V23D-01
Magmatic MORB Volatiles, Seafloor Hydrothermal Systems and Abiotic Organic Synthesis
A plausible model for the origin of the observed C-O-H volatiles observed in MORB glasses is that they were incorporated in primary melts of the upwelling mantle. Based on the observed ferric/ferrous ratios in MORB glass, it is probable that the MORB source mantle contained diamond or graphite, depending on pressure. If true, then during partial mantle melting the graphite/diamond would react with FeO1.5 in garnet/spinel and clinopyroxene to form CO2 which would dissolve in the melt as carbonate ion. Using equation of state models for CO2 activity and ferric/ferrous ratios in the magma it is possible to model the amount of carbonate dissolved in the basaltic magma as a function of the degree of melting (Holloway and O'Day, 2000). The results require that rising MORB magma will become saturated in CO2 at depths much greater than those proposed for MORB magma chambers. Conversely H2O values observed in MORB glasses are far below saturation. However as CO2 reaches saturation and exsolves from the melt the low fO2 imposed by the low ferric/ferrous ratio results in a high H2/H2O ratio in the exsolving supercritical fluid. We have shown that fluids with this composition produce methanol (CH3OH) in the presence of magnetite at seafloor hydrothermal P-T conditions in a flow-through system (Voglesonger, et al., 2001) and that aqueous methanol solutions react in montmorillonite clay interlayers to form a wide variety of complex hydrocarbon molecules, the most abundant being hexamethyl benzene (Williams, et al., 2005). Methyl stearate (C17H35COOCH3) was also observed in moderate amounts. Holloway, J. R. and P. A. O'Day (2000). "Production of CO2 and H2 by Diking-Eruptive Events at Mid-Ocean Ridges: Implications for Abiotic Organic Synthesis and Global Geochemical Cycling." International Geology Review 42: 673-683. Voglesonger, K. M., J. R. Holloway, E. E. Dunn, P. J. Dalla-Betta and P. A. O'Day (2001). "Experimental Abiotic Synthesis of Methanol in Seafloor Hydrothermal Systems." Chemical Geology 180: 129-139. Williams, L. B., B. C. Canfield, K. M. Voglesonger and J. R. Holloway (2005). "Organic molecules formed in a primordial womb." Geology 33: 913-916.
V23D-02
Towards Determining the Upper Temperature Limit to Life
Determining the upper temperature limit to life is key to defining the habitable regions of our planet, understanding the origin of life, and it is an important guide in our search for life elsewhere. Recent studies of hydrothermal vent environments challenge previous known limits with laboratory cultures reaching 121°C, and evidence for microbial communities even within the hottest interior walls of black smoker chimneys. Studies focused on examining the most extreme conditions under which life thrives, survives, and expires are inherently challenging because of the difficulty in directly accessing the hottest portions of the deep biosphere and because of our inability to adequately reproduce in situ environmental conditions in the laboratory. To begin to address these challenges, novel in situ microbial incubators were deployed into the walls of active black smoker chimneys on the Endeavour Segment of the Juan de Fuca Ridge. The incubators contained 3-4 discrete chambers. Each chamber hosted nine thermocouples and some incubators contained OsmoSamplers for continuous time-series sampling of hydrothermal fluids within the chambers. The incubators were deployed for periods of 1.5 months to one year, with reinstrumentation of some sites annually since 2002. The incubators routinely record sharp and well defined temperature gradients within each of the chambers that vary from near seawater values in the most outer chambers to end member conditions (200°C) within the interior walls. Fourier transform analyses indicate that diurnal and semi-diurnal tidal forcing results in small temperature perturbations in all chambers; much longer term perturbations (tens of days) that reach up to 50°C likely reflect localized fracturing events in the subseafloor and fresh injection of hotter fluids. Co-registered microbial community analyses of material recovered from newly precipitated mineral surfaces from within the chambers on cm-scales across the temperature and chemical gradients show a high diversity of 16S archaeal phlotypes and evidence for cells within the hottest portions of the chambers. These initial results again bring into question whether life can exist at temperature in excess of 150°C. It is a tribute to John Holloway that we were able to conduct this study. The authors give him a heartfelt thanks for his enthusiastic encouragement during this project and especially for his numerous pointed and enthusiastic reviews, which where likely instrumental in getting this project funded. http://www.visions05.washington.edu/science/investigations/micro_survival.html
V23D-03
3-D Crystal Tectonics of Red Coral (Corallium Rubrum)
A combination of analytical techniques (petrographic microscopy, SEM, TEM, and EMP) has been used to characterize the internal physico-chemical structure of the red coral (corallium rubrum) skeleton. A section normal to the skeleton axis shows an inner medullar zone with a bulbous-tip cross shape, surrounded by a large circular domain composed of concentric rings (width of each ring ca 150 microns). Growth rings are revealed by the cyclic variation of concentration of the organic matter (OM) and oscillations of the Mg/Ca ratio. Experiments carried out in natural environment show that the detected growth rings are annual. Thus, both oscillations of concentration of OM and Mg/Ca ratio can be used to determine the age of the red coral colonies, some of which can be as old as a few tens (or even a few hundreds) of years. Concentric ring are riddled and display a succession of wavelets (wavelength ca 300 microns). The internal structure of each wavelet is complex, both physically and chemically: it is formed by the accumulation of strata with locally tortuous interfaces due to the presence of micro protuberances (ca 30 microns). This interlocked structure confers an exceptional stiffness on the red coral skeleton. Interfaces between strata sometimes display sharp discontinuities indicating interruption of the mineralizing process. This fact has important consequences on the ability of the whole structure to register external forcings with accuracy. SEM and TEM studies show that each stratum is made of submicron crystalline units (ca 200 nm) organized or not in polycrystalline fibers or blades (ca 1 to 10 microns). Porosity can be observed at all scales between the various structural units. HRTEM studies show that in spite of displaying single crystal scattering behavior, the submicron crystalline units are made of 2-5 nm nanodomains with intercalated nanopores. We interpret the nanodomains as nanograins aggregated by a mechanism of oriented attachment. The red coral skeleton is an example of hierarchically organized organic-inorganic composite material, with five levels of organization. This complex material exhibits structural and compositional order on length scales from the nm to the cm. It is an example of nanoparticles as building blocks for the bottom-up fabrication of complex superstructures with exceptional mechanical properties.
V23D-04
The Importance of Crust-derived CO2 in Mafic Magmas: Evidence from Italian Volcanoes
Volatile abundances in magma are commonly considered to be inherited from the melting conditions in the mantle and subsequently modified by degassing processes during magma rising. The addition of volatiles of crustal origin during magma transfer toward the surface has been proposed only in a few instances but no quantitative assessment has shown that such a process can affect volatile emissions of active volcanoes to any significant extent. Here, we illustrate the overwhelming effect of carbonate assimilation on CO2 degassing from several Italian volcanic centers. Extensive interactions between magmas and the several-kilometers-thick sedimentary carbonate basement are documented in Central-Southern Italy by abundant high temperature skarn xenoliths occurring in the eruptive products. By means of carbonate assimilation experiments and mass balance calculations, we estimate that the main trends in major element compositions observed Mt. Vesuvius eruptive products of the last 25 ka are consistent with 6-9wt percent of carbonate assimilation at 6-12 km depth. Important assimilation degrees were also deduced for some volcanoes of the Roman Province. Such process introduces several wt percent of CO2 in the magma, which largely exceeds CO2 solubility in molten silicate and significantly contributes to the important CO2 degassing reported for these volcanic areas. We show that both the emission rates and the carbon isotopic compositions of the degassed CO2 are consistent with decarbonation during assimilation. Several other volcanic centers recognized to be emplaced over thick carbonate sedimentary successions are characterized by important CO2 degassing (e.g. Mt. Etna, Volcan Popocatépetl, Merapi, Lascar Volcano, Erebus) and show magmatic skarn xenoliths in their eruptive products as evidence of decarbonation during high temperature magma-limestone interactions. When measured, the carbon isotopic composition of CO2 emitted by the fumaroles of these volcanoes strongly deviates from the typical mantle-derived magmatic signature and probably indicates an important contribution from sedimentary carbonates. This degassing mechanism, largely neglected so far, needs to be considered for interpreting volcanic gas emissions and its contribution to global CO2 emissions from volcanoes into the atmosphere has to be estimated.
V23D-05
Modelling Sulfur Content in Volcanic Gases of Mars
Both landed experiments and remote sensing instruments on spacecraft sent to Mars have shown the Martian regolith to be uniformly rich in sulphur, with abundances typically exceeding 2 orders of magnitude those of common sedimentary or igneous rocks on Earth. Various hypotheses have been put forward to explain such an enrichment, including remnants of primitive chondritic material during accretion and volcanic degassing, but none has been quantitatively evaluated. Here, by performing thermodynamic calculations on sulphur partitioning between molten iron core and magma ocean, we show that the Martian mantle has a sulphur content at least 3-4 times higher than that of its Earth counterpart due to more oxidant conditions during core-mantle separation. The high FeO content of the martian basalts implies that although sulphur-rich, these magma are not sulphide saturated at depth. As a result, partial melting of Martian mantle yields basaltic magmas with 3-10 times more sulphur than those of primitive mid-ocean ridge basalts on Earth. Calculations of gas compositions in equilibrium with Martian and Earth like basalts (MORB) at near surface conditions show that volcanic gases on Mars have 10-100 times more sulphur than those on Earth. In addition, the lower pressure of lava delivery on Mars relative to Earth suggests that Martian volcanic exhalations, although equilibrated under low fO2 conditions, may have been dominated by SO2 not H2S, ie unlike those on Earth. Higher sulphur contents and SO2 scrubbing by aquifers, both may have produced widespread acidic conditions in any putative ocean, which were unparalleled on Earth, and prevented carbonate precipitation on Mars. Altogether, our results show that the strength of sulphur degassing, and thus the compositions of planetary atmospheres, may have widely varied between planets of the Solar system, depending in particular upon the redox state prevailing during accretion or its modification thereafter : present day Io volcanism could ressemble what was once Mars magmatism.
V23D-06
Oxygen Fugacity Variation in Martian Meteorites: Carbon Buffering in the Martian Mantle?
Calculations of fO2 in the martian meteorites result in a 5 log fO2 unit range in values relative to the FMQ buffer. If graphite is present in a planetary mantle, the effect of pressure on C-CO-CO2-CH4 equilibria is such that relative oxygen fugacity would increase with pressure. Given the range of fO2 estimates for martian meteorites, it is worthwhile considering whether this could be achieved by polybaric C-CO- CO2-CH4 equilibria in the martian mantle. Only 40-80 ppm C is required to keep peridotite mantle buffered at the C-O surface. Indeed, studies of martian meteorites have shown that Mars is a volatile-rich planet, and C may be an important constituent of the volatile budget. If C buffering at variable pressure is controlling oxygen fugacity in the martian mantle, it would require formation of ALH 84001 at lowest pressures, shergottites at intermediate pressures, and nakhlite and Chassigny parent melt formation at highest pressures, all of which is consistent with knowledge of these meteorites. Carbon buffering is thus a simpler explanation for oxygen fugacity variations in the martian mantle and crust, than by hydrous reservoirs or by crustal assimilation. Furthermore, a small activity of CH4 would be associated with a C-CO-CO22-CH4 gas in the martian mantle, and volcanic degassing of this species could account for the small amount of methane detected recently at the surface of Mars.
V23D-07
Ventilation of CO2 from a reduced mantle and the climate history of Mars
A fundamental question regarding Martian history is the storage of carbon in the Martian interior and the processes and fluxes leading to ventilation of carbonaceous volatiles to the Martian crust and atmosphere. Evidence for liquid water on the Martian surface during the Late Noachian and Hesperian epochs (<3.7 Ga) likely requires a substantial accumulation of greenhouse gases in the Martian atmosphere. Most commonly, it is assumed that this gas was principally volcanogenic CO2. A key consideration regarding the extraction of C during partial melting of the Martian mantle is that the Martian mantle is thought to be reduced, with fO2 between IW and IW+1. Therefore, carbon likely resides chiefly as graphite, and extraction of CO2 from a graphite- saturated mantle can be estimated using thermodynamic models developed for terrestrial basalts by John Holloway. For melts derived from a graphite-saturated mantle at IW and IW+1, calculated liquids will have ~100 ppm and ~1000 ppm CO2, respectively, assuming basalt extraction at 1300 °C and 1 GPa. These concentrations do not change appreciably if temperature and pressure were both higher. Formation of the 50 km thick Martian crust at 4.5 Ga would therefore have liberated an atmosphere of 0.4-4 bars of CO2. Only the higher value is sufficient to maintain a strong greenhouse on early Mars, suggesting either that the Martian mantle must not be more reduced than IW+1 or that gases other than volcanogenic CO2 were responsible for the earliest Martian greenhouse atmosphere. It is sometimes suggested that degassing of CO2 associated with the gigantic (3 X 108 km3) Tharsis magmatic province was responsible for sustaining a significant Martian greenhouse later in Martian history (<3.7 Ga). However, at IW to IW+1, only 40-400 mbars CO2 could have been ventilated. Thus, if these calculations are applicable to the Martian mantle, either the greenhouse gases responsible for sustaining an equable climate compatible with liquid H2O on Mars were not derived chiefly from volcanogenic CO2 or the Martian mantle was more oxidized than IW+1.
V23D-08
Megacryst Petrogenesis in Southern Africa
The Cr-poor megacryst suite is ubiquitous in kimberlites, but its origins are not well understood, including details of the relationship to kimberlite magma. Major and trace element and isotope data for >2000 megacrysts from ~50 kimberlites in southern Africa vary systematically with respect to host kimberlite type, geographic location and tectonic setting. Distinctive differences between Group I and Group II kimberlite megacryst suites confirm a broadly cognate origin. A large range of geochemical differentiation patterns is displayed by individual megacryst suites reflecting the varying influences of melt-rock interaction in the mantle prior to kimberlite eruption. Geographic variations in megacryst composition have been used to construct a deep mantle compositional and thermal structure for southern Africa. This structure suggests the intrusion of deep level mantle magmas in zones of previous tectonism. Variations in the interplay between chemical boundary layer thickness, mantle geotherm, melt-rock ratio, and inferred megacryst-magma phase relations can explain many features of the systematic mineralogical and compositional distribution of megacrysts, and other metasomatic products derived from megacryst parent magmas.