A New Insight Into Seawater-Basalt Exchange Reactions Based on Combined δ18O—Δ′17O—87Sr/86Sr Values of Hydrothermal Fluids From the Axial Seamount Volcano, Pacific Ocean

The δ18O values of submarine vent fluids are controlled by seawater-basalt exchange reactions, temperature of exchange, and to a lesser extent, by phase separation. These variations are translated into the δ18O values of submarine hydrothermal fluids between ca. 0 and + 4‰, a range defined by pristine seawater and equilibrium with basalt. Triple oxygen isotope systematics of submarine fluids remains underexplored. Knowing how δ17O and δ18O change simultaneously during seawater-basalt reaction has a potential to improve i) our understanding of sub-seafloor processes and ii) the rock-based reconstructions of ancient seawater. In this paper, we introduce the first combined δ17O-δ18O-87Sr/86Sr dataset measured in fluids collected from several high-temperature smoker- and anhydrite-type vent sites at the Axial Seamount volcano in the eastern Pacific Ocean. This dataset is supplemented by measurements of major, trace element concentrations and pH indicating that the fluids have reacted extensively with basalt. The salinities of these fluids range between 30 and 110% of seawater indicating that phase separation is an important process, potentially affecting their δ18O. The 87Sr/86Sr endmember values range between 0.7033 and 0.7039. The zero-Mg endmember δ18O values span from -0.9 to + 0.8‰, accompanied by the Δ′17O0.528 values ranging from around 0 to −0.04‰. However, the trajectory at individual site varies. The endmember values of fluids from focused vents exhibit moderate isotope shifts in δ′18O up to +0.8‰, and the shifts in Δ′17O are small, about −0.01‰. The diffuse anhydrite-type vent sites produce fluids that are significantly more scattered in δ′18O—Δ′17O space and cannot be explained by simple isothermal seawater-basalt reactions. To explain the observed variations and to provide constraints on more evolved fluids, we compute triple O isotope compositions of fluids using equilibrium calculations of seawater-basalt reaction, including a non-isothermal reaction that exemplifies complex alteration of oceanic crust. Using a Monte-Carlo simulation of the dual-porosity model, we show a range of possible simultaneous triple O and Sr isotope shifts experienced by seawater upon reaction with basalt. We show the possible variability of fluid values, and the causal effects that would normally be undetected with conventional δ18O measurements.

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Apatite Texture, Composition, and O-Sr-Nd Isotope Signatures Record Magmatic and Hydrothermal Fluid Characteristics at the Black Mountain Porphyry Deposit, Philippines

Economic Geology ◽

10.5382/econgeo.4827 ◽

2021 ◽

Author(s):

Ming Jian Cao ◽

Noreen J. Evans ◽

Pete Hollings ◽

David R. Cooke ◽

Brent I.A. McInnes ◽

...

Keyword(s):

Phase Separation ◽

High Temperature ◽

Oxygen Isotope ◽

Hydrothermal Alteration ◽

Hydrothermal Fluid ◽

Isotope Composition ◽

Hydrothermal Fluids ◽

Nd Isotope ◽

O Isotope ◽

Black Mountain

Abstract The trace elemental and isotopic signatures in apatite can be modified during hydrothermal alteration. This study investigates the suitability of apatite as an indicator of the source, chemistry, and evolution of magma and hydrothermal fluids. In situ textural, elemental, and O-Sr-Nd isotope analyses were performed on apatite in thin sections, from fresh and propylitically altered pre- and synmineralized dioritic porphyries from the Black Mountain porphyry Cu deposit in the Philippines. All studied apatite crystals have similar subhedral to euhedral shapes and are homogeneous in the grayscale in backscattered electron images. In cathodoluminescence images, the apatite in fresh and altered rocks displays yellow to yellow-green and green to brown luminescence, respectively. Apatite in fresh rocks has a higher Cl and Mn content, and lower Fe, Mg, Sr, Pb, and calculated XOH-apatite, compared to apatite in altered rocks. The content of F, rare earth elements (REEs), Y, U, Th, and Zr, and the Sr-Nd isotope signatures of apatite from fresh and altered rocks are similar in all apatite grains (87Sr/86Sr = 0.7034–0.7042 vs. 0.7032–0.7043, εNd(t) = 5.3–8.0 vs. 5.1–8.4). The X-ray maps and elemental and oxygen isotope signatures across individual apatite crystals are typically homogeneous in apatite from both fresh and altered rocks. The distinct luminescence colors, coupled with distinct mobile element compositions (Cl, OH, Mn, Mg, Fe, Sr, Pb), indicate modification of primary magmatic apatite during interaction with hydrothermal fluids. The similarities in Sr isotope ratios (87Sr/86Sr = 0.7032–0.7043) but slight differences in O isotope signatures (δ18O = 6.0 ± 0.3‰ vs. 6.6 ± 0.3‰) in apatite from fresh and altered rocks are consistent with the magma and hydrothermal fluids having the same source and suggest significant phase separation in the hydrothermal fluids given that 18O preferentially fractionates into the residual liquid relative to 16O during phase separation. The similarity of immobile element (REE, Y, U, Th, and Zr) contents in both populations of apatite, consistency of textures and Nd isotope compositions, and absence of obvious dissolution-reprecipitation features all suggest that altered apatite retains some magmatic characteristics. The apatite in fresh rocks has oxygen isotope compositions similar to that of zircons from the same sample (δ18O = 5.9 ± 0.3‰), indicating little to no oxygen isotope fractionation between zircon and apatite and that apatite can be a good proxy for the oxygen isotope composition of the magma. Based on the Cl contents of the magmatic and replacement apatite, and assuming their equilibrium with high-temperature magma fluid and replacement hydrothermal fluid, respectively, the calculated Cl content of the early magmatic fluid and the later replacement fluid can be estimated to be 6.4 to 15.1 wt % and ~0.25 ± 0.03 wt %, respectively. This indicates a depletion of Cl from the early high-temperature fluid to the replacement fluid, consistent with phase separation. This study demonstrates that cathodoluminescence, elemental compositions (such as Cl, Mn, Mg, Fe, Sr, Pb) and Sr-O isotope signatures in apatite can be modified during hydrothermal alteration, whereas other components (REE, Y, U, Th, and Zr) and the Nd isotope composition are preserved. These features can be used to constrain the origin, chemistry, and evolution of the primary magma and ore-forming hydrothermal fluids.

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Genesis and tectonic setting of the Bulage Pb – Zn deposit , Inner Mongolia, China: Evidence from geology, fluid inclusions, EMPA, H–O isotope systematics, zircon U–Pb geochronology, and geochemistry

Geological Journal ◽

10.1002/gj.3410 ◽

2018 ◽

Vol 55 (1) ◽

pp. 344-371 ◽

Cited By ~ 6

Author(s):

Jian Li ◽

Wen‐yan Cai ◽

Li‐juan Fu ◽

Xue‐bing Zhang ◽

Ke‐yong Wang ◽

...

Keyword(s):

Fluid Inclusions ◽

Inner Mongolia ◽

Tectonic Setting ◽

O Isotope ◽

Isotope Systematics

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Si-B-O isotope systematics of Qinling Devonian submarine hydrothermal ore-forming systems

Water-Rock Interaction ◽

10.1201/9780203734049-48 ◽

2021 ◽

pp. 195-197

Author(s):

Shao-Yong Jiang ◽

Martin R. Palmer ◽

Yan-He Li ◽

Chun-Ji Xue

Keyword(s):

O Isotope ◽

Isotope Systematics

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Carbon-isotope systematics of Archean Au–Ag vein deposits in the Superior Province

Canadian Journal of Earth Sciences ◽

10.1139/e90-004 ◽

1990 ◽

Vol 27 (1) ◽

pp. 40-56 ◽

Cited By ~ 30

Author(s):

R. Kerrich

Keyword(s):

Hydrothermal Fluids ◽

Superior Province ◽

Ambient Temperatures ◽

Rayleigh Fractionation ◽

Clastic Sediments ◽

Isotopic Equilibration ◽

Vein Deposits ◽

Carbonaceous Rocks ◽

Isotope Systematics ◽

Main Population

Abundant carbonate is a characteristic feature of most Archean mesothermal Au–Ag vein deposits, but the source of the C is controversial. For Superior Province deposits collectively, the maximum variation of average δ13C values is from −9.0 ± 0.7‰ (1σ, n = 19; Darius) to −0.6 ± 1.6‰ (1σ, n = 7; Cochenour–Willians), and limiting δ13C values are−13.6 and + 1.3‰. At the deposit scale, Fe dolomites in nongraphitic lithologies are for the most part isotopically uniform, where δ13C = −3.4 ± 0.4 (1σ) (Hollinger), −3.2 ± 0.3 (McIntyre), −4.7 ± 1.7 (Dome), −2.8 ± 0.6 (Buffalo Ankerite), −3.6 ± 0.5 (Macassa), −3.2 ± 0.3 (Bousquet), −5.4 ± 0.9 (Lamaque), and −5.3 ± 0.5‰ (Hasaga): the restricted individual ranges of δ13C values imply a corresponding uniformity to the ambient temperature and δ13CΣC of the ore-forming fluids.Within individual deposits, small systematic variations of δ13C carbonate arise from (i) interaction of hydrothermal fluids with carbonaceous rocks, (ii) immiscible separation of CO2 + CH4, or (iii) Rayleigh fractionation effects. Positive shifts in δ13C result from buffering of the fluid to lower Eh by reaction with reduced C, whereas negative shifts reflect partial isotopic equilibration between 13C-depleted C (δ13C ≈ −26‰) and aqueous hydrothermal C species. Transient immiscibility of CO2 + CH4 acts to precipitate carbonates enriched relative to the main population of Fe dolomites. The δ13C values of carbonates in unmineralized alteration halos (−2.2 ± 1.1‰, n = 42) at the McIntyre deposit are enriched in 13C relative to the main gold-bearing vein systems (δ13C = −3.2 ± 0.3‰): the enrichment is attributed to a Rayleigh fractionation accompanying progressive consumption of CO2 as hydrothermal fluids infiltrate laterally from veins into wall rocks. Fe dolomite and calcite are variably enriched in 18O with respect to equilibrium quartz-carbonate fractionations for ambient temperatures of 270–340 °C. Carbonate δ18O values diminish in an irregular manner with depth, converging on values of ~11‰ (Fe dolomite, 6800 ft (2073 m), McIntyre). Variable degrees of oxygen-isotope disequilibrium represent overprinting of carbonates by post-Archean brines in the Canadian Shield.Synvolcanic vesicle calcite in three groups of metabasalts (δ13C = −4.3 ± 2.1; −2.8 ± 1.5; −2.7 ± 1.3‰) and calcite in two groups of clastic sediments (−6.4 ± 1.8; −4.6 ± 2.5‰) remote from deposits are systematically depleted of 13C relative to average Precambrian limestones (~0 ± 1‰), owing to the involvement of CO2 derived from 13C-depleted organic matter. Consequently, calcite in greenstone belt supracrustal rocks is not restricted to approximately 0‰. The total spread of average δ13CFe dol values (−9.0 ± 0.7 to −0.6 ± 0.6‰) in the Au deposits, which goes in hand with a geographical provinciality in O-, Sr-, and Pb-isotope compositions of the ore-forming fluids, is too large to be accounted for by mantle CO2 (−6 ± 2‰) or magmatic CO2 (−6 ± 2‰) alone but rather is interpreted as reflecting generation of hydrothermal fluids in crustal or subcreted rocks heterogeneous in terms of the distribution of 13C-enriched (carbonate) and 13C-depleted (reduced C) lithologies.

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