| Abstract |
Introduction: Although still debated, the origin of water on Earth is usually ascribed to a late veneer of hydrous primitive bodies [1]. The isotopic composition of water hydrogen (D/H ratio) in a diversity of primitive extraterrestrial materials can help to assess the nature of the late veneer agent. We present measurements of D/H ratios of hydrated silicates in carbonaceous chondritic microclasts (CCMs) recently identified within achondrite meteorites [2]. These CCMs have been interpreted to be fossil micrometeorites [2]. Results: Measurements of D/H ratios in the matrix of 6 CM2-like and 6 CR2-like CCMs were performed by Secondary Ion Mass Spectrometry following the procedure described in [3]. D/H ratios from CM2-like carbonaceous chondritic microclasts matrix range from 117.6 × 10-6 to 171.0 × 10-6, with an average value of 132.7 × 10-6. D/H ratios from CR2-like carbonaceous chondritic microclasts matrix range from 112.8 × 10-6 to 203.8 × 10-6, with an average value of 169.4 × 10-6. D/H ratios in CM2-like microclasts compare well to bulk CM2 carbonaceous chondrites [4]. D/H ratios in CR2-like microclasts are significantly lower than D/H measured in bulk CR2 chondrites [4]. This discrepancy probably arises from the fact that bulk measurement techniques on meteorites take into account D-enriched organics in addition to water. The average value of all the microclasts’ population is D/H = 152.0 ± 4.8 × 10-6. Discussion: The CCMs D/H average value compares favourably to the terrestrial value (149 ± 3 × 10-6). This match suggests that fossil micrometeorites could have endowed Earth with water, provided that the micrometeorite flux on early Earth was large enough. This possibility will be discussed at the conference, in the light of flux data, and other constraints to be met such as the PGE abundance. References: [1] F. Robert, Science 293, 1056--1058 (2001). [2] M. Gounelle et al., Geochim. Cosmochim. Acta In press (2002). [3] C. Engrand et al., Meteoritics Planet. Sci. 34, 773--787 (1999). [4] J. F. Kerridge, Geochim. Cosmochim. Acta 49, 1707--1714 (1985). |
