How is Helium Hosted in the Mantle? - A Case Study of Exhumed Peridotite from the Twin Sisters Ultramafic Complex

Mantle noble gases trapped in peridotites are useful because they shed light on processes such as solid/melt partitioning, mantle/atmosphere evolution, and Earth's primordial noble gas composition. Yet the distribution of Helium (He) amongst potential hosting sites in mantle peridotites is poorly established. He is thought to be extremely incompatible in the mantle, and is often hosted in fluid inclusions in volcanic minerals and glasses. Alternatively, previous studies of mylonitized peridotites suggest that the majority (>80%) of the He is hosted in either grain boundaries or lattices (Recanati et al., 2013; Kurz et al., 2009). Unlike these studies, our work focuses on specimens that exhibit no evidence of grain-size reduction sampled from the Twin Sisters Ultramafic Complex (TSC) of the North Cascades in Washington, USA. Quarrying related to Pleistocene and Holocene glaciation has exposed virtually unaltered peridotite. Vacuum crushing of large grains (>180 micron diameter) and fusion of whole-rock material ground to ~20 micron diameter reveal He isotope ratios of up to ~3.5 Ra, confirming a mantle origin for most of the He. High He concentrations (~100 x10^-9 cc-STP/g) in fused whole-rock powder compared to crushed grains suggest that >75% of the He is hosted in intralattice space rather than in fluid inclusions. Serpentinization, which is low in the TSC (typically 0-15 vol%) is focused along joints and faults that crosscut all magmatic features (Kruckenberg et al., 2013). SEM analyses indicate that the serpentine content is <1 vol% amongst our samples, but consistently present, and may have introduced an additional He component. Although in situ radiogenic He is negligible in these low U-Th rocks, serpentinization may have introduced a mixture of atmospheric and radiogenic He in samples with 3He/4He <1 Ra samples. The high concentration of mantle He, associated neither with fluid inclusions nor grain size reduction, suggests an additional He host must be present in TSC. We posit that grain deformation, evidenced by prominent undulose extinction in the olivines, offers an important reservoir for He in sheared grains. If this is the general state in the mantle, then perhaps He is not as incompatible as most (undeformed) mineral-melt partition coefficient results suggest.