Clinopyroxene Grains Research Articles

Slip and stress in block-in-matrix shear zones: 1. microstructure and mineralogy of a serpentine-filled dilational jog

We document the structural setting, microstructure, and mineralogy of a crack-seal-type dilational jog that developed in the stepover between two faults cutting through a phacoid of massive serpentinite, itself embedded within a serpentinite shear zone at the base of the Dun Mountain Ophiolite, New Zealand. Our outcrop and microstructural measurements allow us to constrain the boundary conditions for a numerical model (part 2) that quantitatively explores the relationships between stress, fault slip, and incremental cracking in block-in-matrix shear zones. The dilational jog is c. 3 cm wide and contains hundreds of crack-seal bands, each c. 20–30 μm wide. Internally, the jog comprises two mineralogically distinct crack-seal domains: serpentine-only domains and serpentine-andradite garnet domains. Additionally, individual crack-seal bands have a double-layer structure: in serpentine-only domains each band comprises a thin (<2 μm) layer of chrysotile and a thicker layer (c. 25 μm) of polygonal serpentine/lizardite, whereas each band in serpentine + andradite domains comprises a thinner (c. 5 μm) layer of microcrystalline andradite and a thicker layer (c. 15 μm) of polygonal serpentine/lizardite. Micro-CT analysis shows that the serpentine + andradite domains have conic or ellipsoidal shapes with long axes subparallel to the inferred jog opening direction, and that andradite is smeared along micro-transform surfaces inside the jog. Our conceptual microstructural model invokes jog formation during progressive serpentinization of the host rock. Incremental crack opening along the jog-wall rock interface promotes relatively rapid initial precipitation of chrysotile or andradite at high fluid:rock ratios. As cracks fill and pressure re-equilibrates, relatively slow growth of polygonal serpentine/lizardite is favoured until the cracks are sealed and the cycle repeats. Our observations suggest that the precipitation of andradite (instead of chrysotile) was controlled both by structural boundaries within the jog (e.g., micro-transform surfaces) and by local element transport (e.g., Ca from serpentinizing clinopyroxene grains in the host rocks) to patches of the crack wall.

A hydrous sub-arc mantle domain within the northeastern Neo-Tethyan ophiolites: Insights from cumulate hornblendites

Being a part of the large-scale Indus-Tsangpo suture zone ophiolites of the Alpine-Himalayan orogenic system, the Neo-Tethyan ophiolites of eastern Arunachal Himalaya, northeast India, are important in understanding the formation and evolution of the Neo-Tethyan lithosphere. This study focuses on the hornblendite dykes occurring within serpentinized peridotites of the Mayodia ophiolites in eastern Arunachal Himalaya, to ascertain the origin and evolution of such hydrous mineral cumulates within the ophiolite complex. Texturally and chemically, the hornblendites contain three types of amphiboles, Amp1 (pargasite), Amp2 (Mg-hornblende), and Amp3 (actinolite). The pargasites occur as large cm-sized phenocrysts and show cumulate textures while Mg-hornblende appears as both intercumulus grains between cumulus pargasites and surrounding partially resorbed clinopyroxene grains. Actinolite occurs along the breakdown zone of pargasite and is associated with other secondary minerals. The hornblendites are characterized by high MgO (~12–20 wt%) and low SiO2 (~42–44 wt%) with depleted light rare earth elements (REE) [(La/Sm)N = 0.29–0.52] and almost flat heavy REE [(Gd/Yb)N) = 0.89–1.11]. Moreover, the samples are characteristically depleted in high field strength elements (Nb, Ta, Ti, Hf) while being enriched in Pb. Geothermobarometric calculations yield high magmatic temperatures of ~950 and ~840 °C for pargasite and Mg-hornblende, respectively, and variable pressures with pargasite having the highest pressures (1.6–1.7 GPa) followed by Mg-hornblende (1.1–1.2 GPa) and finally actinolite (~0.55 GPa). Furthermore, fO2 calculations for the cumulus pargasites revealed highly oxidizing conditions (ΔNNO+2) and water content of parental melts (H2Omelt% = ~14.25 %). Parental melt calculations using whole-rock REE concentrations suggest formation from a hydrous sub-alkaline basaltic melt. Whole-rock initial SrNd isotopic ratios of the hornblendites reveal a primary signature for them, falling in the mantle array in proximity to the high-μ (HIMU) mantle reservoir. All these signatures point towards a magmatic arc affinity for these hornblendites. The host peridotite unit shows highly enriched REE concentrations (ƩREE = 18.13 ppm) with flat REE patterns compared to the other peridotites of the locality, which show depleted signatures. Olivine chemistry of the host peridotite unit depicts primary compositions falling in the mantle array. The olivine-liquid thermometer yields temperatures ~1000 °C, which are much lower than those of peridotites occurring in spreading centers (MOR or back-arc). These evidence suggest that the hornblendites and their host peridotite unit represent an island arc root complex, which further imply the hydrous nature of the Neo-Tethyan sub-arc mantle.

Physico-chemical alteration of pyroxene in coastal environments: Empirical constraints from New Zealand and the Azores

Developing approaches to determine the modes of transport and weathering of mafic detrital minerals in natural sedimentary environments is critical to our understanding of sediment production, dispersal and provenance, as well as carbon capture under natural and enhanced weathering regimes. We integrated the characterisation of morphological and surface textures with the surface composition of recent detrital clinopyroxene grains concentrated in a sandy coastal area from the western North Island of New Zealand and a rocky shore area of Santa Maria Island in the Azores Archipelago. Using the compactness shape descriptor, 341 grains were subdivided into elongated, elongated angular, euhedral, angular and subangular groups, with each group further characterised using morphological and microtextural indicators of abrasion, breakage and dissolution/chemical weathering. In both studied environments, the clinopyroxenes are dominated by elongated to angular shapes with flat cleavage surfaces and conchoidal fractures. This is consistent with dominant subaqueous transport by rivers and longshore currents for New Zealand, and dominant wave action with limited sediment supply by rivers along the rocky shore of Santa Maria. More abundant subabraded and abraded shapes with bulbous and elongated depression microtextures are observed in New Zealand, which indicates additional effects by aeolian transport that are not seen in Santa Maria. Consistent with semi-quantitative EDS spot analyses that document fresh clinopyroxenes surfaces, chemical weathering textures are very rare to absent. In Santa Maria, sampling of recent beach sand was complemented by ∼125 kyr-old palaeobeach clinopyroxenes, but these yielded similar textural results without evidence for chemical weathering. However, the surface of these older clinopyroxenes includes small adhering smectite (typically <50 μm in width and a few μm in thickness) interpreted to reflect incipient cementation with buffering of acidic fluids by more reactive lithic fragments in the deposits. The lack of chemical weathering but pervasive evidence for mechanical breakage of clinopyroxenes indicate that, at the sand size, these minerals break faster than they dissolve due to frequent subaqueous and/or aeolian reworking in the studied high-energy environments and temperate climatic conditions.

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.

The critical role of deformation-assisted melt migration in the formation of oceanic core complexes

Oceanic core complexes provide an accessible window into deep processes occurring at slow and ultra-slow-spreading mid-ocean ridges. We analyse samples from IODP ocean drilling of core complexes at the Atlantis Bank, Atlantis Massif, and near the Kane Transform at the South West Indian and Mid-Atlantic ridges. We correlate secondary minerals, including oxides, with sites of melt migration. We interpret changes to mineral assemblage and microchemistry, reaction textures and melt-pseudomorph microstructures as fingerprints of open-system melt-mediated processes. This micro-scale information is combined with a macro-scale review of legacy mineral chemistry data to show that melt-fluxed rocks share remarkably similar characteristics across the three core complexes investigated. These are rich in oxides and have olivine, orthopyroxene and clinopyroxene grains that are chemically distinct from oxide-poor gabbros. We propose that oceanic crust fluxed with external melt can be recognised by the following key features: (1) high modes of secondary minerals, such as oxides and olivine, (2) microstructural evidence for the former presence of melt, and (3) mineral chemistry differences between primary and secondary olivine, orthopyroxene, clinopyroxene and plagioclase. Importantly, olivine has previously only been reported as primary, that is, having crystallised from magma. However, our results show that gabbros with secondary olivine are reliable indictors of melt–rock interaction during deformation-assisted diffuse melt migration through the gabbroic oceanic crust. Finally, we propose a new model for the formation of oceanic core complexes where deformation-assisted melt migration plays a critical role in strain localisation, exhumation and evolution of the core complex.

Formation and evolution of lithospheric mantle beneath the Yilan-Yitong fault zone, Northeast China: Constraints from the Shangzhi peridotite xenoliths

Despite numerous studies, the formation and evolution of the lithospheric mantle beneath the Yilan-Yitong fault zone (YYFZ), Northeast China, are still poorly constrained. Here, we use in-situ trace elements and Sr isotopes of clinopyroxene, as well as whole-rock platinum group elements and Re-Os isotopes of the Shangzhi peridotite xenoliths (including spinel lherzolite and harzburgite) within the YYFZ to address the above important issues. Clinopyroxenes in one harzburgite characterized by high Zr/Hf (38.0–59.9), Ca/Al (9.3–13.4), and low Ti/Eu ratios (118–334) can be attributed to carbonatite metasomatism. Clinopyroxenes in some samples that is selectively enriched in light rare earth element exhibits lower Zr/Hf (17.0–46.9), Ca/Al (4.0–7.6), but higher Ti/Eu ratios (2219–4538), which may have been metasomatized by a carbonated silicate melt. The Sr contents and 87Sr/86Sr ratios for these clinopyroxene grains from the Shangzhi peridotites increase gradually from core to rim, suggesting the addition of radiogenic Sr from the metasomatic melts. Such metasomatic melts are likely derived from subducted carbonate-bearing oceanic crustal components. The 187Os/188Os ratios of two groups of the Shangzhi peridotites are positively correlated with Al2O3, forming two nearly parallel lines, which indicate that the lithospheric mantle beneath the Shangzhi area was formed by partial melting of a fertile mantle precursor with variable 187Os/188Os during the Mesoproterozoic time (∼1.4 Ga). The dominance of Mesoproterozoic mantle peridotites in the Shangzhi area suggests that asthenosphere upwelling and lithospheric mantle replacement are not common beneath the YYFZ.

Calcium-Rich Ultramafites, Ankaramites, and Clinopyroxene–Porphyric Gabbro of the Birkhin Massif in the Ol’khon Region: Solution of the Problem of Primary Melt and Formation of Intrusion

Abstract ––The paper presents new data on the chemical and mineral compositions of ultramafic rocks and various gabbro of the Birkhin massif in the Ol’khon region. Porphyric phenocrysts and zoned grains of clinopyroxene have been first found and studied in gabbronorite, which made it possible to reconstruct the entire melt crystallization trend from ultramafic to gabbro parageneses. Similar clinopyroxene trends have been established for clinopyroxenites and subvolcanic ankaramites, whose bodies and dikes have been recently discovered within the massif and in its environment. The total petrological data show that the magnesian high-Ca ankaramite melt corresponds in composition to the assumed primary melt for the Birkhin massif.

Surface textures of detrital pyroxenes in coastal dune sands (western Gulf of Mexico, Mexico): Implications for their preservation and geoenvironmental processes

AbstractThis work investigates how the surface textures and morphology of pyroxene grains evolve during their source‐to‐sink history. This study applies to detrital clinopyroxenes concentrated in coastal dune sands of the Gulf of Mexico which were sourced in the Trans Mexican Volcanic Belt then transported and deposited in environments subject to limited chemical weathering. The composition and morphology of the pyroxenes was characterised using single‐grain geochemical analysis and surface texture imagery with a novel approach based on the compactness property to assess the shape of minerals. This reveals heterogenous diopside‐augite populations, displaying mineral morphologies dominantly controlled by impact breakage along cleavages, little physical abrasion along their edges and with limited evidence for chemical weathering. Mechanical surface textures dominate over mechanical/chemical and chemical surface textures. These mechanical surface textures are preserved primarily as flat cleavage surfaces and rounded edges inherited from fluvial‐intertidal and aeolian transport, respectively. Mechanically/chemically induced surface textures are preserved as elongated depressions. Chemical surface textures are sparse and mostly represented by mammillated textures that suggest local dissolution under subaqueous conditions. The scarcity of chemical surface textures is attributed to frequent fragmentation of the clinopyroxenes along cleavages and limited chemical weathering during transport of the observed populations. Clinopyroxene grains in the coastal dune sands primarily retain surface characteristics from fluvial transport. Although the breakage of minerals along cleavages can obscure their original morphology under a weathering‐limited erosion regime, this study shows how surface textures and morphology of pyroxene grains is used to determine episodes of transport and deposition close to volcanic environments. The use of the compactness property as a shape descriptor measurement of particles provides an alternative approach to observe how clinopyroxene remains unaltered despite the high energy conditions of the coastal area.

Chemical composition of detrital chrome spinel and clinopyroxene from the Middle Eocene to lowermost Oligocene sandstones of central Iran (Nain area): Constraints on ophiolite provenance

Studies of relict Upper Cretaceous obducted ophiolites provide information on the evolution of the Neotethys suture belts in central Iran. Studies of the sediment derived from their erosion will complement this information by addressing questions related to sandstone provenance, ophiolite emplacement history, and, perhaps most importantly, the mode and timing of their erosion. The ca. 3200 m thick Middle Eocene to lowermost Oligocene fine- to medium-grained siliciclastic beds of the Akhoreh Formation are superbly exposed along the western border of the Central-East Iranian Microcontinent. They cover the northeastern flank of the Cretaceous Nain ophiolite mélange massif and lie adjacent to the Paleogene Urmieh–Dokhtar Magmatic Arc. The common occurrence of chrome (Cr)-spinel in the sandstones indicates the presence of mafic to ultramafic rocks in the source region. However, the timing, extent and nature of contributions of the potential source rocks to the sedimentary succession are not well understood. We conducted a petrographic and chemical compositional study of Cr-spinel in samples of Akhoreh Formation sandstones to outline the type of ocean lithospheric source. Our results document variable Cr# (0.09 to 0.79) and Mg# (0.21 to 0.88) while Fe3+ and TiO2 (average 0.12 wt%) are generally low. The wide range of Cr# and Mg# indicates provenance from a variety of (ultra-)mafic rock types, including supra-subduction peridotites and partially volcanic rocks of an arc-forearc setting. Cr-spinels are chemically similar to those of the lherzolites, harzburgites and dunites of the Nain ophiolite. While 70 % of the Cr-spinel have Cr# > 0.6, suggesting a harzburgitic peridotite provenance, the minimum Cr# < 0.4, observed in the samples from the base of the succession, were likely sourced from lherzolite. The chemical composition of detrital clinopyroxene grains shows high SiO2 (52.76 wt%), CaO (20.15 wt%), and MgO (16.36 wt%), while FeO has moderate (6.03 wt%), Al2O3 has low (2.23 wt%), and TiO2 has very low (0.25 wt%) concentrations (commonly augite and diopside), indicative of crystallization from subalkaline magma in an arc setting. The provenance data trace the erosion history of the obducted ophiolite and indicate that the Nain supra-subduction ophiolite supplied detritus to the Akhoreh Basin northwards from the Middle Eocene to the earliest Oligocene.

Formation of amphibole lamellae in mantle pyroxene by fluid-mediated metasomatism: A focal plane array FTIR study from the Carpathian-Pannonian region

Abstract Amphiboles in the upper mantle (most frequently pargasitic in composition) have recently gained attention due to their role in the water budget and potential control on the rheology and physical discontinuity of layers of the mantle. Although nominally anhydrous minerals are often analyzed with Fourier transform infrared (FTIR) spectroscopy, amphiboles, especially in natural samples, are only rarely the focus of such studies because of their complex structure and variable composition. In mantle xenoliths, amphibole occurs not only interstitially or forming veins, but also as lamellae within orthopyroxene and/or clinopyroxene grains. The genesis of such lamellae is often ambiguous, as they could either be meta-somatic products, or form by exsolution without an external H2O source upon decreasing P-T conditions and consequent destabilization of hydrous point defects in the host pyroxene. To constrain the origin of amphibole lamellae in pyroxenes, we studied mantle xenoliths from the Carpathian-Pannonian region (CPR) by applying hyperspectral imaging using an FTIR equipped with focal plane array (FPA) detector. Amphibole lamellae are absent in xenoliths of the central part of the CPR, but appear in those from the marginal localities that represent a well-hydrated supra-subduction mantle environment. Some of the lamella-hosting pyroxene grains are in contact with interstitial amphibole, suggesting that the formation of the lamellae is related to the amphibole-producing metasomatism; however, others have no adjacent amphibole. To determine the origin of the amphibole lamellae in pyroxenes without neighboring amphibole, hyperspectral images were used to give an estimation of their volume proportion (0.8–5.1 vol%) in the pyroxenes. Using these volume proportions, we calculated that a bulk water content of ~330–670 (orthopyroxene) and ~740–1430 (clinopyroxene) wt. ppm is needed to be contained in the host grain to be able to facilitate subsolidus exsolution of the observed amount of amphibole lamellae. These water contents are, however, too high for mantle pyroxenes, even for an aqueous-fluid saturated upper mantle. This suggests that the formation of amphibole lamellae is related to a metasomatic event with fluid input from an external water source (e.g., melt/fluid inclusion or metasomatic agent on the grain boundary).

The Microstructural Record of Convection in the Little Minch Sill Complex, Scotland

Abstract Detailed microstructural analysis of three basaltic sills of the Little Minch Sill Complex demonstrates that convection leaves a detectable signature in fully solidified bodies. The presence of dense clusters of equant grains of olivine and clinopyroxene in the central parts of sills can only be accounted for if they formed and were enlarged while suspended in convecting magma, with delayed settling to the sill floor. An associated stratigraphic invariance of plagioclase grain shape is consistent with growth while suspended in convecting magma. These microstructural indicators demonstrate that convection during solidification was vigorous and long-lived in the 135-m-thick picrodolerite-crinanite unit (PCU) of the composite Shiant Isles Main sill and vigorous and likely short-lived in the PCU of the composite Creagan Iar sill. In contrast, convection in the Meall Tuath sill was weak and short-lived: plagioclase grain shape in this sill varies with stratigraphic height, indicative of primarily in situ nucleation and growth at the magma-mush interface, while olivine and clinopyroxene were kept suspended in the overlying convecting magma. The magma in all three sills fractionated during solidification, permitting convection driven by the instability of an upper thermal boundary layer. The comparative vigour and longevity of convection in the Shiant Isles Main sill and the Creagan Iar sill was due to their emplacement above an earlier, still-hot, intrusion, resulting in highly asymmetric cooling.

Petrogenesis of mafic rocks from the Xigaze ophiolite, Southern Tibet: Insights into forearc extension induced by Neotethyan rollback

The Xigaze ophiolite in the central Yarlung Zangbo suture zone (southern Tibet) provides evidence of the tectonic processes that formed the Neotethyan Ocean between the Indian and Eurasian plates. In this study, we present zircon U Pb geochronological, mineralogical, and geochemical data for mafic rocks from the Xigaze ophiolite to determine their petrogenesis and tectonic environment. The crustal section of the Xigaze ophiolite includes mainly gabbro lenses, amphibolite dikes, dolerite sills or dikes, and basaltic lavas. The absence of a well-developed gabbroic crust and sheeted dike complexes in the ophiolite indicates slow to ultraslow-spreading conditions and a heterogeneous structure of the oceanic lithosphere, which is not atypical for layered ophiolites. Zircons from the amphibolite and dolerite dike reveal crystallization ages of 125.0 ± 1.8 Ma to 123.8 ± 1.3 Ma, respectively. Geochemically, all mafic rocks have mid-ocean ridge basalt (MORB)-like affinities with enrichment in large ion lithophile elements (LILEs, U) relative to high field strength elements (HFSEs, Nb, Ta and Ti) and light rare earth elements (LREEs). They also exhibit high εNd(t) values ranging from +8.4 to +8.8 at 206 Pb/ 204 Pb = 16.75–17.64. Moreover, chromite and clinopyroxene grains from the mafic rocks have a wide compositional spectrum and are mainly plotted in the supra-subduction zone (SSZ) fields. These features indicate that MORB-like magmatism was triggered by decompression melting of the upwelling asthenospheric mantle with minor mass transfer from the subducting slab. The Xigaze ophiolite lacks boninite, high‑magnesium andesites, island arc tholeiites, and calc-alkaline lavas, which indicates that it is incompatible with the complete Izu-Bonin-Mariana forearc sequence. Combined with previous studies, we speculate that the Xigaze ophiolite represents an ancient SSZ-type oceanic lithosphere formed in the Gangdese forearc during the Early Cretaceous, and the upper plate extension is linked to the southward retreat and rollback of the Neotethyan slab. Thus, the Xigaze and other Neotethyan SSZ ophiolites with no systematic change in composition with stratigraphic age provide new insights into the diversity of melt evolution in the forearc crust. • Amphibolite and dolerite dikes in the Xigaze ophiolite were formed during the Early Cretaceous. • The mafic rocks were derived from upwelling asthenosphere with minor slab input. • The Xigaze ophiolite originated from the Gangdese forearc setting. • The SIR model appears likely to be oversimplified for variable forearc ophiolites.

Protogenetic clinopyroxene inclusions in diamond and Nd diffusion modeling—Implications for diamond dating

Abstract Diamonds are witnesses of processes that have operated in Earth's mantle over more than 3 b.y. Essential to our understanding of these processes is the determination of diamond crystallization ages. These cannot be directly determined on diamond, but they can be calculated using radiogenic isotopic systematics of suitable minerals included in a diamond. This method relies on the assumption that the mineral inclusions were in isotopic equilibrium with the diamond-forming medium. We evaluated the validity of Sm-Nd ages yielded by clinopyroxene inclusions by combining crystallographic orientation analyses and Nd diffusion modeling at the relevant conditions for Earth's cratonic mantle. We investigated the crystallographic orientation relationships (CORs) for 54 clinopyroxene inclusions within 18 diamonds from South Africa and Siberia. Clinopyroxene inclusions in some diamonds showed specific CORs with their hosts, indicating possible syngenesis. Other samples had clusters of clinopyroxene inclusions sharing the same orientation but no specific orientation relative to their hosts, indicating that the inclusions are older than the diamond (i.e., they are protogenetic). Diffusion modeling in the temperature range typical for lithospheric diamonds (900–1400 °C) showed that resetting of the Sm-Nd isotopic system in clinopyroxene grains larger than 0.05 mm requires geologically long interaction with the diamond-forming fluid/melt (&amp;gt;3.5 m.y. at average temperature of ~1150 °C). Depending on inclusion size and temperature regime, protogenetic clinopyroxene inclusions may not fully reequilibrate during diamond-formation events. We suggest that small clinopyroxene inclusions (&amp;lt;0.2 mm) that equilibrated at temperatures higher than 1050–1080 °C may be the most suitable for age determinations.

Mineralogical, Textural and Chemical Characteristics of Ophiolitic Chromitite and Platinum Group Minerals from Kabaena Island (Indonesia): Their Petrogenetic Nature and Geodynamic Setting

This contribution presents the first systematic mineralogical study of chromite composition, silicates and PGM (platinum group minerals) by electron microprobes in the podiform chromitite of Kabaena Island (Indonesia) mined in the past. The main target of this study is to understand the petrogenetic nature of the parental melt from which the chromitites of Kabaena Island precipitated and, indirectly, define the geodynamic tectonic setting of their emplacement. The evolution of PGM, from the magmatic stage to low-temperature processes, is also discussed. The variation of the Cr# = Cr/(Cr + Al), being comprised between 0.65 and 0.75, is similar to the podiform-type chromitite and indicates the absence of Al-rich chromitite. The calculated composition of the parental melt varies from arc to MORB (mid-ocean ridge basalts). Several grains of olivine and clinopyroxene have been found in the silicate matrix or included in fresh chromite. Olivine shows a composition typical of a hosted mantle, and clinopyroxene is similar to those analyzed in the forearc of an SSZ (supra-subduction zone). Small PGM, varying in size from 1 to 10 μm, occur in the chromitites. The most abundant PGM is laurite, which has been found included in fresh chromite or in contact with ferrian chromite along the cracks in the chromite. Laurite forms polygonal crystals, and it occurs as a single phase or in association with clinopyroxene and amphibole. Tiny blebs of Ir-Os alloy (less than 2 μm across) have been found associated with grains of awaruite in the serpentine gangue of the chromitites. The composition of the investigated chromitites suggests that they formed in the mantle of a forearc ophiolite. All the discovered grains of laurite are considered to be magmatic in origin, i.e., entrapped as solid phases during the crystallization of chromite at temperatures above 1000 °C and a sulfur fugacity below sulfur saturation. Iridium–osmium alloys are secondary in origin and represent a low-temperature, around 400 °C, exsolution product.

Kinetic controls on Sc distribution in diopside and geochemical behavior of Sc in magmatic systems

Scandium is important in modern technology and is regarded as a strategic metal in many countries. It is highly dispersed in Earth’s crust and rarely forms independent minerals. Clinopyroxene is the most important Sc-bearing mineral in some world-class deposits hosted in mafic–ultramafic intrusions, which are also the major source of laterite-hosted Sc deposits. However, the factors controlling Sc distribution in minerals have been little explored, impeding the understanding of the geochemical behavior of Sc and why it is common in some clinopyroxene grains. The newly discovered Mouding Sc deposit in SW China is hosted in a zoned intrusion composed, from core to rim, of monzogabbro, syenogabbro, gabbro, magnetite clinopyroxenite, and clinopyroxenite. Clinopyroxene in the intrusion is diopsidic in composition with high Sc contents (80–105 ppm). In-situ trace element mappings of diopside crystals reveal homogeneous, zoned, swallow-tailed, and hourglass internal Sc distribution patterns. These patterns can be produced through kinetically controlled incorporation of Sc on different crystal faces. The preferential substitution of Sc can take place on the {1 0 0}, {1 1 0} and {0 1 0} prism faces because of the high flexibility of the octahedral M1 protosites. The fast growth of diopside, which facilitates kinetically controlled crystallization, is dominated by textural coarsening and promoted by the hydrous parental magmas with low viscosities and active convection. The active flow and efficient interstitial communication of the magma can direct compatible elements from the magma into clinopyroxene, thus favoring formation of Sc-rich grains. Our study provides a feasible way to study intra-grain variations of Sc in minerals and emphasizes that kinetic effects may play a critical role in Sc distribution and enrichment in hydrous magmatic Sc deposits. We also show that disequilibrium crystallization may be more pervasive than previously thought, and the hourglass zoning of clinopyroxene can provide valuable information on this process.

Compositional Evolution of Interstitial Liquid After Onset of Abundant Fe–Ti Oxide Crystallization in Crystal Mush: Insights from Late-Stage Microstructures and Mineral Compositions of the Bijigou Layered Intrusion, Central China

ABSTRACT Compaction is important to the expulsion and compositional modification of interstitial liquid of crystal mush in the solidification of layered intrusions hosting Fe–Ti oxide mineralization. However, the compaction manner and compositional evolution of interstitial liquid after the onset of abundant Fe–Ti oxide crystallization in crystal mush are still enigmatic. In this study, we collected samples from a drill core profile across the major Fe–Ti oxide-rich segment of the Bijigou intrusion, a large and differentiated layered intrusion in Central China, and carried out a detailed study on the late-stage magmatic microstructures of rocks and crystal size distributions (CSDs) and spatial distribution patterns (SDPs) of clinopyroxene and plagioclase to examine the compaction manner of interstitial liquid. The Fe–Ti oxide-rich segment is composed of, from the base upward, a Fe–Ti oxide ore layer (~80 m), a troctolite unit (~105 m), an oxide gabbro unit (~153 m) and an apatite–oxide–gabbronorite unit (~165 m). The CSD curves for both plagioclase and clinopyroxene grains of the samples from the Fe–Ti oxide ore layer display gentle slopes for small crystals and consistent slopes for large crystals. In addition, there is a positive correlation of the SDP R-value (ratio of observed and predicted nearest neighbor distance of plagioclase/clinopyroxene) and mineral mode except plagioclase/clinopyroxene of the samples. The interstitial liquid was likely expelled by intensive pressure solution concomitant with crystallization of abundant Fe–Ti oxides, evidenced by the positive correlation of aspect ratio and alignment factor of plagioclase and the low fraction of trapped liquid (FTL) of 4–6% for the Fe–Ti oxide ore layer. The cumulus plagioclase of the Fe–Ti oxide ore layer was partially dissolved during pressure-solution compaction, leading to ambient Fe-rich interstitial liquid becoming Al-enriched in local places so that abundant amphibole and hercynite are present in the replacive symplectites adjacent to cumulus plagioclase. Above the Fe–Ti oxide ore layer, plagioclase grains in the samples from the troctolite unit show consistent slopes of the CSD curves and negative correlation of SDP R-value and mineral mode in the SDP plot, indicating that the interstitial liquid was mainly expelled by mechanical compaction, with FTL ranging from 7 to 9%. The CSD curves for clinopyroxene grains of the samples from the oxide gabbro unit have been modified by Ostwald ripening, but the expulsion of interstitial liquid in this unit is also attributed to mechanical compaction due to higher compaction rate than accumulation rate. This study demonstrates that the compaction manner of interstitial liquid can change from pressure solution to mechanical compaction with dramatically diminishing crystallization of Fe–Ti oxides. The pressure-solution compaction accompanying with large amounts of Fe–Ti oxides can remarkably modify the composition of interstitial liquid. This study also demonstrates that a combined measurement of CSDs and SDPs for cumulus minerals can be used to quantify the compaction manner of interstitial liquid during solidification of layered intrusions.

The formation of high-Sr/Y plutons in cordilleran-arc crust by crystal accumulation and melt loss

Bulk-rock data are commonly used in geochemical studies as a proxy for melt compositions in order to understand the evolution of crustal melts. However, processes of crystal accumulation and melt migration out of deep-crustal, crystal-rich mush zones to shallower storage regions raise questions about how faithfully bulk-rock compositions in plutons approximate melt compositions. This problem is particularly acute in the lower crust of arcs, where melt reservoirs are subject to periodic melt extraction that leaves behind a cumulate residue. Here, we examine bulk-rock data from the perspective of high-Sr/Y plutonic rocks in the lower crust of a well-exposed Early Cretaceous cordilleran-arc system in Fiordland, New Zealand. We test the validity of using high-Sr/Y bulk-rock compositions as proxies for melts by comparing bulk-rock compositions to melts modeled from &amp;gt;100 major- and trace-element analyses of 23 magmatic clinopyroxene grains from the same samples. The sampling locations of the igneous clinopyroxenes and encompassing bulk rocks are distributed across ~550 km2 of exhumed lower crust and are representative of Mesozoic lower-crustal arc rocks in the Median batholith. We confirm that bulk-rock data have characteristics of high-Sr/Y plutons (Sr/Y &amp;gt;50, Na2O &amp;gt;3.5 wt%, Sr &amp;gt;1000 ppm, and Y &amp;lt;20 ppm), features that have been previously interpreted to indicate the presence of garnet as a residual or fractionating phase. In contrast to bulk rocks, igneous clinopyroxenes have low Sr (&amp;lt;100 ppm), high Y (25–100 ppm), and low molar Mg# [100 × Mg/(Mg + Fe)] values (60–70), which are consistent with derivation from fractionated, low-Sr/Y melts. Chondrite-normalized rare-earth-element patterns and Sm/Yb values in clinopyroxenes also show little to no evidence for involvement of garnet in the source or in differentiation processes. Fe-Mg partitioning relationships indicate that clinopyroxenes are not in equilibrium with their encompassing bulk rocks but could have been in equilibrium with melt compositions determined from chemometry of coexisting igneous hornblendes. Moho-depth calculations based on bulk-rock Sr/Y values also yield Moho depths (average = 69 km) that are inconsistent with Moho depths based on bulk-rock Ce/Y, contact aureole studies, Al-in-horn- blende crystallization pressures, and our modeled clinopyroxene crystallization pressures. These data indicate that most Mesozoic high-Sr/Y bulk rocks in the lower crust of Fiordland are cumulates formed by plagioclase + amphibole + clinopyroxene accumulation and interstitial melt loss from crystal-rich mush zones. Our data do not support widespread fractionation of igneous garnet nor partial melting of a garnet-bearing source in the petrogenesis of these melts. We speculate that melt extraction and the production of voluminous cumulates in the lower crust were aided by unusually high heat flow and high magma addition rates associated with an Early Cretaceous arc flareup. We conclude that bulk-rock compositions are poor proxies for melt compositions in the lower crust of the Median batholith, and geochemical modeling of these high-Sr/Y bulk rocks would overemphasize the role of garnet in their petrogenesis.

Mineral Chemistry and Trace Element Composition of Clinopyroxenes from the Middle Cambrian Ust’-Sema Formation Ankaramites and Diopside Porphyry Basalts and the Related Barangol Complex Intrusions, Gorny Altai, Russia

The origin and geodynamic settings of the Ust’-Sema Formation and the Barangol Complex are some of the most controversial issues in the Early Paleozoic history of the Altai–Sayan Fold Belt. The Ust’-Sema Formation volcanic rocks are enriched in high-Ca clinopyroxene phenocrysts and were classified as ankaramites and diopside porphyry basalts. In this work, we first present LA-ICP-MS analyses of the clinopyroxenes, along with studies of the petrography, mineral composition, and whole-rock chemistry of the Ust’-Sema Formation and related Barangol Complex rocks. An LA-ICP-MS clinopyroxene study showed a slight depletion of light rare-earth elements (La/Yb)N = 0.1–1.0 (on average 0.4); and strong depletion of the high-field-strength elements (Zr, Hf, and Nb) and large-ion lithophile (Rb) elements. An Sr anomaly showed a positive correlation with Mg#. Major- and trace-element composition of the clinopyroxene cores show that these clinopyroxene grains were not captured from the mantle rocks as previously assumed and that the Ust’-Sema Formation and the Barangol Complex rocks were formed from magma with island arc characteristics. The increased titanium and light rare-earth element contents in the phenocryst rims from Biyka volcano suggest an active interaction of the ankaramitic magma with rocks or melts of OIB type.

Geochemical variability among stratiform chromitites and ultramafic rocks from Western Makran, South Iran

The geochemical compositions of minerals from the Moho transition zone of ophiolites potentially can help to understand the magmatic evolution of the ophiolites, and subsequent mantle-melt interactions. The Jurassic-Late Cretaceous Makran ophiolite of south Iran comprises one of the most extensive tracts of oceanic crust which were scraped off and preserved in an accretionary complex. The Makran ophiolite records traces of MORB-OIB-type magmatism during the Jurassic, but mostly supra-subduction zone magmatic activity during the Late Cretaceous. Despite a few geochemical studies on the crustal rocks, the nature and geochemical signatures of the mantle rocks from this ophiolite remain controversial. The Sorkhband mantle-crust transition zone underlying crustal cumulates in the western Makran consists of stratiform chromitites, harzburgites, chromite-rich dunites and dunites, with crosscutting dikes of olivine websterite and olivine clinopyroxenite. Major- and trace-element compositions of clinopyroxene grains in olivine websterite and clinopyroxenite dikes indicate crystallization from melts similar to boninites and low-Ti fore-arc basalts. Spinel compositions in olivine websterite and clinopyroxenite dikes suggest crystal fractionation from boninitic or high- Mg# magmas have played a major role in the genesis these rocks.We propose a two-stage model for the formation of the Sorkhband dunites including (1) supra-subduction zone-related melt infiltrates through harzburgites in the mantle-crust transition zone to dissolve peridotite orthopyroxene and leave dunites with high forsterite-NiO olivines, and (2) boninitic melts accumulate and react with surrounding harzburgites to crystallize cumulate dunites with low-Mg# olivine and high-Ti spinels. We conclude that there have been temporal changes in the composition of mantle melts in the fore-arc mantle section of the Makran ophiolite during the initial subduction of the Neotethyan ocean beneath the Lut block during the Late Cretaceous.

Mineralogical and Re-Os isotope constraints on fluid– and melt–rock interactions and the origin of mantle peridotites from the Amdo ophiolite, northern Tibet

Ophiolites from the Bangong–Nujiang Suture Zone in the central part of the Tibetan Plateau represent relics of Neo-Tethyan oceanic lithosphere. Here we present the results of a detailed mineralogical and Os isotopic study of mantle peridotites of the Amdo ophiolite, which provide new insights into the origin of the peridotites and their diverse melt/fluid–peridotite reactions. The Amdo ophiolite comprises mantle peridotites, mafic–ultramafic cumulates, massive gabbros, diabases, and pillow basalts. The mantle peridotites are mainly refractory harzburgites, composed of olivine (Ol, 65%–85%), orthopyroxene (Opx, 12%–35%), clinopyroxene (Cpx, <4%), and Cr-spinel (Cr-Spl, ~2%). Most olivine grains (Ol-I) are protogranular, locally enclosed in orthopyroxene and Cr-spinel, and have high Fo values (91.1–92.9) and NiO contents (0.29–0.44 wt%), typical of residual origin. In contrast, interstitial olivines (Ol-II) have low Fo values (67.2–84.7) and NiO contents (<0.05 wt%), but extremely high MnO contents (0.28–1.65 wt%), suggesting metasomatism by carbonate-bearing fluids. Hypidiomorphic-granular clinopyroxene grains (Cpx-I) have high Mg# values (~94.5) and low Al2O3 (average 1.26 wt%) and Na2O (<0.19 wt%) contents, typical of residual phases after melting. However, clinopyroxene coronas (Cpx-II) around orthopyroxene have higher Na2O contents (0.24–1.06 wt%), indicative of mantle metasomatism by melts. Osmium isotopic compositions in the harzburgites vary from subchondritic to suprachondritic (187Os/188Os ratios: 0.11716–0.13284). The subchondritic Os isotopic compositions (0.11716–0.12367) correspond to TRD model ages of 0.76–1.56 Ga, suggesting Mesoproterozoic mantle depletion events. On the other hand, several harzburgites with low Os (0.29–0.86 ppb) and suprachondritic 187Os/188Os ratios (0.12823–0.13284) reflect the overprinting of original Os budgets and Os isotopic compositions by percolating melts or fluids. The combined mineralogical and Os isotopic compositions suggest that the Amdo peridotites represent a mixture of ancient subcontinental lithospheric mantle (SCLM) and Mesozoic oceanic lithosphere, which were subsequently infiltrated by carbonate-bearing fluids and silicate melts in a supra-subduction zone.