Melt-rock Interaction Research Articles

Fabrics and Origin of Troctolites in the Keketoukeleke Ultramafic–Mafic Complex, South Altyn Tagh, Northwest China

ABSTRACTTroctolite is relatively rare compared with other ultramafic–mafic rocks, but its origins and rheological deformation are significant for understanding melt–rock interactions and fractional crystallization of mafic magmas. The Keketoukeleke ultramafic–mafic complex in the South Altyn Tagh, northwest China, consists mainly of dunite, coarse‐ and fine‐grained troctolite. Based on petrographic observations, major and trace element variations of the dunites and troctolites, and low olivine Zr/Y and Ti/Y ratios, we propose that the troctolites were formed by fractional crystallization and late localized water‐bearing melt injection. In addition, the olivine in the dunites has A‐ and E‐type fabrics, whereas the olivine in the troctolites has a weak fabric and plagioclase has a pronounced fabric with the [010] axes aligned subnormal to the foliation and [100] axes subparallel to the lineation. The results suggest that the troctolite rheological deformation was concentrated mainly in plagioclase, and the olivine only underwent grain rotation or grain boundary slip. Furthermore, the dating of baddeleyite in troctolite suggests they crystallized at 378.6 ± 2.3 Ma, suggesting the Keketoukeleke ultramafic–mafic complex emplaced the continental crust extension after deep subduction–exhumation process of South Altyn.

Zircon Coupled Dissolution–Precipitation Replacement During Melt–Rock Interaction Modifies Chemical Signatures Resulting in Misleading Ages

ABSTRACTMelt migration through Earth's crust drives well‐documented melt–rock reactions, locally changing rock assemblage and geochemistry. However, melt–zircon interaction remains understudied. We report on three zircon‐melt interaction events from the Pembroke Granulite, New Zealand. Primary zircon from gabbroic gneiss which was subject to minor post‐emplacement melt migration and primary zircon from younger dykes exhibit straightforward microstructures, microchemistry, and age data. In contrast, zircon from melt‐mediated reaction halos adjacent to the dykes and from melt‐fluxed high‐strain zones display dissolution modification of grains, micro‐porosity and blurred or truncated internal zoning typical of replacement by coupled dissolution‐precipitation. Replaced zircon domains show changed rare earth element patterns and redistributed or lost radiogenic Pb that generates ambiguous apparent spot date arrays, smeared over tens of millions of years. We conclude that the metamorphism and three melt–rock interaction events were brief, and the arrays misrepresent the true age and duration of the metamorphism. Pb‐loss persisted beyond the metamorphism, with porosity and inclusions formed during coupled dissolution‐precipitation making replaced zircon domains more susceptible to subsequent Pb‐loss compared to the structurally intact, primary magmatic zircon in the host gabbroic gneiss or dykes. We recommend conducting high‐resolution microstructural investigations upon recognition of spot date arrays observed in single samples to rule out the possibility of spurious arrays resulting from coupled dissolution‐precipitation.

Mesoproterozoic depleted spinel peridotites metasomatized by high-K hydrous melt in the Patagonian back-arc

Here we report the first investigation of spinel-bearing mantle xenoliths from Los Gemelos volcano, Canquel Plateau, Patagonia. They are highly-depleted Mesoproterozoic (ReOs model ages of 1.3–0.9 Ga) harzburgites and clinopyroxene-poor lherzolites characterized by typical indicators of partial melt extraction, such as low whole-rock Al2O3 and CaO contents (<1.5 wt%), high Mg# of silicate phases (90–95), and Cr-rich spinel (Cr# 0.20–0.42). Depletion of incompatible highly siderophile elements (Re, Ru, Pd) relative to the primitive upper mantle supports high degrees of melt extraction. The occurrence of phlogopite and K-rich alkaline glass veins with high- and low-SiO2 compositions is clear evidence of modal metasomatism. The light rare earth element (LREE) enrichment in clinopyroxene, as well as the whole-rock U-shaped REE pattern, confirms cryptic metasomatism. This melt-rock interaction is corroborated by major (i.e., Mg# vs. basaltic elements) and trace (i.e., La/YbN vs. Sr/Y and Ti/Eu; and Sr vs. Ti/Eu) element contents of pyroxenes. The calculated compositions of melts in equilibrium with clinopyroxene coexisting with phlogopite suggest interaction with slab-derived materials. This metasomatism has been generated by a chromatographic fractionation-reaction process, from deep to shallow mantle domains. The percolation of a high-K hydrous magma probably is associated with the upwelling of asthenospheric material through a slab-window, which caused partial melting of oceanic crust and overlying sediments during the Paleocene. The varying intensities of metasomatic imprints recorded by mantle xenoliths from Los Gemelos provide valuable insights into the interaction of slab-derived materials near the lithosphere-asthenosphere boundary, at a distance of ∼600 km from the Andean volcanic arc.

Deformation and melt–rock interaction in the upper mantle: Insights from the layered structure of the Horoman peridotite, Japan

To obtain a better understanding of melt–rock interactions in the upper mantle, microstructural and petrological analyses were conducted on deformed mantle peridotites from the Horoman peridotite complex, Hokkaido, Japan. The Horoman peridotite complex is lithologically heterogeneous and contains various kinds of ultramafic and mafic rocks. We studied an outcrop of 3 × 70 m in size that contains layered spinel harzburgite, plagioclase lherzolite, and mafic rocks. The results indicate that reactive melts migrated preferentially along the foliation in the already deformed peridotite, and that these melt-rich zones became especially prone to further deformation. This inference is supported by (1) the parallelism of the boundaries of rock layers and foliation in the deformed peridotite, and the shape and crystallographic preferred orientations (SPOs and CPOs) of olivine in the peridotites; (2) the diffusive trends of magnesium and modal compositions of pargasite grains near the boundaries between peridotite and mafic layers; (3) variations in the NiO content of olivine crystals; (4) variations in olivine CPOs with orthorhombic (010)[100] slip system patterns and weak fiber-[010] patterns; and (5) the strong pargasite SPOs, the cuspate shapes of the pargasites, and the absence of intercrystallite deformation. The results, combined with previously reported P–T conditions for the Horoman peridotite complex, indicate that the deformed peridotites and mafic rocks with a layered structure represent temperatures of 1050–1150 °C and pressures of 0.7–1.5 GPa. Our results suggest that a decrease in pressure led to the transition from a melt-free to a melt-bearing system with a consequent change in the deformation mechanism, from dislocation creep in the melt-free system to diffusion creep in the melt-bearing system, with strain localization in the fine-grained melt-rich layers. The change in deformation mechanism is likely to have occurred in the uppermost mantle beneath a mid-ocean ridge, where strong rheological contrasts are controlled by spatial variations in the melt fraction.

Composite Melt–Rock Interactions in the Lowermost Continental Crust: Insights from a Dunite–Pyroxenite–Gabbronorite Association of the Mafic Complex from the Ivrea–Verbano Zone (Italian Alps)

Abstract The processes leading to the building of the continental crust through magmatic underplating are fundamentally unknown, mainly because of the rare accessibility to deep level sections of the continental crust. The Italian Alps expose the Permian Mafic Complex, an 8-km-thick gabbronorite–diorite batholith that intruded the lower continental crust during the post-Variscan transtensional tectonics. We present here a petrological and geochemical study of a concentric dunite–pyroxenite–gabbronorite association, called Monte Mazzucco sequence, enclosed at deep levels of the Mafic Complex, thereby allowing us to provide new insights into the magmatic processes driven by emplacement of mantle melts in deep crustal continental areas. The studied sequence includes a ~ 60-m-thick dunite lens, in which olivine (82 mol % forsterite) is associated with accessory Cr-spinel including blebs and lamellae made up of magnetite. The dunite lens is permeated by mm- to cm-scale thick magmatic veins, which range in composition from hornblende lherzolite to olivine hornblendite and hornblende websterite. The lens is mantled by a m-scale ring consisting of amphibole-bearing (≤1 vol %) websterite, and the websterite ring is in turn enclosed by amphibole-free gabbronorites. Both magmatic veins within the dunites and mantling websterites typically include an oxide association of Al-spinel and magnetite. Remarkably, the hornblende websterite veins and the mantling websterites are typically plagioclase-free and include clinopyroxene and amphibole with chondrite-normalized rare earth element patterns characterized by negative Eu anomaly. The mantling websterites display a subtle, gradual outward decrease of Mg# for orthopyroxene, clinopyroxene and accessory olivine, coupled with an increase of the negative Eu anomaly in clinopyroxene and amphibole. The enclosing gabbronorites are amphibole-free and have a chemically evolved signature. We propose a petrogenetic scenario including two major events of melt–dunite interaction. The first resulted from focused reactive melt infiltration and formed the magmatic veins within dunites. The hornblende lherzolite and the olivine hornblendite veins were produced by focused reactive melt migration through dunite grain boundaries, involving dissolution of olivine and recrystallization of Cr-spinel into Al-spinel and magnetite, whereas the hornblende websterite veins crystallized from melts penetrating through narrow fractures and recording earlier plagioclase fractionation. Most likely, the infiltrating melts were overall derived from an evolving H2O-rich magma emplaced below the dunite body. The second event of melt–dunite reactive interaction developed the websterite ring around dunites. We envision that the outermost domain of the dunite body was replaced by websterites in response to reaction with an invading H2O-poor melt, which had previously undergone plagioclase fractionation. The dunite replacement occurred under dynamic conditions, which promoted the reaction progress, thereby leading to total or almost total dissolution of precursor olivine, and started to form the lens shape of the Monte Mazzucco ultramafic association. The gabbronorites closely adjacent to the websterite ring represent the crystallization products of the invading melt.

Evidence for large-scale, long-term highly siderophile element heterogeneities in the Atlantic mantle from Leg 153 and 209 peridotites

Seafloor abyssal peridotites are products after partial melting and melt-rock interaction, providing constraints on upper mantle processes and heterogeneity. Despite extensive studies of ophiolitic peridotites and a burgeoning dataset for dredged abyssal peridotites, limited studies have been undertaken on abyssal peridotite drill core samples. This omission means a lack of comparison between modern ocean mantle lithosphere and that of ophiolites, where field context is possible in the latter, but not the former. Ocean Drilling Program (ODP) Leg 153 near the Kane fracture zone, and ODP Leg 209, near the Fifteen-Twenty fracture zone, both on the mid-Atlantic Ridge but ∼1000 km distant from one another, recovered serpentinized mantle peridotites. We report whole-rock major-, trace- and highly siderophile-element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundances and 187Os/188Os ratios of peridotites from three cores (maximum depth of 200.8 m) from the two Legs to explore mantle heterogeneity within the Atlantic Ocean basin. Peridotites from ODP Leg 153 are relatively fertile, with Al2O3 contents ranging from 1.1 to 2.7 wt%. They are characterized by relatively radiogenic 187Os/188Os (0.1259 ± 0.0017; 2 S.D.) and uniform HSE abundances. The ODP Leg 209 peridotites are highly refractory, with Al2O3 contents ranging from 0.1 to 0.8 wt%, and imply that they record an extensively depleted mantle. They have 187Os/188Os of 0.1207 ± 0.0023 (2 S.D.) and fractionated HSE abundances, indicative of depletion occurring > 1 billion years ago. Marked heterogeneity both in the extent and timing of depletion are therefore evident along a large segment of the mid-Atlantic ridge that produces zero-age basalts. Measurement of Os isotopes in spatially associated mid-ocean ridge basalts (MORB) is challenging due to their sub ng/g Os contents. Instead, Nd isotopes of MORB are used to show that they are unlikely to have formed directly from abyssal peridotite-like sources. Abyssal peridotites therefore represent long-lived refractory residues during mantle upwelling with the sources of MORB being more enriched. Comparison between drilled abyssal peridotites and ophiolitic mantle reveals larger compositional variations in the latter. The overall higher melting extent in ophiolite mantle is likely reflective of a more complex life cycle than the modern oceanic lithosphere.

Nature and evolution of lithospheric mantle beneath the Northeast China: Constraints from the Houtuancun mantle xenoliths

Mantle xenoliths entrained in a newly discovered Cenozoic basalt at Houtuancun within the Yilan-Yitong fault zone, situated beneath the Xing'an Mongolian Orogenic Belt in Northeast China, provide a unique opportunity to constrain the nature and evolution of the lithospheric mantle beneath this region. The mantle xenoliths are predominantly spinel lherzolite, with rare occurrences of harzburgite, wehrlite, and websterite. The Houtuancun spinel lherzolite and harzburgite are characterized by high forsterite contents in olivines (89.3–91.1), and their clinopyroxene rare earth element patterns range from slightly light rare earth element (LREE)-depleted to LREE-enriched, reflecting variable degrees of partial melt extraction (up to 20%) overprinted by later metasomatism. Despite this, the Fe isotopic composition of the Houtuancun lherzolite and harzburgite exhibits little variation (δ57/54Fe = −0.05–0.09‰), with an average of 0.02 ± 0.09‰ (2SD, n = 9), close to the suggested δ57/54Fe value of 0.04 ± 0.04‰ for the upper mantle, and there is no significant inter-mineral disequilibria. In contrast, the wehrlite has a slightly lower Mg# [=100 ╳ Mg2+/(Mg2+ + Fe2+)] (88.4) and δ57/54Fe (−0.05‰) compared to the spinel lherzolite and harzburgite, reflecting kinetic iron fractionation during melt-rock interaction. The websterite exhibits the lowest MgO (20.6–24.8 wt%), highest Al2O3 content (5.8–7.3 wt%), and the highest δ57/54Fe (0.25–0.28‰) among the studied xenoliths, suggesting an episode of subduction-related silicate melt circulation through the upper mantle. The results demonstrate compositional heterogeneity of the mantle beneath the Xing'an Mongolian Orogenic Belt in Northeast China. The observed heterogeneity may result from a combination of varying degrees of melt extraction of the subcontinental lithospheric mantle and metasomatism by melts related to the subduction of the (Paleo)-Pacific plate.

Origin and multi-episodic melt/fluid interactions revealed by the subducted serpentinites in the North Qilian Belt, NW China: Implications for the compositional heterogeneity of the fossil oceanic mantle

In nature, serpentinites act as reservoirs of water and fluid-mobile elements and play an essential role in arc magmatism, fluid metasomatism, and elemental circulation in subduction zones. This paper presents new mineral and whole-rock geochemical analyses of the Qingshuigou serpentinite massif and discusses its petrogenesis, tectonic setting, and mantle metasomatism. The chemical composition (e.g., REE, Y, Mg, and Al) and petrological observations (e.g., pseudomorph and hourglass textures) of serpentines demonstrate that the serpentinites’ protoliths belong to the peridotites. Their degrees of partial melting were estimated to be 15–20 % using Yb, Cr, Ti, and HREEs, indicating that these peridotite protoliths were residual in origin after high degrees of mantle melting. They were relics of the subducted lithospheric mantle in the subduction zone, based on their geochemical similarities with subducted serpentinites. Notably, the Qingshuigou serpentinites show several geochemical fingerprints of melt-rock interactions, such as the U-shaped REE patterns, elevated Zr/Ti, Zr/Hf, and Nb/Ta ratios; this led us to propose that the peridotite protoliths of the serpentinites were not merely melting remnants but were significantly modified by melt metasomatism. Also, they experienced two stages of fluid/rock interactions in the subduction zone: early-stage serpentinization at shallow depths and lizardite/chrysotile to antigorite transition at an elevated depth. Combined with the published data of ophiolitic peridotites from the North Qilian Belt, the compositional heterogeneity of these peridotites and associated serpentinites might be associated with the nature of melt/fluid metasomatism under different geodynamic backgrounds.

Melt-rock interaction as a factor controlling evolution of chromite and olivine in dunite - case study from the Kukes Massif (Mirdita ophiolite, Albania)

Melt-rock interaction as a factor controlling evolution of chromite and olivine in dunite - case study from the Kukes Massif (Mirdita ophiolite, Albania)

Fluid–Melt–Rock Interaction during the Transition from Magmatism to HT-UHT-Granulite- and Amphibolite-Facies Metamorphism in the Ediacaran Adrar–Suttuf Metamafic Complex, NW Margin of the West African Craton (Southern Morocco)

Abstract Underplated mafic intrusions ponded at the base of the lower continental crust in extensional settings can experience ultra-high-temperature (UHT) granulite-facies metamorphism during tens of My due to slow cooling rates. These intrusions are also the source of heat and carbonic fluids for regional high-temperature (HT) granulite-facies metamorphism in the continental crust. This work analyses the fluid–melt–rock interaction processes that occurred during the magmatic to HT-UHT-granulite- and amphibolite-facies metamorphic evolution of high-grade mafic rocks from the Eastern Ediacaran Adrar–Suttuf Metamafic Complex (EASMC) of the Oulad Dlim Massif (West African Craton Margin, Southern Morocco). P–T conditions were determined using Ti-in-amphibole thermometry, two-pyroxene and amphibole–plagioclase thermobarometry, and phase diagram calculations. The thermobarometric study reveals the presence of tectonically juxtaposed lower- and mid-crustal blocks in EASMC that experienced decompression-cooling paths from, respectively, UHT and HT granulite-facies conditions at ca. 1.2 ± 0.28 GPa and 975 ± 50°C, and ca. 0.82 ± 0.15 GPa and 894 ± 50°C, to amphibole-facies conditions at ca. 0.28 ± 0.28 GPa and 787 ± 45°C (precision reported for the calibrations at 1 s level). An age for the magmatic to UHT granulite-facies metamorphic transition of 604 Ma was constrained from published SHRIMP Th–U–Pb zircon ages of the igneous protoliths. An amphibole 40Ar–39Ar cooling age of 499 ± 8 Ma (precision at 2 s level) was obtained for the lower-crustal blocks. Amphibole 40Ar–39Ar closure temperatures of 520–555°C were obtained for an age range of 604–499 Ma and an average constant cooling rate of 4.2°C/My, suggesting that the lower-crustal blocks cooled down to the greenschist–amphibolite facies transition in ca. 100 My. During the high-temperature stage, interstitial hydrous melts assisted textural maturation of the rock matrix and caused incongruent dissolution melting of olivine and pyroxenes, and, probably, development of An-rich spikes at the grain rims of plagioclase, and local segregation of pargasite into veins. Subsequent infiltration of reactive hydrous metamorphic fluids along mineral grain boundaries during cooling down to amphibolite-facies conditions promoted mineral replacements by coupled dissolution-precipitation mechanisms and metasomatism. Ubiquitous dolomite grains, with, in some cases, evidence for significant textural maturation, appear in the granoblastic aggregates of the high-grade mafic rocks. However, calculated phase relationships reveal that dolomite could not coexist with H2O–CO2 fluids at HT-UHT granulite- and low-medium P amphibolite-facies conditions. Therefore, it is proposed that it may have been generated from another CO2-bearing phase, such as an immiscible carbonatitic melt exsolved from the parental mafic magma, and preserved during cooling due to the prevalence of fluid-absent conditions in the granoblastic matrix containing dolomite. The lower-crustal mafic intrusions from EASMC can represent an example of a source of heat for granulitisation of the mid crust, but a sink for carbon due to the apparent stability of dolomite under fluid-absent conditions.

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

Exploring the tectono-magmatic evolution of intraoceanic fore-arc setting during subduction initiation: perspectives from trace and platinum group element systematics of the Jijal ultramafic arc system, NE Pakistan

ABSTRACT The Jijal Complex is considered one of the largest Neo-Tethyan remnants in the intra-oceanic Cretaceous–Palaeogene Kohistan Arc of Pakistan. This complex largely consists of gabbros, peridotites, and chromitites. Previous studies documented the genesis of gabbros and chromitites in detail; however, detailed geochemistry and magma genesis were lacking. In this investigation, we present whole-rock geochemical and platinum group elements (PGEs) data of Jijal peridotites (dunites and harzburgites). The objectives of this study are to elucidate magmatic processes and tectonic regimes involved in the formation of peridotites, factors influencing the concentration and dispersion of PGEs in them, and their tectonic evolution. The investigated peridotite samples are depleted in magmaphile major elements (Al2O3: 0.23–0.57, TiO2: 0.01–0.04, and CaO: 0.08–0.69 wt%), relative to the primitive mantle values, reflecting melt-rock interaction during their magmatic evolution. In addition, rare-earth element (REE) contents in harzburgites are relatively higher compared to the dunites; however, both rock types exhibit depletion with respect to the chondrite-normalized values. The obvious negative anomalies of Nb, Zr, and enriched LREE and large ion lithophiles collectively substantiate their association with depleted arc-type mantle components, metasomatism, and melt/fluid-rock reactions after partial melting episodes. Bulk-rock and geochemical modelling of PGEs data of the peridotites suggest their shallow depth spinel-bearing depleted heterogeneous mantle source and generation from refractory mantle residuum along with boninitic signatures in a supra-subduction zone (SSZ). The PGE’s geochemistry additionally reveals that partial melting, fractionation, metasomatism, and sulphur under-saturation were the key factors that controlled the concentration and distribution of PGEs in the studied rocks. The boninitic features of the Jijal peridotites are equated with intraoceanic fore-arc followed by subduction initiation, and melt-rock reactions in a hydrated forearc mantle, indicating robust evidence of mantle depletion and pervasive refertilisation in an embryonic Neo-Tethyan arc system. Tectonically, the investigated rocks encapsulate vestiges of the Cretaceous Tethyan Ocean and record multifaceted history from boninitic to slab-proximal island arc affinity in compliance with an intraoceanic SSZ fore-arc regime coherent with the subduction inception.

Mineral carbonation of peridotite fueled by magmatic degassing and melt impregnation in an oceanic transform fault

Most of the geologic CO2 entering Earth's atmosphere and oceans is emitted along plate margins. While C-cycling at mid-ocean ridges and subduction zones has been studied for decades, little attention has been paid to degassing of magmatic CO2 and mineral carbonation of mantle rocks in oceanic transform faults. We studied the formation of soapstone (magnesite-talc rock) and other magnesite-bearing assemblages during mineral carbonation of mantle peridotite in the St. Paul's transform fault, equatorial Atlantic. Clumped carbonate thermometry of soapstone yields a formation (or equilibration) temperature of 147 ± 13 °C which, based on thermodynamic constraints, suggests that CO2(aq) concentrations of the hydrothermal fluid were at least an order of magnitude higher than in seawater. The association of magnesite with apatite in veins, magnesite with a δ13C of -3.40 ± 0.04‰, and the enrichment of CO2 in hydrothermal fluids point to magmatic degassing and melt-impregnation as the main source of CO2. Melt-rock interaction related to gas-rich alkali olivine basalt volcanism near the St. Paul's Rocks archipelago is manifested in systematic changes in peridotite compositions, notably a strong enrichment in incompatible elements with decreasing MgO/SiO2. These findings reveal a previously undocumented aspect of the geologic carbon cycle in one of the largest oceanic transform faults: Fueled by magmatism in or below the root zone of the transform fault and subsequent degassing, the fault constitutes a conduit for CO2-rich hydrothermal fluids, while carbonation of peridotite represents a vast sink for the emitted CO2.

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.

Contrasting Cu isotopes in mid-ocean ridge basalts and lower oceanic crust: Insights into the oceanic crustal magma plumbing systems

Magma plumbing systems exert strong controls on the formation and evolution of the oceanic crust during crust accretion. However, plumbing system dynamics based largely on mid-ocean ridge basalts (MORBs) are extremely difficult to constrain because the MORBs are aggregated melts and original information may have been blurred. Copper isotopic compositions of lower crustal gabbroic cumulates effectively archive early-stage histories because sulfides constitute the dominant Cu budget of gabbroic cumulates and are largely isolated from subsequent magma filtering processes. Here, we present Cu isotopes of a suite of gabbroic cumulates from the ultraslow-spreading Southwest Indian Ridge (Hole U1473A), and MORBs from the South Mid-Atlantic Ridge and East Pacific Rise. The δ65Cu of the MORBs from this study (+0.04 ‰ to +0.19 ‰) match those of previous MORB analyses, indicating uniform δ65Cu at various spreading rates. In contrast, the δ65Cu of gabbroic rocks vary significantly (−1.14 ‰ to 0.87 ‰), exceeding by far the range known for peridotites. The Cu budget of these gabbroic rocks is hosted in sulfides and the mantle-like S isotopic compositions of sulfides indicate their origin of igneous processes. These results thus suggest that magma accumulation and sulfide segregation would lead to notable Cu isotopic fractionation in the lower oceanic crust. We propose that repeated recharging of primitive melts and efficient mixing of melts within the plumbing system can buffer the removal of early 63Cu-rich sulfides from sulfide-saturated MORBs. The discrepancy in δ65Cu between lower oceanic crust and MORBs is a natural consequence of continuous replenishment that occurs concurrently with melt-rock interaction, mixing, crystallization and extraction, irrespective of the oceanic settings. The consistent δ65Cu in MORBs (0.09 ± 0.08 ‰) and komatiites (0.06 ± 0.06 ‰) further indicates that their Cu isotopes reflect the mean mantle source composition, providing a robust δ65Cu for bulk silicate Earth (BSE) of 0.08 ± 0.08 ‰ (2sd). Accordingly, we propose that in open sub-ridge plumbing systems, other incompatible element isotopes and ratios of elements with similar incompatibility in MORBs as Cu isotopes suggest, could also represent mean mantle source compositions.

Whole-rock and petrological properties of mantle peridotites from Kızıldağ (Hatay) ophiolite, tracing melt-rock interaction process

Kızıldağ ofiyoliti (G-Türkiye) Neotetis okyanusunun güney kolunda oluşmuş okyanusal litosfer kalıntısı kayaçlar topluluğu olarak bilinmektedir. Ofiyolit istifi tabandan tavana doğru manto peridotitleri, ultramafik-mafik kümülatlar, izotrop gabrolar, levha dayk kompleksi ve yastık şekilli bazaltik örtü kayaçları şeklindedir. Manto peridotitleri harzburjit bileşimindedir ve değişen oranlarda serpantinleşme gözlenmektedir. Manto peridotitlerinin tüm kayaç ana oksit-iz element ve Lantanit Grubu Element (LGE) içerikleri harzburjitlerin değişen derecelerde kısmi ergime kalıntısı kayaçlar olduğuna işaret etmektedir. Harzburjitler düşük tüm-kayaç Al ve Ca kompozisyonlarına sahiptir ve SSZ peridotitlerin tüketilmiş karakterini yansıtmaktadır. Tüketilmiş manto harzburjitleri, bazı büyük iyon yarıçaplı elementler (LILE) ve hafif lantanit grubu elementler (HLGE) bakımından zenginleşme gösterdiği tespit edilmiştir ve bu durum yitim sırasında meydana gelen ergiyik manto etkileşimine işaret etmektedir. Harzburjitlerin tüm kayaç MgO içerikleri ile CaO ve Al2O3 içerikleri kıyaslandığında bu kayaçların %20-30 arası kısmi ergime kalıntısı kayaçlar olduğu görülmektedir. Ayrıca kısmi ergime sırasında uyumsuz davrandığı bilinen Ti ve Yb içeriklerindeki zenginleşmeler ergiyik-kayaç etkileşimine işaret etmektedir.

Basalts from MAR at 13°15′–13°40′N: What mixed?

We present new data on chemical composition of basalt glasses collected within the geochemical anomaly of the Mid-Atlantic ridge at 13°25′–13°40′N, where fresh quenched lavas occur dominantly in asymmetric off-axial structures. The collected samples indicate wide compositional ranges from strongly depleted D-MORB with 0.04 wt% K2O, K2O/TiO2 = 0.08 and (La/Sm)N = 0.69 to the most enriched E-MORB with 0.92 wt% K2O, K2O/TiO2 = 0.42 and (La/Sm)N = 2.45. Clear correlations are traced between incompatible element ratios (K2O/TiO2, La/Sm, Nb/Zr), concentrations of MgO, K2O, H2O, and 87Sr/86Sr ratio, suggesting variable contribution to the compositional diversity of the glasses from two distinct source materials, one of which is depleted mantle. Testing the hypothesis of pyroxenite-derived melts as an enriched end-member failed to reproduce the observed REE patterns, (Sr/Zr)N, and (Ba/Nb)N ratios, and the K2O–K2O/TiO2 relations. Two alternative hypotheses – melts derived from melting remnants of continental crust (1) or oceanic core complex (OCC) gabbro (2) – are suggested based on the complementary character of the REE patterns of the basalt glasses and partial melts compositions calculated at 1000 °C/50 MPa from either bulk continental crust as estimated by Taylor and McLennan (1995) (1) or OCC gabbro sampled in the study area (2). Complementarity of the two sources with the D-MORB holds also for the other geochemical parameters: (Sr/Zr)N vs (Ba/Nb)N, (La/Sm)N vs (Rb/Nb)N, and for the K2O–K2O/TiO2 relations. The first hypothesis calls upon partial melting of continental crustal material preserved in the vicinity of ridge-transform fault system, while the second suggests melt-rock interaction within the off-axial structures of the pre-existed gabbro assemblages, with concurrent melting of the wall-rock material and dissolution of the resulting melt in the intruded magma. Both scenarios, operating either separately or jointly, might contribute to the MORB geochemical variability documented in the present study.

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.

Trace elements in abyssal peridotite olivine record melting, thermal evolution, and melt refertilization in the oceanic upper mantle

Trace element concentrations in abyssal peridotite olivine provide insights into the formation and evolution of the oceanic lithosphere. We present olivine trace element compositions (Al, Ca, Ti, V, Cr, Mn, Co, Ni, Zn, Y, Yb) from abyssal peridotites to investigate partial melting, melt–rock interaction, and subsolidus cooling at mid-ocean ridges and intra-oceanic forearcs. We targeted 44 peridotites from fast (Hess Deep, East Pacific Rise) and ultraslow (Gakkel and Southwest Indian Ridges) spreading ridges and the Tonga trench, including 5 peridotites that contain melt veins. We found that the abundances of Ti, Mn, Co, and Zn increase, while Ni decreases in melt-veined samples relative to unveined samples, suggesting that these elements are useful tracers of melt infiltration. The abundances of Al, Ca, Cr, and V in olivine are temperature sensitive. Thermometers utilizing Al and Ca in olivine indicate temperatures of 650–1000 °C, with variations corresponding to the contrasting cooling rates the peridotites experienced in different tectonic environments. Finally, we demonstrate with a two-stage model that olivine Y and Yb abundances reflect both partial melting and subsolidus re-equilibration. Samples that record lower Al- and Ca-in-olivine temperatures experienced higher extents of diffusive Y and Yb loss during cooling. Altogether, we demonstrate that olivine trace elements document both high-temperature melting and melt–rock interaction events, as well as subsolidus cooling related to their exhumation and emplacement onto the seafloor. This makes them useful tools to study processes associated with seafloor spreading and mid-ocean ridge tectonics.

The water-poor and reduced mantle beneath the Neo-Tethys Ocean: Tectonic setting of the western Yarlung-Tsangpo ophiolites

Water contents and deformation behaviors of minerals in mantle peridotites are diagnostic features to distinguish the physical conditions between mid-ocean ridges and subduction zones, which could serve as effective indicators for discriminating the tectonic setting of ophiolites. In this study, we report a systematic dataset of petrology, geochemistry, water contents and deformation behaviors for the mantle peridotites from the Cuobuzha ophiolite in the western Yarlung-Tsangpo suture zone (YTSZ), southern Tibet, to provide new insights into the long-standing debate for the tectonic setting of ophiolites therein. Geochemical data revealed that the Cuobuzha mantle peridotites experienced low to moderate degrees of partial melting (∼10–15%), although some harzburgites have been intensively modified by late-stage melt-rock interactions. The Cuobuzha peridotites, on the other hand, are characterized by low orthopyroxene water contents (46–95 wt ppm in average), clear olivine crystallographic preferred orientations of A- and E-types, and low oxygen fugacity (logfO2, QFM −2.55 to −0.81, where QFM means quartz-magnetite-fayalite buffer), thus indicative of the occurrence of high-temperature, water-depleted, and reduced mantle in this ophiolite. The features above of the Cuobuzha mantle peridotites are quite similar to those of abyssal peridotites at modern mid-ocean ridges and systematically different from the relatively refractory, water-rich, and oxidized characteristics typical of forearc peridotites in suprasubduction zones. Based on the data from this study and a compiled dataset for other ophiolites from the western YTSZ, we revealed that mantle peridotites with a water-poor and reduced nature are common in these ophiolites, thus providing strong evidence supporting for their origination from a mid-ocean ridge of a mature ocean basin. From the perspective of mantle peridotites, the ophiolites in the western YTSZ now geographically distributed in two branches (i.e., the northern and southern sub-belts) might have formed in a single ocean basin of the Neo-Tethys and were dismembered by late-stage tectonic events to their current locations.