Sr behaviour during hydrothermal alteration of oceanic gabbros exposed at Hess Deep : implications for 87SR/86SR compositions as a proxy for fluid-rock interaction.

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dc.contributor.authorKirchner, Timo
dc.contributor.supervisorGillis, Kathryn M.
dc.date.accessioned2011-05-26T15:05:42Z
dc.date.available2011-05-26T15:05:42Z
dc.date.copyright2011en_US
dc.date.issued2011-05-26
dc.degree.departmentSchool of Earth and Ocean Sciences
dc.degree.levelMaster of Science M.Sc.en_US
dc.description.abstractMid-ocean ridge hydrothermal systems are known to extend to deep levels of the oceanic crust, including the plutonic section, but little is known about the timing and nature of fluid-rock interactions at these levels. To investigate the temporal and spatial characteristics of hydrothermal alteration in the lower crust, this study investigates a suite of hydrothermally altered (<5 to >20% hydrous alteration) gabbroic rocks recovered from the Hess Deep Rift, where 1.2 Ma fast-spreading East Pacific Rise crust is well-exposed. These samples were altered to amphibole-dominated assemblages with chlorite-rich samples occurring in a restricted region of the field area. Hornfels, indicative of reheated, previously altered rocks, are clustered in the central part of the field area. The entire sample suite has elevated 87Sr/86Sr (mean: 0.70257±0.00007 (2σ), n=16) with respect to fresh rock (0.7024). Bulk rock 87Sr/86Sr is strongly correlated with percentage of hydrous alteration and weakly correlated with bulk rock Sr content. The distribution of Sr in igneous and metamorphic minerals suggests that greenshist-facies alteration assemblages (chlorite, actinolitic amphibole, albitic plagioclase) lose Sr to the fluid while amphibolite-facies secondary assemblages (secondary hornblende, anorthitic plagioclase) take up Sr. The temperature-dependent mobilization of Sr in hydrothermal systems has implications for the 87Sr/86Sr and ultimately fluid/rock ratio calculations based on the assessed 87Sr/86Sr systematics. Considering Sr behaviour, minimum fluid/rock ratios of ~1 were calculated for the plutonic section. Due to the large uncertainty regarding fluid Sr composition at depth and the sensitivity of fluid/rock ratio calculations on this parameter, a model combining the sheeted dike complex and the plutonic section to one hydrothermal system is introduced, yielding a fluid/rock ratio of 0.5. This value may be more realistic since the fluid composition entering and exiting the sheeted dike complex is better constrained. The regional distribution of hornfelsed material with elevated 87Sr/86Sr suggests that fluid ingress into the upper plutonics at Hess Deep occurred on-axis in a dynamic interface of a vertically migrating axial magma chamber (AMC) and the base of the hydrothermal system.en_US
dc.description.scholarlevelGraduateen_US
dc.identifier.urihttp://hdl.handle.net/1828/3314
dc.languageEnglisheng
dc.language.isoenen_US
dc.rights.tempAvailable to the World Wide Weben_US
dc.subjectmid-ocean ridgeen_US
dc.subjecthydrothermal systemsen_US
dc.subjectgabbroen_US
dc.subjectEast Pacific Riseen_US
dc.subjectCocos-Nazca spreading centeren_US
dc.subjectSren_US
dc.subjectSr isotopesen_US
dc.subjectamphiboliteen_US
dc.subjecthornfelsen_US
dc.subjectmagma chamberen_US
dc.subjectlower oceanic crusten_US
dc.subjectoceanic crusten_US
dc.subjectplutonic sectionen_US
dc.subjectgabbroic sectionen_US
dc.subjectamphiboleen_US
dc.subjectchloriteen_US
dc.subjectplagioclaseen_US
dc.subjectheat fluxen_US
dc.subjectfluid fluxen_US
dc.subjecthydrothermal fluiden_US
dc.subjectwater-rock interactionen_US
dc.subjectfluid-rock interactionen_US
dc.subjectmetamorphismen_US
dc.subjectgeochemistryen_US
dc.titleSr behaviour during hydrothermal alteration of oceanic gabbros exposed at Hess Deep : implications for 87SR/86SR compositions as a proxy for fluid-rock interaction.en_US
dc.typeThesisen_US

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