What does the CM chondrite Mineralogic Alteration Index really mean?

Introduction: CM chondrites contain minerals that formed by interaction between liquid water and anhydrous minerals [1]. How this process progressed on the parent body is important for understanding how to unravel the effects of aqueous alteration in these meteorites. Studies undertaken to qualify [1] and quantify [2] the extent of alteration found in the CMs resulted in a mineralogic alteration index (MAI) [2]. The MAI is a numerical representation of the Fe 3+ /Si ratio in matrix phyllosilicates in CM meteorite falls. It has been found to correlate with carbonate [3] and sulfate [4] oxygen isotopic composition. One key question has been determination of the extent of terrestrial contamination within these meteorites. Discussion and Implications: Figure 1 is a plot of MAI vs Year of Fall [5] for all the CM chondrites for which an MAI was determined [2]. MAI has a strong positive correlation with Year of Fall (R 2 = 0.93); that is, the more altered meteorites are older. There is no such systematic correlation between Year of Fall and matrix or bulk 17 O, 18 O [6], water content or D [7]. Although not as robust, a correlation also exists between Year of Fall and 17 O in sulfate and carbonate, two minerals that are susceptible to interaction with liquid water. The fact that MAI and carbonate and sulfate 17 O track with Year of Fall indicates that interaction with the terrestrial atmosphere may take place even if the meteorite is recovered immediately after its fall. The MAI may be providing a measure of terrestrial contamination. References: [1] McSween H.Y. 1979 Geochim. Cosmochim. Acta. 43:1761-1770. [2] Browning L.B. et al. 1996 Geochim. Cosmochim. Acta. 60:2621-2633. [3] Benedix et al. 2003 Geochim. Cosmochim. Acta 67:1577-1588. [4] Airieau S. et al. 2001. Abstract #1744 32 nd Lunar Planet. Sci. Conf. [5] Grady M.M. 2000. Catalogue of Meteorites. [6] Clayton R.N. and Mayeda T. 1999. Geochim. Cosmochim. Acta. 63:2089-2104. [7] Eiler J.M. and Kitchen N. 2004. Geochim. Cosmochim. Acta. 68:13951411.