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

• Main Author: Elkins, Lynne J.
• Format: Thesis
• Language: English
• Published: Massachusetts Institute of Technology and Woods Hole Oceanographic Institution 2009
• Subjects: Petrogenesis; Basalt; Polarstern (Ship) Cruise ARK V/Ib; Polarstern (Ship) Cruise ARK VII/1; Poseidon (Ship) Cruise POS185; Poseidon (Ship) Cruise POS210/1; Poseidon (Ship) Cruise POD221; Poseidon (Ship) Cruise POS229; Poseidon (Ship) Cruise POS291; Poseidon (Ship) Cruise POS326; Trident (Ship) Cruise TR139; Endeavor (Ship: 1976-) Cruise EN26; Professor Logatchev (Ship) Cruise; Oden (Ship) Cruise 11; Arctic; Gakkel Ridge; Knipovich Ridge; Kolbeinsey; Kolbeinsey Ridge; Trident; Ocean Island
• Online Access: https://hdl.handle.net/1912/2855

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 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. ...