Sr-Nd isotope systematics in 14-28 Ma low-temperature altered mid-ocean ridge basalt from the Australian Antarctic Discordance, Ocean Drilling Program Leg 187

[1] The effects of low-temperature alteration on the Rb-Sr and Sm-Nd isotope systems were investigated in 14–28 Ma mid-ocean ridge basalts recovered during Ocean Drilling Program (ODP) Leg 187 from the Australian Antarctic Discordance through comparison of pristine glass and associated variably altered basalts. Both Nd and Sm are immobile during low-temperature alteration, and 143Nd/144Nd displays mantle values even in heavily altered samples. In contrast, 87Sr/86Sr and Rb concentrations increase during seawater-rock interaction, which is especially apparent in single samples with macroscopically zoned alteration domains. The increase in 87Sr/86Sr roughly correlates with the visible degree of alteration, indicating a higher seawater/rock ratio in the more altered samples. Sr concentrations, however, do not systematically increase with increasing degree of alteration, most likely reflecting exchange of Sr in smectite interlayer sites. The degree of alteration in the uppermost oceanic crust of the Australian Antarctic Discordance is independent of crustal age. A comparison with literature data for young and old altered oceanic crust suggests that most low-temperature alteration occurs within a few million years after formation of the oceanic crust, probably reflecting greater fluid flux through the crust during its early history as a result of higher permeability and increased fluid circulation near the ridge

Across-arc geochemical variations in the Southern Volcanic Zone, Chile (34.5- 38.0°S): Constraints on Mantle Wedge and Input Compositions

Crustal assimilation (e.g. Hildreth and Moorbath, 1988) and/or subduction erosion (e.g. Stern, 1991; Kay et al., 2005) are believed to control the geochemical variations along the northern portion of the Chilean Southern Volcanic Zone. In order to evaluate these hypotheses, we present a comprehensive geochemical data set (major and trace elements and O-Sr-Nd-Hf-Pb isotopes) from Holocene primarily olivine-bearing volcanic rocks across the arc between 34.5-38.0°S, including volcanic front centers from Tinguiririca to Callaqui, the rear arc centers of Infernillo Volcanic Field, Laguna del Maule and Copahue, and extending 300 km into the backarc. We also present an equivalent data set for Chile Trench sediments outboard of this profile. The volcanic arc (including volcanic front and rear arc) samples primarily range from basalt to andesite/trachyandesite, whereas the backarc rocks are low-silica alkali basalts and trachybasalts. All samples show some characteristic subduction zone trace element enrichments and depletions, but the backarc samples show the least. Backarc basalts have higher Ce/Pb, Nb/U, Nb/Zr, and Ta/Hf, and lower Ba/Nb and Ba/La, consistent with less of a slab-derived component in the backarc and, consequently, lower degrees of mantle melting. The mantle-like δ18O in olivine and plagioclase phenocrysts (volcanic arc = 4.9-5.6 and backarc = 5.0-5.4 per mil) and lack of correlation between δ18O and indices of differentiation and other isotope ratios, argue against significant crustal assimilation. Volcanic arc and backarc samples almost completely overlap in Sr and Nd isotopic composition. High precision (double-spike) Pb isotope ratios are tightly correlated, precluding significant assimilation of older sialic crust but indicating mixing between a South Atlantic Mid Ocean-Ridge Basalt (MORB) source and a slab component derived from subducted sediments and altered oceanic crust. Hf-Nd isotope ratios define separate linear arrays for the volcanic arc and backarc, neither of which trend toward subducting sediment, possibly reflecting a primarily asthenospheric mantle array for the volcanic arc and involvement of enriched Proterozoic lithospheric mantle in the backarc. We propose a quantitative mixing model between a mixed-source, slab-derived melt and a heterogeneous mantle beneath the volcanic arc. The model is consistent with local geodynamic parameters, assuming water-saturated conditions within the slab

Geochemical variations in the Central Southern Volcanic Zone, Chile (38-43°S): The role of fluids in generating arc magmas

We present new Sr-Nd-Pb-Hf-O isotope data from the volcanic arc (VA, volcanic front and rear arc) in Chile and the backarc (BA) in Argentina of the Central Southern Volcanic Zone in Chile (CSVZ; 38-43°S). Compared to the Transitional (T) SVZ (34.5-38°S; Jacques et al., 2013), the CSVZ VA has erupted greater volumes over shorter time intervals (Völker et al., 2011) and produced more tholeiitic melts. Although the CSVZ VA monogenetic cones are similar to the TSVZ VA samples, the CSVZ VA stratovolcanoes have higher ratios of highly fluid-mobile to less fluid-mobile trace elements (e.g. U/Th, Pb/Ce, Ba/Nb) and lower more- to less-incompatible fluid-immobile element ratios (e.g. La/Yb, La/Sm, Th/Yb, Nb/Yb), consistent with an overall higher fluid flux and greater degree of flux melting beneath the CSVZ stratovolcanoes compared to the CSVZ monogenetic centers and the TSVZ VA. The CSVZ monogenetic centers overlap the TSVZ in Sr and Nd isotopes, but the stratovolcanoes are shifted to higher Sr and/or Nd isotope ratios. The Pb isotopic composition of the CSVZ overlaps the TSVZ, which is clearly dominated by the composition of the trench sediments, but the CSVZ monogenetic samples extend to less radiogenic Pb isotope ratios. δ18Omelt from the CSVZ stratovolcano samples are below the MORB range, whereas the CSVZ monogenetic and the TSVZ samples fall within and slightly above the MORB range. The Nd and Hf isotopic ratios of the CSVZ VA extend to more radiogenic compositions than found in the TSVZ VA, indicating a greater contribution from a more depleted source. These correlations are interpreted to reflect derivation of fluids from hydrothermally altered oceanic crust and/or serpentinized upper mantle of the subducting plate. CSVZ BA basalts largely overlap TSVZ BA basalts, displaying less or no subduction influence compared to the VA, but some CSVZ BA basalts tap more enriched mantle, possibly subcontinental lithosphere, with distinctively lower Nd and Hf and elevated 207Pb/204Pb and 208Pb/204Pb isotope ratios

Boron isotope geochemistry and U-Pb systematics of altered MORB from the Australian Antarctic Disordance (ODP Leg 187)

Boron and Pb isotopic compositions together with B–U–Th–Pb concentrations were determined for Pacific and Indian mantle-type mid-ocean ridge basalts (MORB) obtained from shallow drill holes near the Australian Antarctic Discordance (AAD). Boron contents in the altered samples range from 29.7 to 69.6 ppm and are extremely enriched relative to fresh MORB glass with 0.4–0.6 ppm B. Similarly the δ11B values range from 5.5‰ to 15.9‰ in the altered basalts and require interaction with a δ11B enriched fluid similar to seawater ∼ 39.5‰ and/or boron isotope fractionation during the formation of secondary clays. Positive correlations between B concentrations and other chemical indices of alteration such as H2O CO2, K2O, P2O5, U and 87Sr/86Sr indicate that B is progressively enriched in the basalts as they become more altered. Interestingly, δ11B shows the largest isotopic shift to + 16‰ in the least altered basalts, followed by a continual decrease to + 5–6‰ in the most altered basalts. These observations may indicate a change from an early seawater dominated fluid towards a sediment-dominated fluid as a result of an increase in sediment cover with increasing age of the seafloor. The progression from heavy δ11B towards lighter values with increasing degrees of alteration may also reflect increased formation of clay minerals (e.g., saponite). A comparison of 238U/204Pb and 206Pb/204Pb in fresh glass and variably altered basalt from Site 1160B shows extreme variations that are caused by secondary U enrichment during low temperature alteration. Modeling of the U–Pb isotope system confirms that some alteration events occurred early in the 21.5 m.y. history of these rocks, even though a significant second pulse of alteration happened at ∼ 12 Ma after formation of the crust. The U–Pb systematics of co-genetic basaltic glass and variably low temperature altered basaltic whole rocks are thus a potential tool to place age constraints on the timing of alteration and fluid flow in the ocean crust

Chemical and isotopic compositions of altered MORB from ODP Holes 187-1154A, 187-1155B, and 187-1160B

Boron and Pb isotopic compositions together with B-U-Th-Pb concentrations were determined for Pacific and Indian mantle-type mid-ocean ridge basalts (MORB) obtained from shallow drill holes near the Australian Antarctic Discordance (AAD). Boron contents in the altered samples range from 29.7 to 69.6 ppm and are extremely enriched relative to fresh MORB glass with 0.4-0.6 ppm B. Similarly the d11B values range from 5.5‰ to 15.9‰ in the altered basalts and require interaction with a d11B enriched fluid similar to seawater ~39.5‰ and/or boron isotope fractionation during the formation of secondary clays. Positive correlations between B concentrations and other chemical indices of alteration such as H2O CO2, K2O, P2O5, U and 87Sr/86Sr indicate that B is progressively enriched in the basalts as they become more altered. Interestingly, d11B shows the largest isotopic shift to +16‰ in the least altered basalts, followed by a continual decrease to +5-6‰ in the most altered basalts. These observations may indicate a change from an early seawater dominated fluid towards a sediment-dominated fluid as a result of an increase in sediment cover with increasing age of the seafloor. The progression from heavy d11B towards lighter values with increasing degrees of alteration may also reflect increased formation of clay minerals (e.g., saponite). A comparison of 238U/204Pb and 206Pb/204Pb in fresh glass and variably altered basalt from Site 1160B shows extreme variations that are caused by secondary U enrichment during low temperature alteration. Modeling of the U-Pb isotope system confirms that some alteration events occurred early in the 21.5 Ma history of these rocks, even though a significant second pulse of alteration happened at ~12 Ma after formation of the crust. The U-Pb systematics of co-genetic basaltic glass and variably low temperature altered basaltic whole rocks are thus a potential tool to place age constraints on the timing of alteration and fluid flow in the ocean crust

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