PROGRESS IN ZONED GARNET SM-ND GEOCHRONOLOGY

The great potential for direct dating of discrete portions of growth zoned garnets was demonstrated twenty years ago in a series of pioneering contributions. At the Boston University TIMS Facility, recent advances in: (1) analysis of very small Nd aliquots, (2) partial dissolution cleansing of inclusions, (3) chemically contoured microsamplling of garnet growth zones, have been integrated to permit a more full exploitation of the Sm-Nd geochronologic record of zoned garnets. To demonstrate the precision, accuracy, and spatial-temporal resolution that is possible, we report zoned garnet data from three locations: (1) garnet-biotite-chlorite schist from a shear zone in the Tauern Window, Austria, (2) garnet-bearing pelitic schist from Townshend Dam, Vermont, (3) garnet-bearing blueschist from Sifnos, Greece. The Austrian sample yielded 12 concentric growth ages within a 6cm garnet. With starting crushed/sieved/frantzed garnet zone masses ranging from 24 to 142 mg, individual age precisions ranged from ±0.4 to ±0.7 Ma (2SD). The age pattern is consistent with constant volumetric growth spanning 7.6 Myr punctuated by two brief pulses of accelerated growth. The Vermont sample yielded three concentric growth ages from each of two ~2.0cm garnets in the same rock. With starting crushed/sieved/frantzed masses ranging from 33 to 146 mg, age precisions mostly ranged from ±0.7 to ±1.7 (2SD) with one more at ±3.7. Ages from the two crystals are in good agreement indicating a 5.5 Myr growth span. The Sifnos sample includes three concentric growth ages from two ~1.4cm garnets in the same rock. With starting crushed/sieved/frantzed masses ranging from 53 to 135 mg, age precisions mostly ranged from ±0.7 to ±1.8 Myr (2SD) with one more at ±4.8. All six ages from the two crystals give the same age indicating brief growth duration of <1.1 Myr. The consistency of multiple crystals from the same rock, and the consistency of intracrystal age patterns with core-to-rim growth geometry indicate age accuracy. Differences in attainable age resolution among samples relates to differences in garnet Nd concentration and inclusion density. Experience has shown that each new suite of garnets requires preliminary evaluation of these factors and adjustment of cleansing procedures to establish, and maximize, geochronologic expectations.