Добірка наукової літератури з теми "Arc volcano redox"

The Middle Jurassic A6 Anomaly is located 30 km southeast of Eskay Creek, north-central British Columbia and consists of thick, altered felsic igneous rocks overlain by a mafic volcano-sedimentary package. Lithogeochemistry on igneous rocks, X-ray diffraction on altered felsic units, and electron probe microanalysis and secondary ion mass spectrometry on illite and quartz were applied to explore the volcanogenic massive sulfide (VMS) potential, characterize alteration, and determine fluid conditions at the A6 Anomaly. Lithogeochemistry revealed calc-alkaline rhyodacite to trachyte of predominantly FII type, tholeiitic basalts with Nb/Yb < 1.6 (i.e., Group A), and transitional to calc-alkaline basalts and andesites with Nb/Yb > 2.2 (i.e., Group B). The felsic units showed weakly to moderately phyllic alteration (quartz–illite with minor orthoclase and trace chlorite–pyrite–calcite–barite–rutile). Illite ranged in composition from illite/smectite (K = 0.5–0.69 apfu) to almost endmember illite (K = 0.69–0.8 apfu), and formed from feldspar destruction by mildly acidic, relatively low temperature, oxidized hydrothermal fluids. The average δ18O composition was 10.7 ± 3.0‰ and 13.4 ± 1.3‰ relative to Vienna Standard Mean Ocean Water for illite and quartz, respectively. Geothermometry involving illite composition and oxygen isotope composition on illite and quartz yielded average fluid temperatures of predominantly 200–250 °C. Lithogeochemical results showed that the A6 Anomaly occurred in a late-Early to Middle Jurassic evolving back-arc basin, further east then previously recognized and in which transitional to calc-alkaline units formed by crustal assimilation to enriched Mid-Ocean Ridge Basalt (EMORB) (i.e., felsic units, Group B), followed by thinning of the crust resulting in tholeiitic normalized MORB basalts (i.e., Group A) with a minor crustal component. The alteration assemblage is representative of distal footwall alteration, and metal transport in this zone was limited despite favorable temperature, pH, and redox state, indicating a metal depleted source (i.e., felsic units).

ABSTRACT The Early Jurassic Polaris Alaskan-type intrusion in the Quesnel accreted arc terrane of the North American Cordillera is a zoned, mafic-ultramafic intrusive body that contains two main styles of magmatic mineralization of petrologic and potential economic significance: (1) chromitite-associated platinum group element (PGE) mineralization hosted by dunite (±wehrlite); and (2) sulfide-associated Cu-PGE-Au mineralization hosted by olivine (±magnetite) clinopyroxenite, hornblendite, and gabbro-diorite. Dunite-hosted PGE mineralization is spatially associated with thin discontinuous layers and schlieren of chromitite and chromitiferous dunite and is characterized by marked enrichments in iridium-subgroup PGE (IPGE) relative to palladium-subgroup PGE (PPGE). Discrete grains of platinum group minerals (PGM) are exceedingly rare, and the bulk of the PGE are inferred to reside in solid solution within chromite±olivine. The absence of Pt-Fe alloys in dunite of the Polaris intrusion is atypical, as Pt-enrichment of dunite-hosted chromitite is widely regarded as a characteristic feature of Alaskan-type intrusions. This discrepancy appears to be consistent with the strong positive dependence of Pt solubility on the oxidation state of sulfide-undersaturated magmas. Through comparison with experimentally determined PGE solubilities, we infer that the earliest (highest temperature) olivine-chromite cumulates of the Polaris intrusion crystallized from a strongly oxidized ultramafic parental magma with an estimated log f(O2) &gt; FMQ+2. Parental magmas with oxygen fugacities more typical of volcanic arc settings [log f(O2) ∼ FMQ to ∼ FMQ+2] are, in turn, considered more favorable for co-precipitation of Pt-Fe alloys with olivine and chromite. More evolved clinopyroxene- and hornblende-rich cumulates of the Polaris intrusion contain low abundances of disseminated magmatic sulfides, consisting of pyrrhotite and chalcopyrite with minor pentlandite, pyrite, and rare bornite (≤12 wt.% total sulfides), which occur interstitially or as polyphase inclusions in silicates and oxides. The sulfide-bearing rocks are characterized by strong primitive mantle-normalized depletions in IPGE and enrichments in Cu-PPGE-Au, patterns that resemble those of other Alaskan-type intrusions and primitive arc lavas. The absolute abundances and sulfur-normalized whole-rock concentrations (Ci/S, serving as proxy for sulfide metal tenor) of chalcophile elements, including Cu/S, in sulfide-bearing rocks are highest in olivine clinopyroxenite. Sulfide saturation in the relatively evolved magmas of the Polaris intrusion, and Alaskan-type intrusions in general, appears to be intimately tied to the appearance of magnetite. Fractional crystallization of magnetite during the formation of olivine clinopyroxenite at Polaris resulted in reduction of the residual magma to log f(O2) ≤ FMQ+2, leading to segregation of an immiscible sulfide melt with high Cu/Fe and Cu/S, and high PGE and Au tenors. Continued fractionation resulted in sulfide melts that were progressively more depleted in precious and base chalcophile metals. The two styles of PGE mineralization in the Polaris Alaskan-type intrusion are interpreted to reflect the evolution of strongly oxidized, hydrous ultramafic parental magma(s) through intrinsic magmatic fractionation processes that potentially promote sulfide saturation in the absence of wallrock assimilation.

Abstract. During the GWADASEIS cruise (Lesser Antilles volcanic arc, February–March 2009) a very high resolution (VHR) seismic-reflection survey was performed in order to constrain Late Quaternary to Present faulting. The profiles we obtained evidence frequent "ponding" of reworked sediments in the deepest areas, similar to the deposition of Mediterranean "homogenites". These bodies are acoustically transparent (few ms t.w.t. thick) and are often deposited on the hanging walls of dominantly normal faults, at the base of scarps. Their thickness appears sufficient to compensate (i.e. bury) co-seismic scarps between successive earthquakes, resulting in a flat and horizontal sea floor through time. In a selected area (offshore Montserrat and Nevis islands), piston coring (4 to 7 m long) was dedicated to a sedimentological analysis of the most recent of these particular layers. It corresponds to non-stratified homogenous calcareous silty sand (reworked calcareous plankton and minor volcanoclastics). This layer can be up to 2 m thick, and overlies fine-grained hemipelagites. The upper centimeters of the latter represent the normal RedOx water/sediment interface. 210Pb and 137Cs activities lack in the massive sands, while a normal profile of unsupported 210Pb decrease is observed in the hemipelagite below, together with a 137Cs peak corresponding to the Atmospheric Nuclear Experiments (1962). The RedOx level was thus capped by a recent instantaneous major sedimentary event considered as post-1970 AD; candidate seismic events to explain this sedimentary deposits are either the 16 March 1985 earthquake or the 8 October 1974 one (Mw = 6.3 and Mw = 7.4, respectively). This leads to consider that the syntectonic sedimentation in this area is not continuous but results from accumulation of thick homogenites deposited after the earthquakes (as observed in the following weeks after Haiti January 2010 event, McHugh et al., 2011). The existence of such deposits suggests that, in the area of study, vertical throw likely results from cumulated effects of separated earthquakes rather than from aseismic creep. Examination of VHR profiles shows that all major co-seismic offsets are recorded in the fault growth sequence and that co-seismic offsets can be precisely estimated. By using a sedimentation rate deduced from 210Pb decrease curve (0.5 mm yr−1) and taking into account minor reworking events detected in cores, we show that the Redonda system may have been responsible for five >M6 events during the last 34 000 yr. The approach presented in this work differs from fault activity analyses using displaced sets of isochronous surfaces and postulating co-seismic offsets. Combining VHR seismic imagery and coring we can decipher co-seismic vs. slow continuous displacement, and thus actually estimate the amplitude and the time distribution of major co-seismic offsets.

This thesis documents data from an active volcano (Sangeang Api) and previously unstudied, extinct volcanoes (Wai Sano and Doro Kota and Doro Kuta) from adjacent sectors of the eastern Sunda Arc, Indonesia. Sangeang Api erupts co-magmatic suites of lavas and cumulate xenoliths. Lavas are ne-normative, silica-undersaturated, volatile-rich, shoshonitic basalts to basaltic-trachyandesites. They have trace-element compositions typical of arc magmas; enrichment in LREE, alkali-earth elements and Sr, depletion in Nb, Ta, Zr and Ti and high U/Th. They are also enriched in fluid-mobile elements (Cl, Ba, Cs, etc.). The cumulate xenoliths are separated into two distinct groups; the cpx+mgt and cpx+ol pyroxenites and the cpx+mgt+plag±amph gabbros. These groups are compositionally distinct, with their chemistry reflecting their cumulate mineralogy and are shown to drive magma evolution in the system by fractional crystallisation. Oxidation state, water contents and pressures of crystallisation are the primary controls on the primary cumulate mineral assemblages of the xenoliths. Sangeang Api magmas degassed as they ascended through the crust, with degassing driving oxidation at depth and reduction more shallowly. Many of the cumulate xenoliths are variably contaminated by melts indicating percolation and incomplete compaction or post crystallisation intrusion. Fe-isotope studies of the Sangeang Api products shows that the lavas record δ⁵⁷Fe compositions (average = 0.099‰ ±0.051) typical of arc settings. Cumulate xenoliths record heavy Fe-isotope compositions compared to the lavas (mean δ⁵⁷Fegabbro = 0.166‰ ±0.051; mean δ⁵⁷Fepyroxenite = 0.109‰ ±0.066). Using published fractionation factors, it is shown that whilst magnetite mineral separates records equilibrium compositions (mean δ⁵⁷Fe = 0.142‰ ±0.072), Fe-Mg silicate mineral separates display significant disequilibrium in their compositions (mean δ⁵⁷Feoliv = -0.313‰ ±0.284, mean δ⁵⁷Feamph = 0.125‰ ±0.081 and δ⁵⁷Fecpx = 0.109‰ ±0.090). Iron isotope disequilibria highlights the pervasiveness of post-crystallisation contaminative processes. Lavas from the Quaternary (~2Ma) D. Kota and D. Kuta, E. Sumbawa, are geochemically and petrologically similar to those from nearby Sangeang Api. However, they are less potassic (high-K calc-alkaline) with smaller enrichments in Ba and Sr, higher Rb/Sr and lower U/Th, highlighting a role for residual amphibole in the mantle source. Wai Sano at the far western end of Flores, is unique in the Sunda Arc, erupting adakites with characteristically low MgO, high Al₂O₃, high SiO₂, low Y and high Sr/Y. However, the Sr-Nd isotope systematics of these lavas suggests that they cannot have been produced by slab melts as is often suggested for adakites. Trace element characteristics and thermodynamic modelling suggest that a possible source of these magmas is through partial melting (10-13%) of basaltic underplates. Ultrapotassic magmatism in the Sunda Arc is confined to the rear-arc and characterised by trace-element and isotopic evidence of mantle metasomatism and the influence of slab tear windows. Slab window development has changed the loci and composition of volcanism on Sumbawa. Thus, highlighting the effects of local tectonic setting on arc volcanoes.
Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Physical Sciences, 2018

The ancient Greek philosopher Empedocles defined our environments using the four basic elements of fire, earth, wind, and water. Although we now know there are at least 118 elements, of which 98 are naturally occurring, these ancient descriptions aptly describe the habitats on Earth that are occupied by oxides and living things. Many oxides that comprise Earth’s surface are born by the fire represented by the massive heat of Earth’s interior as mediated by plate tectonics. This heat produces the igneous rocks found in volcanoes and our major mountain chains. Water weathers these pristine rocks, which are gradually broken down to form earth, which includes the wide diversity of other rock types, soils, and sediments covering the surfaces of our continents and ocean floors. Weathered oxides in the form of dust are blown by wind and enter the atmosphere, where they influence the chemistry of the air we breathe and the rainfall that supports continental life. The chemical transformations of oxides are strongly influenced by all the environmental conditions they encounter in their life cycle (see Chapter 17). Conversely, the interactions between oxides, water, and organisms help define many of the environments that allow life on Earth to thrive. These interactions form the basis for this final chapter of our book. Oxides are present in all our planet’s major environments. In this chapter, we explore each of the environments defined by the ancient Greeks in descending order based on their distance from Earth’s core. The chapter progresses from the stratosphere (air) to continental surfaces (earth) to our oceans (water) and finally to the subsurface environments of subduction zones such as the Marianas Trench (fire). In each section, we highlight reactions involving the two most important classes of oxides in terms of their environmental impact, both of which are weathering products: (1) the clay minerals and (2) the redox-active colloids of iron and manganese oxides. Clay mineral reactions impact colloidal interactions (Chapter 8), ion exchange (Chapter 10), and the sequestration of environmental nutrients and contaminants. Reactions of the redox-active oxidates of iron and manganese are dominant in terms of reversible and often complex electrochemical (Chapter 11) and photochemical (Chapter 13) processes that take place in natural environments.

Sulfur (S) is an important redox element in estuaries because of its linkage with biogeochemical processes such as SO42− reduction (Howarth and Teal, 1979; Jørgensen, 1982; Luther et al., 1986; Roden and Tuttle, 1992, 1993a,b; Miley and Kiene, 2004), pyrite (FeS2) formation (Giblin, 1988; Hsieh and Yang, 1997; Morse and Wang, 1997), metal cycling (Krezel and Bal, 1999; Leal et al., 1999; Tang et al., 2000), ecosystem energetics (King et al., 1982; Howarth and Giblin, 1983; Howes et al., 1984), and atmospheric S emissions (Dacey et al., 1987; Turner et al., 1996; Simo et al., 1997). The range of oxidations for S intermediates formed in each of these processes is between +VI and −II. Many of the important naturally occurring molecular species of S are shown in table 12.1. On a global scale, most of the S is located in the lithosphere; however, there are important interactions between the hydrosphere, biosphere, and atmosphere where important transfers of S occur (Charlson, 2000). For example, coal and biomass burning, along with volcano emissions inject SO2 into the atmosphere, which can then be further oxidized in the atmosphere and removed as SO42− in rainwater (Galloway, 1985). An example of biogenic sulfur formation is the reduction of seawater SO42− to sulfide by phytoplankton and eventual incorporation of the S into dimethylsulfoniopropionate (DMSP). DMSP, in turn, is converted to volatile dimethyl sulfide (DMS; CH3SCH3)m which is emitted to the atmosphere. In the seawater, SO42− represents one of the major ions, with concentrations that range from 24 to 28 mM, which is considerably higher than the concentrations found in freshwaters (∼0.1 mM). This marked difference makes seawater the major input to estuaries and sets up an important gradient in estuarine biogeochemical cycling. In this chapter, the focus will be on the nonanthropogenic biogenic transformations of S that are relevant to biogeochemical cycling in estuarine and coastal waters. Approximately 50% of the global flux of S to the atmosphere is derived from marine emissions of DMS. Oxidation of DMS in the atmosphere leads to production of SO42− aerosols, which can influence global climate patterns (Charlson et al., 1987; Andreae and Crutzen, 1997).

Abstract The Hualgayoc district in the Cajamarca region of northern Peru experienced middle Miocene andesitic to rhyolitic magmatism and magmatic hydrothermal activity that produced Au and Cu mineralization, including the Cerro Corona porphyry Au-Cu, Tantahuatay high-sulfidation epithermal Au, and the AntaKori skarn Cu-Au-Ag deposits. We examined 32 samples from 22 units that encompass the entire igneous rock record in the district. Our new U-Pb dating of 454 zircon grains from these 22 igneous units yielded Concordia ages showing continuous magmatic activity in the district, from 14.8 to 9.7 Ma. Igneous activity in the eastern part of the district took place between 14.8 and 14.0 Ma, including the Cerro Corona intrusive complex that hosts a porphyry Au-Cu deposit. Magmatism in the western part of the district occurred between 13.7 and 11.5 Ma and included the Tantahuatay Volcanic Complex that hosts a high-sulfidation Au deposit. The different styles of mineralization and ages of igneous rocks in the eastern and western parts are accounted for by deeper erosion to the east. Bulk-rock compositions indicate that parental magmas originated from amphibole-rich juvenile lithospheric mantle or lower crust and evolved through amphibole fractional crystallization. Amphibole and zircon compositions indicate that parental magmas of all igneous units in the Hualgayoc district were water-rich, &gt;3 wt % H2O, and oxidized above the fayalite-magnetite-quartz (FMQ) redox buffer. Our observations in the Hualgayoc district suggest that oxidized conditions and high-water contents are necessary to produce porphyry-type mineralization, but that other factors were also critical to form mineralization, such as a shallow depth of magma emplacement, with near-vertical shape of intrusions. These findings are likely applicable elsewhere.

Like many other elements, natural background levels of trace elements exist in crustal rocks, such as shales, sandstones, and metamorphic and igneous rocks (Benjamin and Honeyman, 2000). In particular, the majority of trace metals are derived from igneous rocks, simply based on the relative fraction of igneous rocks in comparison with sedimentary and metamorphic rocks in the Earth’s crust. The release of trace metals from crustal sources is largely controlled by the natural forces of physical and chemical weathering of rocks, notwithstanding large-scale anthropogenic disturbances such as mining, construction, and coal burning (release of fly ash). As discussed later in the chapter, adjustments can be made for anthropogenic loading to different ecosystems based on an enrichment factor which compares metal concentrations in the ecosphere to average crustal composition. Biological effects of weathering, such as plant root growth and organic acid release associated with respiration also contribute to these weathering processes. As some trace metals are more volatile than others, release due to volcanic activity represents another source of metals with such properties (e.g., Pb, Cd, As, and Hg). Just as Goldschmidt (1954) grouped elements (e.g., siderophiles, chalcophiles, lithophiles, andatomophiles) based on similarities in geochemical properties, trace metals also represent a group of elements with similar chemical properties. One particularly important distinguishing feature of these elements is their ability to bond reversibly to a broad spectrum of compounds (Benjamin and Honeyman, 2000). Thus, the major inputs of trace metals to estuaries are derived from riverine, atmospheric, and anthropogenic sources. Although trace elements typically occur at concentrations of less than 1 ppb (part per billion) (or μg L−1, also reported in molar units), these elements are important in estuaries because of their toxic effects, as well as their importance as micronutrients for many organisms. The fate and transport of trace elements in estuaries are controlled by a variety of factors ranging from redox, ionic strength, abundance of adsorbing surfaces, and pH, just to name a few (Wen et al., 1999).

The chemical reactions that affect atmospheric O2 on a multimillion-year time scale involve the most abundant elements in the earth’s crust that undergo oxidation and reduction. This includes carbon, sulfur, and iron. (Other redox elements, such as manganese, are not abundant enough to have an appreciable effect on O2.) Iron is the most abundant of the three, but it plays only a minor role in O2 control (Holland, 1978). This is because during oxidation the change between Fe+2 and Fe+3 involves the uptake of only one-quarter of an O2 molecule, whereas the oxidation of sulfide to sulfate involves two O2 molecules, and the oxidation of reduced carbon, including organic matter and methane, involves between one and two O2 molecules. The same stoichiometry applies to reduction of the three elements. Because iron is not sufficiently abundant enough to counterbalance its low relative O2 consumption/release, the iron cycle is omitted in most discussions of controls on atmospheric oxygen. In contrast, the sulfur cycle, although subsidiary to the carbon cycle as to its effect on atmospheric O2, is nevertheless non-negligible and must be included in any discussion of the evolution of atmospheric O2. In this chapter the methods and results of modeling the long-term carbon and sulfur cycles are presented in terms of calculations of past levels of atmospheric oxygen. The modeling results are then compared with independent, indirect evidence of changes in O2 based on paleobiological observations and experimental studies that simulate the response of forest fires to changes in the levels of O2. Because the sulfur cycle is not discussed anywhere else in this book, it is briefly presented first. The long-term sulfur cycle is depicted as a panorama in figure 6.1. Sulfate is added to the oceans, via rivers, originating from the oxidative weathering of pyrite (FeS2) and the dissolution of calcium sulfate minerals (gypsum and anhydrite) on the continents. Volcanic, metamorphic/hydrothermal, and diagenetic reactions add reduced sulfur to the oceans and atmosphere where it is oxidized to sulfate. Sulfur is removed from the oceans mainly via formation of sedimentary pyrite and calcium sulfate.

Abstract Epithermal, Carlin, and orogenic Au deposits form in diverse geologic settings and over a wide range of depths, where Au precipitates from hydrothermal fluids in response to various physical and chemical processes. The compositions of Au-bearing sulfidic hydrothermal solutions across all three deposit types, however, are broadly similar. In most cases, they comprise low-salinity waters, which are reduced, have a near-neutral pH, and CO2 concentrations that range from &lt;4 to &gt;10 wt %. Experimental studies show that the main factor controlling the concentration of Au in hydrothermal solutions is the concentration of reduced S, and in the absence of Fe-bearing minerals, Au solubility is insensitive to temperature. In a solution containing ~300 ppm H2S, the maximum concentration of Au is ~1 ppm, representing a reasonable upper limit for many ore-forming solutions. Where Fe-bearing minerals are being converted to pyrite, Au solubility decreases as temperature cools due to the decreasing concentration of reduced S. High Au concentrations (~500 ppb) can also be achieved in strongly oxidizing and strongly acidic chloride solutions, reflecting chemical conditions that only develop during intense hydrolytic leaching in magmatic-hydrothermal high-sulfidation epithermal environments. Gold is also soluble at low to moderate levels (10–100 ppb) over a relatively wide range of pH values and redox states. The chemical mechanisms which induce Au deposition are divided into two broad groups. One involves achieving states of Au supersaturation through perturbations in solution equilibria caused by physical and chemical processes, involving phase separation (boiling), fluid mixing, and pyrite deposition via sulfidation of Fe-bearing minerals. The second involves the sorption of ionic Au on to the surfaces of growing sulfide crystals, mainly arsenian pyrite. Both groups of mechanisms have capability to produce ore, with distinct mineralogical and geochemical characteristics. Gold transport and deposition processes in the Taupo Volcanic Zone, New Zealand, show how ore-grade concentrations of Au can accumulate by two different mechanisms of precipitation, phase separation and sorption, in three separate hydrothermal environments. Phase separation caused by flashing, induced by depressurization and associated with energetic fluid flow in geothermal wells, produces sulfide precipitates containing up to 6 wt.% Au from a hydrothermal solution containing a few ppb Au. Sorption on to As-Sb-S colloids produces precipitates containing tens to hundreds of ppm Au in the Champagne Pool hot spring. Sorption on to As-rich pyrite also leads to anomalous endowments of Au of up to 1 ppm in hydrothermally altered volcanic rocks occurring in the subsurface. In all of these environments, Au-undersaturated solutions produce anomalous concentrations of Au that match and surpass typical ore-grade concentrations, indicating that near-saturated concentrations of dissolved metal are not a prerequisite for generating economic deposits of Au. The causes of Au deposition in epithermal deposits are related to sharp temperature-pressure gradients that induce phase separation (boiling) and mixing. In Carlin deposits, Au deposition is controlled by surface chemistry and sorption processes on to rims of As-rich pyrite. In orogenic deposits, at least two Au-depositing mechanisms appear to produce ore; one involves phase separation and the other involves sulfidation reactions during water-rock interaction that produces pyrite; a third mechanism involving codeposition of Au-As in sulfides might also be important. Differences in the regimes of hydrothermal fluid flow combined with mechanisms of Au precipitation play an important role in shaping the dimensions and geometries of ore zones. There is also a strong link between Au-depositing mechanisms and metallurgical characteristics of ores.