Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.456w&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 456
Computer Science
1
Chassigny, Hydrogen, Isotopes, Kaersutite, Mars, Nakhlites, Shergotty, Snc Meteorites
Scientific paper
The D/H ratio of water in the present martian atmosphere is enriched by a factor of 5.15 relative to the Earth [1] (corresponding to a delta-D(sub)SMOW value of +4150 permil). This D enrichment is thought to result from massive escape of hydrogen (relative to heavier deuterium) from the martian upper atmosphere throughout the planet's history [2,3]. A measurement of the D/H ratio of magmatic (interior) water would provide important constraints on the extent of such hydrogen loss because the D/H ratio of the martian interior probably reflects its original value (assumed to be near the terrestrial value of 1.5 x 10^-4) since, unlike the Earth with its subduction zones, recycling of near-surface and atmosphere-derived waters into the martian deep interior is generally considered to be an unimportant process. We have measured the D/H ratios of individual kaersutitic amphiboles contained in magmatic inclusions in the SNC meteorites (thought to be martian samples) with the hope of sampling the water dissolved in the magma at the time the inclusion was trapped. The measurements were made using the PANURGE ion microprobe [4,5]. We have previously reported delta-D values for kaersutites in Chassigny ranging from +853 +- 55 to +1420 +- 47 permil [4]. We have now measured an additional D/H ratio in Chassigny section USNM 624-1 as well as the D/H ratio of one 25-micrometer kaersutite in Shergotty section AMNH 3937-1. The Chassigny value extends the range of delta-D values measured in this sample down to +501 +- 34 permil. In contrast, the Shergotty kaersutite has a delta-D value of -80 +- 31 permil. The Chassigny delta-D values are similar to those measured in weathering products in the nakhlites [6,7] and suggest that although Chassigny is not visibly weathered, D-enriched fluids from the martian crust have interacted with the sample and altered the delta-D values. This scenario could also explain the large range in delta-D measured in the Chassigny kaersutites. The Shergotty delta-D value could reflect martian interior water that has a very similar delta-D to the bulk Earth, consistent with predictions of formation of the terrestrial planets from material with similar bulk D/H. Ion microprobe measurement of the H+/Si+ secondary ion ratio allows estimation of the total water content of the kaersutites. Both the Chassigny and Shergotty kaersutites contain only ~0.5 wt% water, indicating that they contain a significant oxyamphibole component, similar to terrestrial kaersutites. Water contents of the kaersutites have been previously assumed to be between 1.5 and 2 wt%. These high values have been used to infer the amount of water in the melt inclusion magma at the time they were trapped [8,9] and to constrain the depth of kaersutite crystallization on Mars. Since the kaersutites are much less hydrous than previously assumed, these estimates may need revision, although little is known about the effect of water fugacity on the incorporation of a Ti-coupled oxyamphibole component, so that a kaersutite with low hydroxyl abundance does not necessarily imply a less hydrous melt. References: [1] Bjoraker G. L. et al. (1989) In Proc. 4th Int. Conf. Mars, Tucson, 69-70. [2] Owen T. et al. (1988) Science, 240, 1767-1770. [3] Yung Y. L. et al. (1988) Icarus, 76, 146159. [4] Watson L. L. et al. (1993) In LPSC XXIV, 1493-1494. [5] Deloule E. et al. (1991) In Stable Isotope Geochemistry: A Tribute to Samuel Epstein, 53-62. [6] Kerridge J. F. (1988) In LPSC XIX, 599-600. [7] Watson L. L. et al. (1991) In LPI Tech. Rept. 92-02, 156-166. [8] Johnson et al. (1990) GCA, 55, 349366. [9] McSween H. Y. Jr. and Harvey R. P. (1993) Science, 259, 1890-1892.
Epstein Samuel
Hutcheon Ian D.
Stolper Edward M.
Watson L. L.
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