr ratios characterise the pipe peridotite diopsides. High concentrations of K and Rb characterise the fine-grained material concentrated along grain boundaries, particularly around garnet. This material also has higher 87 Sr/ 86 Sr ratios than the omphacite; these contrasts in concentration and isotopic ratio probably arise through contamination of the intergranular medium, with material from the gassy phase entering the inclusions during pipe emplacement. Previous measurements on kimb yielded data biased by the intergranular medium. The mica analyses yield Cretaceous ages, consistent with the geological age for pipe emplacement. The Rb/K and 87 Sr/ 86 Sr ratios in an “ideal” bimineralic eclogite, i.e., in a rock consisting of omphacite and garnet alone, are dominated by the former mineral, and closely resemble the corresponding ratios in abyssal tholeiite basalts. We conclude that: (i) “Ideal” bimineralic eclogite may have a directly parental role in the genesis of abyssal tholeiite basalt magmas, and regions of ideal eciogite are widespread and important in the upper mantle. (ii) Ideal eclogite cannot itself represent primary upper mantle material. The low 87 Sr/ 86 Sr ratios in the omphacite indicate that formation of this rock was a Pre-Cambrian event, assuming that the primary upper mantle material has Rb/Sr ≥0.023.