Sr Isotopic Evolution of the Mantle: Constraints on Accretion, Recycling, and Crustal Growth

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

DePaolo EAR-0208671 The proposed research involves using the isotope geochemistry of ancient igneous rocks to evaluate issues related to the early evolution of the earth - how long did it take for the Earth to form by accretion, how much chemical differentiation did the Earth experience in the first few hundred million years, and how did the Earth's continental crust grow and change through its history? The approach is to use the mineral zircon, which is common in old rocks and can be precisely dated by U-Pb geochronology, to probe the strontium isotopic composition of the Earth's mantle in Precambrian time. The Sr isotopic composition of the Precambrian mantle is highly uncertain because it is difficult to find materials that preserve their original isotopic composition through later episodes of metamorphism and weathering. Strontium isotopic measurements of zircon offer a possible resolution to this problem. Defining the Sr isotopic composition of the Archean and Proterozoic mantle would potentially provide clues about the accretion history for the Earth, whether early continental crust was mafic, and whether ocean floor weathering and subduction zone processes can decouple Sr isotopes from Hf and Nd isotopes in the mantle. The work to be done for this project includes collecting rock samples from appropriate localities, separating zircon and other minerals, and making measurements of the Rb-Sr, Sm-Nd, and O isotopic composition of the zircons. Parallel work must also be done to date the zircons where the age is not known, and to verify that zircon crystals in a particular rock sample have a single age. The analyses will be done by thermal ionization mass spectrometry, laser ablation (for O), and high-resolution ion microprobe (SHRIMP).