Seafloor basalt alteration and chemical change in the ultra thinly sedimented South Pacific
Seafloor basalt alteration and chemical change in the ultra thinly sedimented South Pacific
Determining the relationship between ocean floor basalt alteration and sedimentation is fundamental to understanding how oceanic crust evolves with time. Ocean floor basalts recovered at IODP Sites U1365 (?100 Ma) and U1368 (?13.5 Ma) in the South Pacific have been subjected to remarkably low sedimentation rates (0.71 and 1.1 m/Myr?1, respectively). We report detailed petrographic and geochemical analysis of basalt cores from these sites in order to investigate what impact sediment insulation has on seafloor alteration beyond 10–15 Myr of ocean crust formation. Both sites exhibit low-temperature (<150°C) alteration (e.g., iron-hydroxides, carbonate, and quartz) within a predominantly oxidative regime, albeit with markedly different alteration styles and intensity. Alteration at Site U1365, which is predominantly composed of sheet flows, occurs mainly near sheet flow boundaries and fractures. In contrast, Site U1368 comprises interlayered pillows and thin sheet flows that have been subjected to relatively even levels of alteration. Variation of alteration style and intensity between Sites U1365 and U1368 appear closely tied to lithology and crustal structure. Although alteration-induced elemental changes at both sites are similar in, e.g., increasing K, Rb, U, Ba, and Fe3+ and decreasing Fe2+, Ca, and Ni, they show distinct differences in Th, which is significantly decreased at Site U1365 but relatively constant at Site U1368. At both sites enrichment of LREEs relative to HREEs is ascribed to alteration. The greater vein abundance and notably higher Fe3+/TiO2, K2O/TiO2, LOI/TiO2, and Rb/TiO2 ratios of representative samples at Site U1365 compared to Site U1368 are attributed to increased alteration intensity. This is mirrored by greater overall chemical change (Fe2O3, FeO, CaO, K2O, Li, Rb, Pb, and U) observed at Site U1365 than those of Site U1368 and other DSDP/ODP sites between 6 and 46 Ma. Since both Sites U1365 and U1368 endured only minimal sedimentation, we attribute the differences in overall chemical change across the two sites to duration of exposure to seawater.
seafloor basalt alteration, sediment blanketing, chemical change, hydrothermal alteration, vein, south pacific
3066-3080
Zhang, Guo-Liang
c972ba0e-10e5-4b50-a477-9081c0664479
Smith-Duque, Christopher
f1385032-8c67-40ca-bc6d-6dd3e6c5b013
July 2014
Zhang, Guo-Liang
c972ba0e-10e5-4b50-a477-9081c0664479
Smith-Duque, Christopher
f1385032-8c67-40ca-bc6d-6dd3e6c5b013
Zhang, Guo-Liang and Smith-Duque, Christopher
(2014)
Seafloor basalt alteration and chemical change in the ultra thinly sedimented South Pacific.
Geochemistry, Geophysics, Geosystems, 15 (7), .
(doi:10.1002/2013GC005141).
Abstract
Determining the relationship between ocean floor basalt alteration and sedimentation is fundamental to understanding how oceanic crust evolves with time. Ocean floor basalts recovered at IODP Sites U1365 (?100 Ma) and U1368 (?13.5 Ma) in the South Pacific have been subjected to remarkably low sedimentation rates (0.71 and 1.1 m/Myr?1, respectively). We report detailed petrographic and geochemical analysis of basalt cores from these sites in order to investigate what impact sediment insulation has on seafloor alteration beyond 10–15 Myr of ocean crust formation. Both sites exhibit low-temperature (<150°C) alteration (e.g., iron-hydroxides, carbonate, and quartz) within a predominantly oxidative regime, albeit with markedly different alteration styles and intensity. Alteration at Site U1365, which is predominantly composed of sheet flows, occurs mainly near sheet flow boundaries and fractures. In contrast, Site U1368 comprises interlayered pillows and thin sheet flows that have been subjected to relatively even levels of alteration. Variation of alteration style and intensity between Sites U1365 and U1368 appear closely tied to lithology and crustal structure. Although alteration-induced elemental changes at both sites are similar in, e.g., increasing K, Rb, U, Ba, and Fe3+ and decreasing Fe2+, Ca, and Ni, they show distinct differences in Th, which is significantly decreased at Site U1365 but relatively constant at Site U1368. At both sites enrichment of LREEs relative to HREEs is ascribed to alteration. The greater vein abundance and notably higher Fe3+/TiO2, K2O/TiO2, LOI/TiO2, and Rb/TiO2 ratios of representative samples at Site U1365 compared to Site U1368 are attributed to increased alteration intensity. This is mirrored by greater overall chemical change (Fe2O3, FeO, CaO, K2O, Li, Rb, Pb, and U) observed at Site U1365 than those of Site U1368 and other DSDP/ODP sites between 6 and 46 Ma. Since both Sites U1365 and U1368 endured only minimal sedimentation, we attribute the differences in overall chemical change across the two sites to duration of exposure to seawater.
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e-pub ahead of print date: 14 July 2014
Published date: July 2014
Keywords:
seafloor basalt alteration, sediment blanketing, chemical change, hydrothermal alteration, vein, south pacific
Organisations:
Geochemistry
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Local EPrints ID: 367626
URI: http://eprints.soton.ac.uk/id/eprint/367626
ISSN: 1525-2027
PURE UUID: 00775570-7d5d-427e-8f7a-29b2268ba2af
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Date deposited: 04 Aug 2014 09:20
Last modified: 14 Mar 2024 17:34
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Author:
Guo-Liang Zhang
Author:
Christopher Smith-Duque
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