Upper limits of water contents in olivine and orthopyroxene of equilibrated chondrites and several achondrites

1,2Dennis HARRIES,3Xuchao ZHAO,3Ian FRANCHI
Meteoritics & Planetary Science (in Press) Open Access Link to Article [doi: 10.1111/maps.13980]
1Department of Analytical Mineralogy, Institute of Geoscience, Friedrich Schiller University Jena, Carl-Zeiss-Promenade 10,07745 Jena, Germany
2European Space Resources Innovation Centre, Luxembourg Institute of Science and Technology, 41 rue du Brill, L-4422Belvaux, Luxembourg
3School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
Published by arrangement with John Wiley & Sons

Hydroxyl defects in nominally anhydrous minerals (NAMs) were potential carriers ofwater in the early Solar System and might have contributed to the accretion of terrestrial water.To better understand this, we have conducted a nanoscale secondary ion mass spectrometrysurvey of water contents in olivine and orthopyroxene from a set of equilibrated ordinarychondrites of the L and LL groups (Baszk ́owka, Bensour, Kheneg Ljouˆad, and Tuxtuac) andseveral ultramafic achondrites (Zakøodzie, Dhofar 125, Northwest Africa [NWA] 4969, NWA6693, and NWA 7317). For calibration, we used terrestrial olivine and orthopyroxene with H2Ocontents determined by Fourier transform infrared. Our 99.7% (~3SD) detection limits are 3.6–5.4 ppmw H2O for olivine and 7.7–10.9 ppmw H2O for orthopyroxene. None of the meteoriticsamples studied consistently shows water contents above the detection limits. A few exceptionsslightly above the detection limits are suspected of terrestrial contamination by ferricoxyhydroxides. If the meteorite samples investigated accreted in the presence of small amountsof water ice, the upper limits of water contents provided by our survey suggest that the retentionof hydrogen during thermal metamorphism and differentiation was ineffective. We suggest thatloss occurred through combinations of low internal pressures, high permeability along grainboundaries, and speciation of hydrogen into reduced compounds such as H2and methane,which are less soluble in NAMs than in water.


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