Host Basalt Research Articles

Deconvolution of high and low-temperature alteration processes along the contact zones of basaltic-dike intrusions in basaltic host rocks of different permeabilities – implications for geothermal exploration

Magmatic intrusions serve as crucial heat sources for geothermal systems, facilitating mass transfer, mineral transformations, and elemental exchange, which result in the formation of contact aureoles. While these processes have been extensively studied in large intrusive complexes in ancient geological formations, understanding of their occurrence in active geothermal systems remains limited. The extreme conditions present in active volcanic systems often obscure the geochemical processes occurring within host rock/intrusion zones. Uncertainties persist regarding whether relatively small dike intrusions (∼0.5 m thick) possess sufficient heat content to induce textural or geochemical changes in the surrounding wall rock, and what implications this may hold for future exploratory drilling projects. The analyses in this study were conducted on two distinct outcrops, each featuring 50-cm thick basaltic intrusions within both high- and low-permeability basaltic host rocks. The low permeability host rock hosts high-temperature mineral phases (>800 °C), such as sanidine + hedenbergite + albite-rich plagioclase in the contact zone, which we interpret to have formed during partial melting. Immobile, incompatible trace elements (such as Zr, Nb and La) retain the signatures of partial melting in both outcrops. We demonstrate that low degree partial melting (F ≈ 3–10 %) results in a compositional shift in the host rock from basalt to dacite and/or trachyandesite. Thermal modelling suggests that these small dikes, along with their partial melts in the contact zone, form over a very short period of time (< 1 day), but can elevate the ambient temperature. This type of events play an important role in the development of active geothermal systems. In theory, these small dikes do not pose a significant risk during geothermal drilling, unless they are too small to be detected during geophysical exploration.

Xenolith petrography for identification of substratum lithology of an active volcano: a case study of Lava Kekep, Sundoro Volcano, Central Java

Abstract Sundoro volcano is an active Quaternary stratovolcano in Central Java, located approximately 50 km west of the Merapi volcano. It is attributed to parasitic cones producing basaltic lava flow. We present our findings of sedimentary and igneous xenoliths embedded in the basaltic lava of the Kekep parasitic cone. Xenolith is one of the key materials to help identify various lithologies composing the substratum of a volcano. Twelve thin sections were analysed through detailed petrographic observation. We identified variations of xenolith lithology consisting of sedimentary rock, particularly siltstone, and igneous rocks such as dunite, pyroxenite and lherzolite. Detailed accounts of substratum lithologies comprises of (a) siltstone composed of &gt;0.125-0.5 mm size plagioclase crystals and silicic fine-grained matrix, (b) pyroxenite consists of massive interlocking 1-5 mm size clinopyroxene and few &lt;1 – 1 mm orthopyroxene, (c) dunite consists of 1 - 2 mm phenocryst of plagioclase, pyroxene, and olivine, embedded in a minor groundmass of plagioclase and pyroxene, (d) lherzolite consists of massive interlocking olivine crystals 1-5 mm in size. The analysis results identify gradual or groundmass and sharp contact between basaltic host rock and xenoliths (pyroxenite, lherzolite, dunite and siltstone). Other characteristics of the twelve samples includes (a) &lt;1 mm minerals fragments embedded in silt to clay-grained lithic sandstone, (b) siltstone showing chilled-baked margin in hand sample specimen, (c) identified disequilibrium textures in basaltic lava, and contact between xenolith and host rock. Substratum lithology along the Java arc is likely composed of Tertiary sedimentary rock in the upper part, transitions of granitoid – gabbroic diorite crustal material in the lower part, with ultramafic crustal material under Sundoro.

Carbon Dioxide Capture and Storage (CCS) in Saline Aquifers versus Depleted Gas Fields

Saline aquifers have been used for CO2 storage as a dedicated greenhouse gas mitigation strategy since 1996. Depleted gas fields are now being planned for large-scale CCS projects. Although basalt host reservoirs are also going to be used, saline aquifers and depleted gas fields will make up most of the global geological repositories for CO2. At present, depleted gas fields and saline aquifers seem to be treated as if they are a single entity, but they have distinct differences that are examined here. Depleted gas fields have far more pre-existing information about the reservoir, top-seal caprock, internal architecture of the site, and about fluid flow properties than saline aquifers due to the long history of hydrocarbon project development and fluid production. The fluid pressure evolution paths for saline aquifers and depleted gas fields are distinctly different because, unlike saline aquifers, depleted gas fields are likely to be below hydrostatic pressure before CO2 injection commences. Depressurised depleted gas fields may require an initial injection of gas-phase CO2 instead of dense-phase CO2 typical of saline aquifers, but the greater pressure difference may allow higher initial injection rates in depleted gas fields than saline aquifers. Depressurised depleted gas fields may lead to CO2-injection-related stress paths that are distinct from saline aquifers depending on the geomechanical properties of the reservoir. CO2 trapping in saline aquifers will be dominated by buoyancy processes with residual CO2 and dissolved CO2 developing over time whereas depleted gas fields will be dominated by a sinking body of CO2 that forms a cushion below the remaining methane. Saline aquifers tend to have a relatively limited ability to fill pores with CO2 (i.e., low storage efficiency factors between 2 and 20%) as the injected CO2 is controlled by buoyancy and viscosity differences with the saline brine. In contrast, depleted gas fields may have storage efficiency factors up to 80% as the reservoir will contain sub-hydrostatic pressure methane that is easy to displace. Saline aquifers have a greater risk of halite-scale and minor dissolution of reservoir minerals than depleted gas fields as the former contain vastly more of the aqueous medium needed for such processes compared to the latter. Depleted gas fields have some different leakage risks than saline aquifers mostly related to the different fluid pressure histories, depressurisation-related alteration of geomechanical properties, and the greater number of wells typical of depleted gas fields than saline aquifers. Depleted gas fields and saline aquifers also have some different monitoring opportunities. The high-density, electrically conductive brine replaced by CO2 in saline aquifers permits seismic and resistivity imaging, but these forms of imaging are less feasible in depleted gas fields. Monitoring boreholes are less likely to be used in saline aquifers than depleted gas fields as the latter typically have numerous pre-existing exploration and production well penetrations. The significance of this analysis is that saline aquifers and depleted gas fields must be treated differently although the ultimate objective is the same: to permanently store CO2 to mitigate greenhouse gas emissions and minimise global heating.

Trace Element and Sulfur Isotope Signatures of Volcanogenic Massive Sulfide (VMS) Mineralization: A Case Study from the Sunnhordland Area in SW Norway

The Sunnhordland area in SW Norway hosts more than 100 known mineral occurrences, mostly of volcanogenic massive sulfide (VMS) and orogeny Au types. The VMS mineralization is hosted by plutonic, volcanic and sedimentary lithologies of the Lower Ordovician ophiolitic complexes. This study presents new trace element and δ34S data from VMS deposits hosted by gabbro and basalt of the Lykling Ophiolite Complex and organic-rich sediments of the Langevåg Group. The Alsvågen gabbro-hosted VMS mineralization exhibits a significant Cu content (1.2 to &gt;10 wt.%), with chalcopyrite and cubanite being the main Cu-bearing minerals. The enrichment of pyrite in Co, Se, and Te and the high Se/As and Se/Tl ratios indicate elevated formation temperatures, while the high Se/S ratio indicates a contribution of magmatic volatiles. The δ34S values of the sulfide phases also support a substantial influx of magmatic sulfur. Chalcopyrite from the Alsvågen VMS mineralization shows significant enrichment in Se, Ag, Zn, Cd and In, while pyrrhotite concentrates Ni and Co. The Lindøya basalt-hosted VMS mineralization consists mainly of pyrite and pyrrhotite. Pyrite is enriched in As, Mn, Pb, Sb, V, and Tl. The δ34S values of sulfides and the Se/S ratio in pyrite suggest that sulfur was predominantly sourced from the host basalt. The Litlabø sediment-hosted VMS mineralization is also dominated by pyrite and pyrrhotite. Pyrite is enriched in As, Mn, Pb, Sb, V and Tl. The δ34S values, which range from −19.7 to −15.7 ‰ VCDT, point to the bacterial reduction of marine sulfate as the main source of sulfur. Trace element characteristics of pyrite, especially the Tl, Sb, Se, As, Co and Ni concentrations, together with their mutual ratios, provide a solid basis for distinguishing gabbro-hosted VMS mineralization from basalt- and sediment-hosted types of VMS mineralization in the study area. The distinctive trace element features of pyrite, in conjunction with its sulfur isotope signature, have been identified as a robust tool for the discrimination of gabbro-, basalt- and sediment-hosted VMS mineralization.

Remobilization of carbon in the lithospheric mantle during decratonization

The sub-continental lithosphere mantle (SCLM) contains considerable amounts of carbon and is responsible for significant CO2 emissions via magmatism during major heating and rifting events. However, the role of SCLM in Earth's carbon cycle during other geological processes, such as decratonization, a process of major removal and replacement of cratonic SCLM, remains poorly understood. The North China Craton is an archetype of Archean craton having been decratonized during the Mesozoic with extensive magmatism developed. Here we show that the primary melt of Early Cretaceous Yixian basalts contained abundant CO2 (0.6 wt%), through investigating melt inclusions trapped in olivine crystals. The melt inclusions are alkalic and chemically distinct from their sub-alkalic host basalts, which could be explained by that the primary alkalic melt was reactively modified during magma ascent in the SCLM. The primary melt has geochemical characteristics consistent with partial melting of a veined carbon-rich amphibole clinopyroxenite source in the SCLM formed metasomatically during circum-craton subduction events. The findings highlight that the carbon introduced into the cratonic SCLM during subduction could be subsequently reactivated and transferred to the crust and atmosphere through decratonization-related magmatism, which might represent a major process regarding the SCLM in regulating the terrestrial carbon cycle.

Vanadium rich Fe-Ti oxide and Cu-sulphide mineralization in Paleoproterozoic Mangikhuta volcanics, Central Indian Craton: metallogenic and petrogenetic implications

This study reports for the first time, Fe-Ti-V oxide and Cu-sulphide mineralization in Paleoproterozoic Mangikhuta basalt Formation, central Indian craton. Electron microprobe and laser ablation analyses of Fe-oxides reveal high FeO (45–69 wt %), TiO2 (19–53 wt %), V (1860 - 4990 ppm), Zr (394–3130 ppm), Nb (55–285 ppm) and Zn (324–668 ppm). Interelemental relationships of Fe-oxides reveal their magmatic origin. High concentration of lithophile elements in Fe-oxides besides V and Ni trends in incompatible element plots indicate their origin from mafic melt. High Cu content (269 and 314 ppm) in the host basalt samples along with chalcopyrite mineralization observed during ore petrography indicates sulphide saturation of Mangikhuta magma. The chondrite normalized rare earth element (REE) plots of the host rock samples are overall similar to the earlier reported Mangikhuta REE patterns, which indicates genetic relation of Fe-oxides and Cu-sulphides with Mangikhuta volcanism. Fe-oxide and Cu-sulphide mineralization in Mangikhuta basalt is related to hydrous and oxygen rich arc related mafic melt intrusion into the Khairagarh back arc basin. Sulphide saturation in Mangikhuta basalt was initiated due to precipitation of Fe-oxides from the evolved melt whereas addition of fresh batch of hydrous and oxygen rich melt derived from the arc-related mantle source increased oxygen fugacity of residual melt that resulted in alternate phases of high oxygen and Sulphur fugacity and precipitation of Fe-oxide and Cu-sulphides from the melt.

Geochemical modeling of basic pegmatites from Foz do Iguaçu in the Itaipu region, western Paraná, Brazil

In the westernmost portion of the state of Paraná, in the city of Foz do Iguaçu, basic pegmatites occur within inflated pahoehoe basaltic flows of the Paraná Igneous Province. Both rock groups are composed of similar mineral assemblages which include labradorite, augite, ilmenite and titano-magnetite, but pegmatites are medium to coarse grained, whereas basalts are fine grained. Host rocks are geochemically classified as basalts (s.s.), while pegmatitic segregations are basaltic andesite and basaltic trachy-andesite, according to the TAS [(Na2O + K2O) x SiO2] diagram. Based on the AFM [(Na2O + K2O) – (FeOtot) – (MgO)] diagram, all rocks are included in the tholeiitic series, classified as high-Fe tholeiite basalts in the cation plot [Al x (Fetot + Ti) x Mg]. Variation diagrams indicate enrichment of SiO2, TiO2, FeO, Na2O, K2O e P2O5 coupled with depletion of Al2O3, MgO e CaO from basalts to pegmatites. La/LuN ratios range between 5.4 to 6.5 for basalts and 6.3 to 7.9 for pegmatites reflecting a greater degree of fractionation in the pegmatitic segregations and, therefore, their more evolved character. Plagioclase crystals in pegmatites have higher sodic contents when compared to those found in host basalts, while CaO content in basaltic pyroxenes is higher than those from pegmatitic segregations. Trace element geochemical modeling results show that it is possible to generate basic pegmatites from basalts comprised of 55 wt.% plagioclase, 35 wt.% clinopyroxene and 10 wt.% titanium and iron oxides. The process occurs as fractional crystallization with 50% residual melt.

Interstitial carbonates in pillowed metabasaltic rocks from the Pilbara Craton, Western Australia: A vestige of Archean seawater chemistry and seawater-rock interactions

Calcites hosted in the interpillow void spaces of extremely well preserved, 3.47–3.12 Ga pillow lavas of the Archean Pilbara Craton, Australia, provide new geochemical insights into the composition of Archean seawater and its interaction with basaltic crust. We present a comprehensive dataset of major and trace elements, radiogenic 147Sm-143Nd, 87Rb-87Sr, and stable C-O isotopes for these calcites.Based on their elemental composition and Post-Archean Australian Shale-normalized rare earth element and yttrium (REYPAAS) patterns, two types of calcites can be distinguished. Type I (n = 9) are coarse-grained calcites with elevated Mn concentrations (478–14,790 ppm) and high REY concentrations that match the basaltic host rock compositions. Petrographic textures in combination with geochemical data indicate precipitation from boiling seawater trapped between pillows during basalt eruption. Small light-REYPAAS depletions, only slightly super-chondritic Y/Ho (34.5–39.1), and low δ18O(VSMOW2) (9.25–16.66 ‰) suggest relatively high fluid-rock interactions of boiling seawater with the basaltic host rock. These characteristics are similar to those of interstitial carbonates found in modern mid-ocean ridge basalts. Type II carbonates (n = 6) are fine-grained, low-Mn (13.7–420 ppm) calcites that exhibit comparably high Sr concentrations (825–2,516 ppm) and anoxic modern seawater-like REYPAAS patterns, strong super-chondritic Y/Ho (39.1–55.8) and δ18O(VSMOW2) values of 9.10–13.18 ‰. These features hint to direct calcite precipitation from seawater within the degassing space with little fluid-rock interaction.Almost all (Type I and Type II) of the calcites investigated here, combined with their respective host rocks, yield well-defined Sm-Nd isochron ages for the Apex (3,466 ± 23 Ma), Euro (3,365 ± 43 Ma), Honeyeater (3,187 ± 33 Ma), and Bradley (3,131 ± 24 Ma) formations. Only calcites from the Mt. Ada Basalt and their associated host rocks show a ∼ 100 Ma younger Sm-Nd isochron age (3,361 ± 22 Ma) compared to the U-Pb zircon age of formation.Highly radiogenic initial Sr isotope compositions of the high-Mn calcites (Type I) and low Sr concentrations, suggest Sr remobilization for these calcites. In contrast, the low-Mn calcite (Type II) exhibit higher Sr concentrations and lower 87Sr/86Sr(i) of 0.7010–0.7011 at 3.34 Ga and 0.70137 at 3.12 Ga, suggesting increasing influence of crustal weathering on the composition of Paleoarchean seawater through time, and progressive decoupling from the Archean mantle (87Sr/86Sr(i) = 0.7003–0.7007).

Insight into a rift volcanism with the petrogenesis of ultramafic enclaves and the host basalts: Kula Volcanic Field, Western Anatolia, Turkey

The Quaternary Kula Volcanic Field (KVF), located on an E-W trending Plio-Quaternary horst, consists of at least 80 scoria cones, associated fissural lava flows and maars. Volcanic activity is divided into three main stages as BI (ca. 2-0.9 Ma), BII (ca. 300-50 ka) and BIII (ca. 10-0.7 ka). The volcanic products of BII and BIII contain ultramafic enclaves. They are generally spherical or elliptical varying in size from 1.7), and Nb/U (>31) ratios, reflecting that crustal contamination is insignificant. Petrogenetic modelling shows that Kula enclaves and host rocks are probably originating from 3-5% partial melting of garnet and spinel-peridotite mantle source mixtures. Rb/Sr (0.03-0.10) and slightly high Ba/Rb (9.5-23) ratios for host rocks demonstrate the presence of amphibole and phlogopite in their source, while low Rb/Sr (0.01-0.09) and high Ba/Rb (18-115) reveal that amphibole is present in the source. Estimated T/P conditions of early crystallizations are between 19.5–9.5 kbar, 63–31 km, and 1260–1130 °C for host rocks and 18.3–11.1 kbar, 59–36 km, and 1324–1208 °C for the enclaves, which suggests that the crystallization might have taken place in the lithospheric mantle.

Diffusion-driven Zn and Mg isotope fractionation in magmatic Fe-Ti-Cr oxides and implications for timescales of magmatic processes

Diffusion-driven isotopic fractionation during crystal growth and crystal-melt interaction has important implications for identifying diffusive processes and estimating the timescales of magmatic processes. Especially, Fe-Ti-Cr oxides are common crystalline phases in both relatively unfractionated and highly evolved basaltic magmas and may record the evolution history of the host magmas. High-precision Zn and Mg isotopic compositions for a set of Fe-Ti-Cr oxides and their host lavas in three types of Cenozoic basalts (sodic, transitional and potassic) from northeast China are reported in this study. These oxide crystals exhibit a wide range of chemical compositions (e.g., FeOT = 20.6–49.1 wt%, TiO2 = 1.38–8.70 wt%, Cr2O3 = 16.2–43.1 wt%), reflecting substantial chemical disequilibrium with their host magmas. A large range of δ66ZnJMC-Lyon from −1.12‰ to 0.24‰ and δ26MgDSM-3 from 0.04‰ to 0.80‰ is observed in oxides (n = 17) from all three lava types. Compared with the host basalts, the oxides are enriched in lighter Zn with △66Znoxide-melt (δ66Znoxide–δ66Znmelt) from −1.43‰ to −0.19‰ and heavier Mg with △26Mgoxide-melt (δ26Mgoxide–δ26Mgmelt) from 0.43‰ to 1.13‰. Comparison with existing theoretical work indicates that these isotopic offsets, even if compositional effects of oxides are considered, are too large to be in equilibrium. There are negative correlations between tetrahedral-site Fe2+ + Zn2+ and Mg2+ (R2 = 0.97) and between δ66Zn and δ26Mg (R2 = 0.66) for the oxides, which are best attributed to isotopic fractionation induced by Zn–Mg inter-diffusion, supported by the core-to-rim increase of Zn contents and decline of Mg contents in zoned oxides. Our results thus suggest that inter-diffusion between Mg and Zn can occur during crystal growth and reaction with host magmas, which is commonly observed between Mg and Fe, and this process is accompanied by strong Zn and Mg isotope fractionations. Such fractionations can be utilized to estimate the timescales for the formation of zoned minerals. Numerical simulation yields a short (∼35 days) diffusion interval for the observed Zn and Mg isotopic variations in the investigated oxide crystals, which indicates rapid cooling of the host lavas. Our results also indicate that fractional crystallization of oxides would lead to slight δ66Zn increase and δ26Mg decrease in the residual melts. Such fractionation should be considered while characterizing the Zn and Mg isotopic compositions of evolved magmas with low MgO and Zn contents.

Spectroscopic comparisons of two different terrestrial basaltic environments: Exploring the correlation between nitrogen compounds and biomolecular signatures

Life detection in the solar system relies on the unambiguous identification of signatures of life and habitability. Organic molecules are essential to life as we know it, and yet many organic compounds are ubiquitous in the solar system and can be synthesized abiotically; thus, their presence alone is not indicative of life. On Earth, chemical signatures of life's processes are often left behind in minerals through the biologically induced formation of secondary minerals or intermediary organic complexes. In natural rocks biomolecules and organic species often co-occur with minerals, and their overlapping peaks can create difficulties in interpretation. In the process of identifying the minerals and organic species in our basaltic samples we noticed signatures for cyanates co-occurring with organic molecules. Cyanates are an overlooked group of nitrogen compounds in which C is bonded to N (e.g., OCN− or SCN−) that often co-occur with urea and ammonium in environments where microorganisms are present. These compounds are common in many terrestrial and oceanic environments and play an important role in biogeochemical nitrogen cycling. In natural systems, these compounds form as the result of multiple biogeochemical pathways, often from the interaction of microbes with a chemically active environment. These interactions leave behind signatures in the form biotic breakdown products such as urea or ammonium and organic reaction byproducts that are observable with spectroscopic methods. To explore these relationships, we used field-portable Raman spectrometers and laboratory micro-Raman imaging to characterize and compare samples collected from two different terrestrial basaltic environments, a lava tube on Mauna Loa, Hawaii, dominated by the precipitation of sulfate minerals and a geothermal stream at Hveragil, Iceland dominated by the precipitation of carbonate minerals. The Raman (RS) measurements were complemented by laser induced breakdown spectroscopy (LIBS), Long-wave Infrared (IR) LIBS, with the addition of gas chromatograph mass spectrometry (GC–MS) and inductively coupled plasma-mass spectrometry (ICP-MS) to identify cyanate compounds, biomolecules, and other nitrogenous compounds related to the breakdown or production of cyanate in host basalts and secondary precipitates. The RS data suggest that the reason for RS cyanate signatures in the carbonate samples could be due to luminescence artifacts while those detected in the host basalts may be due to hydrolysis chemistry. The cyanate signatures detected in the lava tube samples dominated by sulfates do not seem to be luminescence artifacts but may in fact be evidence of an active microbial nitrogen cycle. Our results inform the spectroscopic detection of cyanates in planetary analog environments and the challenges in their identification. Further work is needed to understand their potential as biosignatures on other planetary bodies.

Permian-Triassic flood basalts in the pre-Jurassic basement of the Arctic zone of the West Siberian platform

Research subject. Permian-Triassic flood basalts in the basement of the Arctic zone of the West Siberian Platform locate mainly in graben-rift structures. Flood basalts in this region remain to be understudied in comparison with other areas of its distribution, mainly due to the significant depth of their occurrence (4–6 km).Materials and methods. 36 core samples from 11 superdeep and deep boreholes were studied. Isotopic ratios were measured on mass spectrometers NEPTUNE PLUS (Nd, Sm) and TRITON PLUS (Rb, Sr). Bitumen were studied using a Raman spectrometer LabRAM HR800 Evolution. The Raman spectra were deconvoluted (“Peak fitting” procedure), and the bitumen conversion temperature was estimated.Results. About half of the samples of volcanic rocks underwent metamorphism of the prehnite-pumpellite and locally greenschist facies or intense low-temperature hydrothermal alteration. The studied basalts are close to typical flood basalts and are somewhat similar to island-arc volcanic rocks in terms of their geochemical characteristics. For the first time, thin inclusions of bitumen were found in the amygdalae of Permian-Triassic basalts in the superdeep borehole Tyumenskaya SG-6 at a depth of 7310.6 m. A high similarity of the studied volcanics by geochemical characteristics and the isotopic composition of Sr and Nd with the flood basalts of the Siberian platform is shown.Conclusions. The presence of a negative Ta, Nb, Ti anomaly, as well as a negative Ce anomaly, in some of the analyzed samples indicates a possible contamination of the basalts by island arc volcanics and volcanogenic-sedimentary rocks. The temperature of transformation of bitumen in inclusions in basalts from the well Tyumenskaya SG-6 according to Raman spectroscopy is estimated at 150–300°C and generally corresponds to the temperature of metamorphism of the host basalts. The presence of bitumen in the amygdalae may indicate the migration of hydrocarbons through the basalts.

Deciphering metasomatic events beneath Mindszentkálla (Bakony-Balaton Highland Volcanic Field, western Pannonian Basin) revealed by single-lithology and composite upper mantle xenoliths

Single-lithology and composite xenoliths from Mindszentkálla (Bakony-Balaton Highland Volcanic Field) in the Carpathian-Pannonian region record geochemical evolution of the subcontinental lithospheric mantle. The dominant single-lithology xenoliths are orthopyroxene-rich (22 vol% on average) harzburgites. Three composite xenoliths contain either two or more domains including dunite, olivine-orthopyroxenite, orthopyroxenite, apatite-bearing websterite and amphibole-phlogopite-bearing vein. The presence of different lithologies is a result of at least two metasomatic events that affected the lithospheric mantle. The first event resulted in orthopyroxene enrichment thus formed harzburgitic mantle volumes (Group I xenoliths). Major- and trace element distributions of the bulk harzburgites differ from the geochemical trends expected in residues of mantle melting. In contrast, petrographic and geochemical attributes suggest that the harzburgite was formed by silica-rich melt - peridotitic wall rock interactions in a supra-subduction zone. Within the Group I xenoliths, two subgroups were identified based on the presence or lack of enrichment in U, Pb and Sr. Since these elements are fluid mobile, their enrichment in certain Group I xenoliths indicate reaction with a subduction-related fluid, subsequent to the harzburgite formation. The effect of a second event overprints the features of the Group I xenoliths and is evidenced in all domains of two composite xenoliths (Group II xenoliths). The general geochemical character involves enrichment of basaltic major and minor elements (Fe, Mn, Ti, Ca) in the rock-forming minerals and convex-upward rare earth element (REE) patterns in clinopyroxenes. We suggest that the different domains represent reaction products with variably evolved basaltic melts of a single magmatic event. The tectonic background to the formation of Group I xenoliths is likely linked to the subduction of oceanic crust during the Mesozoic–Paleogene. This happened far from the current position of Mindszentkálla, to where the lithosphere, including the metasomatized mantle volume, was transferred via plate extrusion. The Group II xenoliths appear to bear the geochemical signature of a younger (Neogene) basaltic magmatic event, likely the same that produced the host basalt transporting the xenoliths to the surface.

Cu‐Isotope Evidence for Subduction Modification of Lithospheric Mantle

AbstractUltramafic xenoliths from southeastern Arizona, USA, provide evidence for Cu‐isotope heterogeneity in the lithospheric mantle. We report new data on Type I (Cr‐, Mg‐rich) peridotites, but also the first Cu‐isotope data for Fe‐Ti‐Al‐rich Type II pyroxenite (±amphibole) xenoliths. Whole rock δ65Cu values of the pyroxenites and cryptically metasomatized Type I lherzolites range to isotopically heavier compositions than asthenospheric mantle (i.e., up to +1.44‰ and +1.12‰, respectively, vs. ∼0‰ ± 0.2‰). Copper leached from the xenoliths using aqua regia, assumed to be hosted in interstitial sulfides, is even more variable (δ65Cu −0.78 to +3.88‰), indicating considerable isotopic heterogeneity within individual samples. Host basalts have low δ65Cu (−0.23‰ to −1.30‰), so basalt—xenolith interactions are not responsible for the compositional variations observed. While mass‐dependent fractionation may be partly responsible, metasomatism by fluids derived from recycled crustal materials is the predominant control on isotopic variations observed. Amphibole megacrysts and amphiboles separated from Type II amphibole‐bearing clinopyroxenite have normal, mantle‐like 18O/16O ratios but H‐isotope compositions (δ2HSMOW −82‰ to −45‰) that range between that of nominally anhydrous mantle (−80 ± 10‰) and seawater (0‰). Host basalts are also enriched in 34S relative to depleted asthenospheric mantle, having δ34SCDT values up to +8‰, i.e., compositions commonly attributed to a component of recycled seawater or hydrated oceanic crust. These new data suggest that formation of Type II metasomes in the lithospheric mantle beneath the Basin and Range Province was associated with subduction of the Farallon plate and not alkali basalt magmatism associated with Basin and Range extension.

Origin of gem-quality megacrysts in the Cenozoic alkali basalts from the Muling area, northeastern China

Megacrysts hosted by mantle-derived volcanic rocks provide valuable information on the chemical evolution and ascending history of the parental magmatic systems. However, the origin of the megacrysts has still been far from consensus. Here, we present systematic major- and trace-element data of a variety of gem-quality megacrysts in the Cenozoic basalts from Muling area, northeastern China, with the aim of investigating the origin and formational conditions of these megacrysts. The studied megacrysts are composed of clinopyroxene, garnet and orthopyroxene. All megacrysts are optically and compositionally homogeneous. Clinopyroxene megacrysts, with Mg# of 85.3–87.5, contain Al 2 O 3 of 6.64–8.07 wt%, thus can be ascribed as Al-augites. Garnet megacrysts are rich in pyrope component with Mg# of 74.0–76.0 and contain low Cr 2 O 3 less than 0.05 wt%. All orthopyroxene megacrysts are enstatite in composition. According to the color, they can be further divided into two groups (Group 1: greenish brown; Group 2: dark brown) which are apparently different in chemical composition. Group 2 orthopyroxenes have lower Mg# (85.1–86.3) and Cr 2 O 3 (0.09–0.18 wt%) and higher Al 2 O 3 (6.78–7.71 wt%) and TiO 2 (0.20–0.24 wt%), while Group 1 orthopyroxenes are characterized by higher Mg# (89.2–89.8) and Cr 2 O 3 (0.55–0.66 wt%) and lower Al 2 O 3 (5.13–5.93 wt%) and TiO 2 (0.11–0.15 wt%). Clinopyroxene, garnet and Group 2 orthopyroxene megacrysts can be easily distinguished with equivalent phases in accompanied peridotite xenoliths or peridotite xenoliths in other locations through eastern China by their major-element compositions. They show good correlations between Mg# and major-oxide contents (such as Al 2 O 3 and TiO 2 ), indicating their derivation from melts which had undergone variable extent of fractional crystallization. Mg# and trace-element compositions of their equilibrium melts are incomparable with those of host basalts. We infer clinopyroxene, garnet and Group 2 orthopyroxene megacrysts from Muling are most likely produced by fractional crystallization of basaltic melts that formed earlier than host basalts. Pressure estimations suggest clinopyroxene and garnet megacrysts from Muling were formed at 2.0–2.5 GPa, corresponding to a depth of 66–80 km, near the boundary between lithosphere and asthenosphere based on local lithosphere thickness. Compared with orthopyroxenes in accompanied peridotites, Group 1 orthopyroxene megacrysts have slightly lower Mg# and higher CaO and Al 2 O 3 , similar with those produced by basaltic melts-peridotite interaction experiments in which orthopyroxene precipitated with the expense of olivine and clinopyroxene. A revised model (modified from Liu and Ying (2019): Liu, Y.D., Ying, J.F., 2019. Origin of clinopyroxene megacrysts in volcanic rocks from the North China Craton: a comparison study with megacrysts worldwide. International Geology Review 62, 1845–1861.) has been proposed to decipher the formation of variable megacrysts from Muling. • Megacrysts from Muling consist of Opx, Cpx and Grt. • Cpx, Grt and Group 2 Opx are crystallization products of earlier-formed melts. • Group 1 Opx were probably produced by melt-peridotite reaction. • Megacrysts from Muling formed at 2.0–2.5 Gpa, near LAB.

Calcite U–Pb dating of altered ancient oceanic crust in the North Pamir, Central Asia

Abstract. The North Pamir, part of the western syntax of the India–Asia collision zone, preserves remnants of a poorly understood Paleozoic intra-oceanic subduction zone. To constrain the age of this ancient ocean floor, we analyzed calcite phases in vesicular basalt and basaltic volcanic breccia with U–Pb geochronology using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Dating of radial fibrous to equant spary calcite yielded three meaningful Visean–Serpukhovian ages. Rare-earth elements and yttrium (REE + Y) data reveal that the basaltic host rock of the calcite and oxidizing seawater are major sources of trace elements during calcite precipitation. U–Pb ages seem to be independent of REE + Y concentrations. Our results demonstrate the potential of calcite dating to constrain the age of ancient ocean floors. We challenge the hypothesis that a continuous early Paleozoic Kunlun Terrane extended from northern Tibet into the North Pamir.

Mantle Xenoliths from Huanul Volcano (Central-West Argentina): A Poorly Depleted Mantle Source under Southern Payenia

Huanul is a shield volcano with several lava flows hosting mantle xenoliths erupted during the Pleistocene (0.84 ± 0.05 Ma). It is located in the southern part of the Payenia Volcanic Province, which is among the largest Neogene-Quaternary volcanic provinces of South America. The volcanism here has been ascribed as the northernmost expression of the back-arc volcanism of the Andean Southern Volcanic Zone. We present the first petrographic and mineral chemistry study of mantle xenoliths collected from Huanul lavas with the aim of reconstructing directly the mantle source of the Payenia Volcanic Province. Xenoliths are commonly small (&lt;5 cm in radius) but scarcely crossed by basaltic veins. All xenoliths have a fertile lherzolitic modal composition and are equilibrated in the spinel-facies. Most of them exhibit an almost primitive-mantle geochemical affinity, characterized by slightly depleted clinopyroxene REE patterns reproducible by partial melting degrees between 0 and 4% of a PM source. Geothermobarometric P-T estimates of clinopyroxene-orthopyroxene couples form a linear trend between 10 and 24 kbar with constant increase of T from 814 to 1170 °C along a 50–60 mW/m2 geotherm. Evidences of interaction with the host basalts occur as spongy textures in clinopyroxene and reacted spinel, which tend to became more restitic in composition and show chromatographic or complete overprinting of the trace element compositions. The presence of plagioclase and calculated P-T values constrain this melt/rock reaction process between 6 and 14 kbar, during magma ascent, and fit the mantle adiabat model. Calculated melts in equilibrium with the primary clinopyroxenes do not fit the composition of the host basalt and, together with the geothermobarometric estimations, point to an asthenospheric mantle source for the magmatism in southern Payenia. The PM geochemical affinity of the xenoliths of Huanul is an extremely rare finding in the South America lithospheric mantle, which is commonly extensively refertilized by subduction-derived melts.

Partitioning Behaviors of Cobalt and Manganese along Diverse Melting Paths of Peridotitic and MORB-Like Pyroxenitic Mantle

Abstract The Co, Mn, Fe, and Ni contents of olivine phenocrysts and host basalts are sensitive to source mantle lithology, which suggests they may be used to constrain the processes of mantle melting and identify basalts formed from non-peridotitic (i.e. pyroxenitic) mantle sources. Here, we use a new comprehensive, forward model involving multiple parameters to simulate partitioning of Co and Mn during partial melting of the mantle in different tectonic settings: (1) polybaric continuous melting of peridotite mantle in mid-ocean ridges can generate melts that show decreasing Co and Mn with increasing degrees of melting so that the mid-ocean ridge basalts (MORBs) contain ~39–84 μg/g Co and ~900–1600 μg/g Mn; (2) flux-melting of the mantle wedge in subduction zones tends to produce a melt that has Co increasing from ~24 to 55 μg/g and Mn from ~500 to 1110 μg/g with increasing temperature; (3) melts produced by isobaric melting of the subcontinental lithospheric mantle are also sensitive to increasing temperature and have ~35–160 μg/g Co and ~800–2600 μg/g Mn; (4) decompression melting of peridotite related to the mantle plume generates melts containing ~45–140 μg/g Co and ~1000–2000 μg/g Mn, and the abundances of these metals decrease with increasing degrees of melting; and (5) partitioning behaviors of Co, Mn, and Ni during decompression melting of MORB-like pyroxenite contrast with those during decompression melting of peridotite due to the different mineralogy and compositions in mantle lithologies, and the MORB-like pyroxenite-derived melt is metal-poor with ~25–60 μg/g Co, ~290–1600 μg/g Mn, and ~160–340 μg/g Ni. Although high-Ni, low-Mn forsteritic olivine phenocrysts and high melt Fe/Mn ratio have been proposed as diagnostic indicators of pyroxenitic components in the mantle, our models show that these features can be also generated by melting of peridotite at greater depth (i.e. a high pressure and temperature). To quantify the effect of high-pressure melting of peridotite on these diagnostic indicators, we modeled the correlations of melt Fe/Mn and olivine Co, Mn, and Ni contents with melting depth along the decompression melting path of a thermal plume. When Fe/Mn ratios of basalts and/or compositions of olivine phenocrysts deviate significantly from our modeled correlation lines, high-pressure melting of peridotite cannot explain these data, and the existence of pyroxenitic component in the mantle source is likely required. The pyroxenite-derived melt is modeled to be Ni-poor, but mixing with a peridotite-derived melt can strongly increase the partition coefficient of Ni between olivine and mixed melt, resulting in the generation of high-Ni olivine phenocrysts in plume-associated magmatic suites.

CO2 storage in the Antarctica Sub-Continental Lithospheric Mantle as revealed by intra- and inter-granular fluids

The investigation of the role played by CO2 circulating within the mantle during partial melting and metasomatic/refertilization processes, together with a re-consideration of its storage capability and re-cycling in the lithospheric mantle, is crucial to unravel the Earth's main geodynamic processes. In this study, the combination of petrology, CO2 content trapped in bulk rock- and mineral-hosted fluid inclusions (FI), and 3D textural and volumetric characterization of intra- and inter-granular microstructures was used to investigate the extent and modality of CO2 storage in depleted and fertile (or refertilized) Sub-Continental Lithospheric Mantle (SCLM) beneath northern Victoria Land (NVL, Antarctica). Prior to xenoliths entrainment by the host basalt, the Antarctic SCLM may have stored 0.2 vol% melt and 1.1 vol% fluids, mostly as FI trails inside mineral phases but also as inter-granular fluids. The amount of CO2 stored in FI varies from 0.1 μg(CO2)/g(sample) in olivine from the anhydrous mantle xenoliths at Greene Point and Handler Ridge, up to 187.3 μg/g in orthopyroxene from the highly metasomatized amphibole-bearing lherzolites at Baker Rocks, while the corresponding bulk CO2 contents range from 0.3 to 57.2 μg/g.Irrespective of the lithology, CO2 partitioning is favoured in orthopyroxene and clinopyroxene-hosted FI (olivine: orthopyroxene = 0.10 ± 0.06 to 0.26 ± 0.09; olivine: clinopyroxene = 0.10 ± 0.05 to 0.27 ± 0.14). The H2O/(H2O + CO2) molar ratios obtained by comparing the CO2 contents of FI to the H2O amount retained in pyroxene lattices vary between 0.72 ± 0.17 and 0.97 ± 0.03, which is well comparable with the values measured in olivine-hosted melt inclusions from Antarctic primary lavas and assumed as representative of the partition of volatiles at the local mantle conditions. From the relationships between mineral chemistry, thermo-, oxy-barometric results and CO2 contents in mantle xenoliths, we speculate that relicts of CO2-depleted mantle are present at Greene Point, representing memory of a CO2-poor tholeiitic refertilization related to the development of the Jurassic Ferrar large magmatic event. On the other hand, a massive mobilization of CO2 took place before the (melt-related) formation of amphibole veins during the alkaline metasomatic event associated with the Cenozoic rift-related magmatism, in response to the storage and recycling of CO2-bearing materials into the Antarctica mantle likely induced by the prolonged Ross subduction.

A pre-injection assessment of CO2 and H2S mineralization reactions at the Nesjavellir (Iceland) geothermal storage site

The injection of water dissolved CO2 and H2S into basalts into the Nesjavellir geothermal system (Iceland) is to begin in 2022. This study is a pre-injection investigation assessing the likely response of the fluid-rock system to the gas charged water injection. The target aquifer has a temperature of < 200 °C at the injection well, but the temperature increases to ∼300 °C towards the center of the geothermal field where the production wells are located. The aquifer has current in-situ pH values of 6.7–7.7 and CO2 and H2S concentrations of 30.1–1079 and 60.4–505 ppm, respectively. These pre-injection aquifer fluids are saturated with respect to numerous sulfide minerals but undersaturated with respect to the major carbonate minerals. The fluid during the anticipated pilot carbon and sulfur charged water injection is expected to have a temperature of ∼84 °C, a pH of ∼4.9 and dissolved CO2 and H2S concentrations of 1223 and 480 ppm, respectively. Geochemical modelling confirms that the injection of CO2 and H2S charged fluids will dissolve the altered basaltic host rock near the injection well followed by the precipitation of secondary minerals including sulfides and carbonates further from the well. Calculations suggest about 70% and 100%, respectively, of this injected CO2 and H2S will be mineralized between the injection and production wells. The increasing of the CO2 and H2S content of the injection fluid will increase mineralization efficiency if the increased acidity of this fluid increases the mass of basalt dissolution in the subsurface. Carbon, sulfur and helium isotope systematics and abundances imply that large part of CO2 and H2S emitted from the Nesjavellir powerplant, as well as those released from the geothermal fluids naturally originate from magmatic sources. Mass balance considerations suggest that the currently planned dissolved gas injection into the Nesjavellir system will negligibly affect the CO2 and H2S budget of the aquifer. However, efforts to maximize the mineralization efficiency when upscaling this carbon storage system should be made to limit possible increase in reservoir fluid CO2 concentration.