Olivine Grains Research Articles

Iron isotopes of Chang'e-5 soil and mineral components: Implications for post-eruption processes on lunar surface

Due to rapid magma cooling and extensive space weathering, significant disequilibrium crystallization and secondary modification widely occur in lunar mare basalt after its eruption on the lunar surface. In this study, we conducted bulk and in-situ Fe isotope analyses to investigate the post-eruption processes on Chang'e-5 (CE-5) samples. The CE-5 soil shows a minor elevation of δ56Fe value (∼0.05 ‰) relative to the CE-5 basalt clasts. Correlations between Ni and Cu contents with δ56Fe values suggest that the minor increase in the δ56Fe from the CE-5 basalt to soil primarily occurred during evaporation caused by meteorite impacts. Such isotopic variation between CE-5 basalt and soils is notably lower than what is observed for Apollo samples and reflects the low maturity of CE-5 soils. This is consistent with the low Is/FeO value constrained by magnetic approaches. Therefore, measuring the δ56Fe values of lunar soil is suitable to evaluate the degrees of maturity for lunar soils due to space weathering. In-situ analyses of δ56Fe reveal significant variations in different grains of olivine (δ56Fe: −0.57 to −0.17 ‰) and ilmenite (−0.06 to +0.42 ‰) and also in their interior (mainly for olivine). These δ56Fe variations in minerals can be ascribed to the disequilibrium crystallization of lava flow and fast cooling, which is consistent with conclusions based on petrologic observations such as its extensive differentiation and silicate liquid immiscibility. Therefore, the post-eruption processes on the lunar surface could lead to significant variations in isotopic compositions at different scales of basalts, which in turn record the history of late-stage magma evolution and space weathering on the lunar surface.

Ethanolamine ices: Experiments in simulated space conditions

Laboratory experiments on the interactions between complex organic molecules, interstellar dust, and ultraviolet (UV) radiation are crucial to understanding the physicochemical mechanisms that lead to the synthesis of the observed interstellar complex organic molecules (iCOMs), and to search for new molecular species not yet observed in the gas phase of the interstellar medium (ISM). We aim to study the role of a new, recently discovered interstellar molecule, ethanolamine (EtA, NH CH CH OH), in surface chemistry in the ISM. In the laboratory, thanks to a combination of temperature programmed desorption (TPD) experiments and electron ionization (EI) mass spectrometry analyses, we studied the thermal desorption of pure ethanolamine and its mixture with water from nanometric amorphous olivine grains cooled down to 10 K, with or without UV irradiation. Ethanolamine was found to be stable, even in the presence of water, when irradiated with UV light. The presence of olivine grains strongly modified the TPD curves, trapping the molecule up to about 295 K, meaning that the precursors of some biological molecules could be retained on the grains even in the innermost parts of protoplanetary disk. We then identified a series of products formed when the molecule was irradiated onto the dust substrate. Of particular interest is the fact that irradiation of ice containing ethanolamine, a molecule known to be present in the ISM, can produce more complex and astrobiologically interesting species. Furthermore, our results further our understanding of existing observational data.

Geochemical characteristics of olivine and clinopyroxene in parental mafic–ultramafic rocks from the Yuanshishan Ni-Co laterite deposit in Qinghai Province, NW China

Yuanshishan is a Ni-Co laterite deposit that occurs in the Lajishan tectonic zone of the South Qilian orogenic belt, NW China. Ni–Co mineralization took place during the weathering of the parental mafic–ultramafic rocks. Little is known about the petrogenesis of these parent rocks. Therefore, field and petrographic investigations, together with in situ major and trace element analyses, were carried out on olivine and clinopyroxene from unweathered mafic–ultramafic rocks in the Yuanshishan Ni-Co deposit. The results show that olivine (Ni 963–2674 ppm, Co 52.7–245 ppm) is richer in Co and Ni than clinopyroxene (Ni 155–203 ppm, Co 23.1–32.5 ppm) in the Yuanshishan mafic–ultramafic rocks. The chromite (Ni 98.8–892 ppm, Co 836–1062 ppm) show higher Co and Ni than clinopyroxene, but has lower Ni but higher Co than olivine. Therefore, olivine and chromite are two main Co and Ni rich minerals in the parent rocks of Yuanshishan. A compositional comparison of olivine and clinopyroxene from the Yuanshishan deposit to those from other magmatic sulfide deposits reveals that the Yuanshishan parent magma is relatively poor in Ni and rich in Co. Olivine in the dunite of Yuanshishan has a quite high Fo value (95.2–98.3). The Yuanshishan olivine grains also show a positive correlation between Ni content and Ni/Co or Co content, and the correlation with Fo value is not obvious. The Co content shows a negative correlation with the Ni content and Fo value, which are consistent with the mantle olivine characteristics. Furthermore, the Li, Sc, V contents in olivine are lower than 10 ppm, indicating that the magma originated from the mantle. The Al, Si, Ti, Ca, and Na contents in clinopyroxene indicate that the original magma may have been subalkaline basaltic melt. Using an olivine-spinel thermometer, the Yuanshishan magma crystallization temperatures are calculated to be 1256–1376 °C. The AlⅣ:AlⅥ ratio (0.7–6.0) and Eu/Eu* value (0.9–1.4) of the clinopyroxene indicate that the Yuanshishan mafic–ultramafic rocks were formed under conditions of low to medium pressure and high oxygen fugacity. Moreover, the geochemical discriminant diagrams of clinopyroxene indicate that these rocks possibly formed in an island arc setting.

Textural and chemical variations in peridotite induced by hydrous melt migration in mantle under the back-arc spreading

This study presents the structural and petrological characteristics of mantle peridotites in relation to the rock-hydrous melt reaction found in the Hayachine ultramafic complex, NE Japan. We conducted sampling, microstructural observations, crystal-orientation analyses, and major element composition analyses of the major constituent minerals in the peridotites. We used 17 serpentinized peridotites that preserved better mantle textures. The peridotites are composed of lherzolite to harzburgite, consisting of olivine, orthopyroxene, clinopyroxene, amphibole, and spinel. The peridotites were classified into two textures according to olivine grain size: coarse-grained (ca. 2–3 mm) and fine-grained (ca. 0.3–0.5 mm). Fine-grained peridotites were newly found in this study and were characterized by aggregates of orthopyroxene and amphibole with a large number of spinel inclusions. Based on olivine crystal-preferred orientation (CPO), we found that the coarse-grained peridotites were further classified into Group 1 (A type CPO) and 2 (AG type CPO) and the fine-grained peridotites were accordingly classified into Group 3 (weak CPO). The systematic continuity in the chemical compositions of the minerals suggests that Group 1 peridotites partially melted to form Group 2 peridotites, followed by Group 3 peridotites, due to further reaction of Group 2 peridotites with hydrous melts. These textural and chemical variations in the peridotites could have resulted from rock-hydrous melt reactions under back-arc spreading and subsequent processes.

Geochemistry of pallasite olivine and the origin of pallasites

I have done major element analyses by electron microprobe, in-situ trace element analyses by laser ablation inductively coupled plasma mass spectrometry, and instrumental neutron activation analyses on bulk samples of olivine grains separated from main-group and Eagle-Station pallasites. Most main-group pallasite olivines have homogeneous Fe/Mg yet have varying Fe/Mn. Those few with anomalously ferroan olivine have Fe/Mn within the range of other main-group pallasites. High-temperature redox process coupled with diffusional exchange resulted in the homogeneous compositions of most main-group pallasites; simple diffusional exchange alone is insufficient. The Eagle-Station pallasites have Fe/Mn twice that of main-group pallasites with similar Fe/Mg, a result of having roughly half the Mn content. The Ni/Co ratio of main-group pallasite olivines is relatively constant and was imposed by the same high-temperature redox/diffusion process that established Fe/Mg-Fe/Mn relationships. Variability in trace lithophile element contents within individual pallasites and within individual olivine grains, coupled with very low contents for some that are inconsistent with formation from a magma, indicate that the current mm-sized olivine grains were recrystallized from a fragmental olivine breccia; grain fragments from different portions of an original dunitic mantle were juxtaposed in the breccia. Main-group pallasites are dimict breccias formed of fragmented and mixed monomict dunite breccia and metallic breccia that were formed in the walls and floor of a large basin that penetrated the mantle of their parent asteroid. Limited data indicate that Eagle-Station pallasites may have been formed by a similar process. Given that Eagle-Station and main-group pallasites were formed in distinct regions of the early Solar System, the pallasite forming process likely was common in early Solar System history.

Transformational faulting in Mn2GeO4 from olivine to wadsleyite structure: Implications for physical mechanism of deep-focus earthquakes

High-pressure and temperature deformation experiments interfaced with acoustic emission (AE) monitoring have been conducted to study transformational faulting in Mn2GeO4 olivine, which transforms to the β phase, isostructural to wadsleyite. Metastable Mn2GeO4 olivine exhibits a marked embrittlement behavior at temperatures between 800 and 1100 K, emitting numerous AEs. At each temperature, brittle deformation is characterized by a two-stage process: (1) a “preparation” stage with numerous diffusedly located low-magnitude AEs and large b values (>2), and (2) a failure stage where larger-magnitude AEs form a planar distribution with b values about 1. Microstructure analysis reveals extensive kink band development in olivine grains in the recovered samples. Kink band boundaries (KBBs), with a typical thickness of ∼100 nm, are filled with a nanometric β-Mn2GeO4 “gouge”. A dense array of secondary shear localizations is often present within the kink bands, suggesting significant shear deformation therein. The combined observations suggest that faulting in metastable Mn2GeO4 olivine is a self-similar process, from grain-scale to the sample-scale. Both observed embrittlement behavior and the microstructure of metastable Mn2GeO4 olivine are essentially identical to those in Mg2GeO4 olivine we have reported previously, indicating that the physical mechanism of faulting in metastable olivine is insensitive to the specific crystallographic structure of the high-pressure phase. The low b values (about 1) observed in the faulting process in our experiments are similar to those of deep focus earthquakes in cold subduction zones. Our observed mechanism explains deep focus seismicity in cold metastable mantle wedges, provided that the self-similarity assumption holds to geological scales.

NWA 11562: A Unique Ureilite with Extreme Mg-rich Constituents

A comprehensive study of an ungrouped achondrite meteorite, North West Africa (NWA) 11562, was conducted, involving petrology, mineralogy, and mass-independent chromium isotopic composition. NWA 11562 comprises 34.9 vol% olivine, 56.1 vol% pyroxenes, 6.7 vol% Fe–Ni metal and oxides, 2.2 vol% carbon, and 0.2 vol% anhydrite. The oxygen isotopic composition (δ 18O = 6.24‰ ± 0.13‰ and Δ17O = −1.81‰ ± 0.03‰; Meteoritical Bulletin Database) and chromium isotopic compositions (ε 54Cr = −0.82 ± 0.10, 2SE) are consistent with NWA 11562 being a ureilite, and petrographic observations show features similar to those of the common ureilite group meteorites. Olivine (core: Fo99.0 ± 0.1; rim: Fo98.9 ± 0.1) and pyroxene (orthopyroxene: Mg# 99.0 ± 0.2; clinopyroxene: Mg# 99.1 ± 0.7) core compositions are more magnesian than in any previously known ureilite and lack the characteristic reduction rims of ureilites. Rounded small olivine grains within NWA 11562 indicate that the meteorite experienced impact and associated melting. Combined with the characteristic Fe/Mn ratio (3.84 ± 0.16) and Mg# (99.0 ± 0.1) of olivine cores, we suggest that NWA 11562 represents a more Mg-rich ureilite than any previously reported. NWA 11562 has a high 55Mn/52Cr ratio, and when combined with literature data, it plots on a well-defined 53Mn–53Cr isochron, providing a more accurate age, i.e., 4566.7 ± 0.8 Ma, overlapping previous work. This age may represent the result of early partial melting of the ureilite parent body.

Effect of crustal stress state on magmatic stalling and ascent: case study from Puyehue-Cordón Caulle, Chile

The Southern Volcanic Zone (SVZ) in Chile is an active continental arc with a complex history of volcanism, where a range of magmatic compositions have been erupted in a variety of styles. In the Central SVZ, both monogenetic and polygenetic volcanoes exist, in close proximity to the Liquiñe-Ofqui Fault System (LOFS), but with variable local stress states. Previous studies have inferred varying crustal storage timescales, controlled by the orientation of volcanic centres relative to the N-S striking LOFS and σHMax in this region. To assess the relationship between volcanism and crustal stress states affected by large-scale tectonic structures and edifice controls, we present whole rock geochemical data, to ensure consistency in source dynamics and crustal processing, mineral-specific compositional data, thermobarometry, and Fe–Mg diffusion modelling in olivine crystals from mafic lavas, to assess ascent timescales, from the stratovolcanic edifice of Puyehue-Cordón Caulle and proximal small eruptive centres. Textural observations highlight differences in crystal maturation timescales between centres in inferred compression, transpression, and extension, yet source melting dynamics remain constant. Only samples from the stratovolcanic edifice (in regional compression) preserve extensive zonation in olivine macrocrysts; these textures are generally absent from proximal small eruptive centres in transtension or extension. The zonation in olivines from stratovolcanic lavas yields timescales on the order of a few days to a few weeks, suggesting that even in environments which inhibit ascent, timescales between unrest and eruption of mafic magmas may be short. Significantly, high-resolution compositional profiles from olivine grains in the studied samples record evidence for post-eruptive growth and diffusion, highlighting the importance of careful interpretation of diffusion timescales from zoned minerals in more slowly cooled lavas when compared with tephra samples.

Quantification of grain boundary mobilities in natural olivine by annealing experiments and full-field modelling

We investigate olivine grain boundary (GB) migration in natural peridotites experimentally annealed at high pressure and high temperature, and couple the experimental observations to full-field grain growth models to provide the distribution of GB mobilities in natural olivine polycrystals. A stack of four slices of natural mylonitic peridotite (Oman ophiolite) was annealed at 1473 K for 5 h under a confining pressure of 300 MPa of argon in a Paterson press. The three sintered interfaces of the stack are characterized using scanning electron microscopy and electron backscatter diffraction (EBSD) to extract a distribution of apparent (2D) GB displacements. Full-field simulations of numerically sintered interfaces are then used to infer the GB mean mobility and distribution in olivine polycrystal allowing forward modelling exercise to reproduce the experimental GB displacement distribution.This yields widely dispersed mobilities, which can be approximated by a log-normal distribution of 10−15.85±2.58m4.J−1.s−1. Both the average and the dispersion of GB mobilities can be explained by silicon grain boundary diffusion. Finally, we demonstrate that the high variability of GB mobilities in olivine implies a decrease of the mean growth rate with time. This elucidates the difficulties of extrapolating experimental grain growth rates to geological timescales and observed microstructures in peridotites.

Mineral chemistry and melt evolution of the mantle wedge peridotites in the Late Cretaceous Zagros Belt ophiolites (Iran): clues for the subduction initiation-induced forearc magmatism

We investigate in this paper mineral compositions and geochemical evolution of the mantle wedge peridotites preserved in the Late Cretaceous Zagros ophiolites of SW Iran. Mantle peridotites above subduction zones commonly experience distinct melting, depletion and refertilization processes as a result of the circulation of fluids derived from subducting slabs and flux melting. Our results reveal that the mantle wedge peridotites in the Zagros ophiolites are characterized mainly by residual and impregnated types. Residual peridotites resulted from early depletion and later refertilization processes, whereas impregnated peridotites developed due to episodic melt impregnations within and across the mantle. Mg#s and NiO contents, spinel Cr#, Mg#, and TiO 2 in olivines, Mg# and Al 2 O 3 contents of orthopyroxenes, and Mg#, TiO 2 and Al 2 O 3 contents in clinopyroxenes of dunites, harzburgites and lherzolites indicate the significant role of re-equilibration processes among different mineral phases and interactions with basaltic melts percolating within the host peridotites. The observed geochemical variations in the mineral chemistry of the Zagros peridotites reflect changes in magma chemistry and fluctuations in the degree of melt extraction and melt–rock interactions within the mantle peridotites. However, our data suggest that Mg–Fe distribution in the spinels of some dunites and harzburgites might also have resulted from subsolidus redistribution and exchange with surrounding olivine grains. Spinel and clinopyroxene phases in gabbroic rocks and ultramafic cumulates within the Zagros ophiolites also show significant variations in their compositions, suggesting that their magmas evolved from MORB-like to IAT, calc–alkaline and boninite suites, typical of subduction initiation-generated melts. Hence, the Zagros ophiolites present a case study of time-progressive melt evolution of the forearc oceanic lithosphere. Supplementary material: Datasets of the mineral chemistry and melt evolution of the mantle wedge peridotites in the Late Cretaceous Zagros Belt ophiolites (Iran) are available in tabular and figurative form at https://doi.org/10.6084/m9.figshare.c.7093528 . Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Oxygen isotope study of the Asuka-881020 CH chondrite II: Porphyritic chondrules

Oxygen isotope ratios and elemental compositions of porphyritic chondrules and olivine and pyroxene fragments in the Asuka-881020 CH chondrite were analyzed. The oxygen isotope ratios inside individual porphyritic chondrules are homogeneous within the uncertainty, except for relict grains of olivine and low-Ca pyroxene that have distinct oxygen isotope ratios. The average oxygen isotope ratios of the individual chondrules plot along and above the primitive chondrule mineral (PCM) line with Δ17O (=δ17O – 0.52 × δ18O) values from −4.7 ‰ to +4.1 ‰. The olivine and pyroxene fragments, which have Δ17O values ranging from −2.1 ‰ to +3.2 ‰, are likely to be fragments of the porphyritic chondrules.Unlike the non-porphyritic chondrules in CH and CB chondrites and chondrules in other carbonaceous chondrites, the type I and II chondrules do not show a systematic difference in the Δ17O values. Furthermore, the Δ17O values of the type I chondrules increase from −4.7 ‰ to +4.1 ‰ with increasing Mg# (=molar [MgO]/[MgO + FeO] × 100) from 96 to 99. We argue that the positive Δ17O-Mg# trend is explained by an addition of 16O-poor carbon-rich organics as a reducing agent to the relatively 16O-rich precursor silicate, which is a new environment for chondrule formation. This hypothesis is supported by the two lines of evidence observed in the present study. (1) The chondrules and fragments with higher Δ17O values show larger deviations from the PCM line towards low δ18O, suggesting oxygen isotope mass fractionation between the chondrule melt and CO or CO2. (2) Olivine phenocrysts in the chondrules with high Δ17O values contain Ni-poor Fe-metal particles surrounded by silica-rich glass, which may be reduction products during the chondrule formation. Thus, it is suggested that the porphyritic chondrules in CH and CB chondrites have different origins from chondrules in any other chondrite types, even from the non-porphyritic chondrules in CH and CB chondrites.

A Petrological and Microstructural Study of Ten Brachinites: A Cumulate Texture?

AbstractBrachinites are igneous, ultramafic and unbrecciated primitive achondrites mainly composed of olivine and pyroxene, with a partial melt residue or cumulate origin that is still debated. This study presents a petrological and microstructural study of 10 brachinite meteorites to identify igneous and deformation processes responsible for their formation. Detailed microstructural analyses were performed using secondary electron microscopy, electron probe microanalysis, and electron backscatter diffraction. The olivine‐spinel and two‐pyroxene closure temperatures of these brachinites ranged from 814 ± 27 to 909 ± 20°C. Olivine [001] axes coincide with the elongation of olivine grains and the crystallographic preferred orientations (CPO) of clinopyroxene [001] axes, which led us to infer lineation as olivine [001] axes parallel to X for all samples. Olivine CPO displays a strong or medium concentration of [001] axes parallel to X and [010] axes parallel to Z pointing to a B‐type fabric. Limited internal deformation of olivine grains and slip systems identified from low‐angle boundary misorientation analyses are inconsistent with the CPO and grain misorientation distributions are close to those found in untextured rocks. Olivine B‐type fabrics may therefore result not from plastic deformation but rather from rigid crystal rotation or preferential crystal growth during compaction. Olivine subgrain boundary misorientation axes imply limited activation of [001](100) and [100](001) slip systems, which could be due to primary plastic deformation during or after crystal settling. These new results reveal that brachinites developed a cumulate texture and accommodated only weak plastic deformation during early differentiation processes in the parent body.

Potential controls on magmatic Ni-Cu sulfide mineralization in orogenic belts: Constraints from mineral compositions of the Huangshandong and Erhongwa intrusions in eastern Tianshan, NW China

Numerous Permian mafic-ultramafic intrusions are situated in the eastern Tianshan Orogen at the southern margin of the Central Asian Orogenic Belt. These intrusions demonstrate a diverse range of magmatic Ni-Cu sulfide mineralization. Notably, the Huangshandong intrusion hosts a large Ni-Cu sulfide deposit, whereas the Erhongwa intrusion lacks any economically significant sulfide mineralization. Both intrusions exhibit similar lithologies and mainly consist of lherzolite, gabbro, gabbronorite and diorite.To elucidate the controlling factors influencing the disparity in mineralization between two intrusions, analyses were conducted on the compositions of olivine, spinel, pyroxene and amphibole. Olivine grains from the Erhongwa lherzolite have lower Ni (361–943 ppm) contents than those from Huangshandong layered lherzolite (Ni 684–864 ppm) and Huangshandong massive lherzolite (Ni 1218 to 1878 ppm). This discrepancy may be attributed to early sulfide separation occurring at greater depths. Modeling results indicated that for the Huangshandong mantle source, 1 % slab-derived melts and 2 % slab-derived fluids were incorporated; conversely, for the Erhongwa mantle source, approximately 2.5 % slab-derived melts and 0.5 % slab-derived fluids were added.Parental magmas of the Erhongwa intrusion exhibit higher oxygen fugacity and water contents than those of the Huangshandong intrusion. Specifically, the magma of the Erhongwa intrusion undergoes magma evolution within a shallow magma chamber, whereas the magma of the Huangshandong intrusion gains sulfide saturation within a deep magma chamber. This research suggests that factors including mantle source features, suitable timing for sulfide separation, and depth of magma chamber are essential for Ni-Cu sulfide mineralization in mafic-ultramafic intrusions in convergent margin setting.

Isotopic evolution of the inner solar system revealed by size-dependent oxygen isotopic variations in chondrules

The systematic isotopic difference between refractory inclusions and chondrules, particularly for oxygen, has long indicated an isotopic evolution of the solar protoplanetary disk. However, it remains underconstrained whether such evolution continued during chondrule formation. Intrigued by past reports of the size-dependent oxygen isotopic compositions of chondrules in ordinary chondrites (OC), we analyzed type I olivine-rich chondrules of various sizes in two LL3 chondrites. Although most chondrules show positive Δ17O values comparable to bulk ordinary chondrites, a population of smaller (less than about 0.1 mm2 in cross-section), including many isolated olivine grains (sensu lato), are 16O-enriched (with Δ17O values down to −13.2 ‰). Literature data allow the same observation for R chondrites. All sub-TFL type I chondrules (i.e., Δ17O < 0) chondrules have Mg# > 97 mol% while the supra-TFL type I chondrule olivines extend to the formal boundary with type II chondrules (i.e., Mg# = 90 mol%). The sub-TFL chondrules are likely genetically related to isotopically similar aluminum-rich chondrules described in the literature. They therefore must have formed earlier than most OC and R chondrules when the inner disk was still sub-TFL. This interpretation is supported by the presence of similarly 16O-rich relict grains in supra-TFL OC and R chondrules that must be remains of this incompletely recycled precursor material. The non-carbonaceous reservoir was thus still evolving isotopically towards 16O-poorer composition when chondrule formation began, whether by mixing with outer disk material, late accretion streamers and/or an increase in the solid/gas ratio due to magnetothermal disk winds.

Type‐B Crystallographic Preferred Orientation in Olivine Induced by Dynamic Dehydration of Antigorite in Forearc Regions

AbstractThe crystallographic preferred orientation (CPO) of olivine, specifically the type‐B characterized by c‐axes aligned parallel to lineation and b‐axes concentrated perpendicular to foliation, is essential for explaining the trench‐parallel seismic anisotropy in the forearc regions of subduction zones. However, its origin remains a subject of ambiguity and controversy. In this study, we present experimental findings on the formation of a type‐B olivine CPO through the dehydration of foliated serpentinite under a compressive stress at a pressure of 300 MPa and temperature of 700–750°C. Our results reveal a progressive evolution of olivine CPO, transitioning from a type‐C fabric to a type‐B fabric, with increasing grain size and dehydration level. The type‐B CPO observed in coarse‐grained olivine within fully dehydrated samples primarily arises from mechanisms involving anisotropic growth, grain rotation, and oriented coalescence of newly formed, small olivine grains following the decomposition of antigorite under a compressive stress. This study provides the first experimental evidence for a novel, low‐temperature dynamic dehydration mechanism, in contrast to the mechanism of high‐temperature plastic flow, for explaining the development of type‐B olivine CPO in forearc regions. Hence, it contributes significantly to our understanding of the formation of olivine CPO with implications for seismic anisotropy in subduction zone forearcs.

Determination of the Physical Properties of the Protoplanetary Disk Around WW Cha Stars

The main goal of this work is to put a simple model of the spectral energy distribution of binary stars called WW Cha. This model is built up on the extracted data from various telescopes and archives for the target WW Cha stars and then analyzing them using a python environment. The result of the fitting proposes that there are two protoplanetary disks around the WW Cha star, with different physical properties for each disk, such as the size of the inner disk being 10 AU, while the size of the outer disk being 300 AU. The shape of the outer disk is a flaring disk not a flat disk according to the value of the power law for the surface density (1.5). The emission in the disk is caused by small amorphous olivine grains ranging in size from 0.1 to 3000 micrometers.

Diffusion-induced lithium isotopic heterogeneity in olivines from peridotites of an oceanized mantle lithosphere at the Yunzhug ophiolite (central Tibet)

This paper reports lithium concentrations and isotopic compositions of olivines in the oceanized subcontinental lithospheric mantle (SCLM) peridotites of the Tibetan Yunzhug ophiolite. The results show systematic Li isotope changes with distance from the rim of olivine grains. δ7Li values of olivine in dunites decrease from + 10.46 to + 1.33‰ with increasing distance to olivine rim from 26.15 to 124.71 μm. A negative correlation of δ7Li and Li content in olivine from dunite and harzburgite indicates recent diffusive ingress of Li into the peridotites. The extremely heavy Li isotopic composition requires the seawater or seawater alteration endmember in the mixing model, and reveals Li diffusion from seawater into olivine. As in dunites, olivines in a harzburgite sample show similar variations in δ7Li as a function of distance from the grain rim (e.g., 6.01 to 1.73 in sample 14YZ13). We suggest that the behavior of Li in the oceanized SCLM peridotites may be controlled by Li diffusion from seawater, as Li activity in the liquid state is higher than the solid state in transporting Li through the olivines in the peridotites. This study supports that seawater Li diffusion is one of the important factors for the heterogeneity of mantle Li isotopes in ophiolites.

Pliocene to Pleistocene REE-P Metasomatism in the Subcontinental Lithosphere beneath Southeast Asia—Evidence from a Monazite- and REE-rich Apatite-bearing Peridotite Xenolith from Central Vietnam

Abstract Mantle rocks usually contain rare earth elements (REEs) in very low concentrations. Here, we document an occurrence of monazite associated with REE-rich apatites in a carbonate-bearing wehrlite xenolith from central Vietnam. The xenolith displays an equigranular matrix of rounded olivine grains while texturally primary orthopyroxene, clinopyroxene and spinel are notably absent. Scattered within the olivine matrix, two types of domains are present: domain-I contains blocky clinopyroxene grains within a matrix of quenched silicate melt and is associated with a second generation of olivine, small euhedral spinel, and rare grains of carbonates. Domain-II contains irregularly shaped patches of carbonate associated with silicate glass, secondary olivine, spinel, and clinopyroxene. Monazite and apatite occur only in domain-I: very small rounded to elongate monazite I grains are included in primary olivine, partly crosscut by fine glass veinlets, monazite II as large grains up to 300 × 200 μm in size and monazite III as small euhedral and needle-like crystals in silicate glass pools. Apatite I forms lath-shaped to rounded crystals up to 200 × 50 μm in size, whereas apatite II is present within silicate melt pools where it forms euhedral needle-like to equant grains. Monazites show compositional variation mainly with respect to ∑REE2O3 (63–69 wt %) and ThO2 (1.1–5.3 wt %) and only minor variations in P2O5 (29–32 wt %), SiO2 (&amp;lt;0.05–0.4 wt %), and CaO (0.2–0.4 wt %). Apatites are characterized by strongly variable and high REE2O3 and SiO2 contents (4–27 wt % ∑REE2O3, 0.6–6.8 wt % SiO2) as well as with significant Na2O (0.3–1.5 wt %), FeO (0.1–1.8 wt %), MgO (0.2–0.6 wt %) and SrO (0.2–0.9 wt %) contents. F and Cl contents are in the range 1.9–3.0 wt % and 0.2–0.8 wt %, respectively. The textures observed in this wehrlite xenolith are thought to be the result of an interaction of depleted (harzburgitic) mantle with cogenetic silicate and carbonatite melts formed by fractionation-driven immiscibility within a parental SiO2 undersaturated melt characterized by high P, CO2, and REE contents. The immiscibility occurred in the shallow subcontinental lithosphere at T of 700–800 °C and a depth of ~30 km and the melt–rock interaction occurred in two successive and most likely nearly simultaneous events: an initial stage of metasomatism was triggered by the P-REE-CO2-rich agent with low aH2O resulting in the co-precipitation of carbonates as patches and along micro-veins and of phosphates in a peridotitic assemblage. A second stage is characterized by pervasive infiltration of an alkali-rich basaltic melt into the carbonate + phosphate-bearing assemblage. Based on 232Th and 208Pb contents of monazite, a young age of ~2 Ma can be calculated for the timing of the monazite-forming metasomatic imprint. Based on 39Ar-40Ar extrusion ages of the xenolith-hosting alkali basalts of 2.6–5.4 Ma, this indicates that both carbonatite and basaltic melt infiltration must have occurred no more than a few hundred thousand years before extraction of the xenolith to the surface.

Chemical compositions of Fe‐rich relict olivines from cosmic spherules, understanding their links with ordinary and carbonaceous chondrites

AbstractFe‐rich relict olivine grains are found in a small percentage of cosmic spherules, which are studied here to determine the nature of their precursors. We examined 128 Fe‐rich relict olivine grains with Fa &gt;10 mol% from 53 cosmic spherules of different types collected from Antarctica (Antarctica micrometeorites [AMM]) and deep‐sea sediments (DSS) of the Indian Ocean. Fe‐rich olivines identified in cosmic spherules are close analogs of type II chondrule olivines formed in the early solar system. The olivine analysis shows well‐defined trends in molar Fe/Mn versus Fe/Mg with an affinity for ordinary and carbonaceous chondrites. The minor oxides in olivine are in ranges such as MnO ~0.1–0.8 wt%, Cr2O3 ~0–0.7 wt%, CaO ~0–0.6 wt%, and Al2O3 ~0–0.2 wt%, respectively. The chemical composition suggests that the precursors for these Fe‐rich olivine‐bearing cosmic spherules consist of ordinary chondrites (~21%–23%, AMM‐DSS), carbonaceous chondrites (~17%–36%, AMM‐DSS), and a large fraction overlapping both carbonaceous and ordinary chondrites (~41%–62% AMM‐DSS). The elemental ratios Fe/Si/CI and Mg/Si/CI for the Fe‐rich relict olivines ranging between the values 0.5–1.0 and 1.1–1.7 are compatible with IDPs, Comet 81P/Wild 2 as well as the Asteroid Itokawa and Ryugu, which are indistinguishable from carbonaceous and ordinary chondrites. In addition, pyroxene and olivine assemblages in their Fa versus Fs mol% show strong similarities to EOC chondrites. Our results on Fe‐rich relict olivines show that these grains in cosmic spherules are less common than Mg‐rich olivines, which show a narrow range of chemical compositions identical to those from ordinary chondrites and carbonaceous chondrites, indicating a supplementary contribution of an ordinary chondritic component to the micrometeorite source of dust.

Light oxygen isotopic composition in deep mantle reveals oceanic crust subduction before 3.3 billion years ago

Compositional heterogeneity exists in Earth’s deep mantle, which can be caused by the subduction of oceanic slabs. How early this process started on Earth remains highly debated due to the scarcity of early Archean materials with pristine mantle compositional signatures. Here, using the oxygen isotope and elemental compositions of fresh olivine grains in the 3.27-Ga komatiites of the Weltevreden Formation in the Barberton Greenstone Belt in Southern Africa, we discovered two groups of samples with primitive olivine grains. Group I exhibits normal mantle-like δ18O values and high Fo contents (δ18O = 4.9–5.4‰; Fo = 93–95); Group II is characterized by lower δ18O values with slightly lower Fo contents (δ18O = 3.6–4.7‰; Fo = 91–93). These δ18O values correlate with other geochemical proxies of olivine-poor iron-rich pyroxenite sources, indicating that the Weltevreden komatiites were derived from two distinct mantle sources. The existence of the low-δ18O magmas can be best explained by recycling of the altered oceanic crust into deep mantle arguably by subduction, which started 3.3 billion years ago and is responsible for the deep mantle heterogeneity in early Earth.