Degrees Of Melting Research Articles

Experimental Constraints on the Origin of the Lunar High‐Ti Basalts

AbstractHigh‐pressure and high‐temperature experiments were conducted to simulate melting of a hybrid cumulate lunar mantle. The experimental results show that intermediate to high‐Ti lunar pyroclastic glasses (>6 wt% TiO2) can be produced by partial melting of lunar cumulates. High‐Ti basalts are generated when the ilmenite/clinopyroxene ratios in the lunar mantle cumulates are between 1/1 and 4/1, depending on the degree of melting. The presence of an urKREEP component in the mantle cumulate strongly influences Al2O3/CaO of the melts. The experiments provide strong evidence for the model that the compositional diversity of lunar basalts is a consequence of a gravitational overturn of the lunar interior after the lunar magma ocean had solidified. Ilmenite/clinopyroxene in the cumulate mantle, which generates high‐Ti melts at partial melting, do not comprise the ratios in ilmenite‐bearing cumulates (IBC), which crystallized after ∼90% solidification of the lunar magma ocean and indicate local accumulation of ilmenite in the overturned lunar mantle. However, to fully match the natural composition of the most primitive lunar samples, secondary processes such as assimilation are still required.

Size Reduction to Enhance Crystal-to-Liquid Phase Transition Induced by E-to-Z Photoisomerization Based on Molecular Crystals of Phenylbutadiene Ester.

Harnessing the photoinduced phase transitions in organic crystals, especially the changes in shape and structure across various dimensions, offers a fascinating avenue for exact spatiotemporal control, which is crucial for developing future smart devices. In our study, we report a new photoactive molecular crystal made from (E)-2-(3-phenyl-allylidene)malonate ((E)-PADM). When exposed to ultraviolet (UV) light at 365 nm, this compound experiences an E-to-Z photoisomerization in liquid solution and a crystal-to-liquid phase transition in solid crystals. Remarkably, nanoscopic crystalline rods boost their melting rate and degree compared to bulk crystals, indicating that miniaturization enhances the photoinduced melting effect. Our results demonstrate a simple approach to rapidly drive molecular crystals into liquids via photochemical reactions and phase transitions.

Deep, hot,ancient melting recorded by ultralow oxygen fugacity in peridotites.

The oxygen fugacity (fO2) of convecting upper mantle recorded by ridge peridotites varies by more than four orders of magnitude1-3. Although much attention has been given to mechanisms that drive variations in mantle fO2 between tectonic settings1,3,4 and to comparisons of fO2 between modern rocksand ancient-mantle-derived rocks5-10, comparatively little has been done to understand the origins of the high variability in fO2 recorded by peridotites from modern mid-ocean ridge settings. Here we report the petrography and geochemistry of peridotites from the Gakkel Ridge and East Pacific Rise (EPR), including 16 new high-precision determinations of fO2. Refractory peridotites from the Gakkel Ridge record fO2 more than four orders of magnitude below the mantle average. With thermodynamic and mineral partitioning modelling, we show that excursions to ultralow fO2 can be produced by large degrees of melting at high potential temperature (Tp), beginning in the garnet field and continuing into the spinel field-conditions met during the generation of ancient komatiites but not modern basalts. This does not mean that ambient convecting upper mantle had a lower ferric to ferrous ratio in Archaean times than today nor that modern melting in the garnet field at hotspots produce reduced magmas. Instead, it implies that rafts of ancient, refractory, ultrareduced mantle continue to circulate in the modern mantle while contributing little to modern ridge volcanism.

Magmatic (Cr-Ni-PGE) and secondary/hydrothermal (emerald-peridot-rodingite-nephrite jade) mineralization associated with mafic-ultramafic rock complexes of Pakistan

Mafic magmatism has repeatedly taken place in Pakistan throughout its geologic history from the earliest Proterozoic to Recent. Much of it, however, does not have associated metallic mineralization. This paper concerns mineralization associated with mafic-ultramafic plutonic rock occurrences in the western- and northern suture zones of Pakistan, which demarcate the collision boundary between the northwestern Indian plate and Eurasian blocks. We discuss the petrology of the ophiolites and basaltic magma-related chromite deposits, showings of PGE and Ni minerals, and secondary mineral deposits (emerald and peridot gems, rodingite, and nephrite jade) associated with mafic-ultramafic complexes of the two suture zones. Chromite with high Cr#s (68 to 85) occurs in fair quantity and is mostly considered to be genetically related to basaltic magma of subduction-related environments. PGE and nickeliferous minerals are insignificant in quantity; PGE's are formed by a higher degree of partial melting and melt-rock reaction, whereas Ni-sulphides are the product of serpentinization of the ultramafic rocks. Emerald occurs in several localities in the ISZ, whereas the peridot has so far been reported from only one locality. Emerald is associated with Mg-carbonate ± quartz ± fuchsite ± talc ± serpentine, and peridot (Fo 89-92 ) is associated with chrysotile ± magnesite ± talc ± magnetite ± ludwigite. Both are formed via hydrothermal alteration of ultramafic rocks through the introduction of hydrous fluids incorporating CO 2 , Be, B, K, Al, and possibly Na. In view of the undeformed nature of the emerald and peridot and the 23 Ma age of emerald formation, the mineralization appears to post-date deformation and may be of Late Oligocene-Early Miocene age. The metasomatising fluids were either derived from the subducting Indian plate crust underneath the Kohistan arc or were related to 23-26 Ma silicic magmatism in this area of the NW Indian plate. The nephrite jade in the ophiolite mélange is hosted in serpentinites and has disseminated grains of chrome spinel with elevated Fe/(Mg + Fe) ratios (0.10–0.18), implying low-grade metamorphism of ultramafic rocks (protolith of serpentinite) with loss of Al and introduction of SiO 2 during accompanying metasomatism. The rodingite in Dargai ophiolite could have formed by the action of seawater channeled through fractures and cracks in rocks. However, their mineral assemblages indicate that they formed as a result of the influx of Ca, Al, and Na-bearing hydrothermal fluids related to the intrusion of gabbros/sheeted dykes in the serpentinized ultramafic rocks, resulting in Ca-metasomatism.

Arc building and maturation of the Lombok Island, East Sunda Arc

Intra-oceanic subduction zones are major sites of crust–mantle material exchange and crustal growth; however, the processes responsible for the magmatic evolution and transformation of nascent arcs into mature oceanic arcs with thicker crust are debated. We present mineral chemistry, zircon UPb age and Hf isotopic data, and whole-rock chemical and Sr–Nd–Hf–Pb isotopic data for Cenozoic volcanic rocks on Lombok in the East Sunda Arc, Indonesia, to investigate their petrogenesis and arc maturation. SIMS zircon UPb analyses of lavas from southern Lombok yield an early Oligocene age of ∼30 Ma. The lavas can be divided into three groups based on their ages and geochemical characteristics: group I (late Eocene) and II (∼30 Ma) lavas from southern Lombok, and group III lavas (∼3–0 Ma) from northern Lombok. The lavas are characterized by a progressive enrichment in K2O and incompatible element contents, and NdHf isotopic compositions with time. Group I lavas are tholeiitic basalts with low K2O contents (0.15–0.17 wt%) and Indian-MORB like rare earth element (REE) patterns and NdHf isotopic compositions (εNd = +7.4; εHf = +14.5), enrichment in large-ion lithophile elements (e.g., Rb, U, and K), and typical forearc basalt (FAB) geochemical characteristics. Group II lavas are low-K basaltic andesites to dacites that yield flat chondrite-normalized REE patterns [(La/Sm)N = 0.9–1.4], uniform initial 87Sr/86Sr ratios (0.7038–0.7042), and positive εNd(t) (+5.9 to +6.8) and εHf(t) (+13.9 to +14.4) values. Group III lavas are typically medium- to high-K calc-alkaline basalt to andesite and are enriched in light REEs [(La/Sm)N = 1.6–3.3] with uniform initial 87Sr/86Sr ratios (0.7038–0.7042) and slightly low εNd(t) (+3.9 to +5.4) and εHf(t) (+10.5 to +12.5) values. Progressive enrichment in incompatible element contents and NdHf isotopic compositions over time, from FAB to high-K calc-alkaline lavas, reflect the maturation of the East Sunda Arc. Sr–Nd–Pb–Hf isotopic mixing models, along with trace element ratios (e.g., Ba/Th, La/Yb, Th/Yb), suggest that increasing amounts of sediments were incorporated into the mantle sources of the magmas from group I (∼0%) to group II (<0.5%) and group III (0.5%–1.0%) lavas. Partial melting models show that the formation of group I and II lavas can be reproduced by high-degree (∼10%) partial melting of spinel peridotite, whereas the group III lavas were formed by low-degree (∼5%) partial melting of spinel peridotite. We propose that the evolution of Lombok arc magmas was controlled mainly by the fluxes of subducted slab material, along with decreasing degrees of partial melting as a result of crustal thickening over time.

Formation and hydrothermal alteration of a volcanic center: Melt pooling and mass transfers at Langseth Ridge (Gakkel Ridge, Arctic Ocean)

Volcanic centers are characteristic features of ultraslow-spreading mid-ocean ridges, the least-explored parts of the global ridge system. Volcanic centers can provide insights into deep magmatic and metamorphic processes at these ridges. Here, we present data from the largest volcanic center on the Gakkel Ridge, the Langseth Ridge, situated at 60–62°E. Langseth is ∼10 km wide, consisting of three peaks that rise to 585 m water depth, some 3–4 km above the surrounding seafloor. It strikes perpendicular to Gakkel's spreading direction and can be traced for ∼40 km, which translates to an age of ∼8 Myr. Seafloor imaging revealed abundant (pillow) basalt but also fissures and geologic faults across the Langseth Ridge. Basaltic rocks were sampled at all summits and diabase at the slope of the northern summit that dips into the rift valley.Our samples are of normal to depleted mid-ocean ridge basalt composition and exhibit a wide range of major and trace element contents, due to magmatic processes, accumulation of macrocrysts, and hydrothermal alteration. Radiogenic isotope ratios, most notably 143Nd/144Nd and 208Pb/206Pb, trend from typical rift valley compositions to isotopically enriched values with increasing distance to the rift valley. This trend may imply melt pooling from different sources, potentially representing a shift from shallow melting beneath the rift valley to deeper melting of enriched sources and higher degrees of melting underneath Langseth. Mineral compositions and plagioclase sieve textures imply prolonged storage of magma at depth prior to eruption. Hydrothermal alteration occurred over a range of conditions. Basalt from the summits is weakly altered at temperatures ≪100 °C, which likely occurred in situ at the summit sites. Diabase samples experienced chloritization and albitization and display epidote and quartz veins, which formed at >300 °C. These assemblages and temperatures are typical for lower crustal levels and imply uplift of the samples of >1 km. Diabase samples from the Afanasenkov Seamount, another volcanic center on the Gakkel Ridge that we investigated for comparison, were altered under comparable conditions.Our findings suggest a combined volcanic–tectonic origin of the studied volcanic centers, potentially implying that such complexes may generally form due to the interplay of magmatism and tectonics. Researching volcanic centers has the potential to further our understanding of both deep and shallow crustal processes at ultraslow-spreading ridges, providing further insights into the role of these centers as linkages between lithosphere and hydrosphere and the (deep) biosphere they sustain.

Petrogenesis of flood basalts and shield basanite from Mertolemariam- Abamineos area, northwestern Ethiopian plateau: Assessments for mantle source variations

Petrographic and geochemical data (major and trace elements) are presented for Oligocene flood basalts and Miocene shield lavas from the Mertolemariam-Abamineos area, northwestern Ethiopian Plateau to examine the petrogenesis of the erupted magmas and the nature of mantle source compositions. The Mertolemariam flood basalts have mainly aphyric and plagioclase phyric textures. The Abamineos shield lavas have sparsely clinopyroxene phyric to highly plagioclase-clinopyroxene phyric textures. Geochemical classification shows that the Mertolemariam Oligocene flood basalts are sub-alkaline in composition, whereas the Abamineos Miocene shield lavas are highly alkaline in composition. The Abamineos shield has a unique composition (i.e., basanites) compared with all other shield basalts (transitional to alkaline) in the northwestern Ethiopian Plateau. Major and trace element compositions display two distinct trends between Mertolemariam flood basalts and Abamineos shield basanites. Fractional crystallization, partial melting, and crustal contamination of a homogeneous mantle source cannot explain the compositional variations between the Mertolemariam flood basalts and the Abamineos shield basanites. The trace element composition suggests that the Mertolemariam Oligocene flood basalts were more likely generated from the mixing of OIB (mantle plume) and E-MORB (enriched asthenosphere) mantle components. The Abamineos Miocene shield basanites were derived from the mantle plume (OIB) component. LREE/MREE and LREE/HREE indicate that both groups possibly originated from a mantle source within the stability field of spinel and garnet. In comparison, the Mertolemariam flood basalts were formed by a higher degree of partial melting from relatively shallow depths than the Abamineos shield basanites. We propose a scenario that explains the magmatic genesis in the northwestern Ethiopian Plateau: volcanism initiated by the initial arrival of the mantle plume (OIB-like) beneath the lithosphere, which comes across a geochemically fertile and enriched MORB (E-MORB) mantle component in the upper asthenosphere. The hot mantle plume triggered melting of the fertile and enriched MORB, and then melting occurred in the plume (OIB-like) at depth in the stability field of spinel and garnet. Melts from the mantle plume (OIB-like) and E-MORB components mixed to produce sub-alkaline flood basalts during the Oligocene in the northwestern Ethiopian Plateau Subsequently, melts from the advanced upwelling mantle plume (OIB-like) produced Miocene shield lavas in the northwestern Ethiopian Plateau.

Slab melting boosts the mantle wedge contribution to Li-rich magmas

The lithium cycling in the supra-subduction mantle wedge is crucial for understanding the generation of Li-rich magmas that may potentially source ore deposition in continental arcs. Here, we look from the mantle source perspective at the geological processes controlling the Li mobility in convergent margins, by characterizing a set of sub-arc mantle xenoliths from the southern Andes (Coyhaique, western Patagonia). The mineral trace element signatures and oxygen fugacity estimates (FMQ > + 3) in some of these peridotite xenoliths record the interaction with arc magmas enriched in fluid-mobile elements originally scavenged by slab dehydration. This subduction-related metasomatism was poorly effective on enhancing the Li inventory of the sub-arc lithospheric mantle, underpinning the inefficiency of slab-derived fluids on mobilizing Li through the mantle wedge. However, major and trace element compositions of mantle minerals in other xenoliths also record transient thermal and chemical anomalies associated with the percolation of slab window-related magmas, which exhibit an “adakite”-type geochemical fingerprint inherited by slab-derived melts produced during ridge subduction and slab window opening event. As these melts percolated through the shallow (7.2–16.8 kbar) and hot (952–1054 °C) lithospheric mantle wedge, they promoted the crystallization of metasomatic clinopyroxene having exceptionally high Li abundances (6–15 ppm). Numerical modeling shows that low degrees (< 10%) of partial melting of this Li-rich and fertile sub-arc lithospheric mantle generates primitive melts having two-fold Li enrichment (~13 ppm) compared with average subduction-zone basalts. Prolonged fractional crystallization of these melts produces extremely Li-enriched silicic rocks, which may stoke the Li inventory of mineralizing fluids in the shallow crust.

Geochemistry and zircon U-Pb geochronology of diabase, gabbros, anorthosites and ultramafic rocks from the South Andaman Island ophiolite suite on the South-Eastern margin of the Indian plate

The South Andaman Island is located on the southeastern fringe of the Indian plate. Tectonically, the islands are associated with the Burma-Andaman-Java subduction zone of the Indonesian arc system. Several Mesozoic ophiolitic exposures are preserved in the Burma-Andaman-Java subduction zone. This study reports new petrological, geochemical, and zircon U-Pb geochronological data for the diabases, gabbros, anorthosites, and harzburgites of the ophiolite suite exposed in the South Andaman Island. The diabase is composed of plagioclase and clinopyroxene exhibiting ophitic textures. Anorthosite mainly contains plagioclase (> 90 vol%), pyroxenes and Fe-oxides. Gabbros have plagioclases and clinopyroxenes occurring as the major minerals. Harzburgites consist of olivine, orthopyroxene, clinopyroxene and spinel. Geochemical investigation reveals that these rocks were formed through crystal fractionation and accumulation processes from partial melt generated from a spinel lherzolite source. Gabbro, diabase, and anorthosites are formed by ∼1–5% melting of the, whereas depleted harzburgites were generated by slightly higher degrees of partial melting (∼10–20%) of the spinel lherzolite source. Trace element modelling reveals the influence of mid-oceanic ridge basalt (MORB)-melt interaction and subduction-related mantle alterations.The anorthosite and gabbro samples yield zircon 206Pb / 238U weighted mean ages of 93.2 ± 1.0 Ma and 442.5 ± 4.0 Ma, 383.9 ± 3.3 Ma respectively. However, the harzburgite sample records multiple age population including Paleo-Mesoproterozoic grains ranging in age from 1216 to 1931 Ma as well as Neoproterozoic and Phanerozoic grains of 607 Ma, 594 Ma, 237 Ma, 105 Ma, and 85 Ma suggesting a long-lived supra subduction zone in the Burma-Andaman-Java subduction zone where the South Andaman ophiolite suite was formed, evolved, and was emplaced.

From the mantle source to the crustal sink: magmatic differentiation and sulfide saturation of the Paleoproterozoic komatiites of the Central Lapland Greenstone Belt, Finland

Paleoproterozoic (2.05 Ga) komatiites are widespread in the Central Lapland Greenstone Belt (CLGB), northern Finland. Close association with sulfur (S)-rich country rocks and spatiotemporal connection with the Cu-Ni(-PGE) deposits of Kevitsa and Sakatti make these komatiites interesting targets for sulfide deposit exploration. We provide whole-rock geochemical data from Sattasvaara komatiites and combine it with literature data to form a geochemical database for the CLGB komatiites. We construct a model for the komatiites from adiabatic melting of the mantle source to fractional crystallization at crustal conditions. Using MELTS, we calculate three parental melts (MgO = 20.6–25.7 wt%) in equilibrium with Fo92, Fo93, and Fo94 olivine for the CLGB komatiites. Based on REEBOX PRO simulations, these parental melts can form from a single mantle source by different pressures and degrees of melting when the potential temperature is 1575–1700 °C. We calculate ranges of S contents for the parental melts based on the different mantle melting conditions and degrees of melting. We use Magma Chamber Simulator to fractionally crystallize the parental melt at crustal conditions. These simulations reproduce the major element oxide, Ni, Cu, and S contents from our komatiite database. Simulated Ni contents in olivine are compatible with literature data from Kevitsa and Sakatti, hence providing a baseline to identify Ni-depleted olivine in CLGB komatiites and related intrusive rocks. We show that fractional crystallization of the komatiitic parental melt can form either Ni-rich or Cu-rich sulfide melt, depending on the initial Ni and S content of the parental melt.

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.

Fossil metasomatized and newly-accreted fertile lithospheric mantle volumes beneath the Bakony-Balaton Highland Volcanic Field (central Carpathian-Pannonian region)

This study focuses on upper mantle xenoliths from Sabar-hill (Bakony-Balaton Highland Volcanic Field), a newly discovered locality of mantle xenoliths, situated in the central Carpathian-Pannonian region (CPR). The investigated seven peridotite and one pyroxenite xenoliths record two episodes in the geochemical evolution of the local subcontinental lithospheric mantle. The moderately deformed Group I xenoliths reveal modal contents (≥9 vol% orthopyroxene), major element characteristics (depletion in Al2O3 and CaO in orthopyroxenes) and trace element features (e.g., enrichment in U, Pb and Sr in clinopyroxenes) typical in supra-subduction zones (i.e., mantle wedge) xenoliths. All these suggest that the studied xenoliths were formed by silica-rich fluid/melt - peridotitic wall rock interactions. The tectonic background to the formation of Group I xenoliths is likely linked to the subduction of an oceanic slab during the Mesozoic-Paleogene. This may have happened far from the current position of Sabar-hill, to where the lithosphere, including the metasomatized mantle volume, was transferred via plate extrusion. The less deformed, dominantly protogranular and porphyroclastic Group II xenoliths are depleted in light rare earth elements (LREE). In contrast, they are enriched in clinopyroxene (≥11 vol%), incompatible major elements (e.g., Al and Na) and heavy rare earth elements (HREE), by a minimum of four times compared to primitive mantle. All these suggest that Group II xenoliths represent a geochemically fertile mantle partition and the degree of partial melting affected this mantle section was limited (≤7% based on trace elements). Group II xenoliths have higher equilibrium temperatures (1088–1168 °C) compared to Group I xenoliths (1018–1089 °C). The lack of petrographic and geochemical evidence pointing to metasomatism-related annealing in Group II xenoliths suggests that they are derived from a greater depth. It follows that Group II xenoliths could represent prior asthenosphere volumes which recently accreted to the lithosphere. The lithospherization of the upper part of the asthenosphere may have initiated after the tectonic inversion of the region, when compressional tectonic regime beneath the CPR gradually overtook the former extensional one in the last ∼8 Ma. The high bulk structural hydroxyl content in both Group I (22–1699 ppm) and II (55–320 ppm) xenoliths is likely linked to the subduction events that took place in the CPR from the Mesozoic onwards. These events transported significant amounts of fluid (H2O) back to the upper mantle, affecting both the mantle wedge and the asthenosphere.

Geodynamics and Ore Content of Proterozoic Maphites in the Central Part of the Aldan-Stanovoy Shield (Southern North Asian Craton)

The study of diverse mantle-derived igneous complexes is important for interpreting geodynamic events, ore deposits formation mechanisms, and ore-forming fluid sources. Modern studies of orogenic gold deposits in the Precambrian metamorphosed terranes emphasize the importance of subduction-enriched lithospheric mantle in the ore formation processes. Orogenic gold mineralization in the Nimnyr terrane of the Aldan-Stanovoy shield is confined to the outcrops of mafic granulites from the Medvedev complex, intruded and metamorphosed 1.92–1.90 Ga ago at the final stage of the collision process. The Medvedev complex and ore bodies are intersected by non-metamorphosed dolerites of the 1.87 Ga Timpton-Gynym and 1.75 Ga Timpton-Algamai dike belts formed under conditions of post-collisional and intracontinental extension. The mantle-derived igneous complexes, presenting in a variety of geodynamic settings and ore mineral formation stages, make it possible to identify compositional and evolutionary features of the mantle in connection with ore formation processes. To do this, there were determined rock-forming oxide and trace element concentrations in pre-ore mafic granulites of the Medvedev complex and post-ore dolerites. Based on the geochemical data, there was a reconstruction of rock and mantle source type formation conditions. It was found that the rocks of the Medvedev complex are plume-derived. Doleritic melt formation was contributed to by the subduction-enriched lithospheric mantle material. There is a possility of different degrees of source melting and interaction of plume with the enriched lithospheric mantle at the final stage of the collision process. The obtained results can be used to refine the geodynamic models of gold mineralization formation in the central part of the Aldan-Stanovoy shield. There has been proposed one of the standard models.

Spatially heterogeneous discharge of glacial meltwater to drainages surrounding the ablating Coropuna ice cap, Peruvian Andes

ABSTRACT As tropical glaciers recede in response to global warming, glaciated watershed resilience is threatened by the loss of these important water reservoirs. In the Cordillera Occidental, Peru, we investigate the importance of glacial meltwater to mountain groundwater recharge and subsequent discharge to streams surrounding the rapidly retreating Coropuna ice cap. Using stable isotopes, solute concentrations, and radiocarbon activity, our results suggest that in remote ungauged systems, glacial input can be determined via geochemical analysis and a simple mixing model approach. We find that the proportion of glacial meltwater contribution to groundwater and surface waters is spatially heterogeneous. Differences between drainages are attributed to the degree of melting of headwater glaciers and the routing of waters through volcanic strata. These results provide evidence that glacial meltwater recharge is highly variable even within the same mountain block. Furthermore, the effects on streamflow will be disproportionate as the watersheds approach an ice-free state.

Accretion of “young” Phanerozoic subcontinental lithospheric mantle triggered by back-arc extension—the case of the Ivrea-Verbano Zone

The subcontinental lithospheric mantle (SCLM) beneath Phanerozoic regions is mostly constituted by fertile lherzolites, which sharply contrast with cratonic mantle made of highly-depleted peridotites. The question of whether this chemical difference results from lower degrees of melting associated with the formation of Phanerozoic SCLM or from the refertilization of ancient depleted SCLM remains a subject of debate. Additionally, the timing and geodynamic environment of accretion of the fertile SCLM in many Phanerozoic regions are poorly constrained. We here document new geochemical and Nd-Hf isotopic data for orogenic lherzolite massifs from the Ivrea-Verbano Zone (IVZ), Southern Alps. Even though a few Proterozoic Re depletion ages are locally preserved in these mantle bodies, our data reveal that the IVZ lherzolitic massifs were “recently” accreted to the SCLM in the Upper Devonian (ca. 370 Ma) during Pangea amalgamation, with a petrochemical evolution characterized by low-degree (~ 5–12%) depletion and nearly contemporaneous pervasive to focused melt migration. The lithospheric accretion putatively took place through asthenospheric upwelling triggered by Variscan intra-continental extension in a back-arc setting related to the subduction of the Rheic Ocean. We thus conclude that the fertile sections of Phanerozoic SCLM can be accreted during “recent” events of back-arc continental extension, even where Os isotopes preserve memories of melting events in much older times.

A HIMU-like component in Mariana Convergent Margin magma sources during initial arc rifting revealed by melt inclusions

Compositions of island arc and back-arc basin basalts are often used to trace the recycling of subducted materials. However, the contribution of subducted components to the mantle source during initial arc rifting before back-arc basin spreading is not yet well constrained. The northernmost Mariana arc is ideal for studying this because the transition from rifting to back-arc spreading is happening here. Here we report major and trace element and Pb isotopic compositions of olivine-hosted melt inclusions from lavas erupted during initial rifting at 24°N (NSP-24) and compare them with those in active arc front at 21°N and mature back-arc basin at 18°N. NSP-24 high-K melt inclusions have highly radiogenic Pb compositions and are close to those of the HIMU end-member, suggesting the presence of this component in the magma source. The HIMU-like component may be stored in the over-riding plate and released into arc magma with rifting. HIMU-type seamounts may be subducted elsewhere beneath the Mariana arc, but obvious HIMU-type components appear only in the initial stages of arc rifting due to the low melting degree and being consumed during the process of back-arc spreading.

Calcium isotopic composition of the bulk silicate Earth: A komatiite perspective

Calcium (Ca) isotopic composition of the mantle has been previously estimated as δ44/40Ca. However, δ44/40Ca of the mantle records both its stable isotopic composition and excess/deficit in radiogenic 40Ca that can be produced by the decay of 40K. Here, we present the high-precision Ca isotopic compositions of 16 Archean komatiites from five representative localities (Barberton, Munro, Abitibi, Yilgarn, and Taishan). Due to the high degrees of partial melting that led to their formation, these komatiites are used here to simultaneously estimate ε40/44Ca, δ44/40Ca, and δ44/42Ca of the bulk silicate Earth. The komatiites exhibit a resolvable negative ε40/44Ca relative to SRM915a, with an average of −0.6 ± 0.1 (2SE, 95 c.i.% if not specified). Given the lack of significant differences among the samples and the non-zero value of ε40/44Ca, we suggest using δ44/42Ca to report the stable Ca isotopic composition in this study. Excluding one sample (SD6/400), which may have been affected by alteration, the remaining komatiites display relatively homogeneous δ44/42Ca, ranging from 0.39 ± 0.02‰ (2SE, N = 16) to 0.49 ± 0.02‰ (2SE, N = 10), defining a mean of 0.44 ± 0.02‰ (2SE, N = 15). The absence of correlations between δ44/42Ca and LOI, Sr/Nd, (Dy/Yb)N, and (La/Sm)N among the komatiites indicates that post-eruption alteration, the influence of residual garnet, as well as prior melt extraction from the mantle source have had a negligible influence on their stable Ca isotopic compositions. Based on available experimental petrological data and isotope fractionation factors, it is demonstrated that komatiites were insignificantly fractionated from their mantle sources (i.e., <0.02% in δ44/42Ca). Their δ44/42Ca represents the ambient mantle at the time of segregation. Accordingly, we suggest that the average δ44/42Ca and ε40/44Ca of the komatiites measured here are representative of the bulk silicate Earth. Our new estimate can serve as a reference point for Ca isotopes as a two-dimensional tracer for sources and geological processes.

Elemental and oxygen isotopic fractionation recorded in highly vaporized cosmic spherules from Widerøefjellet, Sør Rondane Mountains (East Antarctica)

AbstractUpon passage through Earth's atmosphere, micrometeorites undergo variable degrees of melting and evaporation. Among the various textural and chemical groups recognized among cosmic spherules, that is, melted micrometeorites, a subset of particles may indicate anomalously high degrees of vaporization based on their chemical and isotopic properties. Here, a selection of such refractory element‐enriched cosmic spherules from Widerøefjellet (Sør Rondane Mountains, East Antarctica) is characterized for their petrographic features, major and trace element concentrations (N = 35), and oxygen isotopic compositions (N = 23). Following chemical classification, the highly vaporized particles can be assigned to either the “CAT‐like” or the “High Ca‐Al” cosmic spherule groups. However, through the combination of major and trace element concentrations and oxygen isotopic data, a larger diversity of processes and precursor materials are identified that lead to the final compositions of refractory element‐enriched particles. These include fragmentation, disproportional sampling of specific mineral constituents, differential melting, metal bead extraction, redox shifts, and evaporation. Based on specific element concentrations (e.g., Sc, Zr, Eu, Tm) and ratios (e.g., Fe/Mg, CaO + Al2O3/Sc + Y + Zr + Hf), and variations of O isotope compositions, “CAT‐like” and “High Ca‐Al” cosmic spherules likely represent a continuum between mineral endmembers from both primitive and differentiated parent bodies that experienced variable degrees of evaporation.

Accretion of the Lower Oceanic Crust at Fast-Spreading Ridges: Insights from Hess Deep (East Pacific Rise, IODP Expedition 345)

Abstract At mid-ocean ridges, melts that formed during adiabatic melting of a heterogeneous mantle migrate upwards and ultimately crystallize the oceanic crust. The lower crustal gabbros represent the first crystallization products of these melts and the processes involved in the accretion of the lowermost crust drive the chemical evolution of the magmas forming two thirds of Earth’s surface. At fast-spreading ridges, elevated melt supply leads to the formation of a ⁓6-km-thick layered oceanic crust. Here, we provide a detailed petrochemical characterization of the lower portion of the fast-spread oceanic crust drilled during IODP Expedition 345 at the East Pacific Rise (IODP Holes U1415), together with the processes involved in crustal accretion. The recovered gabbroic rocks are primitive in composition and range from troctolites to olivine gabbros, olivine gabbronorites and gabbros. Although textural evidence of dissolution-precipitation processes is widespread within this gabbroic section, only the most interstitial phases record chemical compositions driven by melt-mush interaction processes during closure of the magmatic system. Comparing mineral compositions from this lower crustal section with its slow-spreading counterparts, we propose that the impact of reactive processes on the chemical evolution of the parental melts is dampened in the lower gabbros from magmatically productive spreading centres. Oceanic accretion thereby seems driven by fractional crystallization in the lower gabbroic layers, followed by upward reactive percolation of melts towards shallower sections. Using the composition of clinopyroxene from these primitive, nearly unmodified gabbros, we estimate the parental melt trace element compositions of Hess Deep, showing that the primary melts of the East Pacific Rise are more depleted in incompatible trace elements compared to those formed at slower spreading rates, as a result of higher melting degrees of the underlying mantle.

Effects of plasma spraying process on microstructure and mechanical properties of Cr2AlC/410 composite coatings

To investigate the influences of Cr2AlC mass fraction and supersonic plasma spraying process on the microstructure and mechanical properties of Cr2AlC reinforced 410 stainless steel composite coatings, the coatings containing different mass fractions of Cr2AlC were prepared and investigated. The composite coating exhibited low porosity and high adhesion strength. The addition of Cr2AlC significantly enhanced the hardness of the composite coatings through particle strengthening. However, when the mass fraction of Cr2AlC was 20%, the aggregation of Cr2AlC resulted in a strong decrease in the coating preparation efficiency, as well as a decline in adhesion strength. In the supersonic plasma spraying process, the Ar flow rate mainly influenced the flight velocity of the particles, while the H2 flow rate and the current mainly affected the temperature of the plasma torch. Consequently, all of them influenced the melting degree of particles and the quality of the coating. The lowest porosity and the highest hardness and adhesion strength could be obtained when the Ar flow rate is 125 L/min, the H2 flow rate is 25 L/min, and the current is 385 A.