Xenolith constraints on deformation conditions and mechanisms in the lower mantle lithosphere

A cylindrical, 100km diameter shear wave splitting and body wave anomaly extends >500km into the mantle beneath central Nevada and has been interpreted as a downwelling lithospheric "drip" or Rayleigh-Taylor (R-T) instability. We investigated a suite of mylonitic and granular mantle xenoliths from an intraplate alkali basalt flow located directly above the instability (Marcath flow, Lunar Crater volcanic field).

Lunar Crater mylonites are dunites, wehrlites and harzburgites and exhibit spectacular deformation microstructures indicative of dynamic recrystallization; granular peridotites are harzburgites and lherzolites. In the mylonites, olivine neoblasts have indistinguishable compositions from coexisting porphyroclasts; the neoblasts are unzoned, demonstrating isochemical dynamic recrystallization. Mylonites have olivine Mg#s of 87-90 in contrast to granular peridotites (Mg#s mostly 90-92). Spinel Cr#s are as high as 68. Pyroxenes have LREE depleted to enriched chondrite normalized patterns. Overall, the compositional data suggest the xenoliths sample distinct, coexisting domains in the mantle lithosphere: refractory melting residues and enriched cumulates.

Application of major- and REE-based geothermometers to mylonitic and granular xenoliths demonstrates uniform temperatures of 1200-1300°C. Recrystallized and porphyroclastic olivine record Ca-in-olivine temperatures 1200°C, suggesting the mylonites formed in the lower lithosphere. Differential stress magnitudes estimated using grain size piezometrey are 50MPa, corresponding to strain rates of 10^-9-10^-6/s and effective viscosities of 10¹³-10¹⁶ Pa s, depending on assumption of wet or dry deformation conditions.

Among all Lunar Crater xenoliths, the mylonites have low Mg#s and pyroxene fractions. These samples suggest that strain in the mantle lithosphere is accommodated within weak, volumetrically minor heterogeneities, consistent with multiphase rock deformation experiments. We interpret the xenoliths as directly sampling the necking R-T instability beneath Lunar Crater. Delamination of mantle lithosphere may be accommodated by weak inclusions in planetary mantles as a normal consequence of ancient melt-rock reaction, mechanical mixing, and underplating.