Petrogenesis of Garnet Lherzolite and Megacrystalline Nodules from the Thumb, Navajo Volcanic Field

Abstract

Garnet Iherzolite and megacrystalline nodules from The Thumb comprise the deepest mantle sample recovered from the diatremes and intrusions of the Four Corners area. Discrepancies between various geothermometers applied to these nodules are believed to reflect zoning of Fe in garnets as well as problems in calibration of the geothermometers. Because the pyroxenes are not zoned, the pyroxene solvus method probably provides more reliable temperature estimates for this nodule suite than methods based on partitioning of Fe and Mg with garnet. The preferred T-P range of the nodules, 950–1230 °C and 32–40 kb, is thought to reflect a localized perturbation of the ambient geotherm by a diapiric or igneous intrusion.