Eclogite Rocks Research Articles

A supervised machine learning procedure for EPMA classification and plotting of mineral groups

An analytical method to automatically characterize rock samples for geological or petrological purposes is here proposed, by applying machine learning approach (ML) as a protocol for saving experimental times and costs.Proper machine learning algorithms, applied to automatically acquired microanalytical data (i.e., Electron Probe Micro Analysis, EPMA), carried out with a SEM-EDS microprobe on randomly selected areas from a petrographic polished thin section, are trained, used, tested, and reported.Learning and Validation phases are developed with literature mineral databases of electron microprobe analyses on 15 main rock-forming mineral groups. The Prediction phase is tested using an eclogite rock from the Western Alps, considered as an unknown sample: randomly selected areas are acquired as backscattered images whose intervals of gray levels, appropriately set in the gray level histogram, allow the automated particle mineral separation: automated separating Oxford Instruments Aztec Feature ® packages and a mineral plotting software are applied for mineral particle separation, crystal chemical formula calculation and plotting.Finally, a microanalytical analysis is performed on each separated mineral particle. The crystal chemical formula is calculated, and the final classification plots are automatically produced for any determined mineral. The final results show good accuracy and analytical ease and assess the proper nature of the unknown eclogite rock sample. Therefore, the proposed analytical protocol is especially recommended in those scenarios where a large flow of microanalytical data is automatically acquired and needs to be processed.

Ferric Iron in Eclogitic Garnet and Clinopyroxene from the V. Grib Kimberlite Pipe (NW Russia): Evidence of a Highly Oxidized Subducted Slab

Abstract Estimates of oxygen fugacity of eclogitic rocks are linked to the redox evolution of the oceanic protolith during subduction and its residence in the lithospheric mantle, and, based on knowledge of pressures and temperatures, allow modelling of the speciation of volatile elements and diamond (or graphite) versus carbonate stability. To date, the oxygen fugacity of mantle eclogites has been shown to vary between −6 (Kasai, Congo and Udachnaya, Siberia) and −0.1 (Udachnaya, Siberia) log units (relative to the fayalite–magnetite–quartz buffer, FMQ), linked to the low Fe3+ contents of garnets. In this study, we investigated the Fe oxidation state of coexisting garnet and clinopyroxene hand-picked out of 17 diamond-free high-MgO and low-MgO mantle eclogites (dated at 2.84 Ga) from the Grib kimberlite pipe (East-European platform). Measured Fe3+/∑Fe values range between 0.03 and 0.19 for garnet and 0.18–0.38 for clinopyroxene, the former being higher than what was measured previously in garnets equilibrated at mantle conditions. The Fe3+/∑Fe of the reconstructed bulk rock ranges between 0.10 and 0.15 for high-MgO eclogites and 0.10 and 0.24 for low-MgO eclogites (with uncertainties of ± 0.02 and ± 0.03 in both cases). Thermobarometric calculations result in equilibration pressures and temperatures of 3.0–5.2 (± 0.4) GPa and 720–1050 (± 60) °C for both high-MgO and low-MgO eclogites, slightly lower than previous P–T estimates of mantle eclogites from the Udachnaya kimberlite pipe (Siberian craton). At these conditions, ∆logfo2 (FMQ) calculated using the available oxythermobarometric model varies from −1.7 to −0.6 log units for high-MgO eclogites and from −2.9 to 0.9 log units for low-MgO eclogites. Samples recording ∆logfo2 (FMQ) ≤ −1 log units overlap with North Slave, West Africa and Udachnaya eclogites, with no difference among eclogite types. The average values of −1.2 (± 0.4) log units for high-MgO and −0.6 (± 1.1) log units for low-MgO eclogites suggest different redox conditions of basaltic protoliths during subduction worldwide. Previous geochemical studies on the same rock samples reported evidence of cryptic metasomatism in both garnet and clinopyroxene that we demonstrate being not recorded by their major elements, while modal metasomatism evidenced by the presence of phlogopite as a product of interaction with a kimberlitic melt only affects the MgO of the bulk rock. Therefore, we suggest that high Fe3+/∑Fe ratios for garnet (> 0.10) and for reconstructed bulk rocks in the case of both low-MgO and high-MgO samples cannot be due to metasomatic interaction with an oxidized fluid, but rather are the consequence of Fe3+ redistribution in an unusually oxidized mafic protolith upon metamorphism. Our results highlight the redox variability of eclogites of Archaean age at conditions more oxidized than present-day mid-ocean ridge basalts (MORBs) and imply an oxidizing nature of the convective mantle source where magma was formed with consequent speciation of C in the form of carbonate fluid explaining, therefore, the lack of eclogitic diamonds in V. Grib kimberlite pipe.

A key role for diffusion creep in eclogites: Omphacite deformation in the Zermatt-Saas unit, Italian Alps

Eclogites commonly form from subducted oceanic crust and as such carry key information on subduction zone rheology. Using a combination of microanalytical techniques on deformed eclogites from the Zermatt-Saas unit of the Italian Alps, we explore the mechanisms which resulted in both an omphacite shape and lattice preferred orientation. Omphacite defines both foliation and lineation in these rocks and displays a strong S-type lattice preferred orientation. Scarcity of microstructures associated with dislocation creep combined with observed sharp asymmetrical chemical zonation in foliated grains suggest diffusion creep plays an important role in the development of omphacite lattice preferred orientation in these eclogites. Modelling of the pressure-temperature conditions possible for observed mineralogy and mineral geochemistry, and textural relationships between omphacite and retrogressive minerals, place the action of diffusion creep, at the latest, by the onset of retrogression of these Zermatt-Saas eclogites. We propose a model of eclogite deformation that occurred initially via small amounts of dislocation creep which moved quickly into a dominant diffusion creep field, particularly as exhumation/retrogression of these eclogite rocks began. This result suggests that diffusion creep can dominate eclogite deformation at high P-T conditions in subduction zones.

Cubic Fe-bearing majorite synthesized at 18–25 GPa and 1000 °C: implications for element transport, subducted slab rheology and diamond formation

The chemistry and mineralogy of slabs subducted into lower mantle control slab rheology and impact the deep volatile cycle. It is known that the metamorphism of little-altered oceanic crust results in eclogite rocks with subequal proportions of garnet and clinopyroxene. With increasing pressure, these minerals react to stabilize pyrope-rich tetragonal majoritic garnet. However, some eclogites contain higher proportions of omphacitic clinopyroxene, caused by Na- and Si-rich metasomatism on the ocean floor or during subduction. The mineralogy of such eclogites is expected to evolve differently. Here, we discuss the results of the crystallization products of omphacitic glass at ~ 18 and ~ 25 GPa and 1000 °C to simulate P–T regimes of cold subduction. The full characterization of the recovered samples indicates evidence of crystallization of Na-, Si-rich cubic instead of tetragonal majorite. This cubic majorite can incorporate large amounts of ferric iron, promoting redox reactions with surrounding volatile-bearing fluids and, ultimately, diamond formation. In addition, the occurrence of cubic majorite in the slab would affect the local density, favoring the continued buoyancy of the slab as previously proposed by seismic observations. Attention must be paid to omphacitic inclusions in sublithospheric diamonds as these might have experienced back-transformation from the HP isochemical cubic phase.

Diffusion Creep of Sodic Amphibole‐Bearing Blueschist Limited by Microboudinage

AbstractTo investigate the mechanical and microstructural properties of mafic blueschists, we conducted deformation experiments on powdered natural blueschist aggregates using the general shear geometry in the Griggs apparatus. Experiments were performed at ∼1.0 GPa and temperatures ranging from 650 to 700°C. The blueschist starting material consists primarily of sodic amphibole and epidote, with minor amounts of quartz, titanite, albite, and white mica. Strain rate stepping experiments provided mechanical data with stress exponents ranging from 1.8 to 2.2. Microstructural analysis of the deformed samples show that the blueschist aggregates were deforming by microboudinage of the sodic amphibole, with a chemically new sodic‐calcic amphibole diffused into the boudin neck. Based on these results, we interpret the samples to have deformed by diffusion creep of the sodic‐calcic amphibole, which was rate‐limited by diffusion into the boudin neck. We developed a microboudinage diffusion creep flow law using a least square regression, with parameters of A = 2.43e11 MPa−n μm s−1, n = 2.0 ± 0.3, m = 1.0, and Q = 384 ± 15 kJ/mol. Extrapolation of the flow law to the blueschist stability field suggests viscosities that are higher than metasedimentary rocks (quartz dislocation creep flow law) and lower than eclogitic rocks (omphacite dislocation creep flow law) consistent with field observations. We also show that this type of deformation mechanism matches observations of natural rocks in paleosubduction zone environments, supporting the application of this flow law to estimate amphibole rheology in modern subduction zones.

Petrology and Tectonic Geophysics of Massive and Foliated Eclogites in the North Qilian Orogenic Belt: Changes in Mineral Composition, Oxygen Fugacity, and Fabric during Exhumation

The North Qilian orogenic belt is a typical area of “cold” subduction of the early Paleozoic oceanic plate, forming a series of high pressure and low temperature metamorphic rock assemblages. Among them, eclogite is a kind of protolith, which is basaltic or gabbro high pressure metamorphic rock, mainly composed of garnet and chlorite which are two kinds of minerals. Eclogites record the entire history of subduction zone metamorphism and later exhumation. Due to the crystal habit and the developed joints, the strength of the pyroxene in the matrix is weak, so it is subjected to the main strain during deformation, whereas garnet tends to show only passive rotational deformation. This paper presents some new results in petrology and tectonic geophysics of eclogite block-like and planar eclogite. The massive and facial eclogite rocks contain eclogite facies mineral assemblages, and the peak temperature and pressure conditions are [Formula: see text]°C and [Formula: see text] GPa, which are consistent with the adjacent eclogite. Combined with the characteristics of in situ Lu-Hf isotopes, Ce4+/Ce3+ ratios of zircons, relative oxygen fugacity, and absolute oxygen fugacity, it is shown that the oxygen fugacity of the granodiorite porphyry (BL023, BLO31, DB048) of the folio chemical and massive eclogite deposits are all located in MH (magnetite-hematite) buffer zone. Through the calculation results of absolute oxygen fugacity of rock mass, it can be seen that the absolute oxygen fugacity of ore-bearing rock mass is significantly higher than that of non-ore-bearing rock mass. This paper systematically summarizes the research progress of the microscopic and ultrastructural deformation of eclogite minerals in high-pressure metamorphic zones, and discusses the changes of mineral composition, oxygen fugidity, and fabric of eclogite deformation characteristics during the recovery of subduction and reentry.

Petrological and Lu–Hf age constraints for eclogitic rocks from the Pam Peninsula, New Caledonia

We report new petrological and geochronological data for eclogitic rocks of the Pouébo Eclogite Mélange (PEM) in the New Caledonian high-pressure/low-temperature (HP/LT) belt. Pseudosections were calculated to link newly determined Lu–Hf dates with P-T conditions corresponding to garnet growth, which started on the prograde path and continued into the eclogite facies to approximately 560 °C and 2.05 GPa, coinciding with omphacite formation. The Lu–Hf garnet dates of 38.27 ± 0.14, 36.5 ± 0.7, and 38.42 ± 0.15 Ma are weighted towards the early part of the garnet growth interval, and thus represent maximum ages of eclogite-facies conditions. These dates are indistinguishable from those reported earlier from Ar–Ar in white mica, which had been interpreted as cooling ages, and they are about 6 Myr younger than those from U–Pb in metamorphic zircon, which had been previously considered to date peak eclogite-facies metamorphism at 44 Ma. The new Lu–Hf dates overlap with Rb–Sr white mica ages of nearby metasomatic veins related to the blueschist-to-eclogite transition. The Lu–Hf and Rb–Sr data imply that eclogite-facies metamorphism in parts of the PEM lasted at least until 38 Ma and is thus younger than previously suggested. This prograde, near-peak event cannot be reconciled with concurrent (41–36 Ma) cooling previously inferred from the Ar–Ar dates. We conclude that both the 38.4–36.5 Ma Lu–Hf and Rb–Sr dates as well as similar Ar–Ar results record near-peak conditions, while the 44 Ma U–Pb zircon rims probably are related to prograde mineral reactions. The HP stage is followed by rapid exhumation and cooling at minimum rates of ~0.7 cm/yr and ~50 °C/Myr.

The 2 Ga eclogites of Central Tanzania: Directly linking age and metamorphism

The ca. 2 Ga retrogressed eclogites in the Usagaran Belt in central Tanzania are among the oldest documented eclogites in the world. As such, they have been used to pinpoint the thermal conditions of the onset of modern style subduction on Earth. Two samples of retrogressed mafic eclogite have been interrogated to reconstruct the metamorphic history of the subducted crust. The samples are several kilometres apart, separated by a mylonitic upper amphibolite-transitional granulite foliation. Lu--Hf garnet geochronology pinpoints eclogite metamorphism at 1994 ± 9 Ma, supported by rutile U--Pb geochronology. Zircon U--Pb geochronology supports subduction metamorphism at ca. 2000 Ma. Mineral equilibria modelling and Zr in rutile thermometry suggests the retrogressed eclogites reached a minimum pressure of 18 kbar at approximately 800 °C, consistent with a warm subduction thermal gradient. These conditions were followed by amphibolite-granulite-facies overprinting. Proposed tectonic models for the formation of the eclogites involve either east-dipping subduction of the Tanzanian Craton margin, or west-dipping subduction of oceanic crust under the Tanzanian Craton. The retrogressed eclogites were likely exhumed via slab breakoff driven buoyancy of the oceanic crust and transferred to shallow crustal levels in a back arc position. This data, combined with data from eclogites from the Eburnian-Transamazonian and Ubende orogens, demonstrate the possible existence of a large subduction system on the margin of west Africa from ca. 2.1–1.9 Ga. This system has led to the preservation of several eclogitic rocks, which suggests that modern style subduction was operating on significant geographic and temporal scales at this time.

Redox state determination of eclogite xenoliths from Udachnaya kimberlite pipe (Siberian craton), with some implications for the graphite/diamond formation

The formation of diamonds within eclogitic rocks has been widely linked to the fate of carbon during subduction and, therefore, referred to conditions of pressure, temperature, and oxygen fugacity (fo2). Mantle-derived eclogite xenoliths from Udachnaya kimberlite pipes represent a unique window to investigate the formation of carbon-free, graphite–diamond-bearing and diamond-bearing rocks from the Siberian craton. With this aim, we exploited oxy-thermobarometers to retrieve information on the P–T–fo2 at which mantle eclogites from the Siberian craton equilibrated along with elemental carbon. The chemical analyses of coupled garnet and omphacitic clinopyroxene were integrated with data on their iron oxidation state, determined both by conventional and synchrotron 57Fe Mossbauer spectroscopy. The calculated fo2s largely vary for each suite of eclogite samples from 0.10 to − 2.43 log units (ΔFMQ) for C-free eclogites, from − 0.01 to − 2.91 (ΔFMQ) for graphite–diamond-bearing eclogites, and from − 2.08 to − 3.58 log units (ΔFMQ) for diamond-bearing eclogites. All eclogite samples mostly fall in the fo2 range typical of diamond coexisting with CO2-rich water-bearing melts and gaseous fluids, with diamondiferous eclogites being more reduced at fo2 conditions where circulating fluids can include some methane. When uncertainties on the calculated fo2 are taken into account, all samples essentially fall within the stability field of diamonds coexisting with CO2-bearing melts. Therefore, our results provide evidence of the potential role of CO2-bearing melts as growth medium on the formation of coexisting diamond and graphite in mantle eclogites during subduction of the oceanic crust.

Petrology, geochemistry and Sm-Nd systematics of the Paleoproterozoic Itaguara retroeclogite from São Francisco/Congo Craton: One of the oldest records of the modern-style plate tectonics

Paleoproterozoic retrogressed eclogite (retroeclogite) occurs in the Itaguara Sequence included in the suture zone formed by collision between the Archean Divinópolis and Campo Belo/Bonfim Complexes in the southern São Francisco Craton, which represents the South American counterpart of the African Congo Craton. The Itaguara retroeclogite contains scarce omphacite and phengite but abundant garnet porphyroblasts embedded in a fine-grained, amphibole, biotite and quartz-bearing matrix. The 2.20 ± 0.05 Ga eclogitization event (garnet and whole rock Sm-Nd isochronic age) of the E-MORB protolith (TDM ~ 2.47 Ga) is recorded by omphacite formation during high-pressure prograde stage in amphibole eclogite facies due to ~70 km depth subduction process. Amphibole eclogite facies metamorphic peak stage of 17–20 kbar and 600–700 °C occurred during ~2.1 Ga continental collision. Tectonic exhumation-related decompression during collision probably triggered partial melting of the eclogitic rock. Finally, decompression late stage estimated between 5 and 8 kbar and 550–650 °C under amphibolite facies overprint during orogenic collapse was responsible for appearance of kelyphitic reaction rims (symplectite) around garnet crystals. As its Paleoproterozoic contemporary analogues from Congo Craton, the Itaguara retroeclogite is one of the oldest records of the modern-style plate tectonics.

Crustal seismic structure beneath the Garhwal Himalaya using regional and teleseismic waveform modelling

SUMMARYWe investigate the crustal seismic structure of the Garhwal Himalayan region using regional and teleseismic earthquake waveforms, recorded over 19 closely spaced broad-band seismic stations along a linear profile that traverses from the Sub Himalayas to Higher Himalayas. The regional earthquake traveltime analysis provides uppermost mantle P- and S-wave velocities as 8.2 and 4.5 km s−1, respectively. The calculated receiver functions from the teleseismic P waveforms show apparent P-to-S conversions from the Moho as well as from intracrustal depths, at most of the seismic stations. These conversions also show significant azimuthal variations across the Himalayas, indicating complex crustal structure across the Garhwal Himalaya. We constrain the receiver function modelling using the calculated uppermost mantle (Pn and Sn) velocities. Common conversion point stacking image of P-to-S conversions as well as receiver function modelling results show a prominent intracrustal low shear velocity layer with a flat–ramp–flat geometry beneath the Main Central Thrust zone. This low velocity indicates the possible presence of partial melts/fluids in the intracrustal depths beneath the Garhwal Himalaya. We correlate the inferred intracrustal partial melts/fluids with the local seismicity and suggest that the intracrustal fluids are one of the possible reasons for the occurrence of upper-to-mid-crustal earthquakes in this area. The results further show that the Moho depth varies from ∼45 km beneath the Sub Himalayas to ∼58 km to the south of the Tethys Himalayas. The calculated lower crustal shear wave velocities of ∼3.9 and 4.3 km s−1 beneath the Lesser and Higher Himalayas suggest the presence of granulite and partially eclogite rocks in the lower crust below the Lesser and Higher Himalayas, respectively. We also suggest that the inferred lower crustal rocks are the possible reasons for the presence and absence of the lower crustal seismicity beneath the Lesser and Higher Himalayas, respectively.

Genesis of Diamondiferous Rocks from Upper-Mantle Xenoliths in Kimberlite

The conditions of genesis of diamondiferous ultrabasic and basic rocks from xenoliths in kimberlite were studied by combining the data from analytical investigations of their mineral phases and experimental results of the study of melting relations in the diamond-forming mineral systems of the upper mantle. The compositions of minerals in some samples of metasomatized diamondiferous eclogite associated with diamond-free eclogite from kimberlite of the Udachnaya pipe (Yakutia) were studied for the first time. The new results obtained in addition to the literature data were applied for generalization of estimates of genetically important characteristics of the chemical compositions of garnets, Ca-clinopyroxenes, and omphacites from diamond-bearing peridotite, pyroxenite, and eclogite. As a result, it was found that quite “fresh” minerals of diamondiferous rocks have typomorphic differences from the same minerals of diamond-free upper-mantle rocks. At the same time, it is significant that the compositions of minerals from diamondiferous rocks and paragenetic inclusions in diamonds are identical. These peculiarities of mineralogy of diamondiferous rocks are genetically significant; based on the mantle–carbonatite theory of the origin of diamond and associated mineral phases, this provides support for the same physicochemical origin of diamonds, minerals of diamondiferous rocks, and paragenetic inclusions in diamonds. Finally, the following genetic conclusions are made. (1) Completely miscible silicate (±oxide)–carbonate melts with dissolved carbon are the parental medium in petrogenesis of diamondiferous ultrabasic and basic rocks. (2) The physicochemically consistent formation of diamondiferous rocks and paragenetic inclusions of peridotitic and eclogitic minerals in diamonds occurred in the common diamond-forming chambers/reservoirs of parental melts; diamond-free peridotite, pyroxenite, and eclogite were the host mantle rocks for such chambers. (3) The origin of continuous series of diamondiferous peridotite–pyroxenite–eclogite rocks is controlled by the fractional ultrabasic–basic evolution of parental melts with exhaustion of olivine and orthorhombic pyroxene via the peritectic reactions. (4) Ascending flows of kimberlite magmas destroyed the parental chambers and captured diamonds with inclusions, individual minerals, their intergrowths, diamondiferous ultrabasic and basic rocks; at the entrance and exit from the chambers, they captured differentiated diamond-free host rocks of the mantle as well. (5) With further ascent from the mantle to the Earth’s crust, the material of diamond-forming chambers and diamond-free mantle was mixed in convecting kimberlite magma and was transported from the mantle to cumulative crustal chambers. (6) Kimberlite magmas were gradually solidified in stationary cumulative chambers with the release of highly compressed fluids; with an increase of pressure up to the critical values, they intruded into the rocks of the roof and ejected kimberlite with xenoliths of diamondiferous and mantle rocks to the surface with the formation of explosion pipes.

Paleoproterozoic oceanic subduction in the North China Craton: Insights from the metamorphic P–T–t paths of the Chicheng Mélange in the Hongqiyingzi Complex

How and when modern-style plate tectonics started in the Earth’s history is one of the most important scientific issues. The Chicheng mélange in the north margin of North China Craton (NCC) presents a new insight into the Paleoproterozoic cold oceanic subduction and the beginning of modern-style plate tectonic regime. The mélange comprises blocks of retrograde eclogites, amphibolites and peridotites in a matrix of metasediments. The retrograde eclogites mainly contain garnet, amphibole and symplectites consisting of fine-grained plagioclase, clinopyroxene and amphibole. Three metamorphic stages were derived, involving the prograde, peak, and post-peak decompression stages. The P–T conditions for the prograde and peak stages range from ~17 kbar/660 °C to >22/~750 °C kbar based on mineral composition isopleths in P–T pseudosections, involving core–to–mantle increasing pyrope and decreasing grossular in garnet and Ti content in amphibole. The post-peak decompression is proposed to be isothermal under melt (fluid)-absent conditions, recorded a pressure of 8–11 kbar on the basis of symplectite assemblages. For the amphibolites pre-peak heating, peak temperature, and post-peak cooling metamorphic stages were derived. They experienced anataxis during this overprinting metamorphism when heated to above the solidus. The peak temperature conditions are constrained to be 5–7 kbar/780–830 °C on the basis of the contours of the maximum Ti in amphibole and anorthite of plagioclase in P–T pseudosections. Zircon U–Pb dating for the selected samples and a summary of published age data reveal that the eclogite facies metamorphism may have occurred at 1.85–1.80 Ga, and the eclogitic rocks exhumed to a lower crust level during or after subduction to be overprinted by multi-phase thermal events at ~1.66 Ga and 460–420 Ma with undefined conditions, and at 360–270 Ma under amphibolite facies. The eclogite facies metamorphism from the Chicheng mélange shows a geothermal gradient of ~9 °C/km, pointing to a cold oceanic subduction at late Paleoproterozoic that occurred along the northern margin of the NCC. The overprinting metamorphism with anataxis recognized from the amphibolites represents a low-pressure type with a geothermal gradient of ~35 °C/km, which was interpreted to occur at 360–270 Ma governed by extension.

Unravelling a Devonian–Triassic seamount chain in the South Tianshan high‐pressure/ultrahigh‐pressure accretionary complex in the Atbashi area (Kyrgyzstan)

Seamounts are common occurrences on the extant oceanic floors of the earth, and accreted single seamounts are also widely distributed on the modern active margins, but recognition of single seamounts is an uneasy job in the ancient orogens, and the detection of a chain of accreted seamounts is even harder, therefore understanding the multiple accretion of ancient seamounts is nearly negligible. A combined analysis of geochronology and geochemistry was conducted on the basic eclogitic rocks from the Atbashi Accretionary Complex in the Kyrgyz South Tianshan, in a bid to unravel the accretion process of a chain of seamounts in the Paleo‐Asian Ocean in the southern Altaids. A set of reliable U–Pb zircon ages of 397.6 ± 8 Ma age for a basaltic andesite, 353 ± 8 Ma for a gabbro, and 317.1 ± 9 Ma for a diorite were obtained in this study. Both unmetamorphosed basic rocks and eclogites carry geochemical signatures of plume‐derived seamounts that were influenced by their respective mantle source. Taken together with previously published age data, the spatial and temporal features of these Atbashi rocks indicate the timing and sequence of formation, subduction, and extrusion of the South Tianshan oceanic remnants in the Early/Middle Devonian to Late Triassic. The integrated data indicate the existence of a seamount chain composed of Early/Middle Devonian–Late Triassic seamounts in the Atbashi Complex. The paleogeographic pattern and analysis of these fragments derived from the ocean‐type seamounts enable the reconstruction of tectonic models of that portray, magmatism, mantle‐derived plumes, subduction and exhumation of this Devonian–Triassic ocean floor.

In situ rutile U-Pb dating based on zircon calibration using LA-ICP-MS, geological applications in the Dabie orogen, China

Rutile U-Pb dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has received increasing attention in geological applications. Previous natural rutile reference materials are insufficient to meet the growing demand of working standards for laser U/Pb dating. In present study, we have developed a new calibration strategy for LA-ICP-MS rutile dating using the most popular zircon as reference material. The matrix-dependent elemental fractionation of rutile and zircon could effectively decrease to a better level by optimized analytical parameters and with large ablation spot size (44–90 µm). The acceptable Pb/U correction coefficient between rutile and zircon could be obtained while the diameter/depth ratio of the crater is above 1 after assessing elemental fractionation of different ablation size. Five rutile were analyzed by zircon calibration, and their ages show strong agreement with those obtained by secondary ion mass spectrometry (SIMS) or thermal ionization mass spectrometry (TIMS) analysis, with both precision and accuracy within ±2%. Two rutiles of eclogitic rocks from the Dabie orogen yield the ages of 207.0 ± 2.1 Ma and 205.6 ± 2.4 Ma, respectively. Both ages are conceivably younger than that previously reported rutile ages (218 Ma) for eclogites. The quite different U-Pb ages of rutile from different localities most likely indicate the different exhumation history of the subducted continental crust. LA-ICP-MS rutile U-Pb dating using zircon as an external calibration reference material would assuage rutile reference material insufficient supply and improve its widespread application.

Sound Velocities in Shock‐Synthesized Stishovite to 72 GPa

AbstractStishovite (rutile‐type SiO2) is the archetype of dense silicates and may occur in postgarnet eclogitic rocks at lower‐mantle conditions. Sound velocities in stishovite are fundamental to understanding its mechanical and thermodynamic behavior at high pressure and temperature. Here, we use plate‐impact experiments combined with velocity interferometry to determine the stress, density, and longitudinal sound speed in stishovite formed during shock compression of fused silica at 44 GPa and above. The measured sound speeds range from 12.3(8) km/s at 43.8(8) GPa to 9.8(4) km/s at 72.7(11) GPa. The decrease observed at 64 GPa reflects a decrease in the shear modulus of stishovite, likely due to the onset of melting. By 72 GPa, the measured sound speed agrees with the theoretical bulk sound speed indicating loss of all shear stiffness due to complete melting. Our sound velocity results provide direct evidence for shock‐induced melting, in agreement with previous pyrometry data.

Brittle/Ductile Deformation of Eclogites: Insights From Numerical Models

AbstractHow rocks deform at depth during lithospheric convergence and what are the magnitudes of stresses they experience during burial/exhumation processes constitute fundamental questions for refining our vision of short‐term (i.e., seismicity) and long‐term tectonic processes in the Earth's lithosphere. Field evidence showing the coexistence of both brittle and ductile deformation at high pressure‐low temperature (HP‐LT) conditions particularly fuels this questioning. We here present 2D numerical models of eclogitic rock deformation by simple shear performed at centimeter scale. To approximate the eclogite paragenesis, we considered the deformed medium as composed of two mineral phases: omphacite and garnet. We run a series of models at 2.0 GPa and 550 °C for different background strain rates (from 10−14 s−1 to 10−8 s−1) and for different garnet proportions (from 0% to 55%). Results show that whole rock fracturing can occur under HP‐LT conditions for strain rates larger than ~10−10 s−1. This suggests that observation of brittle features in eclogites does not necessarily mean that they underwent extreme strain rate. Care should therefore be taken when linking failure of eclogitic rocks to seismic deformation. We also explore the ranges of parameters where garnet and omphacite are deforming with a different deformation style (i.e., frictional vs viscous) and discuss our results in the light of naturally deformed eclogitic samples. This study illustrates that effective stresses sustained by rocks can be high at these P‐T conditions. They reach up to ~1 GPa for an entirely fractured eclogite and up to ~500 MPa for rocks that contain fractured garnet.

A new occurrence of retrogressed eclogite from the Sanbagawa belt of southwest Japan and its significance

AbstractThe present paper reports, for the first time, the occurrence of an omphacite‐bearing mafic schist from the Asemi‐gawa region of the Sanbagawa belt (southwest Japan). The mafic schist occurs as thin layers within pelitic schist of the albite–biotite zone. Omphacite in the mafic schist only occurs as inclusions in garnet, and albite is the major Na phase in the matrix, suggesting that the mafic schist represents highly retrogressed eclogite. Garnet grains in the sample show prograde‐type compositional zoning with no textural or compositional break, and contain mineral inclusions of omphacite, quartz, glaucophane, barroisite/hornblende, epidote and titanite. In addition to the petrographic observations, Raman spectroscopy and focused ion beam system–transmission electron microscope analyses were used for identification of omphacite in the sample. The omphacite in the sample shows a strong Raman peak at 678 cm−1, and concomitant Raman peaks are all consistent with those of the reference omphacite Raman spectrum. The selected area electron diffraction pattern of the omphacite is compatible with the common P2/n omphacite structure. Quartz inclusions in the mafic schist preserve high residual pressure values of Δω1 > 8.5 cm−1, corresponding to the eclogite facies conditions. The combination of Raman geothermobarometries and garnet–clinopyroxene geothermometry gives peak pressure–temperature (P–T) conditions of 1.7–2.0 GPa and 440–540 °C for the mafic schist. The peak P–T values are comparable to those of the schistose eclogitic rocks in other Sanbagawa eclogite units of Shikoku. These findings along with previous age constraints suggest that most of the Sanbagawa schistose eclogites and associated metasedimentary rocks share similar simple P–T histories along the Late Cretaceous subduction zone.

Melting of H2O and CO2-bearing eclogite at 4–6 GPa and 900–1200 °C: Implications for the generation of diamond-forming fluids

Abstract Eclogites play a significant role in geodynamic processes, transferring large amounts of basaltic material and volatiles (chiefly CO2 and H2O species) into the earth's mantle via subduction. Previous studies of eclogite melting focused on two end member systems: either carbonated or hydrous eclogites. Here we focus on the hydrous carbonated eclogitic system in order to define the position of its solidus and determine the near solidus fluid and melt compositions at 4–6 GPa and 900–1200 °C. Experiments were performed on a rocking multi-anvil press. The total dissolved solids in the equilibrated fluids were analyzed following the cryogenic technique using a LA-ICP-MS. H2O and CO2 content were determined by mass balance calculations. Solid phases were chemically characterized using an EPMA. Garnet and clinopyroxene are present in all experiments, assembling the eclogitic rock. A carbonate phase was detected at all temperatures at 4 GPa and at temperatures below 1200 °C at 5 and 6 GPa. Coesite was observed at all pressures below 1200 °C. The solidus was crossed between 1000 and 1100 °C at 4 and 5 GPa. At 6 GPa we observed a relatively smooth decrease in the H2O and CO2 content of the fluid phase with rising temperature, suggesting the presence of a supercritical fluid. The second critical endpoint is thus defined in this system at ∼5.5 GPa and 1050 °C. The composition of fluids and melts reported in this study indicates that the hydrous carbonated eclogite system is a plausible source-rock for high density fluids (HDFs) found in microinclusions in diamonds, specifically for the intermediate compositions along the array spanned between low-Mg carbonatitic HDFs and hydrous-silicic ones. Our results suggest that the whole array reflects melting in a heterogeneous mantle. Melting of water-rich eclogite produces silicic HDFs, carbonate-rich zones will produce carbonatitc HDFs, while source-rocks with varying H2O/CO2 ratios produce intermediate compositions.

エクロジャイト相変成岩中の非平衡成長ざくろ石：グァテマラ・南部モタグア断層帯の例

エクロジャイト相変成岩中の非平衡成長ざくろ石：グァテマラ・南部モタグア断層帯の例