A PETROLOGIC AND ISOTOPIC STUDY OF LODRANITES - EVIDENCE FOR EARLY FORMATION AS PARTIAL MELT RESIDUES FROM HETEROGENEOUS PRECURSORS

We have conducted petrologic, chemical, and isotopic studies of lodran ites in an attempt to constrain their genesis. Lodran, Gibson, Y-79149 1, Y-791493, Y-74357, Y-8002, Y-75274, MAC 88177, LEW 88280, EET 84302 , FRO 90011, and QUE 93148 are classified as lodranites. Lodranites an d acapulcoites are indistinguishable on the basis of oxygen isotopic c ompositions but are distinct in average grain sizes of their mafic sil icates, with lodranites being significantly coarser-grained. Lodranite s exhibit a diverse range of petrologic and mineralogic features: they range widely in mafic silicate compositions (Fa(3-13)), plagioclase ( 0-11.4 vol%), Fe,Ni metal (0.5-20 vol%), and troilite (0.2-5.3 vol%) c ontents; and shock levels (S1-S4). They appear to have experienced hig h peak temperatures and rapid cooling in the temperature range recorde d by metallographic cooling rates (i.e., 700-350 degrees C). The only dated lodranite, Gibson, cooled to Ar closure temperatures at 4.49 +/- 0.01 Ga. Lodranites formed from chemically and isotopically heterogen eous precursors in which the mineral and oxygen isotopic compositions were correlated. Heating of their parent body to temperatures between similar to 1050-1200 degrees C resulted in formation of Fe,Ni-FeS and basaltic partial melts. Depletions of troilite and/or plagioclase in m ost lodranites testify to the removal of some of these partial melts, although melt migration was complex. Lodranites appear to have experie nced a complex cooling history of slow cooling at high temperatures, f ollowed by rapid cooling at intermediate temperatures, possibly relate d to breakup of the parent body. Lodranites were liberated from their parent body during 1-3 impact events, with most having cosmic ray expo sure ages of 5.5-7 Ma. The acapulcoites are samples from the same pare nt body but were heated to lower temperatures and, thus, experienced l ower degrees of partial melting. Copyright (C) 1997 Elsevier Science L td.