Are Colorado Plateau Eclogite Xenoliths Franciscan?: Oxygen Isotope Evidence From Zoned Garnet

Abstract:

Eclogite xenoliths from the Moses Rock diatreme, UT, USA are of controversial (Proterozoic or Phanerozoic) age. In this study, seven garnets from four Moses Rock eclogite xenoliths were analyzed for δ¹⁸O by ion microprobe. Garnet core δ¹⁸O values are 7.8-10.3‰ VSMOW. All samples have a sharp change between cores and rim values of 5.8-6.9‰. These garnets have the first reported oxygen isotope zoning from mantle xenoliths. The core values are well outside the range of garnets equilibrated with the mantle, suggesting that they began growth during subduction from an altered oceanic crustal protolith. Most rim values reach the mantle range. This decrease in δ¹⁸O from core to rim is consistent with continued subduction of the eclogites into the mantle. The failure of some garnet rims to reach mantle δ¹⁸O values may indicate that they did not equilibrate fully with the mantle, or were exposed to a mixed mantle-slab fluid.

Zoning in the samples from this study record a stepped shift from an altered upper oceanic crust protolith, to a mantle-influenced environment. The preservation of zoning in some of the samples from this study suggests that these eclogites were protected within the cool subducting slab and experienced a short mantle residence time. The preservation of cation and oxygen isotope zoning is more consistent with an origin during Franciscan subduction than Proterozoic subduction, unless the zoning is a late feature that formed just prior to volcanic emplacement. This is further supported by the similar patterns of increased pyrope content and decreasing δ¹⁸O found in some Franciscan eclogite garnets (e.g., Errico et al., 2013, CMP).