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dc.contributor.authorWaters, Christopher L.  Concept link
dc.coverage.spatialEast Pacific Rise
dc.date.accessioned2010-07-13T15:35:39Z
dc.date.available2010-07-13T15:35:39Z
dc.date.issued2010-06
dc.identifier.urihttps://hdl.handle.net/1912/3750
dc.descriptionSubmitted 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 June 2010en_US
dc.description.abstractVolcanic accretion at the fast-spreading East Pacific Rise (EPR) occurs over a ~2-4 km wide neo-volcanic zone on either side of the axial summit trough (AST). Eruption ages are critical for understanding the distribution and timing of volcanic and magmatic activity. Uranium series nuclides are susceptible to fractionation by magmatic processes that occur beneath mid-ocean ridges, and the half-lives of 226Ra (1.6 kyrs) and 230Th (75 kyrs) make them ideally suited for determining eruption ages and placing constraints on eruption frequency and temporal changes in magma chemistry. Accordingly, major and trace element, and long-lived radiogenic and 238U-230Th-226Ra isotope compositions were measured in basalts from 9º-10ºN EPR to determine eruption ages and to place temporal constraints on volcanic and magmatic processes. At 9º30’N EPR, 238U-230Th-226Ra compositions indicate that trace elementally and isotopically enriched mid-ocean ridge basalt (MORB) collected off-axis erupted >8 ka and that E-MORB magmatism is interspersed with normal, depleted MORB magmatism. Lava ages are consistent with eruption from the AST and flow down the ridge flanks, which is in contrast to previous studies that suggested E-MORB erupted from off-axis vents. At 9º50’N EPR, discrete eruptive units are distinguished by high precision 238U, 232Th, and 226Ra sample concentrations, but because the resolution of the 230Th-226Ra model age dating technique is ~±1 kyrs, the surprisingly young ages of these lavas prohibit the construction of an explicit, time-constrained lava stratigraphy. Nonetheless, seven different flows identified within 0.8-2.0 km west of the AST imply greater frequency of flows to these distances than previously recognized. Model age dating of ferrobasalts, basaltic andesites, andesites, and dacites sampled from the east limb of the overlapping spreading center at 9º03’N EPR is difficult due to uncertainties in magma residence times. However, (226Ra/230Th) disequilibria indicate recent basaltic volcanism (<<8 ka) up to ~4 km off-axis. The axial graben at the rise crest sources the most recent volcanic activity and is the dominant location for eruption of high-silica magmas. Major element, trace element, 87Sr/86Sr, and (234U/238U) isotope compositions are consistent with the formation of dacite magmas by extensive crystallization, and 238U-230Th-226Ra systematics imply crustal residence times of ~8 kyrs.en_US
dc.description.sponsorshipThis research was made possible by funding from the Academic Programs Office, from the WHOI travel assistance funds, a Goldschmidt student travel grant, and National Science Foundation grants OCE-0623838, OCE-0527053, and OCE-0137325 to K.W.W.S.en_US
dc.format.mimetypeapplication/pdf
dc.language.isoen_USen_US
dc.publisherMassachusetts Institute of Technology and Woods Hole Oceanographic Institutionen_US
dc.relation.ispartofseriesWHOI Thesesen_US
dc.subjectVolcanismen_US
dc.subjectSubmarine geologyen_US
dc.subjectAtlantis (Ship : 1996-) Cruise AT15-7en_US
dc.subjectAtlantis (Ship : 1996-) Cruise AT11-7en_US
dc.subjectAtlantis (Ship : 1996-) Cruise AT15-17en_US
dc.titleTemporal and petrogenetic constraints on volcanic accretionary processes at 9-10 degrees north East Pacific Riseen_US
dc.typeThesisen_US
dc.identifier.doi10.1575/1912/3750


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