The role of the antigorite + brucite to olivine reaction in subducted serpentinites (Zermatt, Switzerland)

Metamorphic olivine formed by the reaction of antigorite + brucite is widespread in serpentinites that crop out in glacier-polished outcrops at the Unterer Theodulglacier, Zermatt. Olivine overgrows a relic magnetite mesh texture formed during ocean floor serpentinization. Serpentinization is associated with rodingitisation of mafic dykes. Metamorphic olivine coexists with magnetite, shows high Mg# of 94–97 and low trace element contents. A notable exception is 4 µg/g Boron (> 10 times primitive mantle), introduced during seafloor alteration and retained in metamorphic olivine. Olivine incorporated 100–140 µg/g H2O in Si-vacancies, providing evidence for low SiO2-activity imposed by brucite during olivine growth. No signs for hydrogen loss or major and minor element diffusional equilibration are observed. The occurrence of olivine in patches within the serpentinite mimics the former heterogeneous distribution of brucite, whereas the network of olivine-bearing veins and shear zones document the pathways of the escaping fluid produced by the olivine forming reaction. Relic Cr-spinels have a high Cr# of 0.5 and the serpentinites display little or no clinopyroxene, indicating that they derive from hydrated harzburgitic mantle that underwent significant melt depletion. The enrichment of Mg and depletion of Si results in the formation of brucite during seafloor alteration, a pre-requisite for later subduction-related olivine formation and fluid liberation. The comparison of calculated bulk rock brucite contents in the Zermatt-Saas with average IODP serpentinites suggests a large variation in fluid release during olivine formation. Between 3.4 and 7.2 wt% H2O is released depending on the magnetite content in fully serpentinized harzburgites (average oceanic serpentinites). Thermodynamic modelling indicates that the fluid release in Zermatt occurred between 480 °C and 550 °C at 2–2.5 GPa with the Mg# of olivine varying from 68 to 95. However, the majority of the fluid released from this reaction was produced within a narrow temperature field of < 30 °C, at higher pressures 2.5 GPa and temperatures 550–600 °C than commonly thought. Fluids derived from the antigorite + brucite reaction might thus trigger eclogite facies equilibration in associated metabasalts, meta-gabbros, meta-rodingites and meta-sediments in the area. This focused fluid release has the potential to trigger intermediate depths earthquakes at 60–80 km in subducted oceanic lithosphere. © 2020, The Author(s).ISSN:1661-8734ISSN:1661-872

Serpentinite dehydration at low pressures

Petrographic observations combined with mineral compositional analyses constrain the phase relations of pro-grade metamorphosed serpentinites in the Bergell contact aureole (Italy). In a 1500 m profile perpendicular to the north-eastern edge of the Bergell intrusion, seven dehydration reactions ran to completion. Three previously undocumented reactions have been identified within 70 m of the intrusive contact: olivine+ anthophyllite= orthopyroxene+ H2O, tremolite+Cr-Al-spinel = olivine + Mg-hornblende+ H2O and chlorite =olivine+ orthopyroxene + Cr-Al-spinel +H2O. Petrological analysis indicates that these reactions occur over a narrow range of pressure and temperature, 300 +/- 30 MPa and 720 +/- 10 degrees C respectively. Computed phase diagram sections reproduce the observed mineral parageneses with one notable exception. Due to the underestimation of aluminium and sodium contents in Ca-amphibole models, plagioclase is predicted above 700 degrees C instead of Mg-hornblende. In comparison with natural grains, the aluminium content of computed chlorite compositions is overestimated for low grade parageneses while it is underestimated near the upper thermal stability limit of chlorite. In the computed sections, Fe partitioning relative to Mg between olivine and other silicates, suggests a clear preference for Fe in olivine, that therefore shows lower Mg#s. In contrast, microprobe analyses of natural mineral pairs indicate that orthopyroxene, Mg-hornblende and anthophyllite have lower Mg#s than equilibrium olivine. The inferred thermal profile of the metamorphic aureole is not consistent with simple heat conduction models and indicates a contact temperature of similar to 800 degrees C, which is 120-230 degrees C higher than previously estimated. Petrography also reveals extensive retrograde overprint of the prograde parageneses within 200 m of the contact. Retrogression is related to metamorphic fluids that were released by dehydration reactions during contact metamorphism and magmatic fluids expelled from the tonalite intrusion. The thermal gradient between the intrusion and the country rocks induced hydrothermal circulation of these fluids throughout the contact aureole, which beyond peak metamorphic conditions caused retrograde overprint of the prograde parageneses. The proposed phase relations for low and high pressures, and in particular, the transition from tremolite to Mg-hornblende, provides a complete representation of hydration and dehydration processes in serpentinites in subduction zones, along deep oceanic transform faults, and at passive continental margins. The latter has new implications, specifically for subduction initiation.ISSN:1661-8734ISSN:1661-872

Correction to: The role of the antigorite + brucite to olivine reaction in subducted serpentinites (Zermatt, Switzerland)

ISSN:1661-8734ISSN:1661-872

Subducting serpentinites release reduced, not oxidized, aqueous fluids

The observation that primitive arc magmas are more oxidized than mid-ocean-ridge basalts has led to the paradigm that slab-derived fluids carry SO2 and CO2 that metasomatize and oxidize the sub-arc mantle wedge. We combine petrography and thermodynamic modelling to quantify the oxygen fugacity (fO2) and speciation of the fluids generated by serpentinite dehydration during subduction. Silicate-magnetite assemblages maintain fO2 conditions similar to the quartz-fayalite-magnetite (QFM) buffer at fore-arc conditions. Sulphides are stable under such conditions and aqueous fluids contain minor S. At sub-arc depth, dehydration occurs under more reducing conditions producing aqueous fluids carrying H2S. This finding brings into question current models in which serpentinite-derived fluids are the cause of oxidized arc magmatism and has major implications for the global volatile cycle, as well as for redox processes controlling subduction zone geodynamics.ISSN:2045-232

Rate of viral rebound according to specific drugs in the regimen in 2120 patients with HIV suppression

Background: It is currently unclear whether the tendency for viral rebound in patients with viral load < 50 copies/ml differs according to the specific drug regimen being used. Methods: To follow 2120 patients in EuroSIDA who had attained < 50 copies/ml on highly active antiretroviral therapy (HAART), without previously virologically failing HAART. Results: The rate of viral rebound (two consecutive values > 400 copies/ml) was 4.9/100 person-years [95% confidence interval (CI), 4.0–5.8] for patients who were naive pre-HAART and 8.0/100 person-years (95% CI, 7.0–9.0) for those who were experienced with nucleoside analogue reverse transcriptase inhibitors (NRTI) pre-HAART. The rate of rebound was significantly higher in those taking nelfinavir than in those taking efavirenz, both in patients who were naive pre-HAART and those who were NRTI experienced [adjusted rate ratios, 2.83 (95% CI, 1.51–5.31) and 2.86 (95% CI, 1.65–5.00), respectively]. Among patients who were naive pre-HAART, those on abacavir had no evidence of a raised risk of viral rebound (adjusted rate ratio 1.17; 95% CI, 0.51–2.69), but in those with pre-HAART NRTI experience the rate was markedly raised (adjusted rate ratio, 4.48; 95% CI, 2.51–8.00). A similar picture was seen when comparing those on nevirapine with those on efavirenz, although the elevated rate ratio in pre-HAART experienced patients was of lower magnitude (adjusted rate ratio, 1.93). There was no strong evidence that rebound rates differed significantly for any NRTI pairs compared with zidovudine/lamivudine. Conclusion: Viral rebound rates in patients who have attained < 50 copies/ml appear to differ according to the specific drugs being used

Sport et société / Animals and the American Imagination

Le comité de rédaction de Transatlantica dédie ce numéro à la mémoire de Naomi Wulf, membre fondateur de notre revue, et responsable de la rubrique Comptes-rendus en civilisation. L'article de Naomi Wulf et Nathalie Caron "Les Lumières américaines: continuités et renouveau", publié dans le numéro 2009:2 de Transatlantica, a reçu le David Thelen Award, décerné par l'Organization of American Historians, le 21 avril 2012. Cet article sera traduit et publié par le Journal of American History