Mineral and gas chemistry of hydrothermal fluids on an ultrafast spreading ridge: East Pacific Rise, 17° to 19°S (Naudur cruise, 1993) phase separation processes controlled by volcanic and tectonic activity

Abstract

During the Naudur cruise in December 1993, 23 dives using the French submersible Nautile were conducted on the axis of the East Pacific Rise between 17° and 19°S where the spreading rate is among the fastest in the ocean (14 to 16 cm/yr). Twenty hydrothermal fluids located at the topographic high of each segment in the axial domain were collected between 2573 and 2669 m depth on three segments centered, respectively, at 17°25′S, 18°15′S, and 18°26′S. The fluids exhibit a very wide range of temperature, chemical, and gas compositions. On the 17°25′S and 18°26′S segments, fluids have quite uniform compositions, low chlorinities, are gas‐enriched and are low in dissolved metals relative to fluids from the 18°15′S segment which show high chlorinities, are less gas‐enriched and show high‐metal concentrations. Chloride and metal depletion associated with gas enrichment is consistent with phase separation. Whereas CH4 endmembers show large variations between sites, the 13CH4 data are very similar, with 13C values in a narrow range −22.0 to −23.9‰ versus pee‐dee belemnite (PDB). δ13CO2 measured in fluids within the 18°15′S and 18°26′S segments are, respectively, −7.9 and −5.8‰ versus PDB, similar to 13C of CO2 trapped in mid oceanic ridge basalts, suggesting a magmatic origin. The variability in fluid composition is linked to the variability of the accretion process observed on the three segments. The uniform venting of low‐chlorinity fluids in the 17°25′S and 18°26′S segments is connected with volcanic activity which causes boiling with preferential venting of vapor‐enriched fluids. High‐salinity fluids are emitted on the 18°15′S segment where the ridge is tectonics‐dominated and subseafloor circulation controlled by faults. Phase‐separated effluents induced by volcanic and tectonic activity are delivered to the deep ocean in this area, as previously observed on the Juan de Fuca Ridge or in the North Fiji Basin Ridge.

Rare earth elements as indicators of hydrothermal processes within the East Scotia subduction zone system

“…Although the effects of phase separation on the major-element chemistry of vent 395 fluids have been well studied (Bischoff and Rosenbauer, 1984; Von Damm and Bischoff, 396 1987;Butterfield et al, 1990;Auzende et al, 1996;Charlou et al, 1996; Coumou et al, 397 2009), the effects on REE behaviour are less well known and, to our knowledge, there are no 398 REE data in the literature for very low Cl fluids from subduction zone settings. REE 399 concentrations tend to be higher in chloride-rich brines and lower in low-chloride fluids 400 , and low-Cl fluids tend to have larger Eu CN /Eu* CN anomalies 401 .…”

“…Chloride forms stronger complexes with the LREEs compared to the HREEs 384 and is likely to be an important driver behind the light REE-enrichment observed in vent 385 fluids from E2 and E9 South on the ESR, and for most mid-ocean ridge hydrothermal 386 systems (Haas et al, 1995;Migdisov et al, 2009 Figure 2). 394Although the effects of phase separation on the major-element chemistry of vent 395 fluids have been well studied (Bischoff and Rosenbauer, 1984; Von Damm and Bischoff, 396 1987;Butterfield et al, 1990;Auzende et al, 1996;Charlou et al, 1996; Coumou et al, 397 2009), the effects on REE behaviour are less well known and, to our knowledge, there are no 398 REE data in the literature for very low Cl fluids from subduction zone settings. REE 399 concentrations tend to be higher in chloride-rich brines and lower in low-chloride fluids 400 , and low-Cl fluids tend to have larger Eu CN /Eu* CN anomalies 401 .…”

The influence of magma supply and eruptive processes on axial morphology, crustal construction and magma chambers

“…Charlou et al [1996] and Auzende et al [1996] have observed hydrothermal vent chimneys predominantly located on faults at the edge of axial summit graben in the region from 18°22'S to l8°37'S. In contrast, where the magma sill is observed to be unusually shallow, at l7°25'S, they observed mainly diffuse venting of seawater heated by cooling lavas.…”

“…These indicators are: the depth of the seafloor at a 1 km spacing obtained from the URI Sea Beam data synthesis [Tighe et al, 1988]; cross-sectional area measured at 1 km intervals along the ridge from Scheirer and Macdonald [1993]; the along-axis mantle Bouguer anomaly calculated at 2 km intervals from Magde et al [1995]; MgO wt% of lavas dredged from the ridge axis from Sinton et al [1991] and samples collected from the youngest on-axis flows by the submersible Nautile [Auzende et al, 1996]; light attenuation in the water column integrated from 100 to 300m above the seafloor in bins of 1.8 minutes(::::: 18 km) from Baker and Urabe [1996]; and the locations along the ridge where Lonsdale [1989] detected an ASC or an axial summit graben using SeaBeam data.…”

Strontium isotope constraints on fluid flow in the sheeted dike complex of fast spreading crust: Pervasive fluid flow at Pito Deep

“…87 Sr/ 86 Sr in Altered Upper Oceanic Crust [27] To further investigate the striking similarity between the extent of Sr-isotopic exchange between the sheeted dike complex and hydrothermal fluids in these three areas we compare the amount of fluid required to produce the observed Sr-isotopic modifications. The composition of the protolith in each of the three areas is slightly different, Charlou et al, 1996]. Vent fluid data from EPR 9 -10°N are zero-Mg end-member fluids, a number of which have negative Sr; therefore the measured fluids would have higher Sr concentrations [Von Damm, 2000].…”