What the Spatial Correlation of He Isotope and Seimic Velocity Anomalies Implies for Rifting and Volatile Sources in Ethiopia and Afar

Abstract:

Helium isotope ratios higher than the upper mantle value of 8 ± 1R_A (R_A = air ³He/⁴He) are unambiguous tracers of deep mantle (plume) volatile input in lavas and geothermal fluids from Ethiopia and Afar. However the significance of the surface distribution of He isotope ratios in terms of plume structure and melt distribution has received little attention. Recent seismic studies of this segment of the East African Rift give greatly improved lateral resolution of velocity anomalies allowing, for the first time, a detailed comparison of He isotope variations and tomographic imaging of melts, which presumably act to supply heat, mass and volatiles to the surface.

To produce a detailed map of He isotope ratios of the region, we generated 94 new high quality He measurements of fluid inclusions in mafic phenocrysts from lavas sampled along (and off) the axis of the Main Ethiopian Rift (MER) and Afar. Our contribution nearly doubles the existing dataset. Now, ~95% of the region from Chamo Lake through Afar including flood basalts on the flank of the MER – an area of ~400 000 km²– falls within 90 km of a He isotope measurement. This allows us to compare the spatial distribution of He isotope ratios from young lavas with the pattern of upper mantle S-wave velocity anomalies (Hammond et al. 2013) to determine how regions of low velocity (high melt content) correlate with He isotope ratios.

We find that regions of higher ³He/⁴He ratios – up to 19 R_A – correlate with anomalously low velocities at 75 km (i.e. shallow mantle) depth, and sites with low He isotope ratios cluster in higher velocity regions. Sustained upwelling and impingement of a deep mantle plume could explain this spatial correlation; however recent seismic evidence suggests shallow decompression melting accounts for most current volcanism in the MER and Afar (Rychert et al. 2012). Elevated He isotope ratios may therefore reflect shallow remobilization of stalled, undegassed plume material in the absence of a persistent deep volatile source. However this would require deep mantle He to remain in the shallow mantle for timescales on the order of 30 Myr., the approximate time since flood basalt cessation. The spatial correlation between He isotope ratios and seismic velocity anomalies therefore has profound implications for volatile cycling in the mantle and release at rift zones.