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Article: Melt-fluid evolution in gabbroic rocks from hess deep

TitleMelt-fluid evolution in gabbroic rocks from hess deep
Authors
Issue Date1996
PublisherOcean Drilling Program, Texas A&M University.
Citation
Proceedings of the Ocean Drilling Program, Scientific Results, 1996, v. 147, p. 213-226 How to Cite?
AbstractMineralogical and fluid inclusion analyses of gabbroic rocks recovered from Site 894 on the intrarift ridge at Hess Deep indicate that the gabbroic sequence is a product of a complex history involving multiple magmatic and hydrothermal events. Pegmatitic patches throughout the core that contain high concentrations of apatite, zircon, oxide minerals, and coarse-grained amphibole are believed to have crystallized under relatively high fθ2 from a hydrous magma. The patches of micropegmatite represent the most evolved compositions sampled at Site 894 and may reflect reequilibration with evolved, volatile-rich fluids that percolated through the gabbroic sequence late in the magmatic history. Analyses of fluid inclusions entrapped within these zones, indicate that 30-40 wt% NaCl ± Fe ± CO2 -rich brines formed either under immiscible conditions in association with the evolved patches, or were exsolved in the absence of a vapor phase directly from the compositionally evolved melts. Migration of the high-salinity fluids may have resulted in the crystallization of apatites that contain up to ~6 wt% chlorine. The transition from magmatic to hydrothermal seawater-dominated conditions in the plutonic sequence is marked by pene tration of fluids that exhibit equivalent fluid salinities that range from -0.1% to 200% of seawater values (3.2 wt% NaCl) and exhibit uncorrected homogenization temperatures that cluster at ~250°C. The extremely low-salinities were most likely formed during supercritical phase separation of seawater at temperatures >407°C. Their preservation requires that the vapor phase remained isolated from the associated brine and that subsequent mixing with seawater did not occur. The compositions of the plutonic-hosted fluids are similar to those of fluids exiting hydrothermal vents, indicating that the fracture networks may repre sent the feeder systems for sustained hydrothermal flow. Migration of these fluids along microfracture and vein networks resulted in the heterogeneous replacement of the gabbroic rocks by greenschist facies mineral assemblages. This alteration is most intense adjacent to vein networks and in cataclastically deformed zones, in which brittle failure facilitated enhanced fluid flow. Sealing of the microfracture and fracture networks with zeolite facies mineral assemblages marks the cessation of fluid circulation in the plutonic sequence. The plutonic sequence may have undergone as much as 2 km of uplift attendant with crustal thinning and propagation of the Cocos-Nazca spreading center.
Persistent Identifierhttp://hdl.handle.net/10722/72844
ISSN
2010 SCImago Journal Rankings: 0.355

 

DC FieldValueLanguage
dc.contributor.authorKelley, DSen_HK
dc.contributor.authorMalpas, JGen_HK
dc.date.accessioned2010-09-06T06:45:38Z-
dc.date.available2010-09-06T06:45:38Z-
dc.date.issued1996en_HK
dc.identifier.citationProceedings of the Ocean Drilling Program, Scientific Results, 1996, v. 147, p. 213-226en_HK
dc.identifier.issn0884-5891-
dc.identifier.urihttp://hdl.handle.net/10722/72844-
dc.description.abstractMineralogical and fluid inclusion analyses of gabbroic rocks recovered from Site 894 on the intrarift ridge at Hess Deep indicate that the gabbroic sequence is a product of a complex history involving multiple magmatic and hydrothermal events. Pegmatitic patches throughout the core that contain high concentrations of apatite, zircon, oxide minerals, and coarse-grained amphibole are believed to have crystallized under relatively high fθ2 from a hydrous magma. The patches of micropegmatite represent the most evolved compositions sampled at Site 894 and may reflect reequilibration with evolved, volatile-rich fluids that percolated through the gabbroic sequence late in the magmatic history. Analyses of fluid inclusions entrapped within these zones, indicate that 30-40 wt% NaCl ± Fe ± CO2 -rich brines formed either under immiscible conditions in association with the evolved patches, or were exsolved in the absence of a vapor phase directly from the compositionally evolved melts. Migration of the high-salinity fluids may have resulted in the crystallization of apatites that contain up to ~6 wt% chlorine. The transition from magmatic to hydrothermal seawater-dominated conditions in the plutonic sequence is marked by pene tration of fluids that exhibit equivalent fluid salinities that range from -0.1% to 200% of seawater values (3.2 wt% NaCl) and exhibit uncorrected homogenization temperatures that cluster at ~250°C. The extremely low-salinities were most likely formed during supercritical phase separation of seawater at temperatures >407°C. Their preservation requires that the vapor phase remained isolated from the associated brine and that subsequent mixing with seawater did not occur. The compositions of the plutonic-hosted fluids are similar to those of fluids exiting hydrothermal vents, indicating that the fracture networks may repre sent the feeder systems for sustained hydrothermal flow. Migration of these fluids along microfracture and vein networks resulted in the heterogeneous replacement of the gabbroic rocks by greenschist facies mineral assemblages. This alteration is most intense adjacent to vein networks and in cataclastically deformed zones, in which brittle failure facilitated enhanced fluid flow. Sealing of the microfracture and fracture networks with zeolite facies mineral assemblages marks the cessation of fluid circulation in the plutonic sequence. The plutonic sequence may have undergone as much as 2 km of uplift attendant with crustal thinning and propagation of the Cocos-Nazca spreading center.-
dc.languageengen_HK
dc.publisherOcean Drilling Program, Texas A&M University.en_HK
dc.relation.ispartofProceedings of Ocean Drilling Program, Scientific Resultsen_HK
dc.titleMelt-fluid evolution in gabbroic rocks from hess deepen_HK
dc.typeArticleen_HK
dc.identifier.emailMalpas, JG: jgmalpas@hku.hken_HK
dc.identifier.authorityMalpas, JG=rp00059en_HK
dc.description.naturelink_to_OA_fulltext-
dc.identifier.hkuros13201en_HK

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