FEATURES OF GARNET AND CLINOPYROXENE IN DIAMONDIFEROUS ECLOGITES FROM THE UDACHNAYA KIMBERLITE PIPE, YAKUTIA: METASOMATOSIS EVIDENCE
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Mineralogy of diamondiferous eclogite xenolites showing metasomatosis evidence from the Udachnaya kimberlite pipe is discussed. The paper also reviews features of diamonds they contain, compositions of primary garnets and omphacites as well as alteration of structural and species compositions of original garnets and clinopyroxenes during metasomatosis. Based on pyrope structure update, two-phase garnet composition is suggested, which is mostly represented by complex pyrope associated with Ca-pyrope. In all samples, primary omphacite is replaced by another clinopyroxene variety depleted in Na2O, which is typical of partial melting products. Geothermometry results suggested that the eclogites formed within a temperature range of 1,000–1,2000 °C. Based on diamond morphology, data on total N content in diamonds and its aggregation, multiple stages of diamond formation in eclogites and the most probable growth of later diamond generations impacted by metasomatizing mantle fluids containing carbon are postulated. It is suggested that certain diamond formation stages probably had a time gap of several hundred million years.Keywords:
Pyrope
Omphacite
Coesite
In the Su-Lu UHP terrane, eastern China, coesite-bearing eclogites have been reported in five areas (Weihai, Yangkou, Rizhao, Donghai and Rongcheng). The presence of a coesite inclusion (70 μm) in garnet in a strongly retrograded eclogite at Zekou (about 40 km southwest of Rongcheng) shows for the first time that this region also underwent coesite-eclogite facies metamorphism. In addition, the garnet of the Zekou eclogite preserves a distinct compositional zonation. The eclogite consists of garnet (20%), symplectite (70%: amphibole+plagioclase) and other accessory minerals (amphibole, rutile, quartz and ilmenite). Garnet porphyroclasts (0.5-1 mm in diameter) show compositional zonation that can be divided into three parts: core (Alm58-59 Sps0.7-0.9 Prp15-17 Adr0.1-0.2 Grs23-24), mantle (Alm55-58 Sps0.5-0.8 Prp14-15 Adr0.1-0.2 Grs25-28), and rim (Alm53-56 Sps0.4-0.6 Prp17-18 Adr0.1-0.2 Grs25-27). Each part has a characteristic inclusion mineralogy: the core part contains rutile, quartz and no coesite, the mantle part contains coesite, quartz-pseudomorphs after coesite, rutile, omphacite and zircon, and the rim part contains rutile. The coesite inclusion (70 μm) is located in the outer mantle part and is associated with radial fractures in the surrounding garnet. The different inclusion assemblages and the chemical zonation from core to rim in garnet porphyroclasts suggest that the Zekou eclogite preserves the information of the prograde stage during UHP metamorphism.
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Abstract A key question concerning the water budget of Earth’s mantle is how much water is actually recycled into the mantle by the subduction of eclogitized oceanic crust. Hydrous phases are stable only in quartz eclogite not coesite eclogite so that water transport to greater depths is mainly governed by structural water in omphacite and garnet. Here we explore if garnet can be used as a proxy to assess the amount of this water. Available data on the water contents of garnet in coesite eclogite vary over orders of magnitude, from a few up to ca. 2000 ppm. By implication, the maximum bulk-rock water contents are unrealistically high (wt% level). New data from the Erzgebirge indicate moderate amounts of structural H2O stored in garnet (43–84 ppm), omphacite (400–820 ppm), and in the bulk coesite eclogite (ca. 280–460 ppm). Higher garnet water contents occur, but these are not primary features. They are related to molecular water in fluid inclusions that can be attributed to eclogite-facies fluid influx postdating the metamorphic peak. Fluid influx also caused the uptake of additional structural water in garnet domains close to fluid inclusions. Such secondary H2O incorporation is only possible in the case of primary water-deficiency indicating that garnet hosted less water than it was able to store. This is insofar astonishing as comparably high H2O amounts are liberated by the breakdown of prograde eclogite-facies hydrous minerals as a result of ultrahigh-pressure (UHP) metamorphism. Judging from Erzgebirge quartz eclogite, dehydration of 5–10% hydrous minerals (±equal portions of zoisite+calcic amphibole) produces 1500–3000 ppm water. We infer that the largest part of the liberated water escaped, probably due to kinetic reasons, and hydrated exhuming UHP slices in the hanging-wall. Depending on the physical conditions, water influx in eclogite during exhumation (1) produces fluid inclusions and simultaneously enhances the structural water content of nominally anhydrous minerals—as in the Erzgebirge—and/or (2) it may give rise to retrograde hydrous minerals. We conclude that eclogite transports moderate quantities of water (several hundred parts per million) to mantle depths beyond 100 km, an amount equivalent to that in ca. 1% calcic amphibole.
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Coesite
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Coesite- and kyanite-bearing eclogites are abundant in the southern part of the Dabie Mountains (southern Dabie terrane-SDT). Two types of eclogites from the SDT (Types III and IV) were selected for detailed paragenetic study. Type III eclogites, most abundant in the northern part of the SDT, occur as blocks in gneisses and marble and contain eclogitic assemblages of omphacite + garnet + phengite + epidote + coesite + kyanite + carbonate + rutile + ilmenite. These minerals exhibit weak compositional zoning and contain few mineral inclusions. Type IV eclogites, mostly in the southern part of the SDT, occur as coherent layers interbedded with gneisses and amphibolites and have assemblages of omphacite + garnet + glaucophane + kyanite + epidote + phengite + quartz + rutile + ilmenite. Garnets of Type IV eclogites exhibit a prograde compositional zoning and have mineral inclusions of paragonite, phengite, epidote, quartz, and rutile in the core and omphacite, barroisite, and Mg-katophorite in the rim. Prograde blueschist facies (~400°C) assemblages were partially preserved in Type IV eclogites. The eclogitic assemblages of both types of eclogite have been partially or completely retrograded to amphibolite and greenschist facies assemblages. Parageneses and compositions of minerals from eclogites indicate that these rocks have undergone a clockwise P-T evolution path. Within the SDT, the temperatures, estimated according to $$K_{D(Cpx-Gt)}$$ and $$K_{D(Gt-Phen)}$$ for eclogites, indicate a systematic decrease from about 770°C in the north to 580°C in the south. Such variation is also evident for pressure estimates, as coesite occurs only in eclogite in the north, whereas the assemblage omphacite + kyanite + quartz (without coesite) is found in the south. This study, together with ultrahigh-pressure mineral assemblages identified in the gneiss-marble country rock, suggests that the continental crust of the SDT has been subjected to a regional ultrahigh-pressure metamorphism as part of a north-dipping subduction zone formed between the Sino-Korean and Yangtze cratons in Triassic time.
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Lawsonite
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Abstract Major and trace elements in omphacite, including hydrogen, were determined in eclogites from two Variscan basement complexes in Germany: Erzgebirge (EG) and Fichtelgebirge (FG). Erzgebirge eclogite is derived from three units, showing different peak pressure (P) and temperature (T) conditions (Unit 1: 840–920°C/≥30 kbar, Unit 2: 670–730°C/24–26 kbar, Unit 3: 600–650°C/20–22 kbar). The peak conditions of FG eclogite (690–750°C/25–28 kbar) resemble those of EG Unit 2. Coesite eclogite occurs in EG Unit 1, and quartz eclogite in all other units. Omphacite from all samples shows four infrared (IR) absorption bands. Two prominent, sharp bands occur at 3,455 ± 10 cm −1 (band II) and 3,522 ± 10 cm −1 (band III). Band II is usually more prominent than band III, except for few samples with low jadeite content. A further, broad band is centred between 3,270 and 3,370 cm −1 (band I) and a fourth, minor band at 3,611–3,635 cm −1 (band IV). Bands II and III are due to hydrogen bound as structural OH − ions in omphacite. In most cases, this also applies to band IV. However, some spectra with extremely large type IV bands reflect phengite inclusions. The ambiguous band I may be due to different H 2 O species (molecular water, structural OH, and water in phengite). Omphacite of quartz eclogite has lower contents of TiO 2 , Zr, Hf, and REE, compared with that from coesite eclogite. By contrast, omphacite in quartz eclogite from both EG (H 2 O sample averages: 465–852 ppm) and FG (546–1,089 ppm) contains the same amount of structural OH (concentrations given in wt.‐ppm H 2 O) as omphacite in coesite eclogite (492–1,140 ppm). The obtained difference in the garnet‐omphacite H 2 O partition coefficient between quartz (0.01–0.03) and coesite eclogite (0.08–0.11) results from different H 2 O contents in garnet (coesite eclogite: 50–150 ppm; quartz eclogite: <2–50 ppm; Gose & Schmädicke, 2018). The total content of structural OH in omphacite is unrelated to its major and trace element composition. However, treating the individual IR bands separately, a relation between OH and mineral composition is observed. The OH amount defined by band II is positively correlated to Ti and tetrahedral Al, and that of band III shows a positive correlation with Ca and a negative one with Na (and jadeite). Both the total OH content of omphacite and the partial contents deduced from individual IR bands are unrelated to PT conditions. This implies that omphacite incorporated its structural H 2 O mainly in the quartz stability field, presumably during initial omphacite growth. Conversely, most OH in garnet was derived from the final breakdown of the last remaining calcic amphibole close to or within the coesite stability field. Our data suggest that coesite eclogite is able to transport a significant amount of H 2 O (average 550 ppm, maximum 730 ppm), corresponding to that in 3–4 vol.% calcic amphibole, via subduction to depths beyond 100 km. However, the majority of water liberated by dehydration reactions during subduction, including the breakdown of 5–10 vol.% eclogite facies and >10 vol.% pre‐eclogitic hydrous minerals, is not preserved in eclogite but liberated to the mantle wedge.
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Coesite
Rutile
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Mineralogy of diamondiferous eclogite xenolites showing metasomatosis evidence from the Udachnaya kimberlite pipe is discussed. The paper also reviews features of diamonds they contain, compositions of primary garnets and omphacites as well as alteration of structural and species compositions of original garnets and clinopyroxenes during metasomatosis. Based on pyrope structure update, two-phase garnet composition is suggested, which is mostly represented by complex pyrope associated with Ca-pyrope. In all samples, primary omphacite is replaced by another clinopyroxene variety depleted in Na2O, which is typical of partial melting products. Geothermometry results suggested that the eclogites formed within a temperature range of 1,000–1,2000 °C. Based on diamond morphology, data on total N content in diamonds and its aggregation, multiple stages of diamond formation in eclogites and the most probable growth of later diamond generations impacted by metasomatizing mantle fluids containing carbon are postulated. It is suggested that certain diamond formation stages probably had a time gap of several hundred million years.
Pyrope
Omphacite
Coesite
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Coesite
Xenolith
Pyrope
Peridotite
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Coesite provides direct evidence for ultrahigh pressure metamorphism. Although coesite has been found as inclusions in zircon in paragneiss of the north Qaidam Mountains, it has never been identified in eclogite. In this contribution, based on petrographic observations and in situ Raman microprobe spectroscopy, coesite was identified as inclusions in garnet of eclogite from the Aercituoshan, Dulan UHP metamorphic unit, north Qaidam Mountains. Coesite is partly replaced by quartz, showing a pali-sade texture. This is the first report on coesite in eclogite from the north Qaidam Mountains, and is also supported by garnet-omphacite-phengite geothermobarometry (2.7―3.25 GPa, 670―730℃). Coesite and its pseudomorphs have not been found in eclogites and associated rocks of other units of the north Qaidam Mountains. Further studies are required to confirm if all metamorphic units in the north Qaidam Mountains underwent the ultrahigh-pressure metamorphism.
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Omphacite
Phengite
Geothermobarometry
Pseudomorph
Dalradian
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A find of coesite in a kyanite graphite-diamond-bearing eclogite xenolith from the Udachnaya-Vostochnaya kimberlite pipe is described in this paper. The coesite relics were found in intensely fractured garnet indicating some influence of the kimberlite melt, which is supported by the typical secondary mineral assemblage around this inclusion. These data indicate that shallower diamond-free coesite rocks (2,7 GPa) underwent metamorphism distinct from diamond-bearing coesite eclogites (~4 GPa). The metasomatic alteration of rock interacting with C-O-H fluid during diamond crystallization may be another possible reason for the missing coesite in diamond-bearing xenoliths.
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