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

Chemical 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

In situ probing of the present-day zircon-bearing magma chamber at Krafla, Northeastern Iceland

Active felsic magmatism has been rarely probed in situ by drilling but one recent exception is quenched rhyolite sampled during the 2009 Iceland Deep Drilling Project (IDDP). We report finding of rare zircons of up to ∼100 µm in size in rhyolite glasses from the IDDP-1 well products and the host 1724 AD Viti granophyres. The applied SHRIMP U-Th dating for both the IDDP and the Viti granophyre zircons gives zero-age (±2 kyr), and therefore suggests that the IDDP-1 zircons have crystallized from an active magma intrusion rather than due to the 20–80 ka post-caldera magmatic episodes recorded by nearby domes and ridges. Ti-in-zircon geothermometer for Viti granophyre reveals zircon crystallization temperatures ∼800°C–900°C, whereas IDDP-1 rhyolite zircon cores show Ti content higher than 100 ppm, corresponding to temperatures up to ∼1,100°C according to the Ti-in-zircon thermometer. According to our thermochemical model at such elevated temperatures as 1,100°C, rhyolitic magma cannot be saturated with zircon and zircon crystallization is not possible. We explain this controversy by either kinetic effects or non-ideal Ti incorporation into growing zircons at low pressures that start to grow from nucleus at temperatures ∼930°C. High temperatures recorded by IDDP-1 zircon together with an occurrence of baddeleyite require that the rhyolite magma formed by partial melting of the host granophyre due to basaltic magma intrusion. Zr concentration profiles in glass around zircons are flat, suggesting residence in rhyolitic melt for >4 years. In our thermochemical modeling, three scenarios are considered. The host felsite rocks are intruded by: 1) a basaltic sill, 2) rhyolite magma 3) rhyolite sill connected to a deeper magmatic system. Based on the solution of the heat conduction equation accounting for the release of latent heat and effective thermal conductivity, these data confirm that the rhyolite magma could be produced by felsic crust melting as a result of injection of a basaltic or rhyolite sill during the Krafla Fires eruption (1975 AD)

Corrigendum to "Mineralogical and geochemical features of the Allan Hills tephra, South Victoria Land: Implications for mid-Pleistocene volcanic activity in Antarctica" [Polar Sci. 23 (2020) 100505]

International audienceThe authors regret that the Supplementary Material description and the Acknowledgements have not been included in the published version

In Situ Analysis of Copper Alloys by Femtosecond Laser Ablation Inductively Coupled Plasma Mass Spectrometry: Constrains on Matrix Effects

International audienceDirect analysis of copper-base alloys using laser ablation techniques is an increasingly common procedure in cultural heritage studies. However, main discussions remain focused on the possibility of using non-matrix matched external reference materials. To evaluate the occurrence of matrix effects during in situ microanalysis of copper-base materials, using near infrared femto-second laser ablation techniques (NIR fs-LA-ICP-MS), two bronzes, i.e., (Sn-Zn)-ternary and (Sn)-binary copper-matrix reference materials, as well as a reference synthetic glass (NIST-SRM-610) have been analyzed. The results have been compared to data obtained on a sulfide-matrix reference material. Similar values in relative sensitivity averages of 63 Cu, 118 Sn and 66 Zn, as well as in 118 Sn/ 63 Cu and 66 Zn/ 63 Cu ratios were obtained, for all analyzed matrix types, i.e., copper-base-, silicate-, and sulfide-reference materials. Consequently, it is possible to determinate major and minor element concentrations in copper alloys, i.e., Cu, Sn and Zn, using silicate and sulfide reference materials as external calibrators, without any matrix effect and over a wide range of concentrations (from wt.% to ppm)

Carbonyl as a possible carrier of platinum in magmatic fluids

International audienceCO forms the most stable compounds with bonding (stronger than C2H4, C2H2 and others) with native PGE and some cations including Au +. We find experimentally that pressure of 50-200 MPa stabilizes platinum carbonyls to the magmatic temperatures of 800-950 o C [1]. We performed an experimental study of Pt solubility in CO-bearing fluids at 800-950°C and 0.5-2.0 kbar, using a double capsule technique for fluid generation by the thermal decomposition of MgC2O4, with the fluid trapped in porous welded silica glass and Al2O3 ceramics. Bulk Pt content was measured with electrothermal atomic absorption spectrometry (ETAAS) and LA-ICP-MS. We obtain the first estimates of solubility in CO-CO2 fluid at fO2 near CCO buffer that is no less than 600 ppm at P=200 MPa and T=950 o C. Using of albite glass as a trap allows capturing of the reactive fluid and dissolved carbonyls in the bubbles. Raman spectroscopy of quenching phases in the bubble formed at P=100 MPa and T=950 o C confirms formation of polynuclear carbonyl. Carbonyl of Pt absorbed on carbon was also identified. The pressure dependence of Pt solubility at 950°C between 0.5 and 2 kbar is consistent with the formation of Pt3(CO)6 or Pt3(CO)6 2-(Chini complex) Raman spectroscopy data support this interpretation. LA-ICP-MS analysis demonstrated that iron is transferred by fluid along with Pt. Iron carbonyl is unstable at these PT conditions and neutral complex composed of Fe 2+ and Pt3(CO)6 2-can be responsible for the observed Fe transfer and formation of Pt3Fe. Significant Pt solubility is expected at near solidus temperatures of the upper crustal (P=200-300 MPa) ultramafic-mafic intrusions hosting the largest PGE deposits. Solubility of Pt in the fluid at fO2 near CCO buffer (graphitized rocks) is predicted to be strongly dependent on temperature and at P=500 MPa it will reach hundreds ppm level at the temperatures above 600-700 о C (amphibolite facies metamorphism)

H2O-CO2-S fluid triggering the 1991 Mount Pinatubo climactic eruption (Philippines)

International audienceThe factors that trigger explosive eruptions often remain elusive because of the lack of direct data from representative samples. Here, we report the first micro-Raman spectroscopy measurements of fluid and multiphase inclusions trapped in quartz xenocrysts and microlites from andesitic lavas and basaltic enclaves of the 1991 Mount Pinatubo eruption. Our analyses reveal two-phase H2O-CO2-S inclusions containing a CO2-dominated phase and an aqueous sulfate-bearing liquid phase and, less commonly, anhydrite (CaSO4(solid)). The two fluid phases are low-temperature products of a supercritical H2O-CO2-S fluid which was associated with a hydrous silicate melt prior to eruption. The average density of the CO2 phase is 0.4 +/- 0.2 g/cm(3) at room temperature, corresponding to a supercritical fluid density of 0.6 +/- 0.1 g/cm(3) at the conditions of entrapment at 760-1000 degrees C and up to similar to 260 MPa. For the first time, a dense CO2-bearing fluid is reported in Mount Pinatubo volcanic samples. We suggest that this hybrid H2O-CO2-S fluid originated from mixing between sulfur-rich basaltic and hydrous dacitic magmas, as the former was intruded into and interacted with the pre-eruptive Mount Pinatubo dacite magma reservoir, at depths of at least 10 km. Thermodynamic modeling demonstrates that part of the SO2 liberated from the intruded basaltic magma was consumed via interaction with the aqueous fluid-saturated dacitic magma according to the reaction 4SO(2)(basalt)+ 4H(2)O(dacite)=3HSO(4)(-)+H2S+3H(+), yielding early Cu-rich sulfides, late abundant anhydrite, and SO4-rich apatites, which are commonly found in the Mount Pinatubo dacites. We suggest that this hybrid H2O-CO2-S fluid played an important role in triggering the 1991 climactic eruption

Magmatic differentiation processes at Merapi Volcano: inclusion petrology and oxygen isotopes

Indonesian volcano Merapi is one of the most hazardous volcanoes on the planet and is characterised by periods of active dome growth and intermittent explosive events. Merapi currently degasses continuously through high temperature fumaroles and erupts basaltic-andesite dome lavas and associated block-and-ash-flows that carry a large range of magmatic, coarsely crystalline plutonic, and meta-sedimentary inclusions. These inclusions are useful in order to evaluate magmatic processes that act within Merapi's plumbing system, and to help an assessment of which phenomena could trigger explosive eruptions. With the aid of petrological, textural, and oxygen isotope analysis we record a range of processes during crustal magma storage and transport, including mafic recharge, magma mixing, crystal fractionation, and country rock assimilation. Notably, abundant calc-silicate inclusions (true xenoliths) and elevated δ18O values in feldspar phenocrysts from 1994, 1998, 2006, and 2010 Merapi lavas suggest addition of limestone and calc-silicate materials to the Merapi magmas. Together with high δ13C values in fumarole gas, crustal additions to mantle and slab-derived magma and volatile sources are likely a steady state process at Merapi. This late crustal input could well represent an eruption trigger due to sudden over-pressurisation of the shallowest parts of the magma storage system independently of magmatic recharge and crystal fractionation. Limited seismic precursors may be associated with this type of eruption trigger, offering a potential explanation for the sometimes erratic behaviour of Merapi during volcanic crises