Melt-rock Interaction Research Articles

Retrieving timescales of oceanic crustal evolution at Oceanic Core Complexes: Insights from diffusion modelling of geochemical profiles in olivine

The building of oceanic crust at Oceanic Core Complexes (OCC) has been described as a complex process involving multiple intrusions of magma over a protracted period of time. The migration of primitive magmas (i.e., Mid Ocean Ridge Basalts, MORBs) can lead to melt-rock interactions during reactive porous flow processes through the lithosphere. The timescales of these reactive processes and the subsequent cooling of the modified crystal matrix remain unconstrained. Diffusion modelling has been widely used to retrieve timescales of magmatic processes. In this study, we use diffusion models to constrain (i) the minimum timescales of melt-rock interactions and (ii) the cooling rates of the gabbroic sequence forming the oceanic crust at an OCC. We chose samples of the most primitive olivine-rich troctolites from the gabbroic sequence sampled in IODP Hole U1309D (Atlantis Massif OCC, Mid-Atlantic Ridge 30°N). Olivine-rich troctolites were interpreted as marking local partial assimilation of mantle intervals into the oceanic crust, and thus allowed to understand the dynamics of mantle assimilation and the formation of slow-spreading oceanic crust at the Atlantis Massif OCC. Olivines in olivine-rich troctolites represent relicts of pre-existing mantle olivine, while clinopyroxenes and plagioclases are crystallized during reactive percolation. Olivine chemical compositions show that olivine-rich troctolites inherit chemical heterogeneity from the mantle precursor. Flat geochemical profiles in olivine indicate complete chemical re-equilibration of olivine crystals with the locally modified percolating melt. Exception is made for most Ca profiles that show lower Ca contents at the olivine rim compared to the relative crystal core, as the result of subsolidus cooling. Three-dimensional (3D) diffusion models at magmatic conditions (T = 1210–1300 °C and P = 2 kbar) reveal that complete chemical re-equilibration of 4 mm-size mantle-derived olivine with percolating MORB-type melts can be attained within durations of less than 300 yr. The Ca-in-olivine geospeedometer reveal that cooling rates from ~1200 °C to ~1050 °C are constant downhole and on average 0.004 °C/yr; they are comparable with lower temperature cooling rates (850 °C–250 °C) estimated at the Atlantis Massif OCC. The minimum timescales from 3D models point to rather fast re-equilibration of olivine. The downhole chemical heterogeneity inherited from the precursor mantle, coupled with the timescales of diffusive re-equilibration suggest that the partial assimilation of the upwelling mantle and its incorporation into the oceanic crust occurred in the time-frame of a single melt input. Our cooling data are consistent with building of the oceanic crust at OCCs controlled by continuous uplift, in turn governed by long-lived detachment faults. The latter contribute to the rapid cooling of the assimilated mantle intervals and the magma bodies. Diffusion models of geochemical profiles in olivine from a single crustal section allow to reconstitute the early magmatic processes leading to mantle assimilation and early crystallization of gabbros, and the cooling history of the oceanic crust at OCC from magmatic conditions to hydrothermalism.

Tracking the magmatic response to subduction initiation in the forearc mantle wedge: Insights from peridotite geochemistry of the Guleman and Kızıldağ ophiolites, Southeastern Turkey

The initiation of subduction is associated with sequential magmatic responses that lead to the formation of the forearc lithosphere, yet the detailed characteristics of these magmatic activities are not well constrained. Here we use mineral chemistries and bulk-rock trace-element contents of highly-depleted harzburgites from the Guleman and Kızıldağ ophiolites in Southeast Turkey to examine mantle wedge melting dynamics during subduction initiation. We focus on how different components from the subducting slabs potentially contribute to various stages of magmatism throughout the process. Mineral and bulk-rock compositions of these harzburgites are significantly different from those of abyssal peridotites, suggesting that our harzburgites cannot be explained as residues of anhydrous adiabatic melting and melt-rock interaction at mid-ocean ridges alone. This implies that the petrogenesis of SE Turkey harzburgites involves additional processes and components. Harzburgites with the most depleted heavy-rare earth element (HREE) contents are the ones with the highest abundance of strongly incompatible elements, which can be explained by open-system processes where the peridotites in the mantle wedge experienced melting and infiltration of enriched components simultaneously. Open-system dynamic melting models with continuous flux of sediment-derived melts can account for the observed correlation, but are numerically too low compared to the measured values. Based on the observed fractionation between Zr, Hf, and elements with similar incompatibility (middle REEs), we hypothesized the involvement of amphibolite-derived melt and modeled its numerical trace-element contents. Binary mixing between this hypothetical melt and residues of the former open-system model can coherently account for the majority of the obtained trace-element data. This indicates that magmatic events during subduction initiation likely involve multiple components and occur in multiple stages, and that melt-mantle interaction plays a significant role in oceanic forearc lithosphere formation. Based on our model, we suggest the high (Zr/MREE)N signatures in some boninites and depleted harzburgites found in modern forearcs and ophiolites could be inherited from amphibolite-derived melts. Moreover, the existence of slab melts agrees with current constraints on the reconstructed geothermal gradients during subduction initiation based on the petrology and geochemistry of metamorphic soles.

Contrasting compositions between phenocrystic and xenocrystic olivines in the Cenozoic basalts from central Mongolia: Constraints on source lithology and regional uplift

AbstractTwo Cenozoic prominent features are spatio-temporally associated in central Mongolia, i.e., the continental basalts and regional uplift, but their genesis and relationship remain unclear. This study presents major- and trace-element compositions for olivine phenocrysts and xenocrysts, as well as data of bulk-rock geochemistry and Sr-Nd-Hf isotopes for the host basalts. The studied basalts mostly have trachybasalt compositions with high total alkali (Na2O + K2O = 5.1–8.2 wt%) contents and all display OIB-like trace element patterns (e.g., spikes of Ba, Nb, and Ta and troughs of Th and U) and EM1-like Sr-Nd-Hf isotopic compositions. Compared to the partial melting products of mantle peridotite, these basaltic samples have higher FeO/MnO, Zn/Mn, and Zn/Fe ratios. Meanwhile, phenocrystic olivines are characterized by lower Ca, Mn, Mn/Zn, and Mn/Fe but higher Ni than their counterparts in the peridotitic melts, indicating a pyroxenite-rich mantle source. The above geochemical data suggest that the source of the studied basalts was mainly made up of secondary pyroxenite produced by the reaction of recycled oceanic crust with its ambient mantle peridotite. The calculated magma oxygen fugacities (ΔFMQ-0.26 to +0.42) and mantle melting temperatures (1343–1430 °C) do not support a genetic link with the stagnant Pacific slab or with a deep mantle plume. Instead, the far-field effect of India-Eurasia convergence possibly tapped the upper asthenospheric mantle, subsequent melting of which gave rise to the dispersive Cenozoic basalts. On the other hand, the xenocrystic olivines exhibit zoned textures with high-Fo (up to 92) cores and low-Fo (down to 76) rims, reflecting the melt-rock interaction. Preservation of zoned olivine xenocrysts indicates rapid magma ascent and widespread melt-rock reaction in the mantle lithosphere, which may modify the rheology and accelerate the mechanical erosion of mantle lithosphere. Consequently, mass deficit in the lithosphere could have caused isostatic uplift of central Mongolia in the Cenozoic.

Ultra-depleted melt refertilization of mantle peridotites in a large intra-transform domain (Doldrums Fracture Zone; 7–8°N, Mid Atlantic Ridge)

The Doldrums transform system offsets the Equatorial Mid Atlantic Ridge by ~630 km at 7–8° N. This transform system consists of four intra-transform spreading centers (ITRs) bounded by five transform faults. The northernmost ITR is linked to the MAR axis by a ~ 180 km-long transform. Here, during two R/V A. N. Strakhov expeditions (S06 and S09), mantle peridotites were dredged along the transverse and median ridge of the transform, across the western flank of the ITR valley. Residual harzburgites were mainly sampled along the northern Doldrums transform valley, whereas plagioclase-bearing peridotites showing evidence for melt-rock interaction characterize the ITR domain. Petrological and geochemical observations reinforced by geochemical modelling are used to define the behaviour of trace elements during melt extraction and melt-rock reaction in our rocks. Results suggest that residual peridotites derive from mantle rocks that have undergone a degree of partial melting up to 12%, with melting likely starting at the transition of garnet-spinel stability fields, whereas peridotites which suffered melt-rock reactions have been divided into two types: (i) pl-impregnated peridotites, formed by migration of melts at high porosity and high melt-rock ratio; and (ii) refertilized peridotites, generated at reduced porosity, when small fractions of the same percolating melt crystallized clinopyroxene and minor plagioclase. We suggest that the refertilizing agent was a melt highly depleted in incompatible trace elements, in turn produced by an ultra-depleted mantle source. This mantle experienced previous degrees of melt extraction at the ridge axis, before being transposed laterally along the transform where it melted a second time during the opening of the intra-transform spreading segment.

Diverse origins of pyroxenite xenoliths from Yangyuan, North China Craton: implications for the modification of lithosphere by magma underplating and melt-rock interactions

We present detailed major-, trace-element and Sr-Nd-Mg isotopic geochemistry for a suite of pyroxenite xenoliths entrained in Oligocene basalts in Yangyuan county, North China Craton, to reveal their origins and to further constrain the evolution of the lithospheric mantle beneath this area. The Yangyuan pyroxenites exhibit a lithologic spectrum ranging from orthopyroxenite, through orthopyroxene-rich websterite and clinopyroxene-rich websterite to clinopyroxenite. Their equilibrium temperatures (874–975 °C), which are within the temperature range of the upper mantle and lower crust, suggest that they came from the crust-mantle transition zone. The Yangyuan pyroxenite xenoliths can be subdivided into three types based on the compositions of their constituent clinopyroxene. Clinopyroxenes in the type I pyroxenites are characterized by high Mg# (88.3–92.0; Mg# = Mg/(Mg + Fe2+)*100) and Cr2O3 contents (0.86–1.63 wt.%). Their resemblance to those from the peridotite xenoliths, together with their light rare earth element (LREE) enrichment, indicates that the type I pyroxenites should be produced by lithological heterogeneity in the mantle, which may have including metasomatism. Clinopyroxenes in the type II pyroxenites have intermediate Mg# (81.0–86.7) and Cr2O3 contents (0.53–0.73 wt.%), which are similar to those of strongly metasomatized mantle-derived Fe-lherzolite xenoliths. Their LREE-enriched compositions, as well as the high Ni contents of the orthopyroxenes and clinopyroxenes, suggest that the type II pyroxenites were produced by melt-peridotite interactions. Clinopyroxenes from type III pyroxenites have the lowest Mg# (65.2–76.5) and Cr2O3 contents (0.05–0.38 wt.%), which are comparable to those formed by fractional crystallization of primary basalts, suggesting that they are the products of fractional crystallization. The high Mg#, Ti/Eu ratios, the low (La/Yb)N and Ca/Al ratios, and Rb and Ba contents of the clinopyroxenes, as well as the depleted Sr-Nd isotopes of most of the Yangyuan pyroxenites, suggest that asthenosphere or juvenile lithospheric mantle-derived silicate melts were probably involved in their formation, whereas the enriched Sr-Nd isotopes of sample YY17-5 indicate that its source may have mixed with melts derived from recycled ancient lower crust. The light and variable Mg isotopic compositions (δ26Mg = −0.23 to −0.46‰) of the Yangyuan pyroxenites suggest contributions from a subducted oceanic slab. The diverse origins of these pyroxenite xenoliths provide evidence that the deep lithosphere beneath Yangyuan has been modified by multiple melt-rock interactions and magma underplating.

Highly siderophile elements and Os isotope constraints on the genesis of peridotites from the Kızıldağ ophiolite, southern Turkey

In this contribution, we present whole-rock major and trace element chemistry, including rare earth elements (REE), highly siderophile elements (HSE), and Re-Os isotope data, together with mineral chemistry for the harzburgites and dunites from the Kızıldağ ophiolite of SE Turkey, in an attempt to better constrain its petrogenesis. Both the harzburgites and dunites have refractory compositions with CaO contents <0.90 wt%, Al2O3 contents <0.90 wt% and TiO2 contents <0.01 wt%, indicating that they experienced high degrees of partial melting in the supra-subduction zone (SSZ) setting. Besides, the concave-shaped REE patterns and positive anomalies in some incompatible elements (e.g., Cs, Rb and Pb) suggest that the harzburgites and dunites have also experienced interactions with percolating melts after partial melting. Based on the quantitative melt-rock interaction model, the addition of sulfides derived from the percolating magmas could be responsible for the variable HSE contents and suprachondritic Pd/Ir and Pt/Ir ratios observed in some of the harzburgite samples. The extremely low HSE contents in the dunites can be explained by interaction with S-undersaturated melts. Enrichments of Os and Ru in two dunites could be attributed to the precipitation of a higher amount of spinel grains along with the possible Os-Ru microparticles. The significantly wide range of Os isotopic ratios in the harzburgites (187Os/188Os(90 Ma) = 0.12152–0.13584; γOs(90 Ma) = −4.31 to 6.96) and dunites (187Os/188Os(90 Ma) = 0.12690–0.13267; γOs(90 Ma) = −0.08 to 4.47) seems to be consistent with the multi-stage evolution of the oceanic upper mantle. The low Os isotopic ratios (sub-primitive mantle) in some harzburgite samples suggest the lithospheric mantle has experienced ancient melting processes (>1.0 Ga).

Melt-rock interaction in the subarc mantle: records from the plagioclase peridotites of the southern Palawan Ophiolite, Philippines

ABSTRACT The interaction between migrating melts and the upper mantle largely affects the composition of the ascending melt and residual peridotites. In island arcs, melt-rock interactions and products which involved highly depleted mantle peridotites are still largely undocumented despite their petrological importance. In this contribution, the petrographic and geochemical signatures of refractory and refertilized peridotites comprising the mantle section of the southern Palawan Ophiolite, Philippines are investigated. The peridotites are dominantly comprised of residual spinel harzburgites with minor dunite and plagioclase peridotites. Spinel harzburgites are similar to other highly depleted residual mantle materials of other suprasubduction zone ophiolites. Plagioclase peridotites, which are cut by gabbroic intrusions, preserve distinct petrological characteristics indicating melt-rock interaction. The melts preserved as gabbroic dikes have signatures transitional between mid-ocean ridge and island arc, and are comparable to back-arc basin lavas. We therefore propose that the plagioclase peridotites were formed due to the reaction between highly depleted mantle peridotites beneath an intraoceanic arc and migrating BABB-like magmas possibly during the incipient stages of back-arc rifting.

Suprasubduction-zone origin of the podiform chromitites of the Bir Tuluhah ophiolite, Saudi Arabia, during Neoproterozoic assembly of the Arabian Shield

The ultramafic section of a dismembered ophiolite is exposed at Bir Tuluhah, in the north-central part of the Arabian Shield. It is penetratively serpentinized and locally carbonate-altered to talc‑carbonate and quartz‑carbonate rocks (listvenite) along shear zones and fault planes. Despite the high degree of mineral replacement, preserved mesh and bastite textures and fresh relics of primary Cr-spinel and olivine show that the protoliths were mainly harzburgite with minor dunite, with sparse massive chromitite bodies of various forms and sizes. Olivine inclusions in the chromitite lenses have higher forsterite content and NiO concentrations than fresh olivine relics in the host harzburgites and dunites, due to subsolidus re-equilibration. Cr-spinels in the chromitites have higher Cr# (0.74–0.82) than those hosted in dunite (0.72–0.76) or harzburgite (0.55–0.66). The scarce Cr-spinel crystals in harzburgite that have Cr# < 0.6 are interpreted to represent the population least affected by melt-rock interaction. The chromitite bodies are interpreted to have formed just below the contact between the oceanic crust and mantle sections (i.e., the petrologic Moho). The primary olivine (high Fo and Ni content) and Cr-spinel core compositions (high Cr# and low TiO2 content) of the Bir Tuluhah serpentinized peridotite are typical of modern supra-subduction zone (SSZ) fore-arc peridotites and consistent with crystallization from boninitic magma. The multistage petrogenesis leading to the chromitite bodies begins with moderate to high degrees of melt extraction from the protoliths of the serpentinized harzburgites, followed by reaction with melt compositions that evolved from tholeiite to boninite and left dunite residues. The massive Cr-rich chromitites in the Bir Tuluhah ophiolite are most probably the residues of such interaction between depleted harzburgite and ascending melts; mixtures of the reacted melts formed boninites, which became saturated with chrome-rich spinel and crystallized chromite pods before ascending past the Moho. We offer a novel thermodynamic model of this mixing and reaction process that quantifies the yield of Cr-spinel.

Melt-rock interaction between granitic pegmatites and hosting amphibolites from the Chiavenna Ophiolitic Unit (Tanno Pegmatitic Field, Central Alps, North Italy)

The Tanno pegmatitic field, placed southward of Chiavenna (Central Alps, Sondrio, Italy), develops a large number of subplanar dykes that crosscut the Chiavenna Unit, an ophiolitic complex mainly composed, in the study area, of amphibolite rocks. This study focuses on the contact between a pegmatitic dyke and the amphibolitic country rock. We distinguished four zones across the contact: I) inner amphibolite, II) contact amphibolite, III) contact pegmatite, IV) inner pegmatite. The inner amphibolite, not affected by melt-rock interaction, is composed of amphibole, phlogopite, ilmenite, titanite and rutile. Two amphibole generations occur, both of them showing a patchy compositional zoning. Amphibole I are Mg-hornblende, whereas Amphibole II have a pargasitic composition. The contact amphibolite shows an enrichment of mica belonging to the phlogopite-biotite series, titanite and the presence of fluorapatite and plagioclase (Ab45-60), that is absent in the inner amphibolite. Close to the contact, amphiboles display no zoning and gain an Mg-horneblenditic composition. The contact pegmatite has quartz, albitic plagioclase, garnet (almandine-spessartine series), muscovite, K-feldspar and fluorapatite. It shows a comb texture, with elongation of plagioclase crystals normal to the contact itself. Far from the contact, the inner pegmatite has an increasing grain-size and a less organized texture. In this zone several accessory phases occur, including gahnite, columbite-(Fe), monazite-(Ce), xenotime-(Y), uraninite and betafite. Whole rock analyses suggest that a chemical exchange, concerning both major elements and trace elements, occurred between the pegmatitic melt and the hosting amphibolite. A considerable increase of SiO2, Na2O and, to a lesser extent, of Al2O3 is observed from the amphibolite towards the pegmatite; K2O and CaO show a decrease at the same extent. The REE pattern in the pegmatite highlights an enrichment in HREE at the contact. Mineral chemistry confirms this trend with variations observable in plagioclase, gradually more albitic from the amphibolite to the pegmatite. Mineralogical characters and geochemical features allow to classify the Tanno pegmatite in the LCT (Lithium, Cesium, Tantalium) family. Based on the metamorphic peak conditions reported from the Lepontine Dome the ambient conditions during pegmatite intrusion were ca. 550°C and 5 kbar. The reduced thermal difference between pegmatite and wall rock explains the diffuse contact observed by X-ray micro-computed tomography. The collected data suggest a chemical interaction between melt and wall rock, according to the following reaction taking place in the amphibolite Amphibole I + Amphibole II + Ilmenite + Pegmatitic melt → Amphibole III + Plagioclase + Phlogopite + Titanite + Fluorapatite

Neoproterozoic and Cretaceous mantle oxidation states: Controls and heterogeneity through time

To estimate the oxygen fugacity (fO2) of the Neoproterozoic and Cretaceous suprasubduction zone mantle, and to evaluate the possible secular changes in the upper mantle oxidation state, the compositions of spinel, olivine and orthopyroxene of Neoproterozoic (Egypt and Saudi Arabia) and late Cretaceous (Iran) mantle rocks were determined. For accurate estimation of fO2, spinel ferric iron was calculated after correcting the electron microprobe data using a set of spinel standards for which the ferric iron content was measured by Mӧssbauer spectroscopy. The Neoproterozoic samples record strongly heterogenous fO2 values ranging from moderately oxidized (FMQ +0.54) to ultra-reduced (FMQ -4.73) for harzburgites, from highly oxidized (FMQ +1.49) to moderately reduced (FMQ -0.60) for dunites as well as one highly reduced (FMQ -1.61) value for chromitite. Such heterogeneity is not apparent in the late Cretaceous harzburgites that record fO2 values ranging from slightly oxidized (FMQ +0.45) to moderately reduced (FMQ -0.85). The fO2 of the Neoproterozoic forearc mantle is most easily explained by melt-mantle interaction and deep-mantle recycling, while that of the late Cretaceous forearc mantle can be attributed to variable degrees of melt-mantle interaction. The estimated fO2 values of Neoproterozoic/Cretaceous mantle unaffected by melt-rock interaction and deep-mantle recycling, and published values of Precambrian and Modern mantle suggest a consistent upper mantle oxidation state from Proterozoic to present day.

Genesis of pyroxenite veins in supra-subduction zone peridotites: Evidence from petrography and mineral composition of Egiingol massif (Northern Mongolia)

Swarms of orthopyroxenite and websterite veins are found within Egiingol residual SSZ peridotite massif of Dzhida terrain (Central Asian Orogenic Belt, Northern Mongolia). The process of Egiingol pyroxenite veins formation is investigated using new major and trace element analyses of pyroxenite minerals, calculations of closure temperatures and composition of equilibrium melt. The pyroxenites show abundant petrographic and geochemical evidence for replacement of the residual peridotite minerals by ortho- and clinopyroxene due to melt-rock interaction. Relics of peridotite olivines are found in pyroxenites, Cr^# of spinel increases from peridotites to pyroxenites, and compositions of ortho- and clinopyroxene change from peridotite to pyroxenite. The authors show that calculated equilibrium melts for investigated pyroxenites are very similar to compositions of boninite lavas from the Dzhida terrain. Therefore, formation of pyroxenite veins most likely resulted from percolation of boninite melts through the Egiingol peridotites. Orthopyroxenite veins formed at first, followed by websterite veins. Thus, the authors assume that pyroxenite veins represent the channels for boninitic melts migration in supra-subduction environment.

Origin of pyroxenites in the oceanic mantle and their implications on the reactive percolation of depleted melts

Pyroxenites are diffuse in fertile mantle peridotites and considered an important component in the mantle source of oceanic basalts. They are rarely documented in abyssal and ophiolitic peridotites representing residual mantle after melt generation, and few studies defining their origin are to date available. We present a field-based microstructural and geochemical investigation of the pyroxenite layers associated with depleted peridotites from the Mt. Maggiore ophiolitic body (Corsica, France). Field and petrographic evidence indicate that pyroxenite formation preceded the melt–rock interaction history that affected this mantle sector during Jurassic exhumation, namely (1) spinel-facies reactive porous flow leading to partial dissolution of the pyroxenites, and (2) plagioclase-facies melt impregnation leading to [plagioclase + orthopyroxene] interstitial crystallization. Pyroxenes show major element compositions similar to abyssal pyroxenites from slow-spreading ridges, indicative of magmatic segregation at pressures higher than 7 kbar. Both the parental melts of pyroxenites and the melts involved in the subsequent percolation were characterized by Na2O-poor, LREE-depleted compositions, consistent with unaggregated melt increments. This implies that they represent the continuous evolution of similarly depleted melts leading to different processes (pyroxenite segregation and later melt–rock interaction) during their upward migration. To support the genetic relation and the continuity between the formation of pyroxenites and the subsequent melt–rock interaction history, we modeled all the documented processes in sequence, i.e.: (1) formation of single-melt increments after 6% mantle decompressional fractional melting; (2) high-pressure segregation of pyroxenites; (3) spinel-facies reactive porous flow, (4) plagioclase-facies melt impregnation. The early fractionation of pyroxenites leads to a decrease in pyroxene saturation that is necessary for the subsequent reactive porous flow process, without any significant change in the melt REE composition.

Geochemical characterization of mineralized albitite from Paleoproterozoic Bhukia IOCG‐IOA deposit of Aravalli‐Delhi Fold Belt, Rajasthan, western India: Genetic linkage to the gold (±Cu ± U) mineralization

Paleoproterozoic Bhukia gold deposit is located within the Debari Group of Aravalli Supergroup, Rajasthan, northwestern India. The major lithounits that are associated with gold–graphite (±Cu ± U) mineralization exposed in Bhukia are albitite, carbonates, and quartzite which have undergone upper greenschist to amphibolite facies metamorphism. Ore mineralization viz. pyrrhotite, arsenopyrite, chalcopyrite, pyrite, graphite, magnetite, and goethite in the study area are associated with NNW–SSE trending shear zone. Nuggets of gold are identified in the thick gossan zones that occurs parallel to the shear zone. The gold–sulfide lodes of the Bhukia deposits are largely confined either to marble or albitized marble and pure monomineralic albitite. The mineralization that is associated with the albitite, follows the boundaries of albite grains which confirms the role of these weak zones for the mobilization of mineralizing fluid and concentration of metal ions. Hence, geochemical studies of albitite would help to evaluate the petrogenesis, likely magmatic affinity, tectonic setting, controls of the mineralizing fluid, source, and their linkage to the gold (±Cu ± U) metallogeny. Bhukia albitite is characterized by Na–Al–rich and K‐poor compositions along with metaluminous nature. Geochemical analysis of various key elements suggests that albitite is the product of low‐K calc‐alkaline magma series with variation in Nb/Ta ratio indicating their fractionation during the melt–rock interaction within the shear zone. Geochemical signatures suggest the role of upper continental crust in channelizing the mineralized fluids that generated albitite with felsic to intermediate composition in and around Bhukia deciphered from their low Nb/Y ratios. Lower concentration of compatible elements (Ni and Cr) and higher concentration of incompatible elements (Th, La, and Nd) with SiO2 suggest that evolution of albitite rock is related to the fractional crystallization process. Zr/Hf ratios of 9–35 with the average of 18 suggest that albitite formed under magmatic–hydrothermal transitional environment. Rare Earth Elements (REE) patterns of albitite samples are moderately fractionated showing Light Rare Earth Elements (LREE)‐enrichment with a rather steep slope and exhibit distinct negative Eu anomalies (0.42–0.71, average 0.57). The enrichment of LREE in comparison to the Heavy Rare Earth Elements (HREE) suggests a more evolved source, i.e., process of melting at shallow environments. Tectonic setting of albitite samples suggest their origin in a volcanic arc region that underwent rifting. Detailed geochemical characterization unravelled complex evolutionary history of albitite in the Bhukia area, which attests derivation of the parental melt possibly from an enriched SCLM (Sub‐continental lithospheric mantle) source similar to most of the IOCG‐IOA deposits worldwide.

Major and trace elements exchanges during fluid-rock interaction at ultraslow-spreading oceanic lithosphere: Example of the South West Indian Ridge (SWIR)

The smooth seafloor area, located between 62 and 65°E along the ultraslow-spreading South West Indian Ridge, is dominated by exhumed and altered mantle rocks, with very minor basalts and gabbros. This amagmatic region of the global ridge system is an ideal place to study long-lived fluid-rock interactions affecting peridotites and the associated mass transfers. The smooth seafloor area is composed of two corridors, west and east, perpendicular to the ridge axis allowing to establish profiles of chemical element concentrations with distance from the ridge axis. Studied serpentinites are moderately to completely serpentinized with various secondary mineralogical assemblages: predominantly serpentine (lizardite/antigorite) and magnetite associated to accessory minerals such as chlorite, tremolite, talc, carbonates, ferritchromite, sulfides, and Fe-(hydro-)oxides (hematite and goethite). LOI, serpentinization and iron oxidation degrees (Fe3+/Fetot) all increase with distance from the ridge axis in the western corridor and can thus be used as proxys to evaluate mass transfer during serpentinization. Bulk Al2O3 and CaO concentrations are inherited from partial melting and melt-rock interaction, while Fe2O3, MgO, SiO2 and Na2O contents were slightly modified during long-term low-temperature fluid-rock interactions. Concentrations in trace elements, such as Li, B, La, Pr and some FME increase with distance from the ridge and then with the serpentinization degree. During early stages of detachment faulting, a few samples interacted with high-temperature hydrothermal fluids leading to an increase in Fe2O3T, Na2O, REE and FME contents. However, the absence of sulfides in these samples, likely replaced by hematite and goethite, suggest late alteration or circulation of S-poor brines. Late fluid-rock interaction under oxidizing conditions leads to the formation of iron oxyhydroxides (IOH), hematite and goethite, in serpentinites and to a concomitant enrichment in some trace elements (La, Pr, As, Sb and Cd). Evaluation of major and trace elements mass transfer of the smooth seafloor serpentinites shed new light on the consequences of fluid-rock interactions during long-lived exhumation and hydration history.

An SPO-induced CPO in composite mantle xenoliths correlated with increasing melt-rock interaction

Melt-rock interaction, a general term describing various processes including cryptic and modal metasomatism and melt-rock reaction, is the main process responsible for modifying chemical composition of previously melt-depleted mantle peridotite. However, the underlying mechanisms of melt-rock interaction and its effect on mantle rheology, particularly in natural peridotites, is poorly constrained. Composite xenoliths are natural examples of melt-rock interaction as they contain pyroxenitic veins interpreted as evidence of passage of melt through peridotite at high pressures. Here, we present new mineral chemistry (major, trace element, and water contents of olivine and pyroxenes) and microstructural data on a suite of composite xenoliths from the Neogene Hannuoba basalt, North China Craton. We show that despite having experienced high melt/rock ratios, olivines and pyroxenes contain very low water contents (<10 ppm and <100 ppm, respectively). In contrast, melts calculated to be in equilibrium with clinopyroxene show enriched trace element signatures suggesting the infiltrating melt had a crustal origin. Microstructural data corroborate a key role for melt infiltration in causing a systematic shift in olivine crystallographic preferred orientation (CPO) from initially A-type to AG-type with increasing melt/rock ratio. By describing the olivine grain shape with respect to the crystal reference frame, we show that as pyroxene mode (and hence melt/rock ratio) increases, olivine grains appear to rotate with their flattest (0 1 0) faces aligning with the melt flow plane, resulting in an olivine CPO controlled by its shape-preferred orientation (SPO). Previously, such an SPO-induced CPO was only demonstrated in shallow magmatic environments such as mafic intrusions, in mafic lavas, and in high-pressure/high-temperature experiments. Such a finding in the deep lithosphere is important as it suggests that dislocation creep may not always play a major role in CPO development, particularly when melt is involved.

An experimental study of peridotite dissolution in eclogite-derived melts: Implications for styles of melt-rock interaction in lithospheric mantle beneath the North China Craton

Interaction between eclogite-derived melt and peridotite has played an important role in modifying the cratonic lithospheric mantle and generation of high-Mg igneous rocks. To better understand the effect of peridotite physical state on melt-peridotite interaction, we conducted reaction experiments of lherzolite with two basaltic andesites and a ferro-basalt at temperatures of 1300 °C, 1375 °C, and 1425 °C and a pressure of 2 GPa using the reaction couple method. At 1300 °C, when lherzolite is subsolidus, its dissolution rate is slow, and it is mineralogically and texturally unchanged. Garnet and clinopyroxene precipitate at the melt-rock interface. The low-temperature reaction enriches the melt with SiO2 and Na2O, and depletes the melt with Al2O3, FeO, and CaO. At 1375 °C and 1425 °C, when the lherzolite is partially molten, the dissolution and re-equilibration rates are fast due to grain-scale process that involves dissolution, precipitation, and reprecipitation. Dissolution of olivine and precipitation of orthopyroxene produce a melt-bearing orthopyroxene-rich lithology at the melt-rock interface followed by a melt-bearing harzburgite and a melt-bearing lherzolite. The lithology near the interface can be an orthopyroxene-rich harzburgite or an orthopyroxenite depending on the reacting melt composition. The high-temperature reaction produces melt with increased MgO, FeO, CaO, and Mg#, and decreased SiO2, Al2O3, and Na2O. The dissolution rate obtained from the experiments is used to assess the survival of mantle xenoliths in the early Cretaceous high-Mg diorites. Combined with field observations from mantle samples, the experimental results provide insight into the style of eclogite-derived melt and peridotite interaction in the lithospheric mantle beneath the North China Craton. At the lithosphere-asthenosphere boundary, the reaction is dominated by the high-temperature regime, which produces orthopyroxene-rich lithologies such as orthopyroxene-rich harzburgite and orthopyroxenite-veined peridotite. Above the lithosphere-asthenosphere boundary where peridotite is subsolidus, the reaction is characterized by the low-temperature regime, which produces garnet-bearing or garnet-rich lithologies such as garnet pyroxenite, garnetite, and high-Mg granulite. Interactions between eclogite-derived melts and peridotite in the two regimes are responsible for the geochemical features of the early Cretaceous high-Mg igneous rocks from the North China Craton.

Pacific Lithosphere Evolution Inferred from Aitutaki Mantle Xenoliths

Abstract Highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), major and trace element abundances, and 187Re–187Os systematics are reported for xenoliths and lavas from Aitutaki (Cook Islands), to investigate the composition of Pacific lithosphere. The xenolith suite comprises spinel-bearing lherzolites, dunite, and harzburgite, along with olivine websterite and pyroxenite. The xenoliths are hosted within nephelinite and alkali basalt volcanic rocks (187Os/188Os ∼0·1363 ± 13; 2SD; ΣHSE = 3–4 ppb). The volcanic host rocks are low-degree (2–5%) partial melts from the garnet stability field and an enriched mantle (EM) source. Pyroxenites have similar HSE abundances and Os isotope compositions (Al2O3 = 5·7–8·3 wt %; ΣHSE = 2–4 ppb; 187Os/187Os = 0·1263–0·1469) to the lavas. The pyroxenite and olivine websterite xenoliths directly formed from—or experienced extensive melt–rock interaction with—melts similar in composition to the volcanic rocks that host the xenoliths. Conversely, the Aitutaki lherzolites, harzburgites and dunites are similar in composition to abyssal peridotites with respect to their 187Os/188Os ratios (0·1264 ± 82), total HSE abundances (ΣHSE = 8–28 ppb) and major element abundances, forsterite contents (Fo89·9±1·2), and estimated extents of melt depletion (&amp;lt;10 to &amp;gt;15%). These peridotites are interpreted to sample relatively shallow Pacific mantle lithosphere that experienced limited melt–rock reaction and melting during ridge processes at ∼90 Ma. A survey of maximum time of rhenium depletion ages of Pacific mantle lithosphere from the Cook (Aitutaki ∼1·5 Ga), Austral (Tubuai’i ∼1·8 Ga), Samoan (Savai’i ∼1·5 Ga) and Hawaiian (Oa’hu ∼2 Ga) island groups shows that Mesoproterozoic to Neoproterozoic depletion ages are preserved in the xenolith suites. The variable timing and extent of mantle depletion preserved by the peridotites is, in some instances, superimposed by extensive and recent melt depletion as well as melt refertilization. Collectively, Pacific Ocean island mantle xenolith suites have similar distributions and variations of 187Os/188Os and HSE abundances to global abyssal peridotites. These observations indicate that Pacific mantle lithosphere is typical of oceanic lithosphere in general, and that this lithosphere is composed of peridotites that have experienced both recent melt depletion at ridges and prior and sometimes extensive melt depletion across several Wilson cycles spanning periods in excess of two billion years.

Evaluating the roles of melt-rock interaction and partial degassing on the CO2/Ba ratios of MORB: Implications for the CO2 budget in the Earth’s depleted upper mantle

Abstract Carbon content in the Earth's depleted upper mantle has been estimated in previous studies using CO2/Ba ratios of CO2 undersaturated depleted mid-ocean ridge basalt (D-MORB) glasses and melt inclusions. However, CO2/Ba ratios in CO2 undersaturated MORB may not necessarily record those of the mantle source, as they may be affected by (1) assimilation of Ba-rich plagioclase-bearing rocks in the oceanic crust and (2) CO2 degassing through partial degassing and mixing. In this study, we evaluate these effects on the CO2/Ba ratios as well as other volatile to refractory trace element ratios (H2O/Ce, F/Nd, Cl/K, and S/Dy) in D-MORBs using the compositions of olivine-hosted melt inclusions and glasses from the Siqueiros and Garrett transform faults. The Siqueiros and Garrett melt inclusions are CO2 undersaturated and highly depleted in incompatible trace elements, and their average CO2/Ba ratios show relatively large ranges of 90 ± 34 and 144 ± 53 respectively. A subset of melt inclusions in lavas from both transform faults show potential signatures of contamination by plagioclase-rich rocks, such as correlations between major elements contents (e.g., FeO, Al2O3, and MgO), and trace element ratios (e.g., Sr/Nd). We find that (1) assimilation fractional crystallization (AFC) of gabbro into D-MORB and (2) mixing between partial melts of gabbro and D-MORB can reproduce the observed range in Sr/Nd ratios as well as the general trends between major elements. However, we find that these processes had limited effects on the CO2/Ba ratio of the melt inclusions and it is unlikely that they can account for the observed range in the CO2/Ba ratio. On the other hand, while a partial degassing and mixing model can generate melts with large range of CO2/Ba ratios (as proposed by Matthews et al. (2017)), it cannot reproduce the Pearson correlation coefficients between CO2/trace element and 1/trace element ratios observed in the Siqueiros and Garrett melt inclusions. Instead, when analytical uncertainties on the elemental concentrations are considered, a model without partial degassing can adequately reproduce the majority of the observed range in CO2/Ba ratio and Pearson correlation coefficients. Hence, we postulate that the Siqueiros and Garrett melt inclusions are undegassed and use their average CO2/Ba ratios to estimate the Siqueiros and Garrett mantle source CO2 contents (21 ± 2 ppm and 33 ± 6 ppm respectively). We also evaluate the effects of shallow level crustal processes on H2O/Ce, F/Nd, Cl/K, and S/Dy ratios, and after which we filter those effects, we estimate the H2O, F, Cl and S contents in the mantle sources of the Siqueiros (40 ± 8 ppm, 8 ± 1 ppm, 0.22 ± 0.04 ppm, and 113 ± 3 ppm) and Garrett (51 ± 9 ppm, 6 ± 1 ppm, 0.27 ± 0.07 ppm, and 128 ± 7 ppm) melt inclusions.

Calcium isotope sources and fractionation during melt-rock interaction in the lithospheric mantle: Evidence from pyroxenites, wehrlites, and eclogites

To investigate the effects of melt metasomatism on Ca isotope fractionation in the upper mantle, we report data for xenoliths of mantle rocks including composite peridotites (lherzolites and coexisting pyroxenite veins), wehrlites as well as eclogites and their minerals. The δ44/40Ca in clinopyroxenes (cpx) are ~0.3‰ lower than in coexisting garnets in eclogites from the Udachnaya kimberlite in the central Siberian craton, likely due to equilibrium isotope fractionation controlled by mineral structure. The δ44/40Ca in the pyroxenite veins (cumulates from mafic melts) and their host peridotites from Tariat in Mongolia range from 0.88‰ to 1.05‰ and are close to the Bulk Silicate Earth (BSE) value (0.95 ± 0.05‰). Calcium-Fe-rich low-orthopyroxene lherzolites and wehrlites (LW series) from Tok in the SE Siberian craton were formed by reaction of lherzolites and harzburgites with percolating silica-undersaturated melts. The δ44/40Ca (0.65‰–0.87‰) in the Tok peridotites are negatively correlated with Ca/Al, (Gd/Yb)N and positively with Mg# (indices of melt-rock reaction degrees). The δ44/40Ca ratios in the LW series rocks are lighter than the BSE estimate, and similar to δ44/40Ca in their host basalt and an olivine-cpx cumulate (0.70‰–0.76‰). These observations on the Tok suite can be explained by reaction of refractory peridotite protoliths with melts derived from mixed eclogite and peridotite sources. Overall, the δ44/40Ca variations in eclogites, wehrlites, and various rocks related to silicate melts in the mantle, demonstrate that melt metasomatism plays an important role in producing mantle Ca isotope heterogeneity. The data for coexisting garnet and cpx in eclogites outline a potential for garnet to affect Ca isotope inter-mineral and solid-melt fractionation in the mantle.

The field and microstructural signatures of deformation‐assisted melt transfer: Insights from magmatic arc lower crust, New Zealand

AbstractMelt must transfer through the lower crust, yet the field signatures and mechanisms involved in such transfer zones (excluding dykes) are still poorly understood. We report field and microstructural evidence of a deformation‐assisted melt transfer zone that developed in the lower crustal magmatic arc environment of Fiordland, New Zealand. A 30–40 m wide hornblende‐rich body comprising hornblende ± clinozoisite and/or garnet exhibits 'igneous‐like' features and is hosted within a metamorphic, two‐pyroxene–pargasite gabbroic gneiss (GG). Previous studies have interpreted the hornblende‐rich body as an igneous cumulate or a mass transfer zone. We present field and microstructural characteristics supporting the later and indicating the body has formed by deformation‐assisted, channelized, reactive porous melt flow. The host granulite facies GG contains distinctive rectilinear dykes and garnet reaction zones (GRZ) from earlier in the geological history; these form important reaction and strain markers. Field observations show that the mineral assemblages and microstructures of the GG and GRZ are progressively modified with proximity to the hornblende‐rich body. At the same time, GRZ bend systematically into the hornblende‐rich body on each side of the unit, showing apparent sinistral shearing. Within the hornblende‐rich body itself, microstructures and electron back‐scatter diffraction mapping show evidence of the former presence of melt including observations consistent with melt crystallization within pore spaces, elongate pseudomorphs of melt films along grain boundaries, minerals with low dihedral angles as small as &lt;10° and up to &lt;60°, and interconnected 3D melt pseudomorph networks. Reaction microstructures with highly irregular contact boundaries are observed at the field and thin‐section scale in remnant islands of original rock and replaced grains, respectively. We infer that the hornblende‐rich body was formed by modification of the host GG in situ due to reaction between an externally derived, reactive, hydrous gabbroic to intermediate melt percolating via porous melt flow through an actively deforming zone. Extensive melt–rock interaction and metasomatism occurred via coupled dissolution–precipitation, triggered by chemical disequilibrium between the host rock and the fluxing melt. As a result, the host plagioclase and pyroxene became unstable and were reacted and dissolved into the melt, while hornblende and to a lesser extent clinozoisite and garnet grew replacing the unstable phases. Our study shows that hornblendite rocks commonly observed within deep crustal sections, and attributed to cumulate fractionation processes, may instead delineate areas of deformation‐assisted, channelized reactive porous melt flow formed by melt‐mediated coupled dissolution–precipitation replacement reactions.