DATING CRUST-MANTLE SEPARATION - RE-OS ISOTOPIC STUDY OF MAFIC XENOLITHS FROM CENTRAL ARIZONA

Mafic deep crustal and shallow upper mantle xenoliths hosted in the Ch ino Valley and Camp Creek latites of central Arizona have Os-187/Os-18 8 (0.216 to 1.51) ratios dramatically higher than values typical of ma ntle peridotite (<0.13). Re-Os model ages for the xenoliths range from 0.57 to 2.3 Ga with a mean of 1.1 +/- 0.6 Ga consistent with the poss ibility that the xenoliths sample Proterozoic loser crust and upper ma ntle underplated to the ca. 1.6-1.8 Ga basement. Combined Os and Sr, N d, Pb data indicate that these xenoliths were not formed by crustal co ntamination of Cenozoic asthenospheric melts, but allow for recent int eraction between old mafic materials in the lithosphere and high Sr-87 /Sr-86, low Nd-143/Nd-144 melts of Proterozoic lithospheric mantle. Th e potassic latite hosts to the xenoliths also have radiogenic Os, indi cating that they are melts of a mafic rather than a peridotitic source in the old lithosphere. These data demonstrate the utility of the Re- Os system in defining mantle separation ages, or the initial time of m elt extraction from a peridotitic mantle, even in cases where the lith ospheric mantle is remelted long after its formation.