UC Davis

The Easter-Salas y Gómez seamount chain (ESC) is a natural laboratory for understanding the chemical heterogeneity in the Earth’s deep interior and the interactions between a mantle plume and spreading center as the Salas y Gómez plume (SyG) is near the East Pacific Rise (EPR), home of the fastest spreading ridge axis observed on Earth. The geochemical variability along the ESC chain was proposed to reflect increased entrainment of the depleted asthenospheric material towards the west as the plume flow is drawn towards the spreading center along lithospheric channels. Furthermore, the ESC is thought to be comprised of two sub-parallel chains, an enriched one that is in the north and a depleted chain in the south. The SyG plume was proposed to be striped with the enriched chain sampling the Pacific Large Low Shear Wave Velocity Province (LLSVP) and the depleted chain sampling the ambient deep Pacific mantle. Noble gases are uniquely useful tracers of mantle geochemical evolution and in identifying primitive material in the plume source. However, there is limited noble gas data along the ESC. Here we report on new helium and neon isotopes from the ESC lavas and combine them with published data on Sr, Nd, Pb isotopic compositions and water abundances to better understand the origin of the geochemical variability along the ESC and the spatial geochemical variability in the ESC.

We find the He isotopes co-vary with other lithophile isotopic compositions and withwater. The 4He/3He values range from a (primitive) value of 39,500 to values that are more radiogenic than MORBs and as high as 111,300. The 20Ne/22Ne values for five of the samples range from atmospheric values of 9.8 to values up to 12.61, which is more characteristic for a plume mantle source and suggests a solar volatile composition. For both the northern and southern chain the 21Ne/22Ne ratios are less radiogenic than MORBs requiring the presence of less degassed material than MORBs in both chains. Neon, therefore, maybe even more sensitive to the presence of primitive material in a plume than helium.

We find that the previously observed transition towards more depleted composition in the west is due to incorporation of the more primitive FOZO component that carries low and less degassed 4He/3He ratios. FOZO is not as depleted as the source of mid ocean ridge basalts (MORBs) but is depleted compared to the recycled enriched components in the eastern part of the chain. Thus, the geochemical trend towards the west is due to the inherent heterogeneities present within the plume source. Furthermore, we see a greater proportion of the FOZO component in the southern chain. Previous work has shown that more primitive He isotopic ratios requires a reservoir that is less degassed and processed via partial melting compared to the MORB source and that the LLSVPs might be a suitable reservoir for such a source. If so, we find that the southern chain preferentially samples material from the LLSVPs.

The relationship between He and incompatible elements like water allows us to refine the juvenile water abundance of the FOZO component from the commonly used value of 750 ppm down to 150-230 ppm, indicating that FOZO is a lot drier than previously determined. While the 20Ne/22Ne value of up to 12.61 requires a solar source for neon, the ?D and He isotopic relationship for the juvenile water in the deep mantle appears to be derived from a chondritic source. Thus, the deep mantle indicates incorporation of volatiles from different sources during Earth’s early accretion.