ALBERT

Description: The dataset contains B, Li, Mg and Sr concentrations and isotopic compositions of black-smoker, acid-sulfate fluids and "hybrid-types" as well as of fresh and altered rocks from different vent fields (DESMOS and North Su) within the Manus Basin, Papua New Guinea. The data is used to understand the controls of their compositional variability. In particular, the formation of acid-sulfate and hybrid smoker fluids is still poorly understood, and their high Mg concentrations are explained either by dissolution of Mg-bearing minerals in the basement or by mixing between unmodified seawater and magmatic fluids. Mg isotope ratios of the acid-sulfate fluids from Manus Basin are seawater-like, which supports the idea that acid-sulfate fluids in this study area predominantly form by mixing between unmodified seawater and a Mg-free magmatic fluid. Changes in the B, Li, and Sr isotope ratios relative to seawater indicate water-rock interaction in all acid-sulfate fluids. Further, the combination of δ7Li with B concentrations of the same fluids links changes in δ7Li to changes in (1) basement alteration, (2) water-to-rock ratios during water-rock interaction and/or (3) the reaction temperature. These isotope systems, thus, allow tracing of basement composition and acid-sulfate driven alteration of the backarc crust, and help increase our understanding of hydrothermal fluid-rock interactions and the behavior of fluid-mobile elements.

Description: The dataset contains boron (B) concentrations and isotope ratios of hydrothermal vent fluids and volcanic rocks from different vent fields within the Manus Basin, Papua New Guinea, and Nifonea volcano, New Hebrides back-arc. The fluids from these settings show a range of salinities, gas contents, acidities, and host rock compositions; many of them are influenced by phase separation and by addition of magmatic volatiles (both CO2 and SO2). Previous studies of hydrothermal vents in arc/back-arc settings suggest that B contents and isotopic composition of vent fluids are controlled by interactions between seawater, basement and sediments, and propose that phase separation and magmatic fluids play only a subordinate role. In our study, we demonstrate that vent fluids with minor magmatic input indeed reflect the interaction between seawater and oceanic crust. In contrast, the low-salinity Nifonea fluids and some of the acid-sulfate fluids from the Manus Basin have higher B contents as expected, whereas other volatile-rich fluids from the Manus Basin show B depletions. The lack of correlation between B contents and the intensity of magmatic fluid influx (CO2 and SO2) may indicate that magma degassing is not responsible for the B enrichments or depletions in these vent fluids. B enrichments might be related to preferential partitioning of B into the vapour phase during phase separation under PT-conditions well above the two-phase curve and critical line (i.e. T 〉〉 Tcritical, P 〉〉 Pcritical). However, this cannot explain the low B concentrations in the vapour-rich vent fluids from the Manus Basin and the low B isotope ratios in the Nifonea fluids. Instead, we propose that B concentrations and isotope ratios in submarine vent fluids largely depend on the residence and reaction time of the vent fluid in the subsurface. In general, all vent fluids are still influenced by water-rock interaction during hydrothermal circulation. However, vent fluids with short residence times define a trend towards lower B concentrations and isotope ratios, which can be explained by mixing between hydrothermal and magmatic fluid, which is similar to the composition of the host rock. In contrast, the B signature of the magmatic fluid can be overprinted due to preferential mobilization of B from the oceanic crust into vapour-rich fluids at longer reaction times. Thus, B may provide a tool for estimating the extent of B leaching and hence hydrothermal alteration in the subseafloor.

Description: Multiple batch experiments (100 °C, 200 °C; 40 MPa) were conducted, using Dickson-type reactors, to investigate Li and B partitioning and isotope fractionation between rock and water during serpentinization. We reacted fresh olivine (5 g; Fo90; [B] = 〈0.02 µg/g; d11BOlivine -14 per mil; [Li] = 1.7 µg/g; d7LiOlivine = +5.3 per mil) with seawater-like fluids (75 ml, 3.2 wt.% NaCl) adjusted with respect to their Li (0.2, 0.5 µg/ml; and d7LiFluid +55 per mil) and B (~10 µg/ml and d11BFluid -0.3 per mil) characteristics. At 200 °C a reaction turnover of about 70% and a serpentinization mineral assemblage matching equilibrium thermodynamic computational results (EQ3/6) developed after 224 days runtime. Characterization of concomitant fluid samples indicated a distinct B incorporation into solid phases ([B]final_200 °C = 55.61 µg/g; DS/FB200 °C = 13.42) and a preferential uptake of the lighter 10B isotope (Delta11BS-F = -3.46 per mil). Despite a low reaction turnover at 100 °C (〈12%), considerable amounts of B were again incorporated into solid phases ([B]final_100 °C = 25.33 µg/g; DS/FB100 °C = 24.2) with even a larger isotope fractionation factor (Delta11BS-F = -9.97? per mil. While magnitude of isotope fraction appears anti-correlated with temperature, we argue for an overall attenuation of the isotopic effect through changes in B speciation in saline solutions (NaB(OH)4(aq) and B(OH)3Cl-) as well as variable B fixation and fractionation for different serpentinization product minerals (brucite, chrysotile). Breakdown of the Li-rich olivine and limited Li incorporation into product mineral phases resulted in an overall lower Li content of the final solid phase assemblage at 200 °C ([Li]final_200 °C = 0.77 µg/g; DS/FLi200 °C = 1.58). First order changes in Li isotopic compositions were defined by mixing of two isotopically distinct sources i.e. the fresh olivine and the fluid rather than by equilibrium isotope fraction. At 200 °C primary olivine is dissolved, releasing its Li budget into the fluid which shifts towards a lower d7LiF of +38.62 per mil. Newly formed serpentine minerals (d7LiS = +30.58 per mil) incorporate fluid derived Li with a minor preference of the 6Li isotope. At 100 °C Li enrichment of secondary phases exceeded Li release by olivine breakdown ([Li]final_100 °C = 2.10 µg/g; DS/FLi100 °C = 11.3) and it was accompanied by preferential incorporation of heavier 7Li isotope that might be due to incorporation of a 7Li enriched fluid fraction into chrysotile nanotubes.

Description: There has been much recent interest in the origin of silicic magmas at spreading centres away from any possible influence of continental crust. Here we present major and trace element data for 29 glasses (and 55 whole-rocks) sampled from a 40 km segment of the South East Rift in the Manus Basin that span the full compositional continuum from basalt to rhyolite (50-75 wt % SiO2). The glass data are accompanied by Sr-Nd-Pb, O and U-Th-Ra isotope data for selected samples. These overlap the ranges for published data from this part of the Manus Basin. Limited increases in Cl/K ratios with increasing SiO2, La-SiO2 and Yb-SiO2 relationships, and the oxygen isotope data rule out models in which the more silicic lavas result from partial melting of altered oceanic crust or altered oceanic gabbros. Rather, the data form a coherent array that is suggestive of closed-system fractional crystallization and this is well simulated by MELTS models run at 0.2 GPa and QFM (quartz-fayalite-magnetite buffer) with 1 wt % H2O, using a parental magma chosen from the basaltic glasses. Although some assimilation of altered oceanic crust or gabbro cannot be completely ruled out, there is no evidence that this plays an important role in the origin of the silicic lavas. The U-series disequilibria are dominated by 238U and 226Ra excesses that limit the timescale of differentiation to less than a few millennia. Overall, the data point to rapid evolution in relatively small magma lenses located near the base of thick oceanic crust; we speculate that this was coupled with relatively low rates of basaltic recharge. A similar model may be applicable to the generation of silicic magmas elsewhere in the ocean basins.