DI31A-2557
Implications of Heterogeneous Sr-Isotopes in Olivines from Samoan Lavas

Wednesday, 16 December 2015
Poster Hall (Moscone South)
Andrew Reinhard, University of California Santa Barbara, Santa Barbara, CA, United States, Matthew G Jackson, University of California Santa Barbara, Department of Earth Sciences, Santa Barbara, CA, United States and Jason Harvey, University of Leeds, Leeds, United Kingdom
Abstract:
In Ocean Island Basalts (OIB), relationships between helium isotopes and heavy radiogenic isotopes, like 87Sr/86Sr, are often used to constrain the evolution of the mantle. In OIB, 87Sr/86Sr is often measured on whole rock powders, while 3He/4He is measured in olivine hosted melt inclusions. Comparing these values is robust so long as both reservoirs have the same 87Sr/86Sr. However, new evidence suggests significant 87Sr/86Sr disequilibrium can exist between OIB whole rocks and the olivines they host.

The data presented show 87Sr/86Sr can vary by ~2000 ppm between the whole rock (0.7089) and olivine hosted melt inclusions (0.7075) in fresh, zero-age Samoan basalts. Additionally, samples with the highest whole rock 87Sr/86Sr also show the greatest disequilibrium between whole rocks and olivines. Importantly, Sr is highly incompatible in the olivine lattice, thus melt inclusions host nearly all Sr in olivines; measuring pooled olivines provides an average 87Sr/86Sr for the melt inclusion population.

The origin of whole rock- olivine 87Sr/86Sr disequilibrium is not well understood. The assimilation of seawater derived components could produce this effect, but is inconsistent with Cl/K data collected on submarine glasses. Whole rock 87Sr/86Sr increases with silica content suggesting 87Sr/86Sr disequilibrium is the result of mixing magmas derived from heterogeneous mantle sources. If olivine crystalizes from a low 87Sr/86Sr reservoir, and later mixes with a higher 87Sr/ 86Sr magma, the signature of the initial magma would be captured in the melt inclusions.

This whole rock- olivine 87Sr/86Sr disequilibrium has the potential to revolutionize our understanding of the relationships between 3He/4He and 87Sr/86Sr in OIB. These relationships are central to chemical geodynamic models, and establishing a new method for measuring both He and Sr isotopes in olivine hosted melt inclusions will provide new frontiers for future research exploring the noble gas evolution of the mantle.

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