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1.
Eclogitic (E-type) and related parageneses of natural diamonds are represented by suites of diamond inclusions and xenoliths of diamondiferous eclogites. Major-element data are presented for 32 coexisting minerals forming 19 bimineralic and trimineralic inclusions from diamonds, including omphacite-orthopyroxene (1 sample), garnet-omphacite (5 samples), garnet-coesite (5 samples), omphacite-coesite (2 samples), garnet-picroilmenite (2 samples), garnet-kyanite (1 sample), omphacite-phlogopite (2 samples), and garnel-omphacite-phlogopite (1 sample). Major-element variations of coexisting minerals are typical of corresponding eclogites. Omphacite with 5.02 wt% Na2O, inter-grown with orthopyroxene with Mg# 83.7, represents the first example of a diamondiferous websterite paragenesis including Na-clinopyroxene. This indicates a broader range in mineral compositions of E-type-related websteritepyroxenite-associated diamonds than known previously. This unique websterite-pyroxenitic mineral assemblage represents a transitional paragenesis between peridotitic or ultramafic (U-type) and E-type parageneses.

Bimineralic eclogites, ilmenite eclogites, coesite + corundum + kyanite eclogites, and grospydites occur not only as sets of inclusions in diamonds but, with a few exceptions (ilmenite and coesite eclogites), also as diamondiferous eclogite xenoliths. The coesite eclogite paragenesis is a significant inclusion suite in diamonds, and was detected in about 15 diamond occurrences worldwide. It represents from 15% to 22% of all E-type diamonds in several occurrences, and thus should not be considered as rare.  相似文献   

2.
 Diamond-bearing eclogites are an important component of the xenoliths that occur in the Mir kimberlite, Siberian platform, Russia. We have studied 16 of these eclogite xenoliths, which are characterized by coarse-grained, equigranular garnet and omphacite. On the basis of compositional variations in garnet and clinopyroxene, this suite of eclogites can be divided into at least two groups: a high-Ca group and a low-Ca group. The high-Ca group consists of high-Ca garnets in equilibrium with pyroxenes that have high Ca-ratios [Ca/(Ca+Fe+Mg)] and high jadeite contents. These high-Ca group samples have high modal% garnet, and garnet grains often are zoned. Garnet patches along rims and along amphibole- and phlogopite-filled veins have higher Mg and lower Ca contents compared to homogeneous cores. The low-Ca group consists of eclogites with low-Ca garnets in equilibrium with pyroxenes with a low Ca-ratio, but variable jadeite contents. These low-Ca group samples typically have low modal% of garnet, and garnets are rarely compositionally zoned. Three samples have mineralogic compositions and modes transitional to the high- and low-Ca groups. We have arbitrarily designated these samples as the intermediate-Ca group. The rare-earth-element (REE) contents of garnet and clinopyroxene have been determined by ion microprobe. Garnets from the low-Ca group have low LREE contents and typically have [Dy/Yb]n < 1. The high-Ca group garnets have higher LREE contents and typically have [Dy/Yb]n > 1. Garnets from the intermediate-Ca group have REE contents between the high- and low-Ca groups. Clinopyroxenes from the low-Ca group have convex-upward REE patterns with relatively high REE contents (ten times chondrite), whereas those from the high-Ca group have similar convex-upward shapes, but lower REE contents, approximately chondritic. Reconstructed bulk-rock REE patterns for the low-Ca group eclogites are relatively flat at approximately ten times chondrite. In contrast, the high-Ca group samples typically have LREE-depleted patterns and lower REE contents. The δ18O values measured for garnet separates range from 7.2 to 3.1‰. Although there is a broad overlap of δ18O between the low-Ca and high-Ca groups, the low-Ca group samples range from mantle-like to high δ18O values (4.9 to 7.2‰), and the high-Ca group garnets range from mantle-like to low δ18O values (5.3 to 3.1‰). The oxygen isotopic compositions of two of the five high-Ca group samples and four of the eight low-Ca group eclogites are consistent with seawater alteration of basaltic crust, with the low-Ca group eclogites representative of low-temperature alteration, and the high-Ca group samples representative of high-temperature hydrothermal seawater alteration. We interpret the differences between the low- and high-Ca group samples to be primarily a result of differences in the protoliths of these samples. The high-Ca group eclogites are interpreted to have protoliths similar to the mid to lower sections of an ophiolite complex. This section of oceanic crust would be dominated by rocks which have a significant cumulate component and would have experienced high-temperature seawater alteration. Such cumulate rocks probably would be LREE-depleted, and can be Ca-rich because of plagioclase or clinopyroxene accumulation. The protoliths of the low-Ca group eclogites are interpreted to be the upper section of an ophiolite complex. This section of oceanic crust would consist mainly of extrusive basalts that would have been altered by seawater at low temperatures. These basaltic lavas would probably have relatively flat REE patterns, as seen for the low-Ca group eclogites. Received: 10 July 1995 / Accepted: 17 May 1996  相似文献   

3.
《Geochimica et cosmochimica acta》1999,63(11-12):1825-1836
Oxygen isotope data have been obtained for silicate inclusions in diamonds, and similar associated minerals in peridotitic and eclogitic xenoliths from the Finsch kimberlite by laser-fluorination. Oxygen isotope analyses of syngenetic inclusions weighing 20–400 μg have been obtained by laser heating in the presence of ClF3. 18O/16O ratios are determined on oxygen converted to CO2 over hot graphite and, for samples weighing less than 750 μg (producing <12 μmoles O2) enhanced CO production in the graphite reactor causes a systematic shift in both δ13C and δ18O that varies as a function of sample weight. A “pressure effect” correction procedure, based on the magnitude of δ13C (CO2) depletion relative to δ13C (graphite), is used to obtain corrected δ18O values for inclusions with an accuracy estimated to be ±0.3‰ for samples weighing 40 μg.Syngenetic inclusions in host diamonds with similar δ13C values (−8.4‰ to −2.7‰) have oxygen isotope compositions that vary significantly, with a clear distinction between inclusions of peridotitic (+4.6‰ to +5.6‰) and eclogitic paragenesis (+5.7‰ to +8.0‰). The mean δ18O composition of olivine inclusions is indistinguishable from that of typical peridotitic mantle (5.25 ± 0.22‰) whereas syngenetic purple garnet inclusions possess relatively low δ18O values (5.00 ± 0.33‰). Reversed oxygen isotope fractionation between olivine and garnet in both diamond inclusions and diamondiferous peridotite xenoliths suggests that garnet preserves subtle isotopic disequilibrium related to genesis of Cr-rich garnet and/or exchange with the diamond-forming fluid. Garnet in eclogite xenoliths in kimberlite show a range of δ18O values from +2.3‰ to +7.3‰ but garnets in diamondiferous eclogites and as inclusions in diamond all have values >4.7‰.  相似文献   

4.
This paper describes a suite of peridotite xenoliths. some carrying diamonds at high grades, from the richly diamondiferous early Proterozoic (1180 Ma) Argyle (AK1) lamproite pipe, in northwestern Australia. The peridotites are mostly coarse garnet lherzolites but also include garnet harzburgite, chromite — garnet peridotite, a garnet wehrlite, and an altered spinel peridotite with extremely Cr-rich chromite. In all cases the garnet has been replaced by a kelyphite-like, symplectic intergrowth of Alrich pyroxenes, Al-spinel and secondary silicates. The peridotites have refractory compositions characterized by high Mg/(Mg+Fe) and depletion in lithophile elements (Al2O3 and CaO < 1%, Na2O0.03%) and high field strength cations such as Ti, Zr, Y, and Yb. Olivines have high Mg/(Mg+Fe) (Mg 91–93 ) and, like olivine inclusions in diamonds from the Argyle pipe, contain detectable amounts of Cr2O3 (0.03%–0.07%) but have very low CaO contents (typically 0.04%–0.05%). Enstatites (Mg 92–94 ) have comparatively high Cr2O3 (0.2%–0.45%) and Na2O (up to 0.18%) but very low Al2O3 contents (0.5%–0.7%). Diopsides (Mg 92–94 , Ca/(Ca+Mg+Fe)=0.37–0.43) are Cr-rich (0.7%–1.9% Cr2O3) and have low Al2O3 (0.7%–2.2%) and Na2O (0.5%–1.6%) contents. Many have high K2O contents, typically 0.1%–0.4% but up to 1.3% K2O in one xenolith. The chromite coexisting with former garnet is Mg-and Cr-rich [Mg/(Mg+Fe2+)=0.68–0.72, Cr/(Cr+Al)=0.72–0.79] whereas chromite in the spinel peridotite is even more Cr-rich (65% Cr2O3, Cr/(Cr+Al)=0.85, resembling inclusions in diamond. One highly serpentinized former garnet peridotite contains a Cr-rich (up to 13% Cr2O3) titanate resembling armalcolite but containing significant K2O (1%–2.5%), CaO (0.6%–2.2%), ZrO2 (0.1%–0.8%), SrO (0.1%–0.3%), and BaO (up to 0.58%): this appears to have formed as an overprint of the primary mineralogy. Temperatures and pressures estimated from coexisting pyroxenes and reconstructed garnet compositions indicate that the garnet lherzolites equilibrated at 1140°–1290° C and 5.0–5.9 GPa (160–190 km depth), within the stability field of diamond. Oxygen fugacties within the diamond forming environment are estimated from spinel-bearing assemblages to be reducing, with f O2 between MW and IW. The presence of significant K in the diopsides from the peridotite xenoliths and in diopsides from heavy mineral concentrate from the Argyle pipe implies metasomatic enrichment of the subcontinental lithosphere within the diamond stability field. The P-T conditions estimated for the Argyle peridotites demonstrate that diamondiferous lamproite magmas incorporate mantle xenoliths from similar depths to kimberlites in cratonic settings, and imply that Proterozoic cratonized orogenic belts can have lithospheric roots of comparable thickness to beneath Archaean cratons. These roots lie at the base of the lithosphere within the stability field of diamond. The xenoliths, the calcic nature of chrome pyropes from heavy mineral concentrate, and the diamond inclusion assemblage indicate that the lighosphere beneath the Western Australian lamproites is mostly depleted lherozolite rather than the harzburgite commonly found beneath Archaean cratons. Nevertheless, the dominance of eclogitic paragenesis inclusions in Argyle diamonds indicates a significant proportion of diamondiferous eclogite is also present. The form, mineral inclusion assemblage, and the C-isotopic composition of diamonds in the peridotite xenoliths suggest that disaggregated diamondiferous peridotites are the source of the planar octahedral diamonds which constitute a minor component of the Argyle production. These diamonds are believed to have formed from mantle carbon in reduced, refractory peridotite (Iherzolite-harzburgite) in contrast to the predominant strongly 13C-depleted eclogitic suite diamonds which contain a recycled crustal carbon component. The source region of the lamproites has undergone long-term (2 Ga) enrichment in incompatible elements.  相似文献   

5.
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6.
Diamonds: time capsules from the Siberian Mantle   总被引：1，自引：0，他引：1  
Diamonds are thought to be “time capsules” from the Earth's mantle. However, by themselves, consisting of nearly pure carbon, diamonds provide little geochemical information about their conditions of formation and the nature of their mantle hosts. This obstacle to studying the origin of diamonds and their hosts can be overcome by using two main approaches that focus on studying: (1) the rocks that contain diamonds, i.e., diamondiferous xenoliths; and (2) mineral inclusions within the diamonds, the time capsule's little treasures, if you will. Diamondiferous xenoliths, their diamonds, and mineral inclusions within the diamonds are the subject of this review, focusing on studies of samples from the Yakutian kimberlites in the Siberian Platform.Studies of diamondiferous eclogite xenoliths significantly enhance our understanding of the complex petrogenesis of this important group of rocks and their diamonds. Such studies involve various geochemical and petrological investigations of these eclogites, including major and trace-element, radiogenic as well as stable isotopic analyses of whole rocks and minerals. The results from these studies have clearly established that the Group A-C eclogites originate from subduction of ancient oceanic crust. This theory is probably applicable worldwide.Within the last several years, our research group at Tennessee has undertaken the systematic dissection (pull apart) of diamondiferous eclogites from Siberia, consisting of the following steps: (1) high-resolution computed X-ray tomography of the xenoliths to produce 3D images that relate the minerals of the xenoliths to their diamonds; (2) detailed dissection of the entire xenolith to reveal the diamonds inside, followed by characterization of the setting of the diamonds within their enclosing minerals; and (3) extraction of diamonds from the xenolith for further investigation of the diamonds and their inclusions. In this last step, it is important that the nature and relative positions of the diamond inclusions are carefully noted in order to maximize the number of inclusions that can be exposed simultaneously on one polished surface. In this modus operandi, cathodoluminescence imaging, plus FTIR/N aggregation and C/N isotopic analyses are performed on polished diamond surfaces to reveal their internal growth zones and the spatial relationship of the mineral inclusions to these zones.Knowledge gained by such detailed, albeit work-intensive, studies continues to add immensely to the constantly evolving models of the origin of diamonds and their host rocks in the Earth's mantle, as well as to lithospheric stability models in cratonic areas. Multiple lines of evidence indicate the ultimate crustal origin for the majority of mantle eclogites. Similar pieces of evidence, particularly from δ13C in P-type diamonds and δ18O in peridotitic garnets lead to the suggestion that at least some of the mantle peridotites, including diamondiferous ones, as well as inclusions in P-type diamonds, may have had a crustal protolith as well.  相似文献   

7.
The primary garnet (pyrope-almandine)-omphacite (Cpx 1, 6.5–7 wt% Na2O)-sulfide (Fe-Ni-Co mss) assemblage of the two diamondiferous eclogite xenoliths studied (U33/1 and UX/1) experienced two mantle metasomatic events. The metasomatic event I is recorded by the formation of platy phlogopite (~ 10 wt% K2O), prior to incorporation of the xenoliths in the kimberlite. The bulk of the metasomatic alteration, consisting of spongy-textured clinopyroxene (Cpx 2A, 1–3 wt% Na2O), coarser-grained clinopyroxene (Cpx 2B, 2–5 wt% Na2O), pargasitic amphibole (~ 0.8 wt% K2O; 3–3.5 wt% Na2O), kelyphite (Cpx 3, mostly <1 wt% Na2O; and zoned Mg-Fe-Al spinel), sodalite, calcite, K-feldspar, djerfisherite (K5.95Na0.02Fe18.72Ni2.36Co0.01Cu4.08S26Cl ) and a small amount of K-Ca-Fe-Mg glass, is ascribed to the metasomatic event II that occurred also in the upper mantle, but after the xenoliths were incorporated in the kimberlite. A pervasive chloritic alteration (mainly clinochlore + magnetite) that overprints earlier assemblages probably took place in the upper crustal environment. The diamonds are invariably associated with secondary clinopyroxene and chlorite, but the diamonds formed before the entrainment of the xenoliths in the Udachnaya kimberlite.Editorial Responsibility: T.L. Grove  相似文献   

8.
D.A Carswell 《Lithos》1974,7(2):113-121
The chemistry of the pyroxenes suggests that the garnet lherzolites enclosed in the Norwegian basal gneisses have equilibrated at depths greater than 70 kilometres along an expected sub-continental geotherm. Such depths are somewhat shallower than the apparent depths of origin of most garnet lherzolite xenoliths in kimberlite pipes. Distribution coefficients for Fe2+/Mg2+ and Mn2+/Mg2+ between coexisting clinopyroxenes and garnets support the slightly lower equilibration temperatures deduced for the Norwegian garnet lherzolites compared with the xenolithic garnet lherzolites in kimberlites.The pressure-temperature equilibration conditions deduced for the Norwegian garnet lherzolites (800–1020°C at 22–37 kbs) contrast with previous estimates (625 ± 30° at 14 kbs) for basic eclogite masses in the Norwegian gneisses. This suggests a possible dual paragenesis of the Norwegian eclogites, with the garnet lherzolites being tectonic slices of the sub-continental upper mantle and the basic eclogites deep crustal metamorphic rocks.  相似文献   

9.
The xenoliths of garnet–clinopyroxene–disthene rocks(grospydites and associated kyanite eclogites) from the Zagadochnayakimberlite pipe in Yakutia have been studied in detail. Contraryto previous data, the presence of a continuous range in thepyrope-grossular series of garnets is shown on the basis ofnumerous X-ray data and 17 chemical analyses of garnets. Thisconclusion is confirmed by the study of separate grains withkyanite intergrowths from the kimberlite heavy fraction, whichare present in the kimberlite as the result of destruction ofgrospydite xenoliths, and possible, of garnet-kyanite rocksalso. A close connection of the calcium content in the garnetwith the sodium content in the coexisting clinopyroxes is alsoshown. An increase in the chemical potential of sodium resultsin the stability of the pryoxene-kyanite assemblage insteadof a garnet of intermediate composition (50 percent of grossular).The interval of the miscibility gap between calcium-rich andcalcium-poor garnets is increased in this way. The data of chemicalanalyses of 14 pyroxenes from the xenoliths indicate that theydiffer in the high aluminium and sodium content from other pyroxenesof eclogite-facies rocks. Chromium-rich bands with a high chromiumcontent in coexisting garnet, pyroxene, and kyantic have beenoccasionally found in the xenoliths. A study has been made ofthe chrome-kyanite with 12.86 per Cr202. The presence of chromium-richminerals in the grospydite xenoliths confrms their connectionwith ultrabasic rocks.  相似文献   

10.
Ferric iron in mantle-derived garnets   总被引：4，自引：0，他引：4  
The oxidation state of a mantle assemblage may be defined by heterogeneous reactions between oxygen and iron-bearing minerals. In spinel lherzolites, the presence of Fe3+ in spinel allows use of the assemblage olivine-orthopyroxene-spinel to define f O 2 at fixed T and P. As a first step towards establishing an analogous reaction for garnet lherzolites, garnets from mantle-derived xenoliths from South Africa and the USSR have been analyzed with 57Fe Mössbauer spectroscopy at 298 and 77K to determine Fe3+/Fe2+ and the coordination state of iron. Garnets from South African alkremites (pyrope+Mg-spinel) and eclogites, as well as garnet-spinel and low-temperature garnet lherzolites from both South Afica and the USSR, have Fe3+/Fe<0.07. In contrast, garnets from high-temperature garnet lherzolites from within the Kaapvaal craton of South Africa have Fe3+/Fe>0.10. Ferric iron is octahedrally coordinated, and ferrous iron is present in the dodecahedral site in all samples. The occurrence of significant Fe3+ in these garnets necessitates caution in the use of geothermometers and geobarometers that are applied to mantle samples. For example, the presence of 12% of the Fe as Fe3+ in garnets can increase temperatures calculated from existing Fe/Mg geothermometers by>200°C. The concomitant increase in pressures calculated from geobarometers that use the Al content in orthopyroxene coexisting with garnet are 10–15 kbar. Results of calculations based on heterogeneous equilibria between garnet, olivine, and pyroxene are consistent with the derivation of the peridotite samples from source regions that are relatively oxidized, between the f O 2 of the FMQ (quartz-fayalite-magnetite) buffer and that of the WM buffer. No samples yield values of f O 2 as reduced as IW (iron-wüstite buffer).  相似文献   

11.
The pseudo-binary system Mg3Al2Si3O12–Na2MgSi5O12 modelling the sodium-bearing garnet solid solutions has been studied at 7 and 8.5 GPa and 1,500–1,950°C. The Na-bearing garnet is a liquidus phase of the system up to 60 mol% Na2MgSi5O12 (NaGrt). At higher content of NaGrt in the system, enstatite (up to ∼80 mol%) and then coesite are observed as liquidus phases. Our experiments provided evidence for a stable sodium incorporation in garnet (0.3–0.6 wt% Na2O) and its control by temperature and pressure. The highest sodium contents were obtained in experiments at P = 8.5 GPa. Near the liquidus (T = 1,840°C), the equilibrium concentration of Na2O in garnet is 0.7–0.8 wt% (∼6 mol% Na2MgSi5O12). With the temperature decrease, Na concentration in Grt increases, and the maximal Na2MgSi5O12 content of ∼12 mol% (1.52 wt% Na2O) is gained at the solidus of the system (T = 1,760°С). The data obtained show that most of natural diamonds, with inclusions of Na-bearing garnets usually containing <0.4 wt% Na2O, could be formed from sodium-rich melts at pressures lower than 7 GPa. Majoritic garnets with higher sodium concentrations (>1 wt% Na2O) may crystallize at a pressure range of 7.0–8.5 GPa. However the upper pressure limit for the formation of naturally occurring Na-bearing garnets is restricted by the eclogite/garnetite bulk composition.  相似文献   

12.
Petrochemistry of eclogites from the Koidu Kimberlite Complex,Sierra Leone   总被引：1，自引：0，他引：1  
Petrography, mineral and bulk chemistry of upper mantle-derived eclogites (garnet and clinopyroxene) from the Koidu Kimberlite Complex, Sierra Leone, are presented in the first comprehensive study of these xenoliths from West Africa. Although peridotite-suite xenoliths are generally more common in kimberlites, the upper mantle sample preserved in Pipe Number 1 at Koidu is exclusively eclogitic, making this the fifth locality in which eclogite is the sole polymineralic xenolith in kimberlite. Over 2000 xenoliths were collected, of which 47 are described in detail that include diamond, graphite, kyanite, corundum, quartz after coesite, and amphibole eclogites. Grossular-pyrope-almandine garnets are chromium-poor (<0.72 wt% Cr2O3) and fall into two distinct groups based on magnesium content. High-MgO garnets have an average composition of Pyr67Alm22Gross11, low-MgO garnets are grossular- and almandine-rich with an average composition of Gross34Pyr33Alm33. Clinopyroxenes are omphacitic with a range in jadeite contents from 7.7 to 70.1 mol%. Three eclogites contain zoned and mantled garnets with almandine-rich cores and pyrope-rich rims, and zoned clinopyroxenes with diopside-rich cores and jadeite-rich rims, and are among a very rare group of eclogites reported on a world-wide basis. The bulk compositions of eclogites have ranges comparable to that of basalts. High-MgO eclogites (16–20 wt% MgO) have close chemical affinities to picrites, whereas low-MgO eclogites (6–13 wt% MgO) are similar to alkali basalts. High-MgO eclogites contain high-MgO garnets and jadeiterich clinopyroxenes. Low-MgO eclogites contain low-MgO garnets, diopside and omphacite, and the group of primary accessory phases (diamond, graphite, quartz after coesite, kyanite, and corundum); grospydites are peraluminous. Estimated temperatures and pressures of equilibration of diamond-bearing eclogites, using the diamond-graphite stability curve and the Ellis and Green (1979) geothermometer, are 1031°–1363° C at 45–50 kb.K D values of Fe-Mg in garnet and clinopyroxene range from 2.3 to 12.2. Diamonds in eclogites are green, yellow, and clear, and range from cube to octahedral morphologies; the entire spectrum in color and morphology is present in a single metasomatized eclogite with zoned garnet and clinopyroxene. Ages estimated from Sm-Nd mineral isochrons range from 92–247 Ma. Nd values range from +4.05 to 5.23. Values of specific gravity range from 3.06–3.60 g/cc, with calculated seismic Vp of 7.4–8.7 km/s. Petrographie, mineral, and bulk chemical data demonstrate an overall close similarity between the Koidu xenolith suite and upper mantle eclogites from other districts in Africa, Siberia and the United States. At least two origins are implied byP-T, bulk chemistry and mineral compositions: low-MgO eclogites, with diamond and other accessory minerals, are considered to have formed from melts trapped and metamorphically equilibrated in the lithosphere; high-MgO eclogites are picritic and are the products of large degrees of partial melting, with equilibration in the asthenosphere. Fluid or diluted melt metasomatism is pervasive and contributed here and elsewhere to the LIL and refractory silicate incompatible element signature in kimberlites and lamproites, and to secondary diamond growth.  相似文献   

13.
Constraints on the origin of mantle-derived low Ca garnets   总被引：3，自引：1，他引：2  
Current hypotheses for the source rock of low Ca garnets hosted in mantle-derived diamonds and xenoliths range from residues of komatiite generation, to subducted serpentinite, to subducted mid-ocean ridge (MORB) harzburgite. Experiments designed to test these hypotheses were undertaken. The stability and compositional variation of garnets at pressures above 4 GPa through the melting interval of hydrous peridotite, in the subsolidus of depleted harzburgite and peridotite compositions, and along the liquidus of aluminium-undepleted and aluminium-depleted komatiites were examined, and compared with petrological data for natural low Ca garnets. Partitioning of Cr between garnet and ultramafic liquid along the liquidus of komatiites and within the melting interval of peridotite, indicates that garnets in mantle residues after single stage Archean ultramafic liquid removal would contain 2 to 4 wt% Cr2O3. Thus, the more Cr-poor population of mantle-derived low Ca garnets, with Cr2O3 less than 4 wt%, could have originated by such a process. Experimental results for other compositions indicate that average cratonic peridotite or its hydrated equivalent is typically too Cr-poor to be the protolith from which low Ca garnets containing greater than 4 wt% Cr2O3 could have crystallized in the upper mantle. Experiments on a spinel harzburgite composition indicate that an extremely Cr-rich protolith (Cr/Cr+Al>0.3) is required to crystallize spinel and Cr-rich low Ca garnets, at pressures deduced for the ultramafic inclusion suite in diamonds (5 to 7 GPa). Natural examples of such Cr-rich protoliths are represented in some ophiolite harzburgites. All the experimental data taken together require that low Ca garnets with greater than 4 wt% Cr2O3 originated from residues that underwent multiple melt extraction. Whether such multi-stage events formed protoliths for low Ca garnets at shallow (i.e. MORB source region) or deep (i.e. komatiite source region) levels in the Precambrian mantle is not completely resolvable. The former environment can better account for the abundance of spinel in many diamonds hosting low Ca garnets, but the latter scenario best explains the presence of low Ca garnets in harzburgite xenoliths with cratonic bulk compositions well removed from typical MORB residues.  相似文献   

14.
Eclogite xenoliths from the mantle have experienced a wide variety of processes and P-T conditions, many of which are recorded in the mineral compositions and textures. Exsolution of garnet from clinopyroxene is one such texture, occurring in a minority of mantle eclogites. New analyses of clinopyroxene and garnet of eclogite xenoliths from kimberlites at Bellsbank (South Africa) and Obnazhënnaya (Yakutia, Russia) are presented here, and these are combined with data from the literature. Exsolution of garnet from clinopyroxene is generally lamellar, although lens-shaped garnets are also present. Major- and trace-element characteristics show a wide range of compositions and include eclogite Groups A, B, and C. Rare-earth element (REE) concentrations of garnet and pyroxene were determined by SIMS, and the REE patterns are subtly different from those in ordinary eclogites. Differences include the absence of prominent Eu anomalies in samples of this study and differences in the slopes of chondrite-normalized REE patterns. It is possible that these signatures are unique to exsolved eclogites, a result of subsolidus elemental partitioning during exsolution. Some reconstructed whole-rock compositions are aluminuous; comparison with ordinary eclogites shows only minor differences, implying a similar origin. If the immediate precursor to the exsolved eclogites was a monomineralic pyroxenite, the excess aluminium was tied up in Tschermak's molecule, although the occasional presence of kyanite exsolution lamellae is indicative of a Ca-Eskola component. Reconstructed pyroxenes from kyanite- and corundum-rich samples contain unrealistic amounts of aluminium for mantle pyroxenes. A protolith (or parental pyroxene) threshold of 24% Al2O3 may exist, above which (as in a plagioclase cumulate) the final assemblage is kyanite- and/or corundum-bearing.  相似文献   

15.
Petrographic and geochemical features of a suite of eclogite xenoliths from the Rietfontein kimberlite that erupted through probable Proterozoic crust west of the Kaapvaal Craton in the far Northern Cape region of South Africa, are described. Group II eclogites dominate the suite both texturally and chemically, but can be subdivided into bimineralic, opx-bearing and kyanite-bearing groups. Temperature estimates from different geothermometers range from 700 to 1,000°C, indicating derivation from relatively shallow mantle depths. Orthopyroxene-bearing eclogites are inferred to originate from depths of 85 to 115 km and lie close to the average cratonic thermal profile for southern Africa. These uppermost mantle temperatures during the late Cretaceous provide evidence for equilibration of the off-craton lithosphere to craton-like thermal conditions following Namaqua-Natal orogenesis. The kyanite eclogites are distinct from the remaining eclogites in terms of both major and trace element compositions and their lesser degree of alteration. Garnets are richer in Ca, and are Cr-depleted relative to garnets from the bimineralic and opx-bearing eclogites, which tend to be more magnesian. Clinopyroxenes from the kyanite eclogites are more sodic, with higher Al2O3 and lower MgO contents than the bimineralic and opx-bearing eclogites. LREE-depletion, positive Sr and Eu anomalies, and the Al-rich, Si-poor bulk composition suggest a plagioclase-rich, probably troctolitic protolith for the kyanite eclogites. In contrast, the major and trace element bulk compositions of the high-MgO bimineralic and orthopyroxene-bearing eclogites are consistent with gabbroic or pyroxenitic precursors, or high-pressure cumulates, rather than mafic to ultramafic lavas. δ18O values for garnets do not deviate significantly from typical mantle values. The observations reported do not discriminate unambiguously between continental and oceanic origins for the various eclogite components in the mantle lithosphere of this region.  相似文献   

16.
Garnet-clinopyroxene geothermometry has been applied to numerous rock types from the blueschist and eclogite terrain of the Sesia zone in the Western Alps. Discrepancies in apparent temperatures of up to 600° C have been found suggesting substantial deviation from ideal behaviour of the garnet-clinopyroxene equilibria in certain assemblages. Assemblages containing jadeitic pyroxenes rather than omphacitic or diopsidic pyroxenes yield very low K D (2.0) values and correspondingly high apparent temperatures (> 1,000° C). The garnets coexisting with high-Na pyroxenes have compositions similar to those coexisting with omphacitic pyroxenes. Departure from ideal behaviour of these garnets is likely to be minor and is accomodated in the formulation of the geothermometer. Numerous examples of coexisting garnet-clinopyroxene pairs indicate that there is no obvious relationship of K D with jadeite content in pyroxenes with jadeite content less than 70%, but at higher values, K D varies inversely with the jadeite content. The dependence of K D upon the jadeite content in the high-Na pyroxenes is believed to be due to the preference of M2 sites for Fe2+ in the enstatite-jadeite substitution (Fe2+MgNa–1Al–1). This substitution is usually very limited and has no demonstrable effect upon X Fe Cpx when the M1 (Fe2++Mg) content of the pyroxenes is high as in omphacitic and diopsidic pyroxenes. However, when the M1 (Fe2++Mg) content is low, as in jadeitic pyroxenes, the non-ideality of the enstatite type exchange has a marked effect on X Fe Cpx and consequently on garnet-clinopyroxene calculations.  相似文献   

17.
Representative sampling of a diamond-bearing basal horizon in the Carnian Stage (Upper Triassic) on the northeastern margin of the Siberian Platform revealed a wide spectrum of indicator minerals, first of all, garnets, whose compositions are the same as in the inclusions in the regional diamonds. Of special interest are garnets of potential eclogite paragenesis with an abnormally high impurity of MnO (0.5–3.2 wt.%), which was earlier detected in more than 20% of garnets present as inclusions in diamonds of northern Quaternary placers and recommended as a new mineralogical criterion for diamond presence. Subcalcic Cr-pyropes of dunite–harzburgite paragenesis were also found in variable amounts, from 0.7 to 3.9 rel.%, in the sample of 973 grains of pyropes of lherzolite and websterite parageneses. Three grains contain 11.9, 12.6, and 16 wt.% Cr2O3, which corresponds to the presence of 30–34% of Mg–Cr-knorringite component. Such pyropes have been revealed for the first time in the study region. Cr-spinels are a mixture of compositions typical of kimberlites and the regional alkali-ultrabasic rocks. All studied samples contain picroilmenites with a variable content of Cr2O3 impurity. Since Mg–Fe–Ca-garnets with Mg# < 35 can be partly hosted in metamorphic rocks of the Anabar Shield, the elevated content of Na2O impurity (> 0.09 wt.%) was also taken into account. The different contents of indicator minerals in the samples might be due to the variable composition of the diamond orebodies. The Carnian placers call for new systematic sampling. Special attention should be given to estimation of the composition of garnets of presumably eclogite paragenesis with elevated contents of TiO2, MnO, CaO, and Na2O and to search for perovskite and Nb-containing rutile. These minerals, together with zircons, are of interest for determining the U–Pb isotopic age of probable diamond orebodies—kimberlites.  相似文献   

18.
Diamonds and eclogites of the Jericho kimberlite (Northern Canada)   总被引：1，自引：1，他引：0  
We studied diamonds and barren and diamondiferous eclogite xenoliths from the Jericho kimberlite (Northern Slave craton). The majority of the diamonds are non-resorbed octahedral crystals, with moderately aggregated N (IaB < 50%, N < 300 ppm) and δ13C = −5 to −41‰. The diamonds belong to “eclogitic” (90% of the studied samples), “websteritic” (7%) and “peridotitic” (3%) assemblages. The Jericho diamonds differ from the majority of “eclogitic” diamonds worldwide in magnesian compositions of associated minerals and extremely light C isotopic compositions (δ13C = −24 to −41‰). We propose that metasomatism triggered by H2O fluids may have been involved in the diamond formation. Multiple episodes of the metasomatism and associated melt extraction of various ages are evident in Jericho eclogite xenoliths where primary garnet and clinopyroxene have been recrystallized to more magnesian minerals with higher contents of some incompatible trace elements and to hydrous secondary phases. The model is supported by the general similarity of mineral compositions in diamondiferous eclogites to those in diamond inclusions and to secondary magnesian garnet and clinopyroxene in recrystallized barren eclogites. The ultimate products of the metasomatism could be “websteritic” diamond assemblages sourced from magnesian eclogites. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

19.
Results of experimental study at 7.0–8.5 GPa and 1300–1900°C of the systems pyrope Mg3Al2Si3O12 (Prp)-Na2MgSi5O12 (NaGrt) modeling solid solutions of Na-bearing garnets, Prp-jadeite NaAlSi2O6 (Jd) in a simplified mode demonstrating melting relations of Na-rich eclogite, and Prp-Na2CO3 are presented. Prp-Na2MgSi5O12 join is a pseudobinary that results from the decomposition of NaGrt on to coesite and Na-pyroxene. Synthesized garnets are characterized by Na admixture (>0.32 wt % Na2O) and excess Si (3.05–3.15 f.u.). Maximal Na2O concentrations (1.5 wt % Na2O) are reached on the solidus of the system at 8.5 GPa. Clear correlation between Na and Si was established in synthesized garnets; this provides evidence for heterovalent isomorphism of the Mg + Al → Na + Si type with the appearance of Na2MgSi5O12 component as a mechanism of such garnet formation. The Prp-Jd join is also pseudobinary that results from the formation of two series of solid solutions: (1) garnet (Prp-NaGrt-majorite) and (2) pyroxene (Jd-clinoenstatite-Eskola molecule), and the appearance of kyanite at the solidus of the system, where garnets with the highest Na2O contents (>0.8 wt %) are formed. In spite of quite a wide field of garnet crystallization (20–100 mol % Prp), garnets with significant sodium concentration (>0.3 wt % Na2O) are formed in a Jd-rich part of the system (20–50 mol % Prp). In the Prp-Na2CO3 system at 8.5 GPa garnet crystallizes in a wide range of starting compositions as a liquidus mineral containing up to 0.9 wt % Na2O. Our experiments demonstrate that melt alkalinity, as well as PT-parameters control the crystallization of Na-bearing majoritic garnets. The results obtained provide evidence for the fact that the majority of natural diamonds with inclusions of Na-bearing majoritic garnets containing <0.4 wt % Na2O were formed in alkaline silicate (carbonate-silicate) melts at a pressure of <7 GPa. Only a small portion of garnets with higher sodium concentrations (>1 wt % Na2O) could be formed at a pressure of >8.5 GPa. 1 This article was translated by the authors.  相似文献   

20.

The first studies of diamonds in eclogitic xenoliths from the Komsomolskaya kimberlite pipe are described. Among round and oval-shaped xenoliths with diamond ingrowths, samples with a garnet content of 40–90% of the xenolith volume dominate. Two eclogite samples contain grains of accessory rutile; a kyanite sample is also revealed. Certain samples contain two or more crystals of diamonds. Diamonds with an octahedral habit and crystals with transitional habits, which belong to an octahedral-rhombic dodecahedral row, dominate in eclogites; there are many variety VIII aggregates. A high concentration of structural nitrogen, commonly in the A form, was registered in most of the crystals. Diamonds with a small content of nitrogen impurities, 40–67% in the B1 form, are present in a number of xenoliths. The calculated temperatures of the formation of eclogitic xenoliths is 1100–1300°C. Diversity in the impurity compositions of diamonds in the same xenolith shows that these diamonds were formed at various times and in different settings. The diamond position in xenoliths, the various level of nitrogen aggregation in the diamonds, and a number of other factors point to the later formation of the diamonds, as compared to minerals of eclogites, from fluid or fluid-melts in the process of metasomatosis.

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