Chemistry of submarine hydrothermal solutions at Guaymas Basin,Gulf of California

Reconnaisance ALVIN dives in the sediment-filled southern trough of the Guaymas Basin found active hot springs with temperatures ranging up to 315°C. High temperature activity is generally restricted to the crests of large mounds that rise out of the flat-lying basin sediments. The chemistry of the hydrothermal waters is distinctly different from that characteristic of sediment-starved, open ocean ridge axes in that the solutions are alkaline, contain ammonium as a major ion and are strongly depleted in the “ore-forming” metals. These compositions are interpreted as the result of reaction of a primary solution, similar in composition to those as 21°N, EPR, with the biogenous sediments overlying the intrusion zone. The pH of this fluid is raised both by the dissolution of carbonate and the addition of ammonium from thermocatalytic cracking of immature planktonic carbon. Metal sulfides are consequently precipitated at depth in the sediment column. The Guaymas Basin is thus the site of active formation of a sediment-hosted massive sulfide mineral deposit; the exiting waters are the “spent” ore-forming fluid. The ammonium data demonstrate that organic carbon (black shale) is, by itself, a sufficient source of alkalinity to induce the precipitation of sulfides from ascending solutions. Since ammonium does not participate directly in these reactions but does form secondary aluminosilicate minerals these latter should constitute a valuable exploration tool in the search for shale hosted deposits.     

Partition geochemistry of hydrothermal precipitates from submarine hydrothermal fields in the Hellenic Volcanic Island Arc

The geochemical study of 5 sediment cores obtained from different shallow hydrothermal fields along the volcanic arc (Methana, Milos, Kos and Yali), revealed that the degree of rock hydrothermal alteration from one area to another is different and is influenced by the physical and geochemical properties of the hydrothermal venting fluids and the type of the rocks in the substrate. The submarine hydrothermal fields in the central Aegean Sea are linked with the Hellenic Volcanic Arc and imprint their hydrothermal influence on the local marine sediments. Hydrothermal venting fluids differ in pH, temperature, gas and metallic element content, intensity of gas and water flux, while rock substrate is variable in terms of thickness and chemistry of the marine sediments and the mineral deposits. The analytical results showed that the lowest values of Fe are observed in Palaeochori Bay (0.72%) and the highest values are found in Bros Thermi (2.72%). The highest Mn concentrations are found in Bros Thermi (407 ppm) and the lowest are found in Yali (29 ppm). Cu and Pb highest concentrations are found in Bros Thermi (21 ppm) and Thiafi Bay (16 ppm), and the lowest in Yali (1 ppm). Zn highest values are found in Bros Thermi (56 ppm) and the lowest values in Kephalos Bay (10 ppm). Finally, the Ca and Mg-richest layers are observed in Kephalos 7.5 and 0.98% respectively and the lowest are observed in Milos (0.01% for Ca) and Yali (0.12% for Mg). The hydrothermal activity presented variations with geological time and hydrothermal suspended particulate matter coming out from the vent outlets also influence the sediment geochemistry. As an example in Methana, high concentrations of Cu and Zn in SPM result in high levels of Cu and Zn in the sediments.     

Chemistry of submarine hydrothermal solutions at 21 °N,East Pacific Rise

The three hydrothermal fields at 21°N latitude, East Pacific Rise, were resampled and an additional one was discovered. Maximum fluid temperatures observed were within a few degrees of 350°C and these waters had concentrations of Mg and sulfate indistinguishable from zero. One field, NGS, which had active 350°C springs in 1979, was inactive when first located in 1981. However, when a chimney was broken open during sampling, water issued at 273°C and continued to flow for at least five days. The chemical composition strongly suggests that these waters cooled conductively from 350°C in the sealed conduit.The major ion data are consistent with the estimates based on extrapolation of the original measurements made on the hot springs from the Galapagos Spreading Center (Edmondet al., 1979a). The fluids have a pH of 3.5 and the sulfide-forming element concentrations show significant inter-field variations. Fe levels range from 0.8 to 2.4 mmoles/kg; the ratio Fe:Mn varies from 0.9 to 2.9 similar to metalliferous sediments on the ridge flanks, but much higher than observed at Galapagos (where sub-surface precipitation of iron sulfides occurs) indicating that the overwhelming proportion of the mass flux from hydrothermal systems occurs at high temperatures. Zn ranges from 40 to 106 μmoles/kg with Cu being substantially lower. Since the ratio of these elements in tholeiites is about unity, there is strong net preferential mobilization of Zn. Lead ranges from 183 to 359 nmoles/kg. Nickel and Be are highly immobile relative to the other trace elements. The abundance of H₂S is about three times that of the total sulfide-forming cations. These data demonstrate that acid solutions at elevated temperatures can transport substantial amounts of ore-forming elements in the presence of large excesses of sulfide.     

现代海底热液区重晶石的地球化学特征及其影响因素

重晶石是海底热液活动产物之一,其化学元素及Sr、S、O同位素组成可以有效地揭示热液流体的演化过程。本文通过分析研究全球23个热液区中重晶石的化学元素和Sr、S、O同位素组成,探讨了热液区重晶石的元素富集特征,以及沉积物、岩浆、微生物对重晶石的影响因素。结果表明：高温比低温使Sr更容易进入重晶石晶格,重晶石晶型和元素组成特征指示了不同的成矿条件;板状重晶石具有较高的SrO/BaO比值,反映了较高温的成矿条件;柱状重晶石具有较低的SrO/BaO比值,反映了较低温的成矿条件。研究推测Ca类质同象替代Ba的程度可能也受温度的影响。重晶石中的⁸⁷Sr/⁸⁶Sr、δ³⁴S、δ¹⁸O值变化范围较大,分别为0.703 73~0.717 49、3.7‰~36.1‰、3.3‰~18.8‰,表明不同热液区成矿流体受沉积物、岩浆作用、微生物作用等多种因素的影响。受沉积物来源影响的Sr、S使重晶石的⁸⁷Sr/⁸⁶Sr值及δ³⁴S值偏高,温度和热液—沉积物反应程度是主要控制因素;受岩浆来源的SO₂的歧化作用和后期H₂S的氧化作用影响重晶石的δ³⁴S值,扩张中心的演化程度是岩浆影响重晶石δ³⁴S值的主要控制因素;微生物还原作用伴随着S、O同位素分馏,直接影响重晶石的δ³⁴S和δ¹⁸O值,沉积环境条件和初级生产力是主要控制因素。

     

Thermodynamic model of ore-forming processes in a submarine island-arc hydrothermal system

A thermodynamic model suggested for ore-forming processes in a hydrothermal system (HS) in an island arc is based on the technique suggested earlier in [1] for simulating ore-forming hydrothermal systems in mid-oceanic ridges. This technique make use of the principle of flow-through multistep reactor and encompasses (a) the region where hydrothermal solutions are generated when seawater interacts with rocks (descending convection branch); (b) the region where material is transported with the solution at decreasing pressure (feeder channel); and (c) the region where the ore material is deposited (orebody). Hydrothermal systems in island arcs exhibit the following distinctive features taken into account in the model: (1) the composition of the host crustal rocks (rocks of mafic-acid composition instead of basalt and serpentinite) and (2) possible significant involvement of magmatic gases in the feeding of the hydrothermal system. The naturally occurring prototype of the simulated system is the hydrothermal system in the caldera of a submarine volcano in an island arc. The model is simulated in a number of variants in which the hydrothermal fluid is exogenic (heated seawater convecting through hot volcanic rocks), magmatic, or mixed (magmatic plus exogenic) is involved. The simulations were carried out using the HCh version 4.3 [2] program package for the multisystem H-O-K-Na-Ca-Mg-Fe-Al-Si-C-S-Cl-Cu-Zn-Pb-As-Sb-Ag-Au at temperatures of 25?C370°C and pressures of 10?C500 bar. The multisystem included 88 possible solid phases and aqueous solution with 95 species. The thermodynamic properties of compounds were calculated using the UNITHERM databank. The model is underlain by the principle of multiwave flow-through multistep reactor (MFTMR) with a starting rock/water (R/W) ratio of 1: 1. As progressively more solution portions passed through the rocks, the participation of fresh rock in the interaction accordingly diminished because the rock material was gradually exhausted in the system. The magmatic fluid had a composition selected based on data on fumaroles at Kudryavyi volcano [3] with a correction for the degassing pressure. The evolution of ore deposition was simulated in compliance with the scheme described in [4], which was implemented using the technology of ??openness from above?? [3]. The model was simulated with various compositions of the host rocks (basalts, andesites, dacites, and rhyolites) and the origin of the fluid (magmatic fluid alone, seawater alone, and variable proportions of both). Our simulation results indicate that the metallogeny (relative enrichment in Pb, As, Sb, or Ag) of island-arc ore deposits is controlled by the abundances of metals in the host rocks predominant in the hydrothermal system. The mineralogy and geochemistry of ores generated in arc hydrothermal systems are predetermined by the effective transport of metalloids (S, As, and Sb) that have a high migration capacity in these systems. Magmatic gases introduced in the hydrothermal systems play dualistic roles in the ore-forming processes. If the hydrothermal fluid in a hydrothermal system is dominated by magmatic components, deposits of native sulfur are formed, and the precipitation of base metal is thereby suppressed because of the high acidity of the generated hydrothermal solutions. The involvement of magmatic gases in an amount of a few percent in a hydrothermal system enhances the overall oregenerating potential of the system in terms of sulfide ores.     

Hydrothermal systems and the emergence of life

The author reviews current thought about life originating in hyperthermophilic microorganisms. Hyperthermophiles obtain food from chemosynthesis of sulfur and have an RNA nucleotide sequence different from bacteria and eucarya. It is postulated that a hyperthermophile may be the common ancestor of all life. Current research efforts focus on the synthesis of organic compounds in hydrothermal systems.     

The role of phosphorus in rhyolitic liquids as determined from the homogeneous iron redox equilibrium

The redox ratio of iron is used as an indicator of solution properties of silicate liquids in the system (SiO–Al₂O₃–K₂O–FeO–Fe₂O₃–P₂O₅). Glasses containing 80–85 mol% SiO₂ with 1 mol% Fe₂O₃ and compositions covering a range of K₂O/Al₂O₃ were synthesized at 1400°C in air (fixed fO₂). Variations in the ratio FeO/FeO_1.5 resulting from the addition of P₂O₅ are used to determine the solution behavior of phosphorus and its interactions with other cations in the silicate melt. In 80 mol% SiO₂ peralkaline melts the redox ratio, expressed as FeO/FeO_1.5, is unchanged relative to the reference curve with the addition of 3 mol% P₂O₅. Yet, the iron redox ratio in the 85 mol% SiO₂ potassium aluminosilicate melts is decreased relative to phosphorus-free liquids even for small amounts of P₂O₅ (0.5 mol%). The redox ratio in peraluminous melts is decreased relative to phosphorus- free liquids at P₂O₅ concentrations of 3 mol%. In peraluminous liquids, complexing of both Fe⁺³–O–P⁺⁵ and Al⁺³–O–P⁺⁵ occur. The activity coefficient of Fe⁺³ is decreased because more ferric iron can be accommodated than in phosphorus-free liquids. In peralkaline melts, there is no evidence that P⁺⁵ is removing K⁺ from either Al⁺³ or Fe⁺³ species. In chargebalanced melts with 3 mol% Fe₂O₃ and very high P₂O₅ concentrations, phosphorus removes K⁺ from K–O–Fe⁺³ complexes resulting in a redox increase. P₂O₅ should be accommodated easily in peraluminous rhyolitic liquids and phosphate saturation may be suppressed relative to metaluminous rhyolites. In peralkaline melts, phosphate solubility may increase as a result of phosphorus complexing with alkalis. The complexing stoichiometry may be variable, however, and the relative influence of peralkalinity versus temperature on phosphate solubility in rhyolitic melts deserves greater attention.     

First experimental determination of iron isotope fractionation between hematite and aqueous solution at hydrothermal conditions

Although iron isotopes provide a new powerful tool for tracing a variety of geochemical processes, the unambiguous interpretation of iron isotope ratios in natural systems and the development of predictive theoretical models require accurate data on equilibrium isotope fractionation between fluids and minerals. We investigated Fe isotope fractionation between hematite (Fe₂O₃) and aqueous acidic NaCl fluids via hematite dissolution and precipitation experiments at temperatures from 200 to 450 °C and pressures from saturated vapor pressure (P_sat) to 600 bar. Precipitation experiments at 200 °C and P_sat from aqueous solution, in which Fe aqueous speciation is dominated by ferric iron (Fe^III) chloride complexes, show no detectable Fe isotope fractionation between hematite and fluid, Δ⁵⁷Fe_{fluid-hematite} = δ⁵⁷Fe_fluid − δ⁵⁷Fe_hematite = 0.01 ± 0.08‰ (2 × standard error, 2SE). In contrast, experiments at 300 °C and P_sat, where ferrous iron chloride species (FeCl₂ and FeCl⁺) dominate in the fluid, yield significant fluid enrichment in the light isotope, with identical values of Δ⁵⁷Fe_{fluid-hematite} = −0.54 ± 0.15‰ (2SE) both for dissolution and precipitation runs. Hematite dissolution experiments at 450 °C and 600 bar, in which Fe speciation is also dominated by ferrous chloride species, yield Δ⁵⁷Fe_{fluid-hematite} values close to zero within errors, 0.15 ± 0.17‰ (2SE). In most experiments, chemical, redox, and isotopic equilibrium was attained, as shown by constancy over time of total dissolved Fe concentrations, aqueous Fe^II and Fe^III fractions, and Fe isotope ratios in solution, and identical Δ⁵⁷Fe values from dissolution and precipitation runs. Our measured equilibrium Δ⁵⁷Fe_{fluid-hematite} values at different temperatures, fluid compositions and iron redox state are within the range of fractionations in the system fluid-hematite estimated using reported theoretical β-factors for hematite and aqueous Fe species and the distribution of Fe aqueous complexes in solution. These theoretical predictions are however affected by large discrepancies among different studies, typically ±1‰ for the Δ⁵⁷Fe _{Fe(aq)-hematite} value at 200 °C. Our data may thus help to refine theoretical models for β-factors of aqueous iron species. This study provides the first experimental calibration of Fe isotope fractionation in the system hematite-saline aqueous fluid at elevated temperatures; it demonstrates the importance of redox control on Fe isotope fractionation at hydrothermal conditions.     

The emergence of Metazoa in the early history of life

After consideration of currently used time scales and their geochronometric calibration, the views of biologists on evolutionary events in the early history of the Metazoa are discussed, mainly on the basis of two examples from the abundant literature. The phylogenetic views of A.V. Ivanov on the origin and early evolution of Metazoa to the level of sponges, Cnidaria and Platyhelminthes, are outlined. A brief discussion of the subsequent coelomate radiation, based on the work of R.B. Clark follows. It is proposed that these evolutionary events, postulated by biologists, occurred in the time span of about 300 Ma preceding the appearance of large Metazoan body fossils in Ediacarian time, about 600-560 Ma ago. Palaeontological data from this pre-Ediacarian, Late Proterozoic time span are limited to few, mostly small trace fossils. The reasons for this paucity and the consequent need to give new dimensions to trace fossil studies are discussed. It is concluded that there is evidence for a pre-Phanerozoic ‘prehistory’ of the Metazoa. A full explanation of the events and processes leading to their diversification and of its chronologic sequence will only be possible when all interacting systems, including the lithosphere, hydrosphere and atmosphere, relevant to their evolution during the appropriate time interval have been more completely elucidated.     

Rates of reaction of pyrite and marcasite with ferric iron at pH 2

The relative reactivities of pulverized samples (100–200 mesh) of 3 marcasite and 7 pyrite specimens from various sources were determined at 25°C and pH 2.0 in ferric chloride solutions with initial ferric iron concentrations of 10^?3 molal. The rate of the reaction:

$\text{FeS}{}_2\text{+ 14}\text{Fe}{}^{\mathrm{3+}}\text{+ 8}\text{H}{}_2\text{O}\text{= 15}\text{Fe}{}^{\mathrm{2+}}\text{+ 2}\text{SO}{}^{\mathrm{2?}}{}_4\text{+ 16}\text{H}{}^{\mathrm{+}}$

was determined by calculating the rate of reduction of aqueous ferric ion from measured oxidation-reduction potentials. The reaction follows the rate law:

where m_Fe³⁺ is the molal concentration of uncomplexed ferric iron, k is the rate constant and $\text{A}\text{M}$ is the surface area of reacting solid to mass of solution ratio. The measured rate constants, k, range from 1.0 × 10^?4 to 2.7 × 10^?4 sec^?1 ± 5%, with lower-temperature/early diagenetic pyrite having the smallest rate constants, marcasite intermediate, and pyrite of higher-temperature hydrothermal and metamorphic origin having the greatest rate constants. Geologically, these small relative differences between the rate constants are not significant, so the fundamental reactivities of marcasite and pyrite are not appreciably different.The activation energy of the reaction for a hydrothermal pyrite in the temperature interval of 25 to 50°C is 92 kJ mol^?1. This relatively high activation energy indicates that a surface reaction controls the rate over this temperature range. The BET-measured specific surface area for lower-temperature/early diagenetic pyrite is an order of magnitude greater than that for pyrite of higher-temperature origin. Consequently, since the lower-temperature types have a much greater $\text{A}\text{M}$ ratio, they appear to be more reactive per unit mass than the higher temperature types.     

Geochemistry of hydrothermal solutions from 9°50′ N at the East Pacific Rise (EPR) within twelve years after the eruption of a submarine volcano

Hydrothermal solutions were examined in a circulation system that started to develop after the 1991 volcanic eruption in the axial EPR segment between 9°45′ N and 9°52′ N. Within the twelve years elapsed after the eruption, the diffusion outflow of hot fluid from fractures in basaltic lavas gave way to focused seeps of hot solutions through channels of hydrothermal sulfide edifices. The example of field Q demonstrates that the concentrations of H₂S decreased from 86 to 1 mM/kg from 1991 to 2003, and the Fe/H₂S ratio simultaneously increased by a factor of 1.7, a fact that can explain the disappearance of the microbial mats, which were widespread at the fields before 1991. The S isotopic composition of H₂S is independent of the H₂S concentration, a fact testifying to the rapid evolution of the hydrothermal system in the early years of its evolution. Carbon in CH₄ from the hot fluid sampled in 2003 is richer in the light ¹²C isotope than carbon in the fluid from the hydrothermal field at 21° N in EPR, which suggests that methane comes to field Q from more than one source. The composition of particulate matter in the hydrothermal solutions indicates that it was contributed by biological material. Experimental solutions with labeled substrates (t < 70°C) show evidence of the active processes of methane oxidation and sulfate reduction. Our results indicate that, during the 12 years of the evolution of the hydrothermal system, the composition of its solutions evolved and approached the compositions of solutions in mature hydrothermal systems in EPR.     

Modelling colloid association with plutonium: The effect of pH and redox potential

An analytical expression that evaluates the effect of pH and the redox potential (E) on Pu-colloid association was studied on a model basis. It includes surface complexation with one type of surface site and its formulation leads to a distribution coefficient (K_d) as a function of the pH (hydrolysis) and E (redox sensitive species). The formulation also considers the values of the stability and hydrolysis constants for all species present in solution and associated at the surface. Correlations between hydrolysis and surface complexation constants reported in the literature have been applied systematically to evaluate sorption of all species for each colloid system. The presence of ligands in solution was also taken into account. The model was applied to study the association of Pu onto colloids coated with AlOH, FeOH or SiOH groups in the presence and in the absence of carbonates in solution. The tests performed with the model suggest that the oxidation of Pu(III) to Pu(IV) has the potential to increase sorption, as demonstrated by the increased K_d values. Under natural conditions Pu may be present at oxidation states of (III)--(VI), and the effect of redox potential is significant because of the differences in the sorption of each oxidation state. When carbonates are present in the solution, the calculated values of distribution coefficient were lower than those calculated in the absence of carbonates, particularly in the case of Pu(VI). The K_d values obtained with the developed model are compared with experimental values reported for the sorption of Pu onto colloids. This model can equally be applied to study the sorption of other redox sensitive elements.     

Comparative study of sampling methods and in situ and laboratory analysis for shallow-water submarine hydrothermal systems

A primary aim for sampling of submarine hydrothermal vents is to minimize inclusion of ambient seawater. Here, we compare the results of three different sampling methods (air displacement, two-valve bottle, and syringes) for shallow submarine systems. Mixing of hydrothermal fluid with seawater is unavoidable; however, calculations based on linear mixing models allow estimation of the hydrothermal fluid end-member composition. The results show that sampling with a two-valve bottle and syringes are the best options because they allow collection of samples with a large proportion of hydrothermal fluid. Additionally, we compare the results of in situ and laboratory analyses of the fluid samples, and demonstrate that determination of chemical composition in situ is the best option for some components, as re-equilibration affects some component concentrations (i.e. bicarbonate). Conversely, silica determination in situ usually underestimates the concentration in the fluid, as it does not account for polymeric silica. Other components can be measured either in the field or in the laboratory.     

海底热水成矿系统中的流体端员与混合过程：来自白银厂和呷村VMS矿床的流体包裹体证据

古今海底热液流体系统是人们关注的重要科学问题,正确识别流体系统中不同来源的流体端员及其混合-分离过程,是深刻理解海底流体系统及火山成因块状硫化物(VMS)矿床成因的关键。本文选择了我国境内两个典型的VMS矿床:甘肃白银厂矿床和四川呷村矿床,分别对上部块状矿带和下部脉状-网脉状矿带进行了系统的流体包裹体研究。研究表明,海底热液成矿流体系统是一个富集CO_2和CH_4的NaCl-H_2O流体系统。在此系统中,至少已鉴别出5种端员流体,即(1)低温 (12wt％NaCl)卤水;(2)高温(>320℃)高盐度(>14.5wt％NaCl)流体和(3)高温(>350℃)中盐度(10～16wt％Nacl)富气流体,以及(4)低温(～100℃)低盐度(2～5wt％NaCl)流体和(5)中温低盐度流体,它们构成了3个相互分离的温度-盐度演变趋势或混合途径。)低温高盐度卤水封存于卤水池中,在呷村矿床,卤水池发育在海底凹陷盆地热液区内,在白银厂矿床,卤水池则发育在海底之下的多孔火山碎屑岩单元及穿透性断裂破碎带内。高温高盐度流体和高温中盐度富气流体均来自矿区下部浅位岩浆房,前者以液态富金属H_2O流体为主,通常与冷海水发生混合;后者以富CO_2和CH_4的气体为主,高温(>450℃)下呈相对独立的气流存在,至到250～260℃才作为液相混入成矿热液流体中。低温低盐度     

The geometry and petrogenesis of hydrothermal dolomites at Navan, Ireland

The dolomites at Navan, Ireland, formed in Courceyan peritidal and shallow-shelf limestones. The dolomite body has a plume-like geometry, cross-cutting both lithological boundaries and diagenetic barriers generated by sea-floor cementation and emergence. The dolomitizing fluids rose parallel to major faults to diffuse laterally through the succession, controlled by variations in permeability that reflect both facies variation on deposition and pre-dolomitization diagenesis. Cathodoluminescent zones reveal three principal stages of dolomite emplacement, separated by dissolution surfaces, with each stage reflecting several changes in the character of dolomitizing solutions. The predominance of dull zones indicates burial rather than surface conditions. The dolomites formed some time after burial in response to an areally limited hydrothermally-driven flow. Isotope values (σ¹⁸O of — 6σ6 to — 10.4%_δ and σ¹³C of — 0σ2 to +2σ5%_δ PDB), and fluid inclusion data, suggest that these fluids had compositions similar to those of Carboniferous seawater. However, these became hotter with time, with temperature increasing from 60 to 160δC. The Navan dolomites are closely associated with Europe's largest zinc-lead deposit. The distribution of the ores follows the same trend as that of the dolomites and paragenetic relationships indicate that dolomitization and mineralization were temporally and genetically related.     

Accretion of Moon and Earth and the emergence of life.

The discrepancy between the impact records on the Earth and Moon in the time period, 4.0-3.5 Ga calls for a re-evaluation of the cause and localization of the late lunar bombardment. As one possible explanation, we propose that the time coverage in the ancient rock record is sufficiently fragmentary, so that the effects of giant, sterilizing impacts throughout the inner solar system, caused by marauding asteroids, could have escaped detection in terrestrial and Martian records. Alternatively, the lunar impact record may reflect collisions of the receding Moon with a series of small, original satellites of the Earth and their debris in the time period about 4.0-3.5 Ga. The effects on Earth of such encounters could have been comparatively small. The location of these tellurian moonlets has been estimated to have been in the region around 40 Earth radii. Calculations presented here, indicate that this is the region that the Moon would traverse at 4.0-3.5 Ga, when the heavy and declining lunar bombardment took place. The ultimate time limit for the emergence of life on Earth is determined by the effects of planetary accretion--existing models offer a variety of scenarios, ranging from low average surface temperature at slow accretion of the mantle, to complete melting of the planet followed by protracted cooling. The choice of accretion model affects the habitability of the planet by dictating the early evolution of the atmosphere and hydrosphere. Further exploration of the sedimentary record on Earth and Mars, and of the chemical composition of impact-generated ejecta on the Moon, may determine the choice between the different interpretations of the late lunar bombardment and cast additional light on the time and conditions for the emergence of life.     

PGE fractionation in seafloor hydrothermal systems: examples from mafic- and ultramafic-hosted hydrothermal fields at the slow-spreading Mid-Atlantic Ridge

The distribution of platinum group elements (PGEs) in massive sulfides and hematite–magnetite±pyrite assemblages from the recently discovered basalt-hosted Turtle Pits hydrothermal field and in massive sulfides from the ultramafic-hosted Logatchev vent field both on the Mid-Atlantic Ridge was studied and compared to that from selected ancient volcanic-hosted massive sulfide (VHMS) deposits. Cu-rich samples from black smoker chimneys of both vent fields are enriched in Pd and Rh (Pd up to 227 ppb and Rh up to 149 ppb) when compared to hematite–magnetite-rich samples from Turtle Pits (Pd up to 10 ppb, Rh up to 1.9 ppb). A significant positive correlation was established between Cu and Rh in sulfide samples from Turtle Pits. PGE chondrite-normalized patterns (with a positive Rh anomaly and Pd and Au enrichment), Pd/Pt and Pd/Au ratios close to global MORB, and high values of Pd/Ir and Pt/Ir ratios indicate mafic source rock and seawater involvement in the hydrothermal system at Turtle Pits. Similarly shaped PGE chondrite-normalized patterns and high values of Pd/Pt and Pd/Ir ratios in Cu-rich sulfides at Logatchev likely reflect a similar mechanism of PGE enrichment but with involvement of ultramafic source rocks.     

Nine requirements for the origin of Earth's life:Not at the hydrothermal vent,but in a nuclear geyser system

The origin of life on Earth remains enigmatic with diverse models and debates.Here we discuss essential requirements for the first emergence of life on our planet and propose the following nine requirements:(1)an energy source(ionizing radiation and thermal energy);(2)a supply of nutrients(P.K.REE.etc.);(3)a supply of life-constituting major elements;(4)a high concentration of reduced gases such as CH_4,HCN and NH_3;(5)dry-wet cycles to create membranes and polymerize RNA;(6)a non-toxic aqueous environment;(7)Na-poor water;(8)highly diversified environments,and(9)cyclic conditions,such as dayto-night,hot-to-cold etc.Based on these nine requirements,we evaluate previously proposed locations for the origin of Earth's life,including:(1)Darwin's "warm little pond",leading to a "prebiotic soup" for life;(2)panspermia or Neo-panspermia(succession model of panspermia);(3)transportation from/through Mars;(4)a deepsea hydrothermal system;(5)an on-land subduct ion-zone hot spring,and(6)a geyser systems driven by a natural nuclear reactor.We conclude that location(6)is the most ideal candidate for the o rigin point for Earth's life because of its efficiency in continuously supplying both the energy and the necessary materials for life,thereby maintaining the essential "cradle" for its initial development.We also emphasize that falsifiable working hypothesis provides an important tool to evaluate one of the biggest mysteries of the universe-the origin of life.