MANTLE XENOLITHS RECORD DEFORMATION ASSOCIATED WITH ACTIVE LITHOSPHERIC DOWNWELLING BENEATH CENTRAL NEVADA

Intensely deformed mylonitic mantle peridotite xenoliths are preserved in Pleistocene flows and cinder cones at Lunar Crater volcanic field in central Nevada. They are spatially and chemically associated with coarse-grained lherzolites and harzburgites with remarkably high two-pyroxene and Ca-in-olivine temperatures (all 1200-1300°C), suggesting they originate from the base of the mantle lithosphere. Here we report results of a chemical and microstructural investigation of 14 previously unstudied mylonitic dunites, wehrlites, and pyroxene-poor harzburgites.

Orthopyroxenes exhibit little evidence for plastic deformation and in some samples show brittle deformation. Extremely flattened porphyroclastic grains and substantial dynamic recrystallization in olivine suggest deformation occurred by dislocation creep. Recrystallized olivine grain sizes are 50-86 µm yielding flow stresses of 43-63 MPa according to a grain size piezometer. Electron microprobe analyses establish that dynamically recrystallized and porphyroclastic olivine in the mylonites have Ca-in-olivine temperatures >1200°C, which we infer to be the deformation condition. Trace elements measured in pyroxenes in coarse-grained and mylonitic samples yield REE-in-two-pyroxene temperatures of 1278-1338°C (n=4), confirming that a high-temperature signature predates entrainment and eruption.

Using our paleostress magnitudes and assuming a hot (1200°C) dry mantle lithosphere implies deformation occurred at strain rates >10^-9/s, too rapid for steady-state lithospheric deformation. We interpret such localized, transient deformation to be a consequence of formation of a mantle lithospheric drip, as suggested by cylindrical shear wave splitting and body wave anomalies beneath Lunar Crater. Extreme strain localization and viscosity reduction occur around diapirs in planetary mantles.