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.

Garnet Iherzolites with coarse texture show a trend from fertile to refractory compositions consistent with variable depletion by partial melting. These nodules are interpreted as previously depleted mantle rocks, most of which were relatively little affected by the thermal perturbation. Garnet Iherzolites with sheared texture are enriched in Fe and Ti relative to the coarse Iherzolites and are suggested to have formed by deformation and metasomatism of the coarse lherzolites during intrusion of magma related to the thermal perturbation. The texturally diverse ‘megacrystalline’ nodules are interpreted as precipitates from this liquid. They are similar to discrete nodules from kimberlite pipes, but have unique characteristics of their own. The above relationships are thought to record the process of deep-seated intrusion and partial crystallization of evolved liquids, possibly related to the host lamprophyre, shortly before the time of eruption.