Chlorine isotope ratios record magmatic brine assimilation during rhyolite genesis

Publisher Copyright: © 2021 The Authors.Magmatic volatile phases within crustal silicic magma domains influence key volcanic processes such as the build up to eruptions and formation of magmatic-hydrothermal ore deposits. However, the extent and nature of fluid-melt interaction in such environments is poorly understood, as geochemical signals in volcanic rocks originating from pre-eruptive volatile processes are commonly overprinted by syn-eruptive degassing. Here, we use δ37Cl as a conservative tracer of brine-melt interaction on a broad suite of silicic volcanic rocks from Iceland. We find that the δ37Cl values of silicic rocks are systematically shifted to more negative values compared to associated basalts and intermediate rocks by up to 2.9 . These large shifts cannot be explained by well known processes inherent to silicic magma genesis, including crustal assimilation, mineral-melt fractionation and syn-eruptive degassing. Instead, we show that low δ37Cl values in silicic rocks can be attributed to assimilation of magmatic brines that are formed and stored in long lived crustal magma mushes. Our results indicate that magmatic brine assimilation is a fundamental, but previously unrecognised part of rhyolite genesis.Peer reviewe

Spatial variations of primordial and recycled noble gases across Iceland

Noble gas (He, Ne, Ar, Kr, Xe) compositions of mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) have been widely used to investigate the geochemical structure and evolution of Earth’s mantle. Many studies provide evidence for the existence of different mantle domains having distinctive chemical and noble gas signatures. Primordial mantle domains have isotopic signatures that have remained largely unmodified since the Earth’s formation, while recycled mantle domains have undergone extensive modification following chemical fractionation during melt extraction and magma degassing, mantle convection, and subduction recycling. Iceland represents a perfect natural laboratory to study the inventory of primordial and recycled noble gases within the mantle thanks to its particular location above a mid-ocean ridge and a mantle plume. In this hybrid setting, melts with a deep OIB-like mantle origin and with near-primordial mantle gas signatures interact and coexist with melts formed at shallower levels that exhibit MORB-like recycled mantle chemical characteristics. On Iceland, chemical and lithological mantle heterogeneities exist on both long and short length scales, and primordial and recycled noble gases signatures can both be present even in a single sample set. We investigated the spatial relationships between Iceland’s primordial and recycled mantle components by combining new high-precision noble gas (He, Ne, Ar, Kr, Xe) analyses of basaltic glass with a large existing dataset of noble gas data from subglacially erupted basalts collected across the Iceland. Here, we present noble gas data for the Western Volcanic Zone (WVZ), one of the most geologically interesting areas of Iceland. The data indicate a significant and consistent lateral variability in the noble gas signatures in relation to the distance from the plume centre. We discuss possible explanations for these variations, ways to improve our systematic understanding of mantle volatile distribution beneath Iceland, and outline future directions of this research

Rubidium isotopic fractionation during magmatic processes and the composition of the bulk silicate Earth

International audienceRubidium is a moderately volatile element with high incompatibility and fluid mobility. Its stable isotopes have great potential in tracing various geological processes. For example, lunar rocks are isotopically heavier than terrestrial ones, suggesting volatile loss by evaporation during or following the formation of the Moon. However, these studies rely on a poorly constrained estimate for the composition of the Earth's mantle and a poor understanding of high-temperature processes which may act to fractionate stable Rb isotopes. It is therefore important to precisely characterize different rock types that sample the Earth's mantle as well as to evaluate the importance of key isotopic fractionation processes. In order to address these issues, we established a high precision analytical method for Rb isotopic measurements using the Nu Sapphire CC-MC-ICP-MS (collision-cell multi-collector inductively coupled plasma mass spectrometer). In addition, we present a series of Rb isotopic data of volcanic rocks from Hekla volcano (Iceland) and MORB (mid-ocean ridge basalt) samples. We show that our method returns a high Rb sensitivity (∼500 V/μg·g-1 for 85Rb) and a long-term reproducibility of 0.03‰ on δ87Rb (the permil deviation of the 87Rb/85Rb ratio from the SRM 984 standard). This method uses a 2 ng/g Rb solution for analyses, allowing us to consume about 10 times less Rb to achieve similar or better precision than previous studies. Using this method, seven geostandards and one synthetic standard return Rb isotopic data consistent with previous work. Twenty-one Hekla volcanic rocks, spanning compositions from basalt to rhyolite, show limited Rb isotopic variation, with δ87Rb values varying from -0.17‰ to -0.07‰, demonstrating that magmatic evolution has an insignificant effect on Rb isotope ratios. A set of MORB samples (n = 15) from different mid-ocean ridges also span a limited Rb isotopic variation, displaying a range similar to the Hekla rock suite (-0.19 to -0.02‰). Combining our new data together with previously reported OIB data gives an average δ87Rb value of -0.12 ± 0.08‰ (2SD, n = 25), representing the current best estimate of the mantle's isotopic composition. Considering the δ87Rb values of the upper continental crust (-0.14 ± 0.01‰, 2SE, n = 73), as inferred from recent measurements of granites, loess and sediments, and assuming this value represents the whole crust, the revised Rb isotopic composition of the bulk silicate Earth (and by extension the bulk Earth, assuming no Rb partitioned into the core) is -0.13 ± 0.06‰ (2SD)

The indium isotopic composition of the bulk silicate Earth

International audienceIndium (In) behaves as a moderately volatile metal during nebular and planetary processes, and its volatility depends strongly on oxygen fugacity. The In isotopic composition of the bulk silicate Earth (BSE) could provide a critical constraint on the nature of Earth's building blocks and mechanisms that lead to its volatile depletion. However, accurately and precisely determining the isotopic composition of In of the silicate Earth is challenging due to its low abundance in igneous rocks and the presence of significant isobaric interferences on its isotopes (e.g., 113Cd+ on 113In+ and 115Sn+ on 115In+). Here, we present a purification procedure for In from rock matrices and report the first dataset of In isotopic compositions of 30 terrestrial igneous rocks, one biotite geostandard, and one carbonaceous chondrite (Allende) measured on a Nu Sapphire collision-cell equipped multi-collector inductively-coupled-plasma mass-spectrometer (CC-MC-ICP-MS) with an external reproducibility of 0.11‰ (2SD). At this level of precision, we find no statistically significant difference in the In isotopic compositions of mid-ocean-ridge basalts (MORB), oceanic island basalts (OIB), and continental flood basalts (CFB). Furthermore, Canary Islands, Iceland and Afar lavas display no analytically resolvable In isotopic variations from basalts to rhyolites. Therefore, In isotope fractionation during igneous processes is smaller than our analytical uncertainty and the In isotopic compositions of basalts are likely to be representative samples of their mantle sources. The twenty-one terrestrial basalts from diverse geological settings have an average δ115In of 0.35 ± 0.07 ‰ (2SD). This value represents the current best estimate of the In isotopic composition of the mantle as well as of the bulk silicate Earth, assuming limited In isotope fractionation during mantle partial melting, and due to the small contribution of the continental crust to the In budget (<5%). This isotopic composition provides a baseline with which to compare with chondrites and differentiated planetary bodies in future studies

Isotope systematics of Icelandic thermal fluids

Thermal fluids in Iceland range in temperature from 440 °C and are dominated by water (> 97 mol%) with a chloride concentration from 20,000 ppm. The isotope systematics of the fluids reveal many important features of the source(s) and transport properties of volatiles at this divergent plate boundary. Studies spanning over four decades have revealed a large range of values for δD (− 131 to + 3.3‰), tritium (− 0.4 to + 13.8 TU), δ¹⁸O (− 20.8 to + 2.3‰),³He/⁴He (3.1 to 30.4 R[subscript A]), δ¹¹B (− 6.7 to + 25.0‰), δ¹³C[subscript ∑ CO₂](− 27.4 to + 4.6‰), ¹⁴C[subscript ∑ CO₂](+ 0.6 to + 118 pMC), δ¹³C[subscript CH₄](− 52.3 to − 17.8‰), δ¹⁵N (− 10.5 to + 3.0‰), δ³⁴S[subscript ∑ S− II] (− 10.9 to + 3.4‰), δ³⁴S[subscript SO₄](− 2.0 to + 21.2‰) and δ³⁷Cl (− 1.0 to + 2.1‰) in both liquid and vapor phases. Based on this isotopic dataset, the thermal waters originate from meteoric inputs and/or seawater. For other volatiles, degassing of mantle-derived melts contributes to He, CO₂ and possibly also to Cl in the fluids. Water-basalt interaction also contributes to CO₂ and is the major source of H₂S, SO₄, Cl and B in the fluids. Redox reactions additionally influence the composition of the fluids, for example, oxidation of H₂S to SO₄ and reduction of CO₂ to CH₄. Air-water interaction mainly controls N2, Ar and Ne concentrations. The large range of many non-reactive volatile isotope ratios, such as δ³⁷Cl and ³He/⁴He, indicate heterogeneity of the mantle and mantle-derived melts beneath Iceland. In contrast, the large range of many reactive isotopes, such as δ¹³C[subscript ∑ CO₂] and δ³⁴S[subscript ∑ S− II], are heavily affected by processes occurring within the geothermal systems, including fluid-rock interaction, depressurization boiling, and isotopic fractionation between secondary minerals and the aqueous and vapor species. Variations due to these geothermal processes may exceed differences observed among various crust and mantle sources, highlighting the importance and effects of chemical reactions on the isotope systematics of reactive elements. Keywords: Iceland; Isotopes; Thermal fluids; Volatile