METAMORPHISM AND AQUEOUS ALTERATION IN LOW PETROGRAPHIC TYPE ORDINARYCHONDRITES

In order to investigate the relative importance of dry metamorphism an d aqueous alteration in the history of chondrules, chondrules were han d-picked from the Semarkona (petrographic type 3.0), Bishunpur (3.1), Chainpur (3.4), Dhajala (3.8) and Allegan (5) chondrites, and matrix s amples were extracted from the first three ordinary chondrites. The th ermoluminescence (TL) properties of all the samples were measured, and appropriate subsets of the samples were analyzed by electron-micropro be and radiochemical neutron activation and the water and H-isotopic c omposition determined. The TL data for chondrules from Semarkona and B ishunpur scatter widely showing no unambiguous trends, although group B1 chondrules tend to have lower sensitivities and lower peak temperat ures compared with group A5 chondrules. It is argued that these data r eflect the variety of processes accompanying chondrule formation. The chondrules show remarkably uniform contents of the highly labile eleme nts, indicating mineralogical control on abundance and volatile loss f rom silicates and loss and recondensation of mobile chalcophiles and s iderophiles in some cases. Very high D/H values (up to similar to 8000 parts per thousand SMOW) are observed in certain Semarkona chondrules , a confirmation of earlier work. With increasing petrographic type, m ean TL sensitivities of the chondrules increase, the spread of values within an individual meteorite decreases, and peak temperatures and pe ak widths show trends indicating that the TL is mainly produced by fel dspar and that dry, thermal metamorphism is the dominant secondary pro cess experienced by the chondrules. The TL sensitivities of matrix sam ples also increase with petrographic type. Chainpur matrix samples sho w the same spread of peak temperatures and peak widths as Chainpur cho ndrules, indicating metamorphism-related changes in the feldspar are r esponsible for the TL of the matrix. The TL data for the Semarkona and Bishunpur matrix samples provide, at best, only weak evidence for aqu eous alteration, but the matrix contains H with approximately terrestr ial D/H values, even though it contains much water. Secondary processe s (probably aqueous alteration) presumably lowered the D/H of the matr ix and certain chondrules. While chondrule properties appear to be gov erned primarily by formation processes and subsequent metamorphism, th e matrix of Semarkona has a more complex history involving aqueous alt eration as a meteorite-wide process.