Basalt petrogenesis beneath slow- and ultraslow-spreading Arctic mid-ocean ridges


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dc.contributor.author Elkins, Lynne J.
dc.coverage.spatial Mohns Ridge
dc.coverage.spatial Knipovich Ridge
dc.coverage.spatial Gakkel Ridge
dc.date.accessioned 2009-06-30T18:30:46Z
dc.date.available 2009-06-30T18:30:46Z
dc.date.issued 2009-02
dc.identifier.uri http://hdl.handle.net/1912/2855
dc.description Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2009 en
dc.description.abstract To explore the ability of melting mafic lithologies to produce alkaline ocean-island basalts (OIB), an experimental 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°C. Partition coefficients range from 0.0083±0.0006 to 0.020±0.002 for Th and 0.0094±0.0006 to 0.024±0.002 for U in Cpx, and are 0.0032±0.0004 for Th and 0.013±0.002 for U in Gt. Forward-melting calculations using these experimental results to model time-dependent uranium-series isotopes do not support the 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 focused case study of 33 young (<10ka) MOR basalts (MORB) from the shallow endmember of the global ridge system, the Kolbeinsey Ridge (67°05’-70°26’N) found that unaltered Kolbeinsey MORB have universally high (230Th/238U) (1.165-1.296) and relatively uniform (230Th/232Th) (1.196-1.324), εNd (8.43- 10.49), 87Sr/86Sr (0.70274-0.70301), εHf (16.59-19.56), and Pb isotopes (e.g. 208Pb/206Pb = 2.043-2.095). 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 (230Th/232Th) correlates systematically with radiogenic isotopic variations. These data are readily reproduced by models for heterogeneous mantle melting. MORB from 85ºE on the global ultraslow-spreading endmember Gakkel Ridge are homogeneously depleted with low (230Th/238U) and high (226Ra/230Th) that lie along a global negative correlation. Arctic data support a global mantle temperature control on mean (230Th/238U). en
dc.description.sponsorship This work was made possible through funding support from the National Defense Science and Engineering Graduate Fellowship Program, from the Academic Programs Office, from the WHOI and MIT student travel assistance funds, and from National Science Foundation grant OCE-0422278 to K.W.W.S. en
dc.format.mimetype application/pdf
dc.language.iso en_US en
dc.publisher Massachusetts Institute of Technology and Woods Hole Oceanographic Institution en
dc.relation.ispartofseries WHOI Theses en
dc.subject Petrogenesis en_US
dc.subject Basalt en_US
dc.title Basalt petrogenesis beneath slow- and ultraslow-spreading Arctic mid-ocean ridges en
dc.type Thesis en
dc.identifier.doi 10.1575/1912/2855

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