The Relationship between Volatiles (CO2, H2O, F, S & Cl) and Noble Gases in Reykjanes Peninsula Lavas, Iceland

The presence of volatiles in the local mantle magma source region has been suggested as a contributor to the observed high melt production rates in the volcanic segments of the island of Iceland. However, the source of volatiles beneath this island remains enigmatic. New volatile (CO2, H2O, F, S and Cl) concentration data for 157 olivine-hosted melt inclusions in concert with noble gas data (He and Ne) from the Reykjanes Peninsula, Iceland, allow for the reconstruction of magma degassing and the distribution of volatiles in the mantle beneath Iceland. Water concentrations in olivine-hosted melt inclusions from this study record a maximum H2O concentration of 1.99 ± 0.06 wt. %. The H2O concentrations are not correlated with any of the major oxides, but are in some cases elevated relative to other incompatible elements. Solubility models we have tested show that olivine-hosted melt inclusions from the Reykjanes Peninsula have considerably less CO2 for the observed H2O, which implicates degassing to be a more important volatile-loss process than diffusion. Volatile concentrations are negatively correlated with R/Ra, a relationship that implies lower H2O concentrations in the primitive high-3He mantle component and higher-water concentrations in samples that approach Mid Ocean Ridge Basalt (MORB) R/Ra values. Water concentrations are highest along the atmosphere-MORB mixing line on a three-neon isotopic plot, and primitive samples plotting along the atmosphere-solar mixing line have lower water concentrations. These relationships suggest that a volatile-enriched recycled component with high H2O and low 3He/4He values has mixed with a primitive mantle component that is low in H2O but has high 3He/4He values. These two end-member compositions are consistent with a plume component and a depleted MORB source component – potentially enriched and fluxed by subducted components – mixing in the mantle beneath Iceland, and melting to produce the observed volcanic products. These findings support previous suggestions for a three-component-mixing model to explain the relationship between He and H2O in Reykjanes Ridge lavas

A record of spontaneous subduction initiation in the Izu–Bonin–Mariana arc

The initiation of tectonic plate subduction into the mantle is poorly understood. If subduction is induced by the push of a distant mid-ocean ridge or subducted slab pull, we expect compression and uplift of the overriding plate. In contrast, spontaneous subduction initiation, driven by subsidence of dense lithosphere along faults adjacent to buoyant lithosphere, would result in extension and magmatism. The rock record of subduction initiation is typically obscured by younger deposits, so evaluating these possibilities has proved elusive. Here we analyse the geochemical characteristics of igneous basement rocks and overlying sediments, sampled from the Amami Sankaku Basin in the northwest Philippine Sea. The uppermost basement rocks are areally widespread and supplied via dykes. They are similar in composition and age—as constrained by the biostratigraphy of the overlying sediments—to the 52–48-million-year-old basalts in the adjacent Izu–Bonin–Mariana fore-arc. The geochemical characteristics of the basement lavas indicate that a component of subducted lithosphere was involved in their genesis, and the lavas were derived from mantle source rocks that were more melt-depleted than those tapped at mid-ocean ridges. We propose that the basement lavas formed during the inception of Izu–Bonin–Mariana subduction in a mode consistent with the spontaneous initiation of subduction

New constraints on the early formation of the Western Dharwar Craton (India) from igneous zircon U-Pb and Lu-Hf isotopes

Co-auteur étrangerInternational audienceThe Western Dharwar Craton (WDC) is an Archean crustal segment for which the earliest stages of developmenthave remained poorly constrained because the oldest identified lithologies are chronologically indistinguishabledespite vastly different compositions and origins (i.e., 3352 ± 110 Ma Sargur-group komatiites and3342 ± 6 Ma Hassan-Gorur TTG gneiss). Indication for older crust come from ancient detrital zircons(3450–3610 Ma), although their genetic link to the WDC is purely conjectural. In order to bring new under-standing to early development of the WDC, we studied orthogneisses around the Holenarsipur Schist Belt (HSB)for their petrography, major-oxide concentrations, zircon U-Pb geochronology, and Lu-Hf isotope systematics.Our results reveal that the WDC igneous record contains crust older than 3350 Ma in the form of a3410.8 ± 3.6 Ma granitic gneiss and inherited zircons with ages ranging from 3295 ± 18 to 3607 ± 16 Mathat were found within a 3178 ± 10 Ma trondhjemitic gneiss and a biotite-rich enclave found within it. Thepresence of muscovite and the peraluminous signature of the granitic gneiss, in spite of mildly-depleted Hfisotopic signature (εHf= +2.2 ± 0.6 at 3410.8 Ma), suggest that this sample formed by reworking of a felsicprecursor with short crustal residence time, possibly marking the beginning of WDC formation. The oldest in-herited zircons display variableεHfranging from +10.4 at 3414 Ma to−2.3 at 3607 Ma that did not seem tohave influenced the Hf isotopic composition of granitoids of the WDC that formed between 3200 and 3410 Ma,except perhaps in the Sargur area. We suggest that the WDC formed remote from continental crust until a crustalblock containing > 3410 Ma zircons was accreted to it∼3200 My ago. This event resulted in the stabilization ofthe WDC which is marked by diapiric granitoids to which the 3178 Ma trondhjemitic gneiss belongs. After3200 Ma, the crustal block together with granitoids formed between 3410 and 3200 Ma buffered the Hf isotopicsignature of newly formed granitoids, hence, indicating that, by then, the WDC already was a stable continentalsegment

Replication Data for: Observation of an environmentally insensitive solid state spin defect in diamond

Engineering coherent systems is a central goal of quantum science. Color centers in diamond are a promising approach, with the potential to combine the coherence of atoms with the scalability of a solid state platform. We report a color center that shows insensitivity to environmental decoherence caused by phonons and electric field noise: the neutral charge state of silicon vacancy (SiV0). Through careful material engineering, we achieve over 80% conversion of implanted silicon to SiV0. SiV0 exhibits spin-lattice relaxation times approaching one minute and coherence times approaching one second. Its optical properties are excellent, with approximately 90% of its emission into the zero-phonon line and near-transform-limited optical linewidths. These combined properties make SiV0 a promising defect for quantum networks

A record of spontaneous subduction initiation in the Izu-Bonin-Mariana arc

The initiation of tectonic plate subduction into the mantle is poorly understood. If subduction is induced by the push of a distant mid-ocean ridge or subducted slab pull, we expect compression and uplift of the overriding plate. In contrast, spontaneous subduction initiation, driven by subsidence of dense lithosphere along faults adjacent to buoyant lithosphere, would result in extension and magmatism. The rock record of subduction initiation is typically obscured by younger deposits, so evaluating these possibilities has proved elusive. Here we analyse the geochemical characteristics of igneous basement rocks and overlying sediments, sampled from the Amami Sankaku Basin in the northwest Philippine Sea. The uppermost basement rocks are areally widespread and supplied via dykes. They are similar in composition and age-as constrained by the biostratigraphy of the overlying sediments-to the 52-48-million-year-old basalts in the adjacent Izu-Bonin-Mariana fore-arc. The geochemical characteristics of the basement lavas indicate that a component of subducted lithosphere was involved in their genesis, and the lavas were derived from mantle source rocks that were more melt-depleted than those tapped at mid-ocean ridges. We propose that the basement lavas formed during the inception of Izu-Bonin-Mariana subduction in a mode consistent with the spontaneous initiation of subduction

Reply to 'Unclear causes for subduction'

Arculus et al. reply — Keenan and Encarnación suggest that the absence of pre-subduction inception basement in the drill core data taken from site U1438 raises ambiguity in our conclusion of spontaneous subduction initiation in the Izu–Bonin–Mariana system