2002 Volume 78 Issue 3 Pages 51-56
We improved the U-Pb zircon dating method employing HR-SIMS by applying i) preparation of precise and homogeneous zircon standards by ID-TIMS, ii) simultaneous analysis of Pb isotopes by multi-collection system, and iii) use of a highly focused primary ion beam as small as 5μm in diameter. Using the improved HR-SIMS technique, U-Pb ages of zircons in an eclogite xenolith from the Colorado Plateau were determined. Zircon occurs as small inclusions (<20μm) in most constituent minerals. Ten zircons were measured, and the concordant analyses of these zircon data yielded a weighted mean value of 65.2±0.7Ma for the age of crystallization. Petrographic and geochemical observations suggest that this zircon age represents the age of initial subduction of basaltic oceanic crust. We conclude that the eclogite xenolith was a fragment of basaltic oceanic crust subducted to >90km depth along a cold geotherm with a subduction rate of >2cm/y in Tertiary.