Terrestrial exposure of a fresh Martian meteorite causes rapid changes in hydrogen isotopes and water concentrations

Determining the hydrogen isotopic compositions and H2O contents of meteorites and their components is important for addressing key cosmochemical questions about the abundance and source(s) of water in planetary bodies. However, deconvolving the effects of terrestrial contamination from the indigenous hydrogen isotopic compositions of these extraterrestrial materials is not trivial, because chondrites and some achondrites show only small deviations from terrestrial values such that even minor contamination can mask the indigenous values. Here we assess the effects of terrestrial weathering and contamination on the hydrogen isotope ratios and H2O contents of meteoritic minerals through monitored terrestrial weathering of Tissint, a recent Martian fall. Our findings reveal the rapidity with which this weathering affects nominally anhydrous phases in extraterrestrial materials, which illustrates the necessity of sampling the interiors of even relatively fresh meteorite falls and underlines the importance of sample return missions

Using dissolved H₂O in rhyolitic glasses to estimate palaeo-ice thickness during a subglacial eruption at Bláhnúkur(Torfajökull, Iceland)

The last decade has seen the refinement of a technique for reconstructing palaeo-ice thicknesses based on using the retained H2O and CO2 content in glassy eruptive deposits to infer quenching pressures and therefore ice thicknesses. The method is here applied to Bláhnúkur, a subglacially erupted rhyolitic edifice in Iceland. A decrease in water content from ~0.7 wt.% at the base to ~0.3 wt.% at the top of the edifice suggests that the ice was 400 m thick at the time of the eruption. As Bláhnúkur rises 350 m above the surrounding terrain, this implies that the eruption occurred entirely within ice, which corroborates evidence obtained from earlier lithofacies studies. This paper presents the largest data set (40 samples) so far obtained for the retained volatile contents of deposits from a subglacial eruption. An important consequence is that it enables subtle but significant variations in water content to become evident. In particular, there are anomalous samples which are either water-rich (up to 1 wt.%) or water-poor (~0.2 wt.%), with the former being interpreted as forming intrusively within hyaloclastite and the latter representing batches of magma that were volatile-poor prior to eruption. The large data set also provides further insights into the strengths and weaknesses of using volatiles to infer palaeo-ice thicknesses and highlights many of the uncertainties involved. By using examples from Bláhnúkur, the quantitative use of this technique is evaluated. However, the relative pressure conditions which have shed light on Bláhnúkur’s eruption mechanisms and syn-eruptive glacier response show that, despite uncertainties in absolute values, the volatile approach can provide useful insight into the mechanisms of subglacial rhyolitic eruptions, which have never been observed

An isotopically distinct hydrogen reservoir in the South Pacific mantle

No abstract available

l-Erdadevelopmentsinordertoimprovethewatercontent determinationinhydrousandnominallyanhydrousmantlephases

International audienceThepreciseknowledgeof thewater content ingeological samples suchas mantleminerals, volcanicglasses andglassyinclusions provestobecrucial informationfortheEarthSciencesbecausewaterhasaconsiderableinfluenceonphysical andchemical properties of Earth'smantleandcrust. Amongthenuclearmicrobeamtechniques, elasticrecoil detectionanalysis(ERDA) hasbeenusedat the nuclearmicroprobeof thePierreSu ̈eLaboratory(LPS) withaverygoodreliabilityforalongtimetodeterminethewatercontent of various materials. Previous ERDAmeasurements gavethetotal Hcontent at amicrometricscaleinthebulkof hydrous geologic samples. However, itwasmoredifficulttocharacterizenominallyanhydrousphases. Recenteffortsallowedustosignificantlyimprove thedifferent steps of theERDAanalysis, fromthesamplepreparationtothedeterminationof theuncertainties of theresultingH concentration. Alargeseriesof verydifferent geological sampleshasbeenmeasured: volcanicglassesandglassyinclusions, synthetic andnatural nominallyanhydrousminerals. Ournewsamplepreparationprotocol limitedthethicknessof thesurfacelayerof H-pol- lution, leadingtoaneasierdifferentiationofthebulkcontribution. SimultaneousPIXEandRBSmeasurementsallowthepreciseloca- tionoftheinterestingareasandalsogiveinformationonthechemical characterizationoftheinvestigatedsamples, withrespecttothe major andminor elements. Theprocessingof thelist-modeacquisitionfiles is madeusingtheRISMINsoftware, allowingamong other interestingfeatures tochecktheabsenceof water loss under thebeam. Wewill present theresults of theH-content measure- ments andshowthe goodagreement betweenour present results andthose obtainedbyFourier transforminfraredspectroscopy (FTIR) for thesameset of samples. Thedetectionlimit was measuredonadehydratednatural SanCarlos olivine((Mg,Fe)2SiO4) equal to130wt ppmH2O(15wt ppmHor 300at ppmH) andthe relative uncertainties onthe water concentrationrange from 10%to15%

Hydrogen partitioning between melt, clinopyroxene, and garnet at 3 GPa in a hydrous MORB with 6 wt.% H2O

International audienceAbstract To understand partitioning of hydrogen between hydrous basaltic and andesitic liquids and coexisting clinopyroxene and garnet, experiments using a midocean ridge basalt (MORB) + 6 wt.% H2O were conducted at 3 GPa and 1,150–1,325°C. These included both isothermal and controlled cooling rate crystallization experiments, as crystals from the former were too small for ion microprobe (SIMS) analyses. Three runs at lower bulk water content are also reported. H2O was measured in minerals by SIMS and in glasses by SIMS, Fourier Transform infrared spectroscopy (FTIR), and from oxide totals of electron microprobe (EMP) analyses. At 3 GPa, the liquidus for MORB with 6 wt.% H2O is between 1,300 and 1,325C. In the temperature interval investigated, the melt proportion varies from 100 to 45% and the modes of garnet and clinopyroxene are nearly equal. Liquid composition varies from basaltic to andesitic. The crystallization experiments starting from above the liquidus failed to nucleate garnets, but those starting from below the liquidus crystallized both garnet and clinopyroxene. SIMS analyses of glasses with [7 wt.% H2O yield spuriously low concentrations, perhaps owing to hydrogen degassing in the ultra-high vacuum of the ion microprobe sample chamber. FTIR and EMP analyses show that the glasses have 3.4 to 11.9 wt.% water, whilst SIMS analyses indicate that clinopyroxenes have 1,340–2,330 ppm and garnets have 98–209 ppm H2O. DH cpx-gt is 11 ± 3, DH cpx-melt is 0.023 ± 0.005 and DH gt-melt is 0.0018 ± 0.0006. Most garnet/melt pairs have low values of DH gt-melt, but DH gt-melt increases with TiO2 in the garnet. As also found by previous studies, values of DH cpx-melt increase with Al2O3 of the crystal. For garnet pyroxenite, estimated values of DH pyroxenite-melt decrease from 0.015 at 2.5 GPa to 0.0089 at 5 GPa. Hydration will increase the depth interval between pyroxenite and peridotite solidi for mantle upwelling beneath ridges or oceanic islands. This is partly because the greater pyroxene/olivine ratio in pyroxenite will tend to enhance the H2O concentration of pyroxenite, assuming that neighboring pyroxenite and peridotite bodies have similar H2O in their pyroxenes

Carbon flux at mid-ocean ridges and CO2/Nb variability in the mantle

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Calibration of the infrared molar absorption coefficients for H in olivine, clinopyroxene and rhyolitic glass by elastic recoil detection analysis

International audienceWe use a nuclear microbeam technique (Elastic Recoil Detection Analysis or ERDA) to measure the H content in 2 rhyolitic glasses, 4 olivines, 4 orthopyroxenes, and 7 clinopyroxenes. These samples have been characterized previously for their OH absorption spectra by infrared (FTIR) spectroscopy. We use ERDA and FTIR data to calibrate the infrared molar absorption coefficients. The blank level of the ERDA method is 102 ± 81 ppm H2O, too great for the analysis of many natural nominally anhydrous minerals (NAMs) from the upper mantle, but applicable to H-rich glasses, natural and synthetic NAMs. Calibration of the molar absorption coefficient ε with the ERDA results gives 95 ± 8 l mol− 1 cm− 1 for rhyolitic glasses (peak height), 34,515 ± 7050 l mol− 1 cm− 2 for olivine and 46,103 ± 5300 l mol− 1 cm− 2 for clinopyroxene (integrated area). The olivine calibration of the present study is intermediate between that of Bell et al. [Bell, D.R., Rossman, G.R., Maldener, J., Endisch, D. and Rauch, F., 2003. Hydroxide in olivine: a quantitative determination of the absolute amount and calibration of the IR spectrum. Journal of Geophysical Research, 108(B2). doi:10.1029/2001JB000679] and that of Libowitzky and Rossman [Libowitzky, E. and Rossman, G.R., 1997. An IR absorption calibration for water in minerals. American Mineralogist, 82: 1111-1115]. For clinopyroxene, the best agreement with the ERDA data is obtained for the Libowitzky and Rossman [Libowitzky, E. and Rossman, G.R., 1997. An IR absorption calibration for water in minerals. American Mineralogist, 82: 1111-1115] calibration (5% lower than the ERDA data). The present calibration also confirms that the Paterson [Paterson, M.S., 1982. The determination of hydroxyl by infrared absorption in quartz, silicate glasses and similar materials. Bulletin de Mineralogie, 105: 20-29] calibration systematically underestimates the OH content in olivine