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Relatively small amounts (typically between 2-200 parts per million) ofpresolar grains have been preserved in the matrices of chondritic meteorites.The measured abundances of the different types of grains are highly variablefrom one chondrite to another, but are higher in unequilibrated chondrites thathave experienced little or no aqueous alteration and/or metamorphic heatingthan in processed meteorites. A general overview of the abundances measured inpresolar grains (particularly the recently identified presolar silicates)contained in primitive chondrites is presented. Here we will focus on the mostprimitive chondrite groups, as typically the highest measured abundances ofpresolar grains occur in primitive chondrites that have experienced littlethermal metamorphism. Looking at the most aqueously altered chondrite groups,we find a clear pattern of decreasing abundance of presolar silicate grainswith increasing level of aqueous alteration. We conclude that the measuredabundances of presolar grains in altered chondrites are strongly biased bytheir peculiar histories. Scales quantifying the intensity of aqueousalteration and shock metamorphism in chondrites could correlate with thecontent in presolar silicates. To do this it would be required to infer thedegree of destruction or homogenization of presolar grains in the matrices ofprimitive meteorites. To get an unbiased picture of the relative abundance ofpresolar grains in the different regions of the protoplanetary disk where firstmeteorites consolidated, future dedicated studies of primitive meteorites,IDPs, and collected materials from sample-return missions (like e.g. theplanned Marco Polo) are urgently required.

The effect of aqueous alteration and metamorphism in the survival of presolar silicate grains in chondrites