VOLATILE AND TRACE ELEMENT CHARACTERISTICS IN SUBDUCTION INFLUENCED XENOLITHS

Determining the trace element and volatile characteristics of mantle-derived xenoliths is crucial to understanding the processes that occur in the Earth's interior. Trace element (Nd, Sr, Pb, REE) and volatile (He, Ar, CO2) concentrations and isotopic ratios of fluids trapped in arc lava xenoliths are useful for identifying the source of the arc lava fluids and provide insight into mantle processes. Xenoliths derived from arc lava, may contain fluids trapped along kink bands and grain cleavages as seen at the Kamchatka Arc. These are believed to be related tectonism, not trapped during crystal formation. Therefore these fluids provide a unique opportunity to study late addition subduction derived fluids. In the case of Kamchatka, these fluids display volatile characteristics [CO2/3He (2x109-3x1010); delta 13C (-16 to -32), 3He/4He (0.6 – 7.3)], and trace element chemistry [REE, 87Sr/86Sr (0.703 -.7041), etc.] that indicates mantle wedge alteration by slab-derived fluids from altered oceanic crust.

Mantle xenoliths associated with the Samoan Archipelago have radiogenic and presumably slab derived Sr, Nd, and Pb consistent with observations made at Kamchatka and other subduction zones. Conversely, the volatile chemistry such as 3He/4He (8-22Ra), CO2/3He (4x108-5x1010), ä13C (-6.2 to -2), 4He/40Ar*, 3He/(U + Th), etc. are not consistent with Kamchatka. In order to better constrain the source of volatiles and subduction added fluids we focused our attention on CO2 concentrations, C isotopic compositions, and CO2/3He ratios, along with He isotopic ratios.

Samoan xenoliths exhibit relatively consistent He isotopic ratios with significant variations in the CO2/3He ratio at a given location. Compared to Kamchatka, Samoan xenoliths have lower CO2/3He, higher 3He/4He ratios, and heavier C isotopic compositions that are not consistent with fluids from subducting slab similar to Kamchatka. The variations in helium isotopes, specifically CO2/3He ratios, suggest a large variation in the amount of added sedimentary component from 1:1 to 1x106. The data for Kamchatka provides a reasonable estimate for CO2/4He of an altered oceanic crustal component. This estimate along with measurable helium and carbon isotope data (4He/3He vs. CO2/3He) and other trace elements (Sr, Ba, La) can be used to estimate the added sediment component from slab related fluids present in Samoan xenoliths.