Rhenium and osmium concentrations and isotopic compositions in altered ocean crust rocks at ODP Site 185-801

Re and Os concentrations and Os isotopic ratios were determined for composite samples prepared from volcanoclastics (VCL) and basaltic flows (FLO) from Jurassic oceanic crust (Ocean Drilling Program Leg 185, Site 801 in the western Pacific), with the aim of determining the effect of seafloor weathering on the Re-Os budget. A supercomposite sample, prepared from a proportionate mixture of the various composite powders, served to represent the average composition of the altered oceanic crust [Kelley, K.A., Plank, T., Ludden, J. and Staudigel, H., (2003). Composition of altered oceanic crust at ODP Sites 801 and 1149, Geochem. Geophys. Geosyst. 4(6) 8910, doi:10.1029/2002GC000435.]. Re contents vary from 0.2 to 1.3 ng/g, and from 2.2 to 3.1 ng/g in the VCL and FLO composites respectively. Os contents vary from 0.005 to 0.047 ng/g in the VCL, and from 0.008 to 0.027 ng/g in the FLO composites. The FLO composites have much higher Re/Os ratios and thus have more radiogenic Os compositions (187Os/188Os = 1.38 to 8.48) than the VCL composites (187Os/188Os = 0.32 to 4.40). The VCL composite from the upper section of the crust shows evidence for substantial Re loss and Os uptake, consistent with oxidative weathering processes. However, Re uptake during weathering processes under more reducing conditions, evident in the FLO samples from throughout the section and to a lesser extent in the lower VCL samples, more than compensates for this Re loss in the upper VCL. Os concentrations were essentially unchanged by these reductive processes. Model age calculations suggest that Re uptake continued for tens of millions of years after crust formation. Abundant secondary pyrite is found throughout the altered Hole 801C crust in zones of restricted seawater flow, and this may have accommodated an important part of the input Re. The Re content of the supercomposite (~2.2 ng/g) is about 1 ng/g higher than would be expected on the basis of its Yb content. If the results from Hole 801C are typical, they suggest that the Re concentration of at least the upper part of the oceanic crust may be nearly doubled during seafloor alteration. Such large extents of Re uptake would have a significant effect on the oceanic Re budget. Furthermore, assuming that they survive passage through the subduction zone, these elevated Re contents would greatly decrease the proportion of subducted oceanic crust required in the source region to explain the radiogenic Os compositions of many ocean island basalts.

Tab. 1: Zircon isotopic data and rock ages from the Antarctic Peninsula

Morphological and U-Pb isotope studies on sedimentary zircons reflect the orogenic evolution of their former host rocks. The orogenic history of detrital zircons from the Trinity Peninsula Formation (TPF) defines the former geological surrounding of the sedimentation basin of the TPF. Same few weil rounded, polycyclic zircons of Precambrian age and Cambrian overprint give hints for an old cratonic source rock. Because of their very low frequency compared with euhedral types, the contribution of an cratonic shield area to the bulk of the sedimentary debris is neglectable low. Euhedral zircons of granitoid origin and Carboniferous age indicate a derivation from an area of widespread Carboniferous intrusions. Except for southern South America and unsurveyed regions in the Antarctic Peninsula itself, no region could deliver zircons with a Carboniferous age record. The only acceptable explanation for the origin of these zircons is a position of the Antarctic Peninsula during the sedimentation of the TPF approximately southwest of southern South America.

Chemical and isotope compositions of rocks from altered ocean crust at DSDP/ODP Sites 417/418

Large-scale compositional domains at DSDP/ODP drill sites 417A, 417D and 418A were analyzed for O, Sr and Nd isotope ratios, and REE, U, K, Rb and Sr abundances, to constrain the bulk chemical composition of the oceanic crust that is recycled at subduction zones. The combination of the three sites gives the composition of the upper oceanic crust in this region over a distance of about 8 km. The d18O(SMOW) and 87Sr/86Sr(meas) of compositional domains 10-100 m in size correlate well, with a range of 7.7-19.2 and 0.70364-0.70744, and mean of 9.96 and 0.70475, respectively. The Rb inventory of the upper crust increases by about an order of magnitude, while Sr contents remain constant. U abundances increase moderately under oxidizing alteration conditions and nearly triple in the commonly reducing alteration environments of the upper oceanic crust. REEs are influenced by alteration only to a small extent, and recycled oceanic crust is similar to MORB with respect to 143Nd/144Nd. Even though the average composition of the upper oceanic crust is well defined, the large scale composition varies widely. Highly altered compositional domains may not have a large impact on the average composition of the oceanic crust, but they may preferentially contribute to fluids or partial melts derived from the crust by prograde metamorphic reactions.

Elemental data for composites from DSDP Sites 417A, 417D, and 418A

While the input of river-alkalinity into seawater is relatively well known, the complementary acidity production is poorly understood. Using the major-element budget of seafloor alteration of the upper 500 m of 120-Ma-old oceanic crust at DSDP/ODP Sites 417A, 417D and 418A in the central western Atlantic, we estimate the acidity flux associated with the low-temperature weathering of the upper oceanic crust. The acidity flux is calculated based on major-element fluxes and charge-balance considerations. The relevant chemical fluxes from seawater to the upper crust are 4.1+-0.1; 1.4+-1.4; 2.2+-0.6 and -12+-2 10**11 mol/yr for K, Mg, Na and silicate-Ca, respectively. The associated acidity flux is (3.5+-3)10**11 eq/y. Relative to continental weathering, these fluxes are significant for K and silicate-Ca, but are minor for Na, Mg and acidity. Thus, riverine fluxes of alkalinity are not significantly balanced by acidity fluxes from low-temperature upper ocean crust alteration.

Mineral an chemical composition of basalts from DSDP Sites 417 and 418, West Atlantic Ocean

Major element composition ranges of closely associated basalt glass-whole rock pairs from individual small cooling units approach the total known range of basalt glass and whole rock compositions at IPOD sites 417 and 418. The whole rock samples fall into two groups: one is depleted in MgO and distinctly enriched in plagioclase but has lost some olivine and/or pyroxene relative to its corresponding glass; and the other is enriched in MgO and in phenocrysts of olivine and pyroxene as well as plagioclase compared to its corresponding glass. By analogy with observed phenocryst distributions in lava pillows, tubes, and dikes, and with some theoretical studies, we infer that bulk rock compositions are strongly affected by phenocryst redistribution due to gravity settling, flotation, and dynamic sorting after eruption, although specific models are not well constrained by the one-dimensional geometry of drill core. Compositional trends or groupings in whole rock data resulting from such late-stage processes should not be confused with more fundamental compositional effects produced in deep chambers or during partial melting.

Efficient algorithms for solving systems of ordinary differential equations for ecosystems modeling /

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