Assessment of shock effects on amphibole water contents and hydrogen isotope compositions: 2. Kaersutitic amphibole experiments

作者：Minitti Michelle E^*; Leshin Laurie A; Dyar M Darby; Ahrens Thomas J; Guan Yunbin; Luo Sheng Nian

来源：Earth and Planetary Science Letters, 2008, 266(3-4): 288-302.

DOI：10.1016/j.epsl.2007.11.012

• Mars
• Martian meteorites
• amphibole
• impact shock
• hydrogen isotopes

To constrain the influence of impact shock on water and hydrogen isotope signatures of Martian meteorite kaersutites, we conducted shock recovery experiments on three terrestrial kaersutite crystals. Homogeneous impact shock to 32 GPa, commensurate with shock levels experienced by Martian meteorite kaersutites, led to increases in kaersutite water contents (Delta H2O-0.25-0.89 wt. %), decreases in Fe3+/Sigma Fe (4-20%), and enrichments in hydrogen isotope composition (Delta D=+66 to +87 parts per thousand) relative to pre-shock values. The latter values represent the largest shock-induced hydrogen isotope fractionations measured to date. These observations are explained most completely by a two-step shock process. First, shock-induced devolatilization led to hydrogen isotope enrichment through preferential loss of H relative to D. Second, reaction of the kaersutite with the ambient atmosphere led to increased water contents and reduced Fe. Fe reduction and water addition via the reaction Fe2+ + OH- <-> Fe3+ + O2- + 1/2H(2) explain the Fe3+/Sigma Fe data and some of the water data, Further water addition mechanisms (irreversible adsorption, shock implantation) are necessary to fully explain the increased water contents. Addition of water from the terrestrial atmosphere, which is isotopically light relative to the experimental kaersutite compositions, means the measured hydrogen isotope enrichments are likely minima. The measured (minimum) levels of hydrogen isotope enrichment are relevant to the hydrogen isotope variability within and among Martian kaersutites, but are minor relative to their absolute delta D values. Alternatively, addition of water from the enriched Martian atmosphere could explain both Martian kaersutite hydrogen isotope variability and absolute 6D values. However, the low Martian kaersutite water contents leave little room for significant water addition. The importance of the ambient atmosphere to the outcome of the shock experiments makes it difficult to translate our results to Mars given the unknown influence of its more tenuous atmosphere on the processes observed in the experiments. Our results suggest that shock is a feasible mechanism for influencing

• 出版日期2008-2-20