Host Eclogite Research Articles

Fluid/rock interaction and mass transfer in continental subduction zones: constraints from trace elements and isotopes (Li, B, O, Sr, Nd, Pb) in UHP rocks from the Chinese Continental Scientific Drilling Program, Sulu, East China

In order to better understand the role of fluids during subduction and subsequent exhumation, we have investigated whole-rock and mineral chemistry (major and trace elements) and Li, B as well as O, Sr, Nd, Pb isotopes on selected continuous drill-core profiles through contrasting lithological boundaries from the Chinese Continental Scientific Drilling Program (CCSD) in Sulu, China. Four carefully selected sample sets have been chosen to investigate geochemical changes as a result of fluid mobilization during dehydration, peak metamorphism, and exhumation of deeply subducted continental crust. Our data reveal that while O and Sr-Nd-Pb isotopic compositions remain more or less unchanged, significant Li and/or B isotope fractionations occur between different lithologies that are in close contact during various metamorphic stages. Samples that are supposed to represent prograde dehydration as indicated by veins formed at high pressures (HP) are characterized by element patterns of highly fluid-mobile elements in the veins that are complementary to those of the host eclogite. A second sample set represents a UHP metamorphic crustal eclogite that is separated from a garnet peridotite by a thin transitional interface. Garnet peridotite and eclogite are characterized by a >10% difference in MgO, which, together with the presence of abundant hydroxyl-bearing minerals and compositionally different clinopyroxene grains demonstrate that both rocks have been derived from different sources that have been tectonically juxtaposed during subduction, and that hydrous silicate-rich fluids have been added from the subducting slab to the mantle. Two additional sample sets, comprising retrograde amphibolite and relatively fresh eclogite, demonstrate that besides external fluids, internal fluids can be responsible for the formation of amphibolite. Li and B concentrations and isotopic compositions point to losses and isotopic fractionation during progressive dehydration. On the other hand, fluids with isotopically heavier Li and B are added during retrogression. On a small scale, mantle-derived rocks may be significantly metasomatized by fluids derived from the subducted slab. Our study indicates that during high-grade metamorphism, Li and B may show different patterns of enrichment and of isotopic fractionation.

Garnet–spinel–corundum–quartz-bearing titanohematite veins in eclogite from the Sulu ultrahigh-pressure terrane: Imprint of a short-lived, high-temperature metamorphic stage

Eclogite-hosted garnet–spinel–corundum–quartz-bearing titanohematite veins and lenses (10–20 cm in width) are described for the first time in the Sulu ultrahigh-pressure metamorphic terrane. Some of the lenses were aligned parallel to the foliation of the host eclogite, suggesting that they were the product of ductile deformation of one titanohematite layer. A vein composed of titanohematite + ilmenite + hematite + spinel + garnet + corundum + quartz + K-feldspar + albite was studied in detail. Ti–Fe oxides account for up to >80% and Al-rich phases for ∼15% of the total volume of this vein. Electron microprobe analyses show that the titanohematite solid solution was made up of 0.75 hematite + 0.25 ilmenite. The unusual mineral assemblage of garnet + spinel + corundum + quartz implies that this vein could have experienced high temperatures (>900 °C). Although the garnets showed well-defined Mg and Mn diffusion zoning in the rim as a result of the high temperature event, slight Mg and Mn growth zoning was preserved in the core. Thus, we suggest that the Sulu UHP terrane could have experienced a short-lived, high-temperature (>900 °C) stage during exhumation. Garnets in the titanohematite vein were characterized by extremely low trace-element contents. Petrological and geochemical features of the veins suggest that they could be metamorphic products of igneous cumulates composed of magnetite + plagioclase ± clinopyroxene.

Metasomatic origin of diamonds in the world's largest diamondiferous eclogite

The world's largest diamondiferous eclogite (8.8 kg, UD-111/02) from the Udachnaya kimberlite, Siberia, is the subject of this investigation. High-resolution X-ray computed tomography (HRXCT), chemistry of host minerals and diamond inclusions, stratigraphy of the diamonds (cathodoluminescence zoning), and carbon isotopes have been obtained. HRXCT images, 3-d models, and subsequent detailed dissection of this Group B eclogite revealed linear distributions of diamonds along alteration zones. Both unusual multiple-diamond clusters with dodecahedral morphologies, and distinctive internal zoning recorded by cathodoluminescence analyses are indicative of multiple stages of diamond growth and resorption in UD-111-02. Carbon-isotope ratios ( δ 13C) of 16 diamonds have a total range of ~ 7‰ (− 2.0 to − 9.2‰), with the majority at − 5.4‰. Diamond core values are variable and include the lightest (− 9.2‰) and heaviest δ 13C (− 2.0‰) values, suggesting that cores formed from distinct carbon sources, or grew under different conditions. In contrast, the rims of all diamonds have similar δ 13C values (average − 5.2‰ with a 2SD of 1.2‰), suggesting that they formed under uniform conditions. Mineral inclusions in diamonds display compositional variations. These lines of evidence, considered together, indicate that diamonds in this eclogite were formed by several events, most likely from multiple metasomatic fluids. Furthermore, it is becoming increasingly evident that in most diamondiferous eclogites, diamonds are not synchronously formed with their host eclogites.

Zircon U-Pb age and Hf isotope composition of a quartz vein in ultrahigh-pressure eclogite from the western Dabie orogen

In this paper, we report U-Pb age and Hf isotope composition of hydrothermal zircons from a quartz vein within an UHP eclogite outcrop from the Hong’an area, western Dabie orogen. These data are used to decipher the age, formation conditions and source of fluid flow during the exhumation of UHP rock. Zircons from the vein have perfect euhedral shape, oscillatory zoning or weak zoning, and very low Th/U ratios (0.03―0.07), indicating that they precipitated from the aqueous fluid responsible for the vein formation. The weighted mean 206Pb/238U age of 224.7 ± 1.3 Ma of these zircons is taken as the best estimated age of the quartz vein formation, and records aqueous fluid flow during the early exhumation stage of the UHP rocks. The zircons in the quartz vein show low Lu/Hf ratios and have similar Hf compositions to those in the host eclogite, indicating an internal source and small-scale fluid transport for veining. Therefore, the Hf isotope composition of hydrothermal zircon from quartz vein can constrain on the nature, forming condition, and origin of high-pressure metamorphic fluids.

U-Pb age, trace-element, and Hf-isotope compositions of zircon in a quartz vein from eclogite in the western Dabie Mountains: Constraints on fluid flow during early exhumation of ultrahigh-pressure rocks

Quartz veins in high-pressure (HP) to ultrahigh-pressure (UHP) rocks are the products of fluid-rock interaction, and thus provide insight into fluid processes in subduction zones. In this paper, we report an integrated study of mineral inclusion, trace-element, U-Pb age, and Lu-Hf isotope compositions of hydrothermal zircon grains from a quartz vein within an UHP eclogite outcrop from the Hong’an area, western Dabie Mountains. These data are used to decipher the age, conditions of formation, and source of fluid for zircon formation during the exhumation of UHP rocks. Zircon grains from the vein have perfect euhedral shape, and show sector zoning or weak zoning, indicating that they precipitated from the aqueous fluid responsible for the vein formation. Raman spectroscopy analysis reveals that the zircon grains contain inclusions of garnet, omphacite, rutile, quartz, and H 2 O, implying that they crystallized from aqueous fluid under HP eclogite-facies conditions. The zircon grains show low Th/U and Lu/Hf ratios, nearly flat HREE patterns, absent Eu anomalies and low LREE contents. These characteristics are consistent with their precipitation in the presence of garnet and epidote, and absence of feldspar, and thus suggest that trace-element concentrations in hydrothermal zircon are controlled by co-precipitation of mineral assemblages. Crystallization temperatures of 670 to 712 °C, which were calculated using the Ti content of zircon, are consistent with their formation under eclogitefacies conditions and may correspond to the temperature of the infiltrating fluid. The weighted mean 206 Pb/ 238 U age of 224.7 ± 1.3 Ma is taken as the best estimate for the age of quartz-vein formation and records aqueous fluid flow during the early exhumation stage of UHP rocks. The zircon grains in the quartz-vein have Hf compositions similar to those in the host eclogite, which demonstrates isotopic equilibrium between fluid and rocks and that the fluid-rock ratio was likely low.

Fluids in deeply subducted continental crust: Petrology, mineral chemistry and fluid inclusion of UHP metamorphic veins from the Sulu orogen, eastern China

The complex vein associations hosted in southern Sulu ultrahigh-pressure (UHP) eclogites contain quartz ± omphacite (or jadeite) ± kyanite ± allanite ± zoisite ± rutile ± garnet. These minerals have chemical compositions similar to those of host eclogites. Inclusions of polycrystalline quartz pseudomorphs after coesite were identified in vein allanite and garnet, and coesite inclusions were found in vein zircon. These facts suggest that the veins together with host eclogites have been subjected to synchronous UHP metamorphism. The vein minerals contain relatively high concentrations of rare earth elements (REE), high-field-strength elements (HFSE) and transition metal elements (TME). A kyanite–quartz vein has a whole-rock composition similar to adjacent UHP metamorphic granitic gneisses. Abundant primary multi-solid fluid inclusions trapped within UHP vein minerals contain complex daughter minerals of muscovite, calcite, anhydrite, magnetite, pyrite, apatite, celestite and liquid and gas phase of H 2O with solids up to 30–70% of the inclusion volume. The presence of daughter minerals anhydrite and magnetite indicates the subduction fluids were oxidizing, and provides a possible interpretation for the high oxygen fugacity of subduction zone magmas. These characteristics imply that the UHP vein minerals were crystallized from supercritical silicate-rich aqueous fluids that were in equilibrium with peak-UHP minerals, and that the fluids in deeply subducted continental crust may contain very high concentrations of silicate as well as HREE, HFSE and TME. Such fluids might have resulted in major fractionation between Nb and Ta, i.e. the UHP fluids have subchondritic Nb/Ta values, whereas the host eclogites after extraction of the fluids have suprachondritic Nb/Ta values. Therefore, voluminous residual eclogites with high Nb/Ta ratios may be the complementary suprachondritic reservoir capable of balancing the subchondritic depleted mantle and continental crust reservoirs.

Fluid flow during exhumation of deeply subducted continental crust: zircon U‐Pb age and O‐isotope studies of a quartz vein within ultrahigh‐pressure eclogite

AbstractQuartz veins in high‐pressure to ultrahigh‐pressure metamorphic rocks witness channelized fluid flow that transports both mass and heat during collisional orogenesis. This flow can occur in the direction of changing temperature/pressure during subduction or exhumation. SHRIMP U‐Pb dating of zircon from a kyanite‐quartz vein within ultrahigh‐pressure eclogite in the Dabie continental collision orogen yields two age groups at 212 ± 7 and 181 ± 13 Ma, which are similar to two groups of LA‐ICPMS age at 210 ± 4 and 180 ± 5 Ma for the same sample. These ages are significantly younger than zircon U‐Pb ages of 224 ± 2 Ma from the host eclogite. Thus the two age groups from the vein date two episodes of fluid flow involving zircon growth: the first due to decompression dehydration during exhumation, and the second due to heating dehydration in response to a cryptic thermal event after continental collision. Laser fluorination O‐isotope analyses gave similar δ18O values for minerals from both vein and eclogite, indicating that the vein‐forming fluid was internally derived. Synchronous cooling between the vein and eclogite is suggested by almost the same quartz–mineral fractionation values, with regularly decreasing temperatures that are in concordance with rates of O diffusion in the minerals. While the quartz veining was caused by decompression dehydration at 700–650 °C in a transition from ultrahigh‐pressure to high‐pressure eclogite‐facies retrogression, the postcollisional fluid flow was retriggered by heating dehydration at ∼500 °C without corresponding metamorphism. In either case, the kyanite–quartz vein formed later than the peak ultrahigh‐pressure metamorphic event at the Middle Triassic, pointing to focused fluid flow during exhumation rather than subduction. The growth of metamorphic zircon in the eclogite appears to have depended on fluid availability, so that their occurrence is a type of geohygrometer besides geochronological applicability to dating of metamorphic events in orogenic cycles.

Trace element-enriched fluids released during slab dehydration: Implications for oceanic slab-mantle wedge transfer

Mafic blueschists of the Tianshan (NW China) display an interconnected network of eclogite-facies veins derived by prograde blueschist dehydration. The eclogite-facies vein-network in the Tianshan blueschist was found to be the product of hydrofracturing induced by fluids released by the breakdown of glaucophane, paragonite and epidote minerals during blueschist–eclogite transition and, thus represent former fluid pathways within a Paleozoic subduction zone. The veins are predominantly composed of omphacite fibers with minor quartz, calcite, and apatite. The transition from blueschist- to eclogite-facies parageneses occurs as “dehydration” halos along some of these veins. The fluids are interpreted to have been derived from the host blueschist as a result of dehydration reactions at peak metamorphic conditions of 480–600 °C and 18–21 kbar (Gao and Klemd, 2001 and John et al., 2006). The low in trace element fluid caused a strong mobilization of LILE, REE, and high field strength elements (HFSE) in those parts of the host rock with which the passing fluid reacted (John et al., 2006). Hence indicating that so-called immobile elements can be mobilized by reactive flow. Furthermore, field evidence shows that rutile occurs in the form of needle-like segregations in an eclogite boudin and as prismatic crystals in an omphacite-bearing vein cross-cutting foliated host eclogites. Textural and geochemical studies indicate that titanium, niobium and tantalum can be mobilized and transported by fluids which were liberated by means of dehydration of the subduction oceanic crust. Vein-rutile precipitation occurred at sites where evidence for an increasing oxygen fugacity in the acting fluid is found, causing a sudden depletion of the HFSE concentration in the fluid. Such a fluid may act as agent for the subduction component seen in the distinct chemistry of normal island-arc magmas.

Postcollisional flow of aqueous fluid within ultrahigh-pressure eclogite in the Dabie orogen

A combined study of chronometric dating and oxygen isotope analysis for minerals from vein and host eclogite as well as regional country-rock gneiss in the Dabie orogen provides a direct constraint on timing of fluid flow in this orogen formed by continental collision. Oxygen isotope ratios of vein minerals are significantly lower than those of the host eclogite, but comparable with those of the regional gneiss. This suggests the veining fluid came from the regional gneiss (i.e. exhumed slab itself) rather than the host eclogite. While zircon U–Pb and phengite Ar–Ar dating yields ages of 214 to 222 Ma for the eclogite and gneiss, the vein gives a quartz–muscovite Rb–Sr isochron age of 181 Ma and a muscovite K–Ar age of 179 Ma. Thus the veining postdates the Triassic ultrahigh pressure metamorphic event, witnessing postcollisional fluid flow after the orogenic cycle of continental collision.

Mineral evolution of a garnet‐pyroxenite nodule within eclogite, eastern Sulu ultrahigh‐pressure metamorphic terrane, East China

AbstractDetailed microtextural observations and bulk chemical analysis were undertaken on a garnet‐pyroxenite nodule within retrograde eclogites from the NE Sulu ultrahigh‐pressure metamorphic (UHPM) terrane. The results suggest that the protolith was a cumulate from a gabbroic body. The nodule consists primarily of coarse clinopyroxene grains with a very high content of the Ca‐Tschermakite molecule. Microscopic observations and back‐scattered electron images (BSE) demonstrate a complicated intergrowth of clinopyroxene, garnet and ilmenite, which represents the peak metamorphic assemblage. The primary clinopyroxene grains are armoured with a thin garnet corona up to 0.5 mm wide that forms an interconnected network. Within the clinopyroxene grains, four sets of garnet lamellae are distributed along crystallographic planes; locally, a vermicular intergrowth of garnet and diopside is developed. Besides the garnet, parallel arrays of ilmenite blebs are common within the clinopyroxene. Hydrous minerals such as amphibole, zoisite and titanite formed at later stages, and replaced diopside, garnet and ilmenite respectively. The P–T conditions determined for the formation of the garnet lamellae indicate that the garnet pyroxenite experienced UHP metamorphism at the same peak P–T condition as its host eclogite. The very high Ca‐Tschermakite content (31–34 mol.%) of the primary clinopyroxene indicates crystallization at about 9–17 kbar and 1250–1450 °C, and together with the microtextural observations, suggests that the protolith of the garnet pyroxenite was a cumulate from a former gabbroic body, in which case, the host eclogite might represent the gabbroic body.

Water content in natural eclogite and implication for water transport into the deep upper mantle

Infrared spectroscopy and ion micro-probe measurements showed that the major constituent minerals of eclogites from the Kokchetav massif, which have been subducted to ∼180 km depths, contain significant amounts of water up to 870 ppm H 2O (by weight) in omphacite, 130 ppm H 2O in garnet and 740 ppm H 2O in rutile. Omphacite shows three hydroxyl absorption bands at 3440–3460, 3500–3530 and 3600–3625 cm − 1 , garnet has a single band at 3580–3630 cm − 1 and rutile has a single sharp band at 3280 cm − 1 . The hydroxyl absorbance at these wavenumbers changes with the crystal orientation in polarized infrared radiation, indicating that the water is structurally incorporated in these minerals. The water contents in omphacite and garnet increase systematically with the metamorphic pressure of the host eclogites. The partitioning coefficient of the water content between coexisting garnet and omphacite is similar in different eclogites, D Grt/Omp∼0.1–0.2, but decreases slightly at high pressure. Based on the mineral proportions of the eclogites, we estimate bulk-rock water content in the eclogites ranging from 3070 to 300 ppm H 2O (by weight). Although hydrous minerals are absent in the diamond-grade eclogite (∼60 kbar and ∼1000 °C), trace amounts of water are incorporated in the nominally anhydrous minerals such as omphacite and garnet. The presence of significant water in these minerals implies that the subducting oceanic crust can transport considerable amounts of water into the deep upper mantle beyond the stability of hydrous minerals. Such water may be stored in the deep upper mantle and have an important influence on dynamics in the Earth's interior.

Microdiamonds, their classification and tectonic implications for the host eclogites from the Dabie and Su-Lu regions in central eastern China

AbstractWe have found >10 in situ microdiamonds in thin sections of eclogites from the Dabie and Su-Lu regions of central eastern China since the first occurrence of microdiamond in eclogites from the Dabie Mountains (DMT) reported in 1992. The microdiamonds are found not only in the central part but also in the northern part of the DMT. Several free crystals have been recovered from the crushed eclogites from the central DMT. Most in situ microdiamonds are inclusions in garnets but a few larger ones are intergranular. Most of the diamondiferous eclogites in the central part of the DMT are associated with coesite. Most importantly, the observation of microdiamonds in northern Dabie lead us to question the supposition that this is a low-P metamorphic terrane. All the diamondiferous eclogites from both the north and central DMT are of continental affinity as demonstrated by their negative εNd values. Therefore, both the north and central eclogite belts in the DMT are considered to be from the deep subducted terrane. Five in situ microdiamonds and two free crystals are first reported in this paper. The dimensions of the in situ microdiamonds are 30–180 µm and the free crystals are up to 400–700 mm across. All the microdiamonds are confirmed as such by Raman spectroscopy. The results of an infrared spectroscopic investigation on two larger free crystals and two in situ microdiamonds show that all the microdiamonds from both the Dabie and Su-Lu regions are mixed types IaA and IaB diamonds and there is no indication of any synthetic microdiamonds in our samples because such synthetic microdiamonds are always rich in type Ib.

Breakdown of lawsonite subsequent to peak UHP metamorphism in the Dabie terrane and its implication for fluid activity

Abundant occurrences of quartz vein within eclogites in the Dabie-Sulu orogen provide us an opportunity to study metamorphic fluid flow during subduction and exhumation of continental crust. It is, however, usually short of petrological constraints on pressure and temperature of vein formation. This study focuses on kyanite-quartz veins within low-T/high-P eclogite in the Dabie terrane that contain unique polycrystalline aggregates, interpreted as pseudomorphs after porphyroblasts of lawsonite. Coesite pseudomorphs were found for the first time in garnet from eclogite, resulting in a revised estimate of peak P-T conditions at 670 °C and 3.3 GPa for the eclogite. This indicates a stability field at graphite/diamond transition, thus upgrading the HP unit to a UHP unit. Neither foliation texture, undulose extinction of quartz nor coesite were observed in quartz veins, although the peak P-T conditions were estimated the same as that in host eclogite in light of thermodynamic calculation based on mineral assemblage in kyanite-quartz veins. Therefore, the formation of the kyanite-quartz veins as well as the breakdown of lawsonite into the kyanite-quartz-zoisite assemblage took place during exhumation subsequent to the peak UHP conditions. In this regard, the continental subduction not only brought the water of water-bearing mineral such as lawsonite into the deep mantle, but also released the water from the mineral during exhumation.

Nature of diamonds in Yakutian eclogites: views from eclogite tomography and mineral inclusions in diamonds

We have performed dissections of two diamondiferous eclogites (UX-1 and U33/1) from the Udachnaya kimberlite, Yakutia in order to understand the nature of diamond formation and the relationship between the diamonds, their mineral inclusions, and host eclogite minerals. Diamonds were carefully recovered from each xenolith, based upon high-resolution X-ray tomography images and three-dimensional models. The nature and physical properties of minerals, in direct contact with diamonds, were investigated at the time of diamond extraction. Polished sections of the eclogites were made, containing the mould areas of the diamonds, to further investigate the chemical compositions of the host minerals and the phases that were in contact with diamonds. Major- and minor-element compositions of silicate and sulfide mineral inclusions in diamonds show variations among each other, and from those in the host eclogites. Oxygen isotope compositions of one garnet and five clinopyroxene inclusions in diamonds from another Udachnaya eclogite (U51) span the entire range recorded for eclogite xenoliths from Udachnaya. In addition, the reported compositions of almost all clinopyroxene inclusions in U51 diamonds exhibit positive Eu anomaly. This feature, together with the oxygen isotopic characteristics, is consistent with the well-established hypothesis of subduction origin for Udachnaya eclogite xenoliths. It is intuitive to expect that all eclogite xenoliths in a particular kimberlite should have common heritage, at least with respect to their included diamonds. However, the variation in the composition of multiple inclusions within diamonds, and among diamonds, from the same eclogite indicates the involvement of complex processes in diamond genesis, at least in the eclogite xenoliths from Yakutia that we have studied.

A metasomatic origin for late Archean eclogitic diamonds: Implications from internal morphology of diamonds and Re-Os and S isotope characteristics of their sulfide inclusions from the late Jurassic Klipspringer kimberlites

The majority of the Klipspringer eclogitic diamonds display complex crystal zonation with zones of N- and H-rich cubic diamond alternating with zones of N-poor octahedral diamond. These growth features were likely related to different levels of carbon supersaturation in the diamond forming fluid(s). The diamonds are younger than their host rock and formed through differential flux of fluids into the eclogite. Re-Os isotope systematics of sulfide inclusions yield an isochron age of 2554 ± 150 Ma. The age overlaps the break-up of the Vaalbara supercontinent, the formation of the Ventersdorp lavas and a major re-activation of the Thabazimbi-Murchinson lineament into which the Klipspringer kimberlites intruded 148 ± 4 Ma ago. While a separate, relatively older generation of Klipspringer diamonds (Westerlund and Gurney, 2004) may be related to a proposed ~2.9 Ga craton-wide eclogitic diamond formation event (Shirey et al. , 2001; 2002), a temporal connection between the current diamonds and the Ventersdorp lavas is inferred. Sulfur isotope compositions (δ34S = −1.8 to +2.4 ‰) of the sulfide inclusions overlap mantle values and large S isotope excursions indicative of altered MORB and associated sediment protoliths are absent. The data are compatible with slightly altered oceanic basalt or sub-lithospheric mantle sources for the S in these inclusions. The radiogenic initial Os isotope composition of the sulfide inclusions (187Os/188Os of 0.187 ± 0.046; γOs = +69) is not compatible with an origin of the PGE in the sulfides from the Ventersdorp lavas. The diamond growth characteristics along with the S and Os isotopic composition of their sulfide inclusions could be accounted for by the remobilization of local fluids in or proximal to the pre-existing eclogite host rock. We propose that the event that generated the Ventersdorp lavas may have raised the ambient lithospheric temperature and triggered fluid remobilization and the formation of the diamonds.

Petrology of two diamondiferous eclogite xenoliths from the Lahtojoki kimberlite pipe, eastern Finland

Diamondiferous Group A eclogites constitute a minor portion of the mantle-derived xenoliths in the eastern Finland kimberlites. They have been derived from the depth interval 150–230 km where they are inferred to occur as thin layers or small pods within coarse-grained garnet peridotites. The chemical and isotopic composition of minerals suggest that they represent (Proterozoic?) mantle-derived melts or cumulates rather than subducted oceanic lithosphere. During magma ascent and emplacement of the kimberlites, the eclogite xenoliths were mechanically and chemically rounded judging from the types of surface markings. In addition, those octahedral crystal faces of diamonds that were partially exposed from the rounded eclogite xenolith became covered by trigons and overlain by microlamination due to their reaction with the kimberlite magma. The diamonds bear evidence of pervasive plastic deformation which is not, however, evident in the eclogite host. This suggests that annealing at ambient lithospheric temperatures has effectively recrystallised the silicates while the diamond has retained its lattice imperfections and thus still has the potential to yield information about ancient mantle deformation. One of our samples is estimated to contain approximately 90,000 ct/ton diamond implying that some diamonds occur within very high-grade pods or thin seams in the lithospheric mantle. To our knowledge, this is one of the most diamondiferous samples described.

An oxygen isotope study of quartz veins within eclogites from the Dabie terrane

The oxygen isotope composition of minerals from quartz veins and host eclogites in the Dabie terrane was measured in order to place geochemical constraints on the origin and transport of metamorphic fluid. The results are discussed together with structural and petrological relationships between quartz vein and wallrock. The quartz veins can be temporally classified into three groups: (1) synmetamorphic vein which would be formed prior to eclogite-facies recrystallization when they were exhumated from mantle depths to deep crustal levels; (2) early retrogressive vein which was formed in the early stage of eclogite exhumation subsequent to the recrystallization, the vein-forming fluid is still relevant to the eclogites; (3) late retrogressive vein which was formed in the late stage of eclogite exhumation from deep crustal to upper crustal levels, oxygen isotope fractionation between vein quartz and host eclogite significantly deviates from equilibrium values and the vein-forming fluid was principally derived from granitic gneiss hosting the eclogites. For the synmetamorphic vein, it appears that local advective transport of fluid is the predominant mechanism in the processes of vein precipitation; the scale of oxygen isotope homogenization within the veins is much larger than that within the associated eclogites. The vein-forming fluid would be derived from the exsolution of dissolved hydroxyls within eclogite minerals due to significant pressure decrease. Fluid flow prior to the eclogite-facies recrystallization and the early retrogression may occur mainly along pressure gradients.

Primary fluids entrapped at blueschist to eclogite transition: evidence from the Tianshan meta-subduction complex in northwestern China

The Tianshan is the first locality worldwide where primary fluids at blueschist to eclogite transition have been documented. Veins containing high-pressure minerals in massive host eclogites transitional to blueschists, or eclogite boudins surrounded by blueschists, indicate that a free fluid phase was present at the time of eclogitization. The high-pressure veins are predominantly composed of omphacite fibers with minor quartz or calcite. The transition from blueschist- to eclogite-facies parageneses occurs as "dehydration" halos around these veins. Clinozoisite, paragonite, glaucophane, and omphacite inclusions preserved in garnet porphyroblasts in wall eclogites document the transformation of blueschist to eclogite. C-axis-parallel, non-planar populations of fluid inclusions were trapped during the growth of omphacite in high-pressure veins and dehydrated wall rocks. Low salinity H2O + NaCl ± solid-bearing inclusions are preserved in omphacite fibers in veins and matrix omphacite of wall rocks. None of the isochores of these low salinity aqueous fluid inclusions intersect peak eclogite-facies metamorphic conditions, suggesting that, although the textural evidence constrains the entrapment of fluid inclusions to peak metamorphic conditions, their densities must have been modified during exhumation. The fluids are interpreted to have been derived from the host blueschist as a result of dehydration reactions such as 13 Gln+5 Czo=9 Prp+26 Jd+12 Di+19 Qtz+15 H2O and Gln+Pg=Prp+3 Jd+2 Otz+2 H2O. The similarity of vein and wall rock mineral compositions, fluid inclusion characteristics and O-isotope data also favor an internal source for the fluids. The major element composition of veins indicate that Si, Na, and Ca-rich aqueous fluids were released during dehydration at a depth of 50±10 km within a Paleozoic subduction zone.

Diamonds and Their Mineral Inclusions, and What They Tell Us: A Detailed “Pull-Apart” of a Diamondiferous Eclogite

For the first time, three-dimensional, high-resolution X-ray computed tomography (HRXCT) of an eclogite xenolith from Yakutia has successfully imaged diamonds and their textural relationships with coexisting minerals. Thirty (30) macrodiamonds (≥1 mm), with a total weight of just over 3 carats, for an ore grade of some 27,000 ct/ton, were found in a small (4 × 5 × 6 cm) eclogite, U51/3, from Udachnaya. Based upon 3-D imaging, the diamonds appear to be associated with zones of secondary alteration of clinopyroxene (Cpx) in the xenolith. The presence of diamonds with secondary minerals strongly suggests that the diamonds formed after the eclogite, in conjunction with meta-somatic input(s) of carbon-rich fluids. Metasomatic processes are also indicated by the non-systematic variations in Cpx inclusion chemistry in the several diamonds. The inclusions in the diamonds vary considerably in major- and trace-element chemistry within and between diamonds, and do not correspond to the minerals of the host eclogite, whose compositions are extremely homogeneous. Some Cpx inclusions possess +Eu anomalies, probably inherited from their crustal source rocks. The only consistent feature for the Cpx crystals in the inclusions is that they have higher K2O than the Cpx grains in the host. The δ13C compositions are relatively constant at −5% both within and between diamonds, whereas δ15N values vary from −2.8% to −15.8%. Within a diamond, the total N varies considerably from 15 to 285 ppm in one diamond to 103 to 1250 ppm in another. Cathodoluminescent imaging reveals extremely contorted zonations and complex growth histories in the diamonds, indicating large variations in growth environments for each diamond. This study directly bears on the concept of diamond inclusions as time capsules for investigating the mantle of the Earth. If diamonds and their inclusions can vary so much within this one small xenolith, the significance of their compositions is a serious question that must be addressed in all diamond-inclusion endeavors.

First report of felsic whiteschist in the ultrahigh-pressure metamorphic belt of Dabie Shan, China

We report the first occurrence of a talc-kyanite assemblage typical of whiteschists in felsic ultrahigh-pressure metamorphic rocks of Dabie Shan, China. The whiteschist assemblage occurs in a leucocratic layer, less than half a meter thick, which crosscuts a coesite-bearing eclogite. Both its geologic setting and geochemistry suggest that the protolith was a felsic dyke, which suffered loss of some elements, in particular alkalies. The whiteschist consists of quartz, minor epidote, kyanite, talc and omphacite. Common accessories are rutile, apatite and zircon. Epidote is zoned, and the core includes quartz pseudomorphs after coesite and contains up to 5 wt% REE. Kyanite includes fresh omphacite with the highest jadeite content ( Jd ≤ 0.60). The host eclogite consists of omphacite (X Jd = 0.44 to 0.37 from core to rim), garnet, quartz, kyanite, epidote, phengite (Si ≤ 3.47 a.p.f.u.) and amphibole, and accessory rutile, apatite, zircon and ilmenite. Fresh coesite occurs in epidote, and its polycrystalline quartz pseudomorphs are included in both omphacite and kyanite. The whiteschist and the host eclogite share the same tectonometamorphic evolution, and show three metamorphic stages: (I) coesite-eclogite-facies stage at P ≥ 2.6–2.7 GPa and T = 710 ± 20°C; (II) early decompression stage at P ≥ 1.5–2.0 GPa and T = 650 ± 30°C; (III) late adiabatic decompression to P = 0.7 GPa and T = 670 ± 40°C.