Basalt Petrogenesis Beneath Slow and Ultraslow-Spreading Arctic Mid-Ocean Ridges

Lynne Elkins, Ph.D., 2009
Kenneth Sims, Advisor

To explore the melting of mafic lithologies to produce OIB, experiments were carried out measuring clinopyroxene-melt and garnet-melt partitioning during silica-poor garnet pyroxenite melting for U and Th at 2.5GPa and 1420-1450°C. Calculations using the resulting partition coefficients (clinopyroxene: 0.0083±0.0006−0.020±0.002 (Th) and 0.0094±0.0006−0.024±0.002 (U); garnet: 0.0032±0.0004 (Th) and 0.013±0.002 (U)) to model time-dependent uranium-series isotopes do not support the presence of pyroxenite in the source of OIB.

To examine mantle heterogeneity and the timing and nature of melt processes, U-Th-Pa-Ra disequilibria, radiogenic isotopes, and trace-element compositions were measured for the slow-spreading Arctic MOR. At the Kolbeinsey Ridge (67°05’-70°26’N), unaltered MORB have universally high (²³⁰Th/²³⁸U) (1.165-1.296) and relatively uniform (²³⁰Th/²³²Th) (1.196-1.324), e_Nd (8.43-10.49), ⁸⁷Sr/⁸⁶Sr (0.70274-0.70301), e_Hf (16.59-19.56), and Pb isotopes (e.g. ²⁰⁸Pb/²⁰⁶Pb = 2.043-2.095). This suggests a homogeneous source and long peridotite melting column produce the thick Kolbeinsey crust. Trace element ratios suggest a young, depleted source. Data from the slow- to ultraslow Mohns and Knipovich Ridges form a sloped array, and (²³⁰Th/²³²Th) correlates with radiogenic isotopes, which suggests heterogeneous mantle melting. MORB from 85ºE on the ultraslow Gakkel Ridge are homogeneously depleted, with low (²³⁰Th/²³⁸U) and high (²²⁶Ra/²³⁰Th). Arctic data support a global mantle temperature control on mean (²³⁰Th/²³⁸U).