Basaltic Inclusions Research Articles

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.

Fracturing and AE characteristics of matrix-inclusion rock types under dynamic Brazilian testing

The fracturing properties and acoustic characteristics of heterogeneous and matrix-inclusion rocks are crucial to the safety of underground infrastructures and mining projects under extreme loading conditions. In this study, elliptically shaped matrix-inclusion specimens were fabricated with cement as the matrix and three rock materials (basalt, marble and sandstone) as inclusions which cover igneous, metamorphic and sedimentary rocks with different strength ratios to the cement. Dynamic Brazilian tests were conducted on the specimens with various inclusion orientations β (i.e. angles between impact direction and inclusion major axis) in the range of 0° and 90° using a split Hopkinson pressure bar (SHPB). By combining digital image correlation (DIC) and acoustic emission (AE) techniques, the integrated system demonstrates well-synchronised dynamic forces, DIC strain fields, and AE signals. The detailed procedures were presented to interpret dynamic mechanical properties, fracturing process and AE characteristics of specimens. Generally, for stronger inclusion materials with higher stiffness and strength, dynamic peak forces, threshold orientation angles of transgranular cracks, and AE counts and duration indicate higher characteristics. For instance, the fracture pattern of basalt inclusions transforms from interfacial crack to transgranular crack with increasing orientation angle β, and the specimens with transgranular cracks exhibit higher dynamic peak forces. It is observed that transgranular cracks are mainly induced by tensile stress with a higher AE peak frequency (200–250 kHz), whereas interfacial cracks contain additional shear cracking having a relatively lower peak frequency (100–150 kHz). Meanwhile, finite element modelling demonstrates that stress fields are affected by the elastic mismatch between inclusion and matrix, while failure modes (i.e. interfacial and transgranular cracks) are mainly governed by the tensile strength of inclusions.

Pre-Eruptive Magma Configurations and Petrogenetic Relationships of the Rattlesnake Tuff, Oregon–Insights From Spectacularly Banded High-Silica Rhyolite Pumices

The 7.1 Ma Rattlesnake Tuff (RST) of eastern Oregon is a widespread and voluminous (>300 km3) ignimbrite composed of 99% crystal poor (≤1%) high-silica rhyolite (HSR) and <1% dacites. Basaltic andesitic to basaltic inclusions within dacites are samples of underpinned mafic magmas. The RST HSR is comprised of five increasingly evolved compositional Groups (E–A), and HSR pumices range from white to dark grey, often co-mingled in spectacular banded pumices. Previously, Groups were interpreted as rhyolites generated by crystal fractionation within a single reservoir, where more evolved rhyolite melts formed from relatively less evolved rhyolite parents. To reassess compositional HSR Groups and their implications for tapping a single or multiple rhyolite reservoirs as well as reevaluating the petrological relationships among groups, we focus on large banded pumices for geochemical analysis. Statistical analysis of existing and new data verified these five compositional Groups and gaps, best characterized by variations in Ba, Eu/Eu*, Eu, FeO*, Hf, and Zr. Wet-liquidus temperatures, storage temperatures, and storage pressures calculated for all HSR Groups indicate similar pre-eruptive conditions (∼6.1–7.5 km depth; storage temperatures of ∼805–895°C). Differentiation trends, trends in storage pressure and temperature, and lack of crystal-rich tuff or country rock corroborate existing models for HSRs that involve a single, density-stratified magma reservoir prior eruption. Density differences are sufficient to prevent convection between layers of HSRs in a single reservoir when water content increases from 2–4 wt% from Groups E–A. However, if HSRs do not represent a liquid line, it is possible to generate HSRs through batch melting of various regional country rock. Yet, HSRs would still accumulate within the same storage zone, where density variations kept HSRs from mixing until eruption when these banded pumices formed. In either scenario, our study underscores the significance of water content and density variations for accumulating rhyolite magmas in a contiguous magma body without mixing. This has implications for other compositionally heterogenous rhyolitic ignimbrites where natural samples do not provide comparable evidence to argue for pre-eruptive confocal storage of different rhyolite magmas as is the case for the Rattlesnake Tuff.

Trace element partitioning between olivine and melt in lunar basalts

Abstract Mineral/melt partition coefficients have been widely used to provide insights into magmatic processes. Olivine is one of the most abundant and important minerals in the lunar mantle and mare basalts. Yet, no systematic olivine/melt partitioning data are available for lunar conditions. We report trace element partition data between host mineral olivine and its melt inclusions in lunar basalts. Equilibrium is evaluated using the Fe-Mg exchange coefficient, leading to the choice of melt inclusion-host olivine pairs in lunar basalts 12040, 12009, 15016, 15647, and 74235. Partition coefficients of 21 elements (Li, Mg, Al, Ca, Ti, V, Cr, Mn, Fe, Co, Y, Zr, Nb, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) were measured. Except for Li, V, and Cr, these elements show no significant difference in olivine-melt partitioning compared to the data for terrestrial samples. The partition coefficient of Li between olivine and melt in some lunar basalts with low Mg# (Mg# < 0.75 in olivine, or < ~0.5 in melt) is higher than published data for terrestrial samples, which is attributed to the dependence of DLi on Mg# and the lack of literature DLi data with low Mg#. The partition coefficient of V in lunar basalts is measured to be 0.17 to 0.74, significantly higher than that in terrestrial basalts (0.003 to 0.21), which can be explained by the lower oxygen fugacity in lunar basalts. The significantly higher DV can explain why V is less enriched in evolved lunar basalts than terrestrial basalts. The partition coefficient of Cr between olivine and basalt melt in the Moon is 0.11 to 0.62, which is lower than those in terrestrial settings by a factor of ~2. This is surprising because previous authors showed that Cr partition coefficient is independent of fO2. A quasi-thermodynamically based model is developed to correlate Cr partition coefficient to olivine and melt composition and fO2. The lower Cr partition coefficient between olivine and basalt in the Moon can lead to more Cr enrichment in the lunar magma ocean, as well as more Cr enrichment in mantle-derived basalts in the Moon. Hence, even though Cr is typically a compatible element in terrestrial basalts, it is moderately incompatible in primitive lunar basalts, with a similar degree of incompatibility as V based on partition coefficients in this work, as also evidenced by the relatively constant V/Cr ratio of 0.039 ± 0.011 in lunar basalts. The confirmation of constant V/Cr ratio is important for constraining concentrations of Cr (slightly volatile and siderophile) and V (slightly siderophile) in the bulk silicate Moon.

Aeolian abrasion of rocks as a mechanism to produce methane in the Martian atmosphere

Seasonal changes in methane background levels and methane spikes have been detected in situ a metre above the Martian surface, and larger methane plumes detected via ground-based remote sensing, however their origin have not yet been adequately explained. Proposed methane sources include the UV irradiation of meteoritic-derived organic matter, hydrothermal reactions with olivine, organic breakdown via meteoroid impact, release from gas hydrates, biological production, or the release of methane from fluid inclusions in basalt during aeolian erosion. Here we quantify for the first time the potential importance of aeolian abrasion as a mechanism for releasing trapped methane from within rocks, by coupling estimates of present day surface wind abrasion with the methane contents of a variety of Martian meteorites, analogue terrestrial basalts and analogue terrestrial sedimentary rocks. We demonstrate that the abrasion of basalt under present day Martian rates of aeolian erosion is highly unlikely to produce detectable changes in methane concentrations in the atmosphere. We further show that, although there is a greater potential for methane production from the aeolian abrasion of certain sedimentary rocks, to produce the magnitude of methane concentrations analysed by the Curiosity rover they would have to contain methane in similar concentrations as economic reserved of biogenic/thermogenic deposits on Earth. Therefore we suggest that aeolian abrasion is an unlikely origin of the methane detected in the Martian atmosphere, and that other methane sources are required.

Petrogenesis of basalt from the middle Okinawa Trough: New insights from olivine‐hosted melt inclusions

Silicate melt inclusions (SMIs) in basalt from the middle Okinawa Trough are investigated in detail to understand the source geochemical characteristics and petrogenesis of the basalt. The middle Okinawa Trough basalts contain abundant SMIs within olivine phenocrysts. The major element compositions of the SMIs are analysed using an electron microprobe. After correcting for olivine postentrapment crystallization, the major element compositions of the SMIs are more variable in terms of SiO2 (49.7–54.8 wt%) and MgO (5.1–7.2 wt%) than those of the host basalt (SiO2, 52.4 wt%; MgO, 5.7 wt%), which could indicate the existence of more primitive melts than erupted bulk rock. The major element compositions of the SMIs indicate that the SMI‐recorded melt evolution process was mainly dominated by the fractional crystallization of olivine. According to laser ablation inductively coupled mass spectrometer analyses, the trace element compositions of the SMIs are similar to those of the host bulk rock and feature obvious enrichment in Pb and large‐ion lithophile elements relative to high field strength elements, rare earth elements, and highly negative Nb and Ta anomalies. The low Ce/Pb ratios, high Pb contents, and trace element distribution patterns in the SMIs are related to contributions from subducted sedimentary components in the magma source. Based on the electron microprobe analyses, the chemical composition of the olivine suggests that the lithology of the basalt source may be pyroxenite. The basalts may have been generated by the partial melting of pyroxenite during interactions between mantle‐wedge peridotite and Si‐rich melts that were released from the subducted slab.

Origin of primitive ocean island basalts by crustal gabbro assimilation and multiple recharge of plume‐derived melts

AbstractChemical Geodynamics relies on a paradigm that the isotopic composition of ocean island basalt (OIB) represents equilibrium with its primary mantle sources. However, the discovery of huge isotopic heterogeneity within olivine‐hosted melt inclusions in primitive basalts from Kerguelen, Iceland, Hawaii and South Pacific Polynesia islands implies open‐system behavior of OIBs, where during magma residence and transport, basaltic melts are contaminated by surrounding lithosphere. To constrain the processes of crustal assimilation by OIBs, we employed the Magma Chamber Simulator (MCS), an energy‐constrained thermodynamic model of recharge, assimilation and fractional crystallization. For a case study of the 21–19 Ma basaltic series, the most primitive series ever found among the Kerguelen OIBs, we performed sixty‐seven simulations in the pressure range from 0.2 to 1.0 GPa using compositions of olivine‐hosted melt inclusions as parental magmas, and metagabbro xenoliths from the Kerguelen Archipelago as wallrock. MCS modeling requires that the assimilant is anatectic crustal melts (P2O5 ≤ 0.4 wt.% contents) derived from the Kerguelen oceanic metagabbro wallrock. To best fit the phenocryst assemblage observed in the investigated basaltic series, recharge of relatively large masses of hydrous primitive basaltic melts (H2O = 2–3 wt%; MgO = 7–10 wt.%) into a middle crustal chamber at 0.2 to 0.3 GPa is required. Our results thus highlight the important impact that crustal gabbro assimilation and mantle recharge can have on the geochemistry of mantle‐derived olivine‐phyric OIBs. The importance of crustal assimilation affecting primitive plume‐derived basaltic melts underscores that isotopic and chemical equilibrium between ocean island basalts and associated deep plume mantle source(s) may be the exception rather than the rule.

Geochemistry of olivine-hosted melt inclusions in the Baekdusan (Changbaishan) basalts: Implications for recycling of oceanic crustal materials into the mantle source

We determined the major and trace element concentrations of olivine-hosted melt inclusions in basalts from the active Baekdusan volcano situated on the border between China and North Korea in order to understand better the nature of the mantle source and the geodynamic processes that gave rise to volcanism at this site. Rehomogenized melt inclusions (after ‘Fe-loss’ correction) can be divided into three groups: a low-Si alkaline group, a high-Si alkaline group, and a high-Si sub-alkaline group. The low-Si group is composed of picrobasalt to basanite, and the high-Si group consists of (trachy)basalt to basaltic (trachy)andesite. The low-Si group has generally higher TiO2, CaO, and P2O5, but lower Al2O3, Na2O, and K2O contents at a given MgO concentration compared with those in the high-Si group. The CaO and P2O5 contents are positively correlated, indicating the presence of a calcium phosphate, probably tuite [γ-Ca3(PO4)2], in the source. The melt inclusions are enriched in light rare earth elements [(La/Yb)N=7.8–30.4]. On a primitive-mantle-normalized incompatible element plot, the low-Si group has positive anomalies in Ba and P compared with typical oceanic island basalt (OIB). On the other hand, the high-Si group exhibits remarkable positive anomalies in Eu, Ba, Rb, K, Pb, P, and Ti, implying that K-hollandite and tuite are essential phases in the source. The high-Si subalkaline group has lower abundances of incompatible elements than the high-Si alkaline group, reflecting different degrees of partial melting from the same source. In contrast to OIB, Baekdusan magmatism is characterized by positively fractionated (Zr/Hf)N ratios, and is best approximated by admixture of partial melts derived from both clinopyroxene-rich eclogite and garnet peridotite. Intraplate volcanism in northeastern Asia is closely associated with deep subduction of the Pacific plate. The subducting Pacific slab flattens and stagnates in the mantle transition zone under northeastern Asia, and this zone may yield a wet plume. Focused mantle upwelling through a gap in the stagnant slab may also be accompanied to the plume responsible for the Baekdusan magmatism. Thermal decomposition of K-hollandite within recycled continent-derived sediments is likely to metasomatize the ambient mantle peridotite above the stagnant slab. As the plume ascends through the upper mantle, the metasomatized mantle and recycled oceanic crustal materials (tuite-bearing eclogite or garnet pyroxenite) entrained by the plume may undergo partial melting, resulting in the volcanism observed at Baekdusan.

Ca/Al of plagioclase-hosted melt inclusions as an indicator for post-entrapment processes at mid-ocean ridges?

The composition of melt inclusions in basalts erupted at mid-ocean ridges may be modified by post-entrapment processes, so the present composition of melt inclusions may not represent their original composition at the time of entrapment. By combining the melt inclusion composition in samples from the South Mid-Atlantic Ridge at 19oS analyzed in this study, and from the Petrological Database, we found that post-entrapment crystallization processes resulted in higher Ca/Al, Mg#[100°—atomic Mg 2+ /(Mg 2+ +Fe 2+ )], MgO and FeO contents, and lower CaO and Al 2 O 3 contents of plagioclase-hosted melt inclusions relative to those hosted in olivine. In addition, melt inclusions hosted in plagioclase with anorthite content larger than 80mol.% had been modified more readily than others. By discussing the relationships between Ca/Al and fractional crystallization, post-entrapment crystallization, and the original melt composition, we propose that Ca/Al can be regarded as an indicator of the effect of post-entrapment processes on melt inclusion composition. Specifically, i) when Ca/Al 1.0, the compositions of melt inclusions do not reflect the original melt composition nor preserve information about the mantle source. According to these criteria, plagioclase-hosted melt inclusions with Ca/Al>1.0 in basalts from the South Mid-Atlantic Ridge at 19oS cannot represent the composition of the melt at the moment of their entrapment.

Magma transfer and degassing budget: Application to the 2009–2010 eruptive crisis of Mt Garet (Vanuatu arc)

Mt Garet, on Gaua Island, is one of the active volcanoes of the Vanuatu arc. We report here a new dataset on lapilli and lava erupted during Mt Garet unrest in 2009–2010 and on products of the older activity of Gaua composite volcano. The present-day magma of Mt Garet is a trachy-andesite (52wt.% SiO2) with relatively high Rb/Th (14.6) and Ba/La (41) ratios compared to the Gaua pre- and syn-caldera series, but typical of the central part of Vanuatu arc. Its mineral assemblage is mainly composed of plagioclase (An86–56) and clinopyroxene (Fs5–16) which display significant chemical variations, patchy zones, surface dissolution, and oscillatory zoning that imply episodes of high undercooling and growth rates. The paragenesis is complemented by Fe–Ti oxides and scarce olivine (Fo72–73). The melt inclusions are ubiquitous and their compositions cover a chemical spectrum from basalt to trachy-andesite. Volatile-rich basaltic inclusions (H2O: 2.7wt.%, S: 0.15wt.%, and Cl: 0.22wt.%) are preserved in Mg-rich clinopyroxene whereas the majority of the melt inclusions is volatile poorer with, ≤1.0wt.% of H2O, ≤0.05wt.% of S, and 0.25–0.27wt.% of Cl. At 1100°C the measured viscosity of anhydrous magma of Mt Garet is 103.5Pas. Adding 0.8 to 2.5wt.% of H2O decreases the melt viscosity by 0.5 to two orders of magnitude. Combining data on bulk rocks, minerals, and their melt inclusions together with the very first published gas fluxes acquired during the same period of activity, we propose that the high sulfur outgassing in 2009–2010 was produced by the degassing of a basaltic magma batch (~0.027km3) emplaced in a shallow reservoir. This scenario would require temperature and H2O-loss driven resorption/crystallization, magma mixing, and exsolution of an early gas phase rich in H2O, and S.We suggest here the 2009–2010 activity to be sustained by the existence of thermal convection driven at the bottom of the magma reservoir by cooling, and in which the bubbles are small enough to be stagnant. The most energetic phases are better explained by an additional gas volume, associated to the crystallization of titanomagnetite microcrysts which significantly enhance bubble nucleation. The ultimate step of crystal growth prior to eruption suggests magma ascent within few hours.

Petrogenesis of Late Cenozoic basalts from North Hainan Island: Constraints from melt inclusions and their host olivines

Melt inclusions and their host olivines in basaltic lavas provide important information about the nature of their mantle source. We present the first analyzed chemical data of olivine-hosted melt inclusions in Cenozoic basalts from the North Hainan Island and report the discovery of both tholeiitic and alkalic melt inclusions in a single rock sample. Cenozoic basalts from the Hainan Island are predominantly tholeiites with only small amounts of alkali basalts. There is a much broader compositional variation in melt inclusions than whole rocks. Compared to partial melts of mantle peridotite, the Hainan basalts have lower CaO, Na2O/TiO2, CaO/Al2O3 and Co/Fe, and higher TiO2, FeO∗, Fe/Mn, Zn/Fe and Zn/Mn. The olivine phenocrysts from the Hainan basalts contain lower Ca and Mn, and higher Ni and Fe/Mn than those of olivines crystallized from partial melts of peridotite. Projections from or towards olivine into the plane CS-MS-A for melt inclusions and whole rocks with MgO >7.5wt% imply that the residual minerals in the source of the tholeiites are mainly clinopyroxene and garnet, possibly with some orthopyroxene, while in the source of the alkali basalts they are dominated by clinopyroxene and garnet. This indicates that a pyroxenite component could serve as the source lithology of the Hainan basalts. The OIB-like trace element compositions, with Ba, Sr, Nb and Ta positive anomalies, and Th and U negative anomalies, of the Hainan basalts suggest that a recycled oceanic crust component was involved in the source of the Hainan basalts. Based on a CMAS projection of primary magma compositions of the whole rocks and melt inclusions, we infer that a stage-2 silica-deficient pyroxenite derived from melt–peridotite reaction or mechanical mixing between recycled oceanic crust and peridotite can serve as the source lithology. Partial melts derived from such a source can match the overall compositions of the Hainan basalts better than those of MORB-eclogite and fertile peridotite. The compositional range from tholeiitic to alkalic basalts is ascribed to decreasing degrees of melting of a similar silica-deficient pyroxenite source, which is consistent with the lower incompatible trace element and REE abundances in the tholeiites compared to the alkali basalts. Both subalkaline and alkaline melt inclusions are present in a single sample, which indicates that magma mixing had occurred in the deep magma chamber prior to eruption below the Hainan Island.

Testing the ureilite projectile hypothesis for the El'gygytgyn impact: Determination of siderophile element abundances and Os isotope ratios in ICDP drill core samples and melt rocks

AbstractThe geochemical nature of the impactites from International Continental Scientific Drilling Project—El'gygytgyn lake drill core 1C is compared with that of impact melt rock fragments collected near the western rim of the structure and literature data. Concentrations of major and trace elements, with special focus on siderophile metals Cr, Co, Ni, and the platinum group elements, and isotope ratios of osmium (Os), were determined to test the hypothesis of an ureilite impactor at El'gygytgyn. Least squares mixing calculations suggest that the upper volcanic succession of rhyolites, dacites, and andesites were the main contributors to the polymict impact breccias. Additions of 2–13.5 vol% of basaltic inclusions recovered from drill core intervals between 391.6 and 423.0 mblf can almost entirely account for the compositional differences observed for the bottom of a reworked fallout deposit at 318.9 mblf, a polymict impact breccia at 471.4 mblf, and three impact melt rock fragments. However, the measured Os isotope ratios and slightly elevated PGE content (up to 0.262 ng g−1 Ir) of certain impactite samples, for which the CI‐normalized logarithmic PGE signature displays a relatively flat (i.e., chondritic) pattern, can only be explained by the incorporation of a small meteoritic contribution. This component is also required to explain the exceptionally high siderophile element contents and corresponding Ni/Cr, Ni/Co, and Cr/Co ratios of impact glass spherules and spherule fragments that were recovered from the reworked fallout deposits and from terrace outcrops of the Enmyvaam River approximately 10 km southeast of the crater center. Mixing calculations support the presence of approximately 0.05 wt% and 0.50–18 wt% of ordinary chondrite (possibly type‐LL) in several impactites and in the glassy spherules, respectively. The heterogeneous distribution of the meteoritic component provides clues for emplacement mechanisms of the various impactite units.

Major and trace element zoning in plagioclase from Kizimen Volcano (Kamchatka): Insights into magma-chamber processes

The data on the geochemistry of the rocks of Kizimen Volcano and results of microprobe studies of major and trace elements in plagioclase grains from acid lavas and basalt inclusions are presented. The characteristics of the Kizimen Volcano are the following: (1) basalt inclusions are abundant in acid lavas; (2) banded, mixed lavas occur; (3) the distribution curves of rare-earth elements of acidic lavas and basalt inclusions intersect; (4) Sr-Nd isotope systematics of the rocks and inclusions do not indicate mixture with crustal material; (5) plagioclase phenocrysts are of direct and reverse zonation; (6) olivine and hornblende, as well as acid and mafic plagioclases, coexist in the rocks. The studies revealed that the rocks are of a hybrid nature and originated in the course of repeated mixture of acid and mafic melts either with chemical and thermal interaction of melts or exclusively thermal ones. Study of the major- and trace-element distribution in zonal minerals provides an informative tool for understanding the history of the generation and evolution of melts in a magma chamber.

Mineralogy, geochemistry and petrogenesis of igneous inclusions within three inactive diapirs, Zagros belt, Shahre-kord, Iran

AbstractThe Kaj-Rostam Abad, Dashtak and Doab diapirs are part of the Precambrian–Cambrian Hormuz series that are rich in igneous inclusions concentrated by dissolution of diapiric salt. They are situated in the Iran–Pakistan salt range and commonly associated with inclusions of basalt, trachyte, andesite, micro-gabbro, gypsum and anhydrite, with lesser amounts of carbonate rocks. The mineral assemblage in these inclusions developed in three stages: (I) magmatic stage (diopside, Ti-augite, kaersutite, plagioclase, apatite, biotite and opaque minerals), (II) late magmatic stage (biotite, quartz, chlorite, albite, calcite, titanite, epidote, actinolite and opaque minerals) and (III) vein mineralization (quartz, chlorite, albite, calcite, garnet, epidote, opaque minerals and actinolite). Clinopyroxene is diopside to Ti-augite. Actinolite, kaersutite, albite and pycnochlorite are constituents of the metasomatic rocks of the area. Chlorite geothermometry yielded a temperature of 330–500 °C for chlorite formation. Clinopyroxene thermobarometry ranges from 960 ≤T≤ 1440 °C and 1 ≤P≤ 10 kbar. The presence of halite-bearing fluid inclusions in hydrothermal quartz veins with homogenization temperatures between 320 and 350 °C points to strong evidence of hydrothermal events. The salinity of these fluids is 39.8–42.7 wt% NaCl. δ18O data on hydrothermal quartz veins range from 14.89 to 22.09 ‰ (SMOW), indicating that the studied samples were affected by fluids originated from sedimentary-evaporitic rocks. Meteoric water that penetrated the evaporitic rocks likely mixed with late magmatic fluids while subjected to magmatic heat, when buried to depths of several kilometres by the Phanerozoic cover sequence. Whole-rock geochemistry data for the studied rocks emphasize their alkaline to sub-alkaline affinities, in a transitional magmatic series.

Plastic Deformation of Upper Mantle Silicate Minerals

Summary Several minerals which from their occurrence in nodular inclusions in basalts and on theoretical grounds are believed to occur in the upper mantle have been deformed experimentally at high temperature and pressure. The glide mechanisms on which olivine crystals in naturally occurring aggregates are deformed operate in the experimentally deformed specimens in addition to other glide mechanisms which occur naturally only in meteorites. The glide mechanisms which predominate in olivine are shown to be a function of temperature and strain-rate. Diopside deforms chiefly by mechanical twinning in the experiments and enstatite by slip and shear-transformation to clinoenstatite.

Early mafic magmatism and crustal anatexis on the Isle of Rum: evidence from the Am Màm intrusion breccia

AbstractThe Rum Igneous Centre comprises two early marginal felsic complexes (the Northern Marginal Zone and the Southern Mountains Zone), along with the later central ultrabasic–basic layered intrusions. These marginal complexes represent the remnants of near-surface to eruptive felsic magmatism associated with caldera collapse, examples of which are rare in the North Atlantic Igneous Province. Rock units include intra-caldera collapse breccias, rhyolitic ignimbrite deposits and shallow-level felsic intrusions, as well the enigmatic ‘Am Màm intrusion breccia’. The latter comprises a dacitic matrix enclosing lobate basaltic inclusions (~1–15 cm) and a variety of clasts, ranging from millimetres to tens of metres in diameter. These clasts comprise Lewisian gneiss, Torridonian sandstone and coarse gabbro. Detailed re-mapping of the Am Màm intrusion breccia has shown its timing of emplacement as syn-caldera, rather than pre-caldera as previously thought. Textural analysis of entrained clasts and adjacent, uplifted country rocks has revealed their thermal metamorphism by early mafic intrusions at greater depth than their present structural position. These findings provide a window into the evolution of the early mafic magmas responsible for driving felsic magmatism on Rum. Our data help constrain some of the physical parameters of this early magma–crust interaction and place it within the geochemical evolution of the Rum Centre.

Hf–W chronometry of the IAB iron meteorite parent body

The IAB iron meteorites are unusual in having abundant silicate inclusions, but the relationship of metal and silicates is not well understood. In order to study the sequence and the timing of events that led to the observed structure of IAB iron meteorites, the Hf–W isotope system was applied to metal and silicate separates from seven IAB iron meteorites. A major metal segregation event on the IAB parent body is dated by the average 182W/ 184W ratio of the IAB metal phases (− 3.1 ± 0.2 ε 182W-units), corresponding to a time of 1.8 + 2.3 / − 2.0 Ma after the formation of Ca,Al-rich inclusions (CAIs). The 182Hf– 182W data for three bulk inclusions from Landes, Copiapo and El Taco define a linear trend interpreted as an isochron, corresponding to an age of 2.5 + 2.3 / − 2.0 Ma after CAI formation. This age most likely dates silicate differentiation on the IAB parent body and is, within analytical uncertainty, indistinguishable from the major metal segregation event. Several silicate inclusions are displaced from the IAB silicate isochron towards less radiogenic W compositions, most likely reflecting thermal events on the parent body. Metal segregation and silicate differentiation occurred early enough, so that these processes could have at least partially been triggered by an internal heat-source (e.g., 26Al). Basaltic inclusions of the Caddo County specimen define an age of 4.6 + 0.7 / − 0.6 Ma, most likely dating a partial melting event on the IAB parent body. Three silicate-separates from another El Taco inclusion yield an age that postdates CAI formation by 10.8 + 2.4 / − 2.0 Ma and the IAB silicate isochron by ~ 8.3 Ma. This prolonged interval of local thermal events on the IAB parent body are best explained by collisional heat sources (i.e. impacts). The Hf–W ages for thermal events on the IAB parent body are within the range of previously reported 129I– 129Xe ages but are significantly older than published 87Rb– 87Sr, 147Sm– 143Nd and most 40 K– 40Ar ages. Most likely, these chronometers were reset during later thermal events that had only a minor effect on the 182Hf– 182W and 129I– 129Xe systems.

The oxidation state of sulfur in synthetic and natural glasses determined by X-ray absorption spectroscopy

Sulfur K-edge X-ray absorption near edge structure (XANES) spectra were recorded for experimental glasses of various compositions prepared at different oxygen fugacities ( fO 2) in one-atmosphere gas-mixing experiments at 1400 °C. This sample preparation method only results in measurable S concentrations under either relatively reduced (log fO 2 < −9) or oxidised (log fO 2 > −2) conditions. The XANES spectra of the reduced samples are characterised by an absorption edge crest at 2476.4 eV, typical of S 2−. In addition, spectra of Fe-bearing compositions exhibit a pronounced absorption edge shoulder. Spectra for all the Fe-free samples are essentially identical, as are the spectra for the Fe-bearing compositions, despite significant compositional variability within each group. The presence of a sulfide phase, such as might exsolve on cooling, can be inferred from a pre-edge feature at 2470.5 eV. The XANES spectra of the oxidised samples are characterised by an intense transition at 2482.1 eV, typical of the sulfate anion SO 4 2−. Sulfite (SO 3 2−) has negligible solubility in silicate melts at low pressures. The previous identification of sulfite species in natural glass samples is attributed to an artefact of the analysis (photoreduction of S 6+). S 4+ does, however, occur unambiguously with S 6+ in Fe-free and Fe-poor compositions prepared in equilibrium with CaSO 4 at 4–16 kbar, and when buffered with Re/ReO 2 at 10 kbar. Solubility of S 4+ thus requires partial pressures of SO 2 considerably in excess of 1 bar. A number of experiments were undertaken in an attempt to access intermediate fO 2s more applicable to terrestrial volcanism. Although these were largely unsuccessful, S 2− and S 6+ were found to coexist in some samples that were not in equilibrium with the imposed fO 2. The XANES spectra of natural olivine-hosted melt inclusions and submarine glasses representative of basalts at, or close to, sulfide saturation show mainly dissolved S 2−, but with minor sulfate, and additionally a peak at 2469.5 eV, which, although presumably due to immiscible sulfide, is 1 eV lower than that typical of FeS. These sulfate and sulfide-related peaks disappear with homogenisation of the inclusions by heating to 1200 °C followed by rapid quenching, suggesting that both these features are a result of cooling under natural conditions. The presence of small amounts of sulfate in otherwise reduced basaltic magmas may be explained by the electron exchange reaction: S 2− + 8Fe 3+ = S 6+ + 8Fe 2+, which is expected to proceed strongly to the right with decreasing temperature. This reaction would explain why S 2− and S 6+ are frequently found together despite the very limited fO 2 range over which they are thermodynamically predicted to coexist. The S XANES spectra of water-rich, highly oxidised, basaltic inclusions hosted in olivine from Etna and Stromboli confirm that nearly all S is dissolved as sulfate, explaining their relatively high S contents.

The oxygen isotopic composition as an indicator of the genesis of “Basaltic” corundum

This paper reports an oxygen isotopic study of corundums and associated minerals from the Yogo lamprophyres (Montana, US), plagioclase-corundum inclusions in alkali basalt (Tunkin depression, Russia), and from modern alluvium of Podgelbanochnyi alkaline basaltic volcano (Primorye, Russia). It is shown that all sapphires genetically related to mafic magmatic rocks have a similar oxygen isotopic composition (the variations of δ18O are within 2.5‰) with most values plotting between +4.5 and +7.0‰ SMOW. The oxygen isotopic ratios in the associated minerals (olivines, pyroxenes, mica, and others) and host rocks are plotted in the same interval. This indicates that the sapphire crystallized during evolution of the parental mafic magma. However, there are xenogenic corundums, which were only transported by basaltic magma to the Earth’s surface. They have a sharply distinct oxygen isotopic composition, which suggests their disequilibrium with the host lavas, and, correspondingly, a different genetic nature.

Experience of modeling the garnet + orthopyroxene + spinel + plagioclase reaction by the method of thermodynamic potential minimization

Relative to the method of phase correspondence, physicochemical modeling of mineral associations based on the method of thermodynamic potential minimization and consideration of solid solutions offers new opportunities [1]. Although the thermodynamic database of end members and the models of mineral solid solutions are not sufficiently accurate, the high potential of the method based on minimization for the solution of several petrological issues is evident. The present paper shows an implication of this method for thermodynamic analysis of the garnet reaction structure, for which oxygen pressure and fugacity cannot be estimated by conventional thermobarometry. Solid inclusions in alkaline basalts of the Konfetka paleovolcano in Primorye [2] contain fragments of fractured garnet crystals replaced by orthopyroxene‐ spinel‐plagioclase symplectite along fractures and the periphery (figure). Bright orange stilpnomelane is developed locally in the reaction rim. The symplectite has a specific composition (Table 1). Orthopyroxene shows appreciable variations in the Al 2 O 3 content: a lower concentration (2.9‐3.4 wt %) at the contact with garnet than at the contact with spinel and plagioclase (9.5‐10.3 wt %). Plagioclase is also heterogeneous. Its composition varies from An 84 to An 60 . At the same time, the Fe mole fraction of orthopyroxene and spinel is nearly constant. However, the spinel composition is characterized by a lower degree of oxidation ( Fe 3+ / Σ Fe = 8.1‐11.8 at %, Table 1). The compositional heterogeneity of minerals emphasizes the nonequilibrium state of the symplectite rim, at least, with respect to the Al 2 O 3 potential. Elucidation of PT constraints of the garnet replacement by the orthopyroxene‐spinel‐plagioclase symplectite can provide insight into the genesis of garnet inclusions. The equilibrium temperature of the orthopyroxene‐spinel system can be estimated approximately by the Liermann‐Ganguly method [3]: