Fengyou Chu

Geochemical constraints on the contribution of Louisville seamount materials to magmagenesis in the Lau back-arc basin, SW Pacific

Seamounts are an integral part of element recycling in global subduction zones. The published trace element and Pb-Sr-Nd isotope data for basaltic lavas from three key segments (Central Lau Spreading Ridge (CLSR), Eastern Lau Spreading Ridge (ELSR), and Valu Fa Ridge (VFR)) of the Lau back-arc basin were compiled to evaluate the contribution of Louisville seamount materials to their magma genesis. Two geochemical transitions, separating three provinces with distinct geochemical characteristics independent of ridge segmentation, were identified based on abrupt geochemical shifts. The origin of the geochemical transitions was determined to be the result of drastic compositional changes of subduction components added into the mantle source, rather than the transition from Indian to Pacific mid-ocean ridge basalt (MORB) mantle, or due to variable mantle fertilities. The most likely explanation for the drastic shifts in subduction input is the superimposition of Louisville materials on ‘normal’ subduction components consisting predominantly of aqueous fluids liberated from the down-going altered oceanic crust and minor pelagic sediment melts. Quantitative estimation reveals that Louisville materials contributed 0–74% and 21–83% of the Th budget, respectively, to CLSR and VFR lavas, but had no definite contribution to the lavas from the ELSR, which lies farthest away from the subducted Louisville seamount chain (LSC). The spatial association of the subducted LSC with the Louisville-affected segments suggests that the Louisville signature is regionally but not locally available in the Tonga subduction zone. Besides, the preferential melting of subducted old Cretaceous LSC crust instead of the old normal Pacific oceanic crust at similar depths implies that elevated temperature across the subduction interface or seamount erosion and rupture were required to trigger melting. A wider implication of this study, thus, is that seamount subduction may promote efficiency of element recycling in subduction zones.

Mantle melting beneath the Southwest Indian Ridge: signals from clinopyroxene in abyssal peridotites

AbstractsThe mineral chemistry and texture of clinopyroxenes in peridotite from the Kingkong tectonic zone of the Southwest Indian Ridge segment in an effort to constrain mantle melting beneath this slow-spreading ridge are reported. There are three types of clinopyroxenes in the abyssal peridotites: coarse-grained, intergranular and exsolved. The compositional variations among these three types suggest that the coarse-grained clinopyroxene is a mantle-derived source. The Al, Na and Ti contents and the Na/Ti ratio of the coarsegrained clinopyroxene may be used to monitor the degree of partial melting, combined with the contradistinction with Spinel Cr#, which is calculated to be between 7.9% and 14.9%, and may represent low degrees of melting in the global ocean ridge system. The along-axis compositional variations in the coarse-grained clinopyroxene suggest that the degree of partial melting is primarily controlled by the transform faults on both sides of the ridge. Nonetheless, the northwestern side of the ridge may be affected by a hypothesised detachment fault as documented by the calculated P-T conditions. Simultaneously high Na and low Ti contents in the coarse-grained clinopyroxene points to mantle heterogeneities along the ridge axis.

Lipid Biomarkers Reveal Microbial Communities in Hydrothermal Chimney Structures from the 49.6°E Hydrothermal Vent Field at the Southwest Indian Ocean Ridge

ABSTRACT Lipid biomarkers were investigated to reveal the microbial life preserved in sulfide and Si-rich chimney from the 49.6°E hydrothermal vent field. In sulfide chimney, iso-/anteiso-fatty acids and H-shaped glycerol dialkyl glycerol tetraethers are the main microbial biomarkers. In Si-rich chimney, monounsaturated fatty acids (C16:1n7, C18:1n7) are the main bacterial biomarkers detected, and crenarchaeol and its isomer are relatively abundant (up to 25% of glycerol dialkyl glycerol tetraethers) archaeol biomarkers. Composition of lipid biomarkers reveals the diversity of microbial communities in different types of chimney structures. Sulfate-reducing bacteria and hyperthermophilic archaea were considered to be the majority microbial life in sulfide chimney, and sulfur-oxidizing bacteria were abundant in Si-rich chimney while archaea in Si-rich chimney and mainly attributed to Thaumarchaeota, which were predominately ammonia oxidizers. Our result suggested that fluid temperature and gaseous components could be the main constraints for the diversity of microbial communities in hydrothermal chimney structures in 49.6°E hydrothermal vent field.

Multi-stage growth and fluid evolution of a hydrothermal sulphide chimney in the East Pacific Ridge 1–2° S hydrothermal field: constraints from in situ sulphur isotopes

Sulphur isotopes can be used as a powerful tool to trace fluid evolution and explore the formation of chimneys. To clarify the in situ S isotopic variations of sulphides at the micro-scale, we analyzed a sulphide chimney collected from the hydrothermal field in the East Pacific Rise 1–2° S using a sensitive high-mass-resolution ion micro-probe for stable isotopes (SHRIMP SI). Three mineral zones can be identified in the chimney: an external outer wall of porous anhydrite and colloform pyrite, an internal middle zone of sub-euhedral pyrite and massive chalcopyrite, and an inner zone of massive pyrite. The δ 34 S V-CDT values of the sulphides fall within the range 1.83–7.51 ‰ (avg. 4.05 ‰, n = 16), and S isotopic values increase from the core (3.09 ‰, n = 3) to the middle (3.78 ‰, n = 11) to the edge (6.99 ‰, n = 2). These results illustrate mineral crystallization processes and the mixing between seawater-derived S and magmatic–hydrothermal fluids during the growth of the chimney. The zones from the edge to the core are characterized by crystal morphologies of colloform/anhedral pyrite to massive pyrite with decreasing δ 34 S values, revealing multi-stage mineral deposition and sulphur isotopic fractionation. In contrast to the increase in δ 34 S values from the core to the edge in one profile (profile A), anomalously low δ 34 S values in fine-grained pyrite relative to chalcopyrite in another profile (profile B) in the middle zone result from S isotopic exchange between seawater SO 4 2− and fluid H 2 S due to different fluid–seawater mixing, possibly caused by variations in permeability and porosity across the chimney.

Zn isotopes in hydrothermal sulfides and their oxidation products along the south mid-Atlantic ridge: evidence of hydrothermal fluid deposition

Significant Zn isotope fractionation occurs during seafloor hydrothermal activities. Therefore, exploring variations in Zn isotope composition affected by hydrothermal fluids and oxidative processes would help to better understand hydrothermal fluid cycling and sulfide deposition on mid-ocean ridges. In this paper, the Zn isotope compositions of different types of sulfides and their oxidation products obtained from hydrothermal fields on the South Mid-Atlantic Ridge (13–15°S) were analyzed using a Neptune plus MC-ICP-MS. The δ66Zn ratios range from –0.14‰ to +0.38‰, and the average δ66Zn ratio is +0.12±0.06‰ (n=21, 2 SD) for all the studied sulfides and oxidation products. The Cu-rich sulfides have a slightly heavier Zn isotope composition (average δ66Zn=+0.19±0.07‰, n=6) than the Zn-rich sulfides (average δ66Zn=–0.02±0.06‰, n=5). The Zn isotope compositions of the oxidation products are similar to those of the Cu-rich sulfides, with an average δ66Zn ratio of 0.14±0.06‰ (n=10, 2 SD). The Zn isotope compositions of all the samples are generally within the ranges of sulfides from hydrothermal fields on other mid-ocean ridges, such as the East Pacific Rise (9°N, 21°N) and the Trans-Atlantic Geotraverse. However, the average Zn isotope composition indicates the presence of significantly lighter Zn isotopes relative to those reported in the literature (average δ66Zn=+0.39‰). The significant enrichment of the Zn-rich sulfides with light Zn isotopes reveals that kinetic fractionation likely occurs during mineral deposition. Furthermore, the Zn isotope compositions of the sulfides and their oxidation products (average δ66Zn=+0.12‰) are significantly lighter than the average Zn isotope composition of the ocean (δ66Zn=+0.5‰), which could further constrain the modern Zn isotope cycle in the ocean by serving as a sink for light Zn isotopes.