Bachelor of Arts
F. Zeb Page
Eclogite, Franciscan, Colorado, Plateau, Navajo, Volcanic, Field, Xenolith, Moses, Rock, Oxygen, Isotope, Metamorphic, Petrology, Garnet, Zoning, Diatreme, Subduction, Accretionary, Wedge, California, Four, Corner
Eclogites can be found in two distinct tectonic settings in the western USA: in the Franciscan Complex as tectonic blocks in a fossil accretionary wedge, and in the Navajo Volcanic Field as xenoliths in diatremes. Intra-crystal oxygen isotope analyses of garnets from these two settings provide important information on the source, composition and timing of fluid interactions in both eclogite settings. Because of the well-documented relationship between Franciscan subduction and the volcanism in the Navajo Volcanic Field, the samples in this study can be directly compared to determine if they share a common origin. The Navajo eclogites in this study have the first intra-garnet oxygen isotope zoning found in any mantle xenoliths. Xenolith garnets record core δ18O values of 7.8 to 10.3‰ consistent with altered upper oceanic crust. Rim values are lower in δ18O with values of 5.8 to 6.9‰, consistent with late garnet growth in equilibrium with slab-contaminated mantle fluids. Three garnets homogenous in δ18O were found. Atoll textures present in one sample record altered upper oceanic crust values in the atoll, with replacement cores and rims consistent with a mantle-dominated fluid. The presence of garnets homogenous in δ18O within a few mm of garnets zoned in δ18O suggests textural control, both intra- and inter-garnet, on oxygen zoning. In particular, a relationship between the extent of healed fracturing and oxygen zoning is observed across all the Navajo eclogite samples. Garnets homogenous in δ18O still record cation zoning, which is inconsistent with homogenization by diffusion. The presence of zoning in both cations and oxygen in these samples suggests a short residence time in the mantle, and a Mesozoic-Cenozoic origin.
Franciscan eclogite garnets record altered upper oceanic crust core δ18O values of 9.3 to 13.3‰. Rim values in these samples range from 6.7 to 9.5‰. Low-δ18O rims record interaction with mantle or serpentinization fluids. High-δ18O rims suggest protection from these fluids due to location and/or channelized fluid flow. Oscillation in δ18O values between cores and rims suggests exposure to changing fluid environments or some movement of these samples within the subduction channel before exhumation. No definitive genetic link was found between the Franciscan and Moses Rock eclogites, however time constraints placed upon the Moses Rock xenoliths suggest that the two may have originated within the same subduction zone.
Hoover, William F., "Eclogites and eclogites: Oxygen isotope evidence of a shared subduction origin for Franciscan eclogites and Moses Rock eclogite xenoliths" (2014). Honors Papers. 294.