Low‐temperature fluid flow through sulfidic sediments from TAG: Modification of fluid chemistry and alteration of mineral deposits

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94945/1/grl9672.pd

Hydroschorlomite in altered basalts from Hole 1256D, ODP Leg 206: The transition from low-temperature to hydrothermal alteration

International audienceHydroschorlomite, a Ti-, Ca-, Fe-rich andraditic arnet present in the deepest cores of basalts (661?749 bsf) drilled in Hole 1256D during Ocean Drilling Program (ODP) Leg 206 (equatorial east Pacific), is reported here for the first time in oceanic crust. Detailed petrological and mineralogical studies by optical microscope, electron microprobe, scanning and transmission electron microscope, and micro-Raman spectroscopy are used to characterize this hydrogarnet and its relationships with other minerals. Hydroschorlomite occurs in Hole 1256D as small (5?50 ?m) anhedral or euhedral crystals associated either with celadonite in black halos adjacent to celadonite veins or with brown saponitic phyllosilicate in brown alteration halos adjacent to veins of saponite and iron oxyhydroxides. Both types of halos are formed at low temperature (less than about 100?C). Textural observations suggest that hydroschorlomite formation is contemporaneous with the phyllosilicates. Hydroschorlomite is rich in CaO (22.5?26.5 wt%), TiO2 (22.0?28.6 wt%), and FeOt (6.2?12.9 wt%) and contains significant F (up to 0.85 wt%) and Zr2O3 (up to 0.34 wt%). The presence of OH suggested by the low total percentages of oxides (95.2?97.3 wt%) is confirmed by the OH vibration at 3557 cm?1 in the micro-Raman spectrum. Chemical mapping indicates that hydroschorlomite is not zoned and is always associated with either celadonitic or saponitic phyllosilicates. Some hydroschorlomite crystals partly include tiny (<10 ?m) skeletal titanomagnetite. The occurrence of hydroschorlomite in Hole 1256D basalts coincides with a general downward increase in temperatures and overall intensity of alteration manifest by the alteration of plagioclase and the occurrence of small amounts of mixed-layer chlorite-smectite. The titanium necessary to form hydroschorlomite is provided by the breakdown of primary tiny (<10 ?m) titanomagnetite, while calcium is provided by the replacement of plagioclase by albite. Hydroschorlomite is thus an indicator of alteration of titanomagnetite under conditions transitional from low-temperature alteration to hydrothermal metamorphism with formation of titanite and may affect magnetic properties of the rocks

Geochemistry of Sublacustrine Hydrothermal Deposits in Yellowstone Lake—Hydrothermal Reactions, Stable-Isotope Systematics, Sinter Deposition, and Spire Formation

Geochemical and mineralogical studies of hydrothermal deposits and altered vent muds from the floor of Yellowstone Lake indicate that these features form due to hydrothermal fluid quenching in shallow flow conduits or upon egress into bottom waters. Siliceous precipitates occur as conduits within the uppermost sediments, as tabular deposits that form along sedimentary layers, and as spires as much as 8 m tall that grow upward from crater-like depressions on the lake bottom. These deposits are enriched in As, Cs, Hg, Mo, Sb, Tl, and W. Variations in major-element geochemistry indicate that subaerial sinters from West Thumb and spire interiors are nearly pure SiO2, whereas sublacustrine conduits are less SiO2 rich and are similar in some cases to normal Yellowstone Lake sediments due to incorporation of sediments into conduit walls. Vent muds, which are hydrothermally altered lake sediments, and some outer conduit walls show pervasive leaching of silica (~63 weight percent silica removal). This hydrothermal leaching process may explain the occurrence of most sublacustrine vents in holes or vent craters, but sediment winnowing by vent fluids may also be an important process in some cases. Stable-isotope studies indicate that most deposits formed at temperatures between 78°C and 160°C and that vent fluids had oxygen-isotope values of –3.2 to –11.6 per mil, significantly higher than lake waters (–*16.5 per mil). Sulfur-isotope studies indicate that vent waters and lake waters are dominated by sulfur derived from volcanic rocks with δ34S ~ 2.5 per mil. Geochemical reaction modeling indicates that spires form from upwelling hydrothermal fluids that are saturated with amorphous silica at temperatures 80°–96°C. Reaction calculations suggest that silica precipitation on the lake bottom is initially caused by mixing with cold bottom waters. Once a siliceous carapace is established, more rapid silica precipitation occurs by conductive cooling. Silicification of thermophilic bacteria is a very important process in building spire structures

Ultramafic clasts from the South Chamorro serpentine mud volcano reveal a polyphase serpentinization history of the Mariana forearc mantle

Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Lithos 227 (2015): 1-20, doi:10.1016/j.lithos.2015.03.015.Serpentine seamounts located on the outer half of the pervasively fractured Mariana forearc provide an excellent window into the forearc devolatilization processes, which can strongly influence the cycling of volatiles and trace elements in subduction zones. Serpentinized ultramafic clasts recovered from an active mud volcano in the Mariana forearc reveal microstructures, mineral assemblages and compositions that are indicative of a complex polyphase alteration history. Petrologic phase relations and oxygen isotopes suggest that ultramafic clasts were serpentinized at temperatures below 200 °C. Several successive serpe ntinization events represented by different vein generations with distinct trace element contents can be recognized. Measured Rb/Cs ratios are fairly uniform ranging between 1 and 10, which is consistent with Cs mobilization from sediments at lower temperatures and lends further credence to the low-temperature conditions proposed in models of the thermal structure in forearc settings. Late veins show lower fluid mobile element (FME) concentrations than early veins, suggesting a deacreasing influence of fluid discharge from sediments on the composition of the serpentinizing fluids. The continuous microfabric and mineral chemical evolution observed in the ultramafic clasts may have implications as to the origin and nature of the serpentinizing fluids. We hypothesize that opal and smectite dehydration produce quartz-saturated fluids with high FME contents and Rb/Cs between 1 and 4 that cause the early pervasive serpentinization. The partially serpentinized material may then be eroded from the basal plane of the suprasubduction mantle wedge. Serpentinization continued but the interacting fluids did not carry the slab-flux signature, either because FME were no longer released from the slab, or due to an en route loss of FMEs. Late chrysotile veins that document the increased access of fluids in a now fluid-dominated regime are characterized by reduced trace element contents with a slightly increased Rb/Cs ratio near 10. This lack of geochemical slab signatures consistently displayed in all late serpentinization stages may indicate that the slab-derived fluids have been completely reset (i.e. the FME excesses were removed) by continued water-rock reaction within the subduction channel. The final stage of diapiric rise of matrix and clasts in the conduits is characterized by brucite-dominated alteration of the clasts from the clast rim inward (independent of the intra-clast fabric relations), which corresponds to re-equilibration with alkaline, low-silica activity fluids in the rising mud.This study was funded through a grant of the DFG to WB (BA 1605/5-1)

Stable isotopic composition of upper oceanic crust formed at a fast spreading ridge, ODP Site 801

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94808/1/ggge303.pd

Oxygen isotope composition of a section of lower oceanic crust, ODP Hole 735B

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95244/1/ggge964.pd

Cycling of sulfur in subduction zones: The geochemistry of sulfur in the Mariana Island Arc and back-arc trough

The sulfur contents and sulfur isotopic compositions of 24 glassy submarine volcanics from the Mariana Island Arc and back-arc Mariana Trough were determined in order to investigate the hypothesis that subducted seawater sulfur ([delta]34S = 21[per mille sign]) is recycled through arc volcanism. Our results for sulfur are similar to those for subaerial arc volcanics: Mariana Arc glasses are enriched in 34S ([delta]34S = up to 10.3[per mille sign], mean = 3.8[per mille sign]) and depleted in S (20-290 ppm, MEAN = 100 ppm) relative to MORB (850 ppm S, [delta]34S = 0.1 +/- 0.5[per mille sign]). The back-arc trough basalts contain 200-930 ppm S and have [delta]34S values of 1.1 +/- 0.5[per mille sign], which overlap those for the arc and MORB. The low sulfur contents of the arc and some of the trough glasses are attributed to (1) early loss of small amounts of sulfur through separation of immiscible sulfide and (2) later vapor-melt equilibrium control of sulfur contents and loss of sulfur in a vapor phase from sulfide-undersaturated melts near the minimum in sulfur solubility at [function of (italic small f)]O2 [approximate] NNO (nickel-nickel oxide). Although these processes removed sulfur from the melts their effects on the sulfur isotopic compositions of the melts were minimal. Positive trends of [delta]34S with 87Sr/86Sr, LILE and LREE contents of the arc volcanics are consistent with a metasomatic seawater sulfur component in the depleted sub-arc mantle source. The lack of a 34S-rich slab signature in the trough lavas may be attributed to equilibration of metasomatic fluid with mantle material along the longer pathway from the slab to the source of the trough volcanics. Sulfur is likely to have been transported into the mantle wedge by metasomatic fluid derived from subducted sediments and pore fluids.Gases extracted from vesicles in arc and back-arc samples are predominantly H2O, with minor CO2 and traces of H2S and SO2. CO2 in the arc and back-arc rocks has [delta]13C values of -2.1 to -13.1[per mille sign], similar to MORB. These data suggest that degassing of CO2 could explain the slightly lower [delta]13C values for some Mariana Trough volcanic glasses, and that incorporation of subduction-derived organic carbon into the Mariana Trough mantle source may not be necessary. More analyses are required to resolve this question, however.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30563/1/0000196.pd

The origin and fate of C during alteration of the oceanic crust

The contents and isotope compositions of water and carbon, including total, reduced, and inorganic (carbonate) C, were studied in 170 My altered oceanic basalts from Ocean Drilling Program Hole 801C in the western Pacific Ocean. Reduced C contents of 0.12–0.29 wt% CO2 and $\delta ^{13}$C values of $-22.6$ to $-27.8‰$ occur throughout the basement section. High total C concentrations in the upper volcanic section (UVS), above 300 m sub-basement, are dominated by inorganic C, and concentrations of both decrease with depth, from 1.92 to 0.57 wt% CO2 and 1.76 wt% CO2 to 0.66 wt% CO2, respectively. The $\delta ^{13}$C of inorganic C in the UVS ($-$0.4 to ${+}1.5{‰}$) indicates precipitation of seawater dissolved inorganic carbon (DIC) through the intensive circulation of seawater. $\delta$D values of $-$59.8 to ${-}17.6{‰}$ in the UVS also result from seawater interaction. In contrast, total C contents in the lower volcanic section (LVS) are low (0.22–0.39 wt% CO2) and dominated by reduced C, resulting in negative $\delta ^{13}$C values for total C ($-$18.7 to ${-}23.5{‰}$). We propose that a proportion of this reduced C could have formed through abiotic reduction of magmatic CO2 at the ridge axis. The contents and $\delta ^{13}$C values of inorganic carbon in the LVS (0.05–0.09 wt% CO2 and $-$10.7 to ${-}9.5{‰}$, respectively) fall in the range characteristic of C in mid-ocean ridge basalt glasses, also suggesting a magmatic origin. $\delta$D values in the LVS (weighted average $= {-}69.3{‰}$) are consistent with magmatic water. Reduced C in the basalts may also have formed through microbial activity at low temperatures, as indicated by previous work showing negative $\delta ^{34}$S values in the basalts.Our results show: (1) that magmatic C can be stored in altered oceanic basalts both as reduced and inorganic C resulting from high-temperature processes at mid-ocean ridges; (2) that microbial activity may add reduced C to the basalts during low-temperature alteration on ridge flanks; and (3) that circulation of cold seawater in the uppermost few hundred meters of basement adds seawater DIC as carbonate to the basalts and filling fractures in the basement. We estimate the content of magmatic C stored in the altered basaltic crust to be 0.126 wt% CO2. Compared with previous estimates, this concentration probably represents an upper limit for magmatic C. This resultant magmatic C flux into the crust, ranging from $1.5\times 10^{12}$–$2\times 10^{12}$ molC${\cdot }$y$^{-1}$ is similar to the outgassing CO2 flux [${\sim }1.32\pm 0.8$–$2.0 \times 10^{12}$ molC${\cdot }$y$^{-1}$, Le Voyer et al., 2019 and Cartigny et al., 2018, respectively]. Further data are needed to better constrain the fraction of magmatic CO2 that does not escape the oceanic lithosphere but remains stored as reduced and inorganic carbon

The origin and fate of C during alteration of the oceanic crust

The contents and isotope compositions of water and carbon, including total, reduced, and inorganic (carbonate) C, were studied in 170 My altered oceanic basalts from Ocean Drilling Program Hole 801C in the western Pacific Ocean. Reduced C contents of 0.12–0.29 wt% CO2 and $\delta ^{13}$C values of $-22.6$ to $-27.8‰$ occur throughout the basement section. High total C concentrations in the upper volcanic section (UVS), above 300 m sub-basement, are dominated by inorganic C, and concentrations of both decrease with depth, from 1.92 to 0.57 wt% CO2 and 1.76 wt% CO2 to 0.66 wt% CO2, respectively. The $\delta ^{13}$C of inorganic C in the UVS ($-$0.4 to ${+}1.5{‰}$) indicates precipitation of seawater dissolved inorganic carbon (DIC) through the intensive circulation of seawater. $\delta$D values of $-$59.8 to ${-}17.6{‰}$ in the UVS also result from seawater interaction. In contrast, total C contents in the lower volcanic section (LVS) are low (0.22–0.39 wt% CO2) and dominated by reduced C, resulting in negative $\delta ^{13}$C values for total C ($-$18.7 to ${-}23.5{‰}$). We propose that a proportion of this reduced C could have formed through abiotic reduction of magmatic CO2 at the ridge axis. The contents and $\delta ^{13}$C values of inorganic carbon in the LVS (0.05–0.09 wt% CO2 and $-$10.7 to ${-}9.5{‰}$, respectively) fall in the range characteristic of C in mid-ocean ridge basalt glasses, also suggesting a magmatic origin. $\delta$D values in the LVS (weighted average $= {-}69.3{‰}$) are consistent with magmatic water. Reduced C in the basalts may also have formed through microbial activity at low temperatures, as indicated by previous work showing negative $\delta ^{34}$S values in the basalts.Our results show: (1) that magmatic C can be stored in altered oceanic basalts both as reduced and inorganic C resulting from high-temperature processes at mid-ocean ridges; (2) that microbial activity may add reduced C to the basalts during low-temperature alteration on ridge flanks; and (3) that circulation of cold seawater in the uppermost few hundred meters of basement adds seawater DIC as carbonate to the basalts and filling fractures in the basement. We estimate the content of magmatic C stored in the altered basaltic crust to be 0.126 wt% CO2. Compared with previous estimates, this concentration probably represents an upper limit for magmatic C. This resultant magmatic C flux into the crust, ranging from $1.5\times 10^{12}$–$2\times 10^{12}$ molC${\cdot }$y$^{-1}$ is similar to the outgassing CO2 flux [${\sim }1.32\pm 0.8$–$2.0 \times 10^{12}$ molC${\cdot }$y$^{-1}$, Le Voyer et al., 2019 and Cartigny et al., 2018, respectively]. Further data are needed to better constrain the fraction of magmatic CO2 that does not escape the oceanic lithosphere but remains stored as reduced and inorganic carbon