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Geochemistry of basalts from IODP site U1365: Implications for magmatism and mantle source signatures of the mid-Cretaceous Osbourn Trough
No full textThe Integrated Ocean Drilling Program site U1365 drilled into the basement of the southwest Pacific crust formed from the mid-Cretaceous Osbourn Trough that rifted apart the Manihiki and Hikurangi Plateaus (the Greater Manihiki). The basalt geochemistry at this site is crucial for understanding the magmatic processes and mantle source of the mid-Cretaceous Osbourn Trough. The recovered fresh basalts were low-K tholeiitic normal (N) and depleted (D) mid-ocean ridge basalt (MORB). Their trace element and Sr–Nd isotope compositions indicate a Pacific-type mantle source rather than any significant influences from the nearby Louisville Seamount Chain or from the Greater Manihiki Plateau. Despite the presence of a plume head underneath the Osbourn Trough at its initial stage, the insignificance of a plume head could be explained by the long-distance (> 1000 km) southward migration of the Osbourn Trough. Lavas at site U1365 vary from low-MgO (< 6.9 wt.%) N-MORB at the bottom to high-MgO (8 wt.% to 9.5 wt.%) D-MORB and, then, to medium-MgO (7.3 wt.% to 8.2 wt.%) N-MORB according to their eruption sequences, which was accompanied by magma mixing in the magma reservoir. The D-MORB group lavas have higher melting degrees than those of N-MORB group based on their concentrations of TiO2, Na2O and CaO corrected for crystallization relative to MgO = 7.8 wt.%. The major element compositions of the high-MgO D-MORB lavas were consistent with partial melting in the spinel–peridotite zone over a pressure interval from ~ 3.1 GPa to 2 GPa in the mantle. The significant overlap of N-MORB and D-MORB in Sr–Nd isotopes suggests that chemical differences between the two groups were derived from the mantle melting processes. Based on comparison with lavas from the East Pacific Rise where a positive correlation of mantle melting degree vs. spreading rate is shown, we suggest that the Osbourn Trough might have a full spreading rate of ~ 140 mm/yr. Thus, the slow ridge-like axial morphology of the Osbourn Trough should be a character of an extinct fast ridge
Seafloor basalt alteration and chemical change in the ultra thinly sedimented South Pacific
No full textDetermining 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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