Basalt Petrogenesis Beneath Slow - and Ultraslow-Spreading Arctic Mid-Ocean Ridges
MASSACHUSETTS INST OF TECH CAMBRIDGE CENTER FOR METEOROLOGY AND PHYSICAL OCEANOGRAPHY
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To explore the ability of melting mafic lithologies to produce alkaline ocean-island basalts OIB, a study was carried out measuring clinopyroxene Cpx- melt and garnet Gt-melt partition coefficients during silica-poor garnet pyroxenite melting for a suite of trace elements, including U and Th, at 2.5GPa and 1420-1450 degrees C. Forward-melting calculations using the results to model time-dependent uranium-series isotopes do not support presence of a fixed quantity of garnet pyroxenite in the source of OIB. To use U-series isotopes to further constrain mantle heterogeneity and the timing and nature of melting and melt transport processes, U-Th-Pa-Ra disequilibria, radiogenic isotopes, and trace-element compositions were measured for the slow-spreading Arctic mid-ocean ridges MOR. A case study of 33 young MOR basalts MORB from the shallow endmember of the global ridge system, the Kolbeinsey Ridge found that unaltered Kolbeinsey MORB have universally high 230Th238U 1.165-1.296 and relatively uniform 230Th232Th 1.196-1.324, SigmaNd, 87Sr86Sr SigmaHf, and Pb isotopes. This suggests a homogeneous mantle source and a long peridotite melting column produces the thick Kolbeinsey crust. Trace element ratios suggest a young, depleted mantle source. Data from the slow- to ultraslow Mohns and Knipovich Ridges north of Kolbeinsey form a sloped array, and 230Th232Th correlates systematically with radiogenic isotopic variations. These data are readily reproduced by models for heterogeneous mantle melting. MORB from 85 degrees E on the global ultraslow-spreading endmember Gakkel Ridge are homogeneously depleted with low 230Th238U and high 226Ra230Th that lie along a global negative correlation. Arctic data support a global mantle temperature control on mean 230Th238U.
- Physical and Dynamic Oceanography
- Geology, Geochemistry and Mineralogy