3HE/22NE VARIATIONS AMONG OCEAN ISLAND, MID-OCEAN RIDGE, AND BACKARC BASALTS

³He and ²²Ne are primordial isotopes inherited from Earth’s accretion. Neither isotope is produced in the mantle or fractionated by partial melting. It has been observed that mid-ocean ridge basalts (MORBs) have relatively high ³He/²²Ne (typically 4 - 10) compared to ocean island basalts (OIBs), whose ³He/²²Ne approach solar nebula and chondritic values (< 2). This discrepancy reflects the existence of a heterogeneous mantle with distinct reservoirs for MORBs and OIBs. Because Earth formed from precursor materials with low, chondritic or solar nebula-like ³He/²²Ne, a mechanism for producing a high ³He/²²Ne MORB source is required. Fractionation of He from Ne has been attributed to the effects of multiple giant impacts which led to outgassing of liquid magma oceans. However, the discrepancy in ³He/²²Ne can also be modeled by plate tectonic cycling and diffusion between dunite channels and surrounding harzburgite wallrock [Dygert et al., 2016]. We compiled data from papers which recorded isotopic compositions for basalt samples from mid-ocean ridges, hotspots, and back-arc basins. In addition to differences in ³He/²²Ne between MORBs and OIBs, we identify discrepancies in the distribution of ³He/²²Ne in OIBs from various localities. While the distributions from Tristian, Reunion Island, and Iceland reflect the expected low ³He/²²Ne (~2-5), those from Hawaii, the Shona Ridge and the Azores Archipelago display higher average ³He/²²Ne (~4-6). Proximity of the Shona Ridge and the Azores Archipelago to the Mid-Atlantic Ridge could result in interactions between mantle plume and the MORB source, leading to a net elevated ³He/²²Ne source. Alternatively, among themselves, OIBs may source distinct mantle reservoirs with variable ³He/²²Ne as suggested by radiogenic isotopes. The kinetic fractionation model of Dygert et al. [2016] predicts that mantle sources which experienced extensive or repeated melting should have high ³He/²²Ne. This is supported by our observations from highly depleted basalts from the Manus backarc basin, which are unusually higher in ³He/²²Ne than MORBs, as well as positive correlations of ³He/²²Ne with radiogenic isotope indices of depletion.

Dygert, N., et al, 2016. Plate tectonic cycling and whole mantle convection modulate Earth’s 3He/22Ne ratio. AGU Fall Meeting, D11A-2343.