Geochemical tracers of processes affecting the formation of seafloor hydrothermal fluids and deposits in the Manus back-arc basin

Craddock, Paul R.

doi:10.1575/1912/2609

Cited by 14 publications

(30 citation statements)

References 77 publications

(210 reference statements)

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“…As a result of investigations of putative abiogenic C 1 -C 4 n-alkanes recovered from saline fracture waters in crystalline basement rocks of the Canadian Shield, SHERWOOD LOLLAR et al (2002) proposed a characteristic H isotope trend with carbon number whereby δ 2 H meth values (often below -400‰) are extremely low relative to the C 2 -C 4 alkanes (δ 2 H ~ -220 to -320‰), which show a trend of increasing δ 2 H with increasing carbon number (Figure 4.8). This trend has subsequently been observed in several other Precambrian shield gases (SHERWOOD LOLLAR et al, 2006, 2008 and is postulated to reflect preferential cleavage of 12 C-1 H bonds relative to 12 C-2 H bonds during the abiogenic polymerization process, putative kinetic effects associated with 13 C/ 12 C fractionation during polymerization.…”

Section: H / 1 H Ratios and Abiogenesis In Igneous Environmentsmentioning

confidence: 52%

“…(Trefry et al, 1994;Craddock, 2008). The analytical uncertainty (2s) was ±5% for Mg, Na, Cl, Ca, K, SO 4 , Br, and F concentrations, ±10% for Sr, Li, Rb, Cs, Fe, Mn and Al concentrations and ±2% for SiO 2 and B concentrations.…”

Section: Methodsmentioning

confidence: 99%

“…The trends presented by SHERWOOD LOLLAR et al (2002, 2006, 2008 have not been observed in purported abiogenic hydrocarbons from higher temperature settings (Figure 4.8), such as those present in vent fluids of the Lost City hydrothermal site or those formed in abiotic synthesis experiments (FU et al, 2007), leading to speculation that this reflects mechanistic differences. Based on the experimental observations of reversible exchange presented here, this discrepancy may actually reflect partial or complete alkane-water H isotope exchange in the latter cases.…”

Section: H / 1 H Ratios and Abiogenesis In Igneous Environmentsmentioning

confidence: 98%

“…Our understanding of key geochemical parameters such as fluid redox and pH, and the mobility and behavior of metals in hydrothermal solutions has also advanced (e.g. SEYFRIED, 1987;SEEWALD and SEYFRIED, 1990;SEYFRIED and DING, 1995;TIVEY et al, 1995;ALLEN and SEYFRIED, 2003;CRADDOCK, 2008;KLEIN et al, 2009;MCCOLLOM and BACH, 2009). While these inorganic aspects of vent fluid geochemistry are reasonably well constrained, what is less well understood is the formation and transformation of carbon species in hydrothermal solutions, especially in ultramafic geologic settings where highly reducing fluids contain high concentrations of CH 4 and other hydrocarbons of purported non-biological (abiotic)…”

Section: Introductionmentioning

confidence: 99%

“…Our understanding of key geochemical parameters such as fluid redox and pH, and the mobility and behavior of metals in hydrothermal solutions has also advanced (e.g. SEYFRIED, 1987;SEEWALD and SEYFRIED, 1990;SEYFRIED and DING, 1995;TIVEY et al, 1995;ALLEN and SEYFRIED, 2003;CRADDOCK, 2008; KLEIN et al, 2009; MCCOLLOM and BACH, 2009 origin (HOLM and CHARLOU, 2001; KONN et al, 2009). In addition, though water-rock interactions in backarc environments may broadly resemble those of basalt-hosted systems in some respects , our understanding of the role of acidic magma-derived solutions in influencing hydrothermal fluid chemistry in these environments is currently limited by a lack of comprehensive geochemical investigations (GAMO et al, 2006;.…”

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confidence: 99%

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Laboratory and field-based investigations of subsurface geochemical processes in seafloor hydrothermal systems

Reeves¹

2010

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This thesis presents the results of four discrete investigations into processes governing the organic and inorganic chemical composition of seafloor hydrothermal fluids in a variety of geologic settings. Though Chapters 2 through 5 of this thesis are disparate in focus, each represents a novel investigation aimed at furthering our understanding of subsurface geochemical processes affecting hydrothermal fluid compositions. Chapters 2 and 3 concern the abiotic (nonbiological) formation of organic compounds in high temperature vent fluids, a process which has direct implications for the emergence of life in early Earth settings and sustainment of present day microbial populations in hydrothermal environments. Chapter 2 represents an experimental investigation of methane (CH 4 ) formation under hydrothermal conditions. The overall reduction of carbon dioxide (CO 2 ) to CH 4 , previously assumed to be kinetically inhibited in the absence of mineral catalysts, is shown to proceed on timescales pertinent to crustal residence times of hydrothermal fluids. In Chapter 3, the abundance of methanethiol (CH 3 SH), considered to be a crucial precursor for the emergence of primitive chemoautotrophic life, is characterized in vent fluids from ultramafic-, basalt-and sediment-hosted hydrothermal systems. Previous assumptions that CH 3 SH forms by reduction of CO 2 are not supported by the observed distribution in natural systems. Chapter 4 investigates factors regulating the hydrogen isotope composition of hydrocarbons under hydrothermal conditions. Isotopic exchange between low molecular weight n-alkanes and water is shown to be facilitated by metastable equilibrium reactions between alkanes and their corresponding alkenes, which are feasible in natural systems. In Chapter 5, the controls on vent fluid composition in a backarc hydrothermal system are investigated. A comprehensive survey of the inorganic geochemistry of fluids from sites of hydrothermal activity in the eastern Manus Basin indicates that fluids there are influenced by input of acidic magmatic solutions at depth, and subsequently modified by variable extents of seawater entrainment and mixing-related secondary acidity production. CHAPTER 1 IntroductionSeafloor hydrothermal systems associated with the generation of oceanic crust represent a dramatic expression of heat and mass transfer from the interior of the Earth. In addition to profound effects on ocean chemistry, convective circulation of seawater through volcanically active oceanic crust leads to the formation of metal sulfide deposits. In addition, seafloor hot springs are often invoked as ideal settings from which early microbial life could have emerged.Since the initial discovery of low temperature venting at the Galápagos Spreading Center in 1977 (CORLISS et al., 1979), our understanding of the composition and diversity of hydrothermal solutions has expanded greatly. During convection through oceanic crust of basaltic composition, O 2 -replete seawater reacts with crustal rock, typically producin...

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Section: H / 1 H Ratios and Abiogenesis In Igneous Environmentsmentioning

confidence: 52%

Section: Methodsmentioning

confidence: 99%

Section: H / 1 H Ratios and Abiogenesis In Igneous Environmentsmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

“…Our understanding of key geochemical parameters such as fluid redox and pH, and the mobility and behavior of metals in hydrothermal solutions has also advanced (e.g. SEYFRIED, 1987;SEEWALD and SEYFRIED, 1990;SEYFRIED and DING, 1995;TIVEY et al, 1995;ALLEN and SEYFRIED, 2003;CRADDOCK, 2008; KLEIN et al, 2009; MCCOLLOM and BACH, 2009 origin (HOLM and CHARLOU, 2001; KONN et al, 2009). In addition, though water-rock interactions in backarc environments may broadly resemble those of basalt-hosted systems in some respects , our understanding of the role of acidic magma-derived solutions in influencing hydrothermal fluid chemistry in these environments is currently limited by a lack of comprehensive geochemical investigations (GAMO et al, 2006;.…”

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confidence: 99%

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Laboratory and field-based investigations of subsurface geochemical processes in seafloor hydrothermal systems

Reeves¹

2010

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Divining gold in seafloor polymetallic massive sulfide systems

2019

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Hydrothermal fluids on the modern seafloor are important carriers of base and precious metals in a wide range of volcanic and tectonic settings. The concentrations and distribution, especially of gold and silver, in associated seafloor massive sulfide (SMS) deposits are strongly influenced by variable source rocks, fluid chemistry, and precipitation mechanisms. Compositional data of 130 SMS deposits around the world show a large range of gold and silver grades, in part reflecting strong buffering of the hydrothermal fluids by their host rocks. Geochemical reaction-path modeling shows that in most cases the investigated hydrothermal fluids are undersaturated with gold and silver, and solubilities can be orders of magnitude higher than the Au and Ag concentrations measured in the corresponding fluids. Precipitation of gold during conductive cooling of mid-ocean ridge black smoker (MOR) fluids occurs at low temperatures but can be very rapid, with > 90% of the gold deposited in the first 25°C of cooling below~150°C. The result is a Zn-Au polymetallic assemblage with Au and Ag deposited at the same time together with Pb and sulfosalts. In ultramafic-dominated (UM) systems, the strongly reduced hydrothermal fluids promote the deposition of gold at higher temperatures and explain the correlation between gold and copper in these deposits. In this case, the lower stability of the AuHS°complex at low ƒO 2 (buffered by fayalite, magnetite, and quartz) results in gold deposition at > 250°C with early bornite and chalcopyrite and before sphalerite and silver, producing a high-temperature Cu-Au assemblage. In sediment-hosted (SED) systems, the much higher pH stabilizes Au(HS) 2 − and keeps gold in solution to very low temperatures, after the precipitation of chalcopyrite, sphalerite, and galena, resulting in Au-poor polymetallic sulfides and very late-stage deposition of gold, commonly with amorphous silica. In arc-related (ARC) systems, gold deposition occurs at somewhat higher temperatures than in the MOR case, in part because the fluids start with higher gold concentrations. This can be explained by probable direct magmatic contributions, and the high ƒO 2 of the fluids, which promotes the solubility of gold at the source. During cooling, gold precipitates at about 160°C with sphalerite, tennantite, silver, and galena, resulting in an Au-rich polymetallic sulfide assemblage. The mixing of hydrothermal fluids with seawater generally causes oxidation and eventually a decrease in the pH at a mixing ratio of 1:1, causing an initial increase in the solubility of gold and silver. This can delay gold deposition from aqueous species to very low temperatures. These complex systematics make prediction of Au and Ag grades difficult. However, important new data are coming to light on the actual concentrations of the precious metals in hydrothermal fluids. In particular, the input of magmatic volatiles and leaching of pre-existing gold can lead to significant increases in the Au and Ag concentrations of the venting fluids and earlier d...

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Enargite-luzonite hydrothermal vents in Manus Back-Arc Basin: submarine analogues of high-sulfidation epithermal mineralization

et al. 2016

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Active and inactive hydrothermal chimneys composed almost entirely of enargite and luzonite, rare minerals in seafloor hydrothermal deposits, were found at the summits of two submarine volcanoes,North Su and Kaia Natai, in the Manus Back-Arc Basin. Detailed mineralogical and geochemical studies revealed that most probably these deposits precipitated at T = 200°–330 °C and high fS2. The negative δ34S values (−8.58 to −3.70‰) of the enargite-luzonite are best explained by disproportionation reactions of magmatic SO2 and suggest that the high fS2 is likely provided by direct magmatic input of SO2 into the hydrothermal system. Fractionation of Cu stable isotopes during the precipitation of enargite-luzonite (δ65Cu ranges from −0.20 to +0.35‰) is inferredtobe associatedwith either Rayleigh-type fractionation, or redox processes (Cu+ oxidation to Cu2+) and the mass balance of dissolved Cu+ and Cu2+ species in the hydrothermal fluid. The trace element composition of enargite and luzonite indicates a temporal fluctuation of the chemistry of the ore-forming fluid with an increase of Fe, Ga, Tl, Au, Hg, Pb and Ag, and decrease of Sb, Sn, Te, Ge and V concentrations with time and points out that this type of deposits is the richest in Au (average 11.9 ppm) and Te (average 169 ppm) among all other types of seafloor metal deposits. In addition to the widespread inorganic precipitation of enargite and luzonite in this setting, there is evidence that this mineralization may be biogenically mediated on the external surfaces of the active vents. Fungi-like filaments mineralized by luzonite imply that the fungi (Dikarya subkingdom) may be implicated in a mechanismof bio-sequestration of As, S and Cu, and provide the initial substrate for luzonite precipitation. The studied enargite-luzonite deposits have characteristics similar to those of subaerial high-sulfidation epithermal mineralization: back-arc basin setting; acid-sulfate and boiling ore-forming fluids; altered (advanced argillic stage) dacitic host rocks; major enargite-luzonite and minor pyrite, barite and S0; δ34S b 0‰. Therefore, they may be considered as submarine analogues of subaerial high-sulfidation epithermal deposits with the potential for concealed porphyry Cu(-Au) mineralization at depth

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Geochemical tracers of processes affecting the formation of seafloor hydrothermal fluids and deposits in the Manus back-arc basin

Cited by 14 publications

References 77 publications

Laboratory and field-based investigations of subsurface geochemical processes in seafloor hydrothermal systems

Laboratory and field-based investigations of subsurface geochemical processes in seafloor hydrothermal systems

Divining gold in seafloor polymetallic massive sulfide systems

Enargite-luzonite hydrothermal vents in Manus Back-Arc Basin: submarine analogues of high-sulfidation epithermal mineralization

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