V41D-01 08:00h
Early Accretion of Water: Implications for the Oxidation State of the Planets.
Introduction: There are two main scenarios that account for the accretion of water to the terrestrial planets: either the water originated outside of the inner solar system and was later delivered to the terrestrial planets by means of some suitable mechanism, or the source of water was local, i.e. it came from the same feeding zone of the Earth's rocky material, and was concomitantly accreted to the planet (Drake, 2004). These two end member scenarios distinguish water both with respect to its source (exogenous - comets, asteroids, phyllosilicates [Cielsa and Lauretta, 2004]), or endogenous - hydrous minerals, water adsorbed on grains [Stimpfl et al. 2004]) and timing of addition to the terrestrial planets (after accretion or concomitant with accretion). If the provenance of water was related to exogenous sources, the terrestrial planets would have accreted from dry material and subsequently obtained water from comets, asteroids, or phyllosilicates. On the contrary, if water came from local sources, then the terrestrial planets would have accreted from wet materials and therefore water would have been present early in the history of these planets. It is important to distinguish between these possibilities because the presence or absence of water while the planets were differentiating would have profound implications in the geochemical evolution of the planetary bodies. For example, it would affect the partitioning behavior of siderophile elements, the melting temperature of the rocks, and of course it could control the evolution of the oxidation state of the planets. Oxidation state of terrestrial planets: An extremely reducing environment characterized the early solar system. Based on the ratio H2O/H2 Jurewicz (1995) proposed a fO2 of about IW-5 for the solar nebula; under this condition very little Fe2+ can exist. However, because planetary material contain Fe2+ and Fe3+ there must have been some mechanism at work that oxidized the metallic iron. If water was present during early differentiation, it could represent the oxidizing agent that might have moved the initial fO2 from solar value to the current one inferred for the mantle of Moon, Vesta, Mars, Venus ($\sim$ IW -1) and Earth (QFM) as recently summarized by Jones (2004). It has been proposed that the reaction: Fe(metal) + H2O(melt) = FeO(silicate melt) + 2H(core) (1) could simultaneously oxidize the mantle and also, in the case of the Earth, explain the density deficit of the core (Okuchi 1997, Righter and Drake 1999). Wet Earth: Stimpfl et al., (2004) have recently shown that adsorption of water gas onto dust grains in the accretion disk could account for 3.5 times the amount of water stored in the terrestrial oceans (5 x 10E24 g). However, if water is used by reaction (1) to oxidize the mantle the initial volatile budget of the Earth needs to be much greater. Righter (2004) recently proposed that ~50 Earth's oceans are required to raise the fO2 of the terrestrial mantle from one consistent to 2% FeO (IW-3 or IW-2) to the proposed 8% FeO for the primitive upper mantle (QFM). However, the results presented by Stimpfl et al. (2004) are preliminary and that more sophisticated modeling could sustain higher adsorption on the grains. Likewise, the results of Righter (2004) do not take into account the unknown effects of pressure and temperature. Coupled with the results of Rubie et al. (2004), accretion of water-bearing grains could plausibly contribute to the current oxidation states of planetary mantles.
V41D-02 08:15h
Partial Melting of Ordinary Chondrite Under Reducing Conditions
A critical parameter in determining the nature and processes of differentiation of planetary materials in the early solar system is oxygen fugacity. Chondrites record a range of oxygen fugacities from approximately 5 log units below the iron-wustite (Fe-FeO) buffer (enstatite chondrites) to close to QFM (some carbonaceous chondrites). Among the equilibrated chondrites, an "oxidation gap" appears to exist between ordinary chondrites and enstatite chondrites, although several groups of unequilibrated carbonaceous chondrites appear to occupy this "gap". Some primitive achondrites fill this gap (e.g. pallasites, acapulcoites, lodranites, winonaites, and silicate-bearing IAB and IIE irons), although the precursors to these groups are poorly known. In this experimental study, we have determined the modification in mineral compositions during partial melting under reducing conditions and explore the idea that the primitive achondrites may be formed through differentiation under reducing conditions of a more oxidized precursor. Partial melting experiments were conducted on an H6 chondrite (Kernouve) under reducing conditions at 1 atm and at 1.3 GPa pressure in a solid media deformation apparatus. In the 1 atm experiments, fO$_{2}$ was buffered by gas mixing and sealed silica tube techniques to values determined from thermodynamic calculations of primitive achondrites; in the deformation experiments, aluminum jackets were used. The experiments suggest that partial melting of an oxidized precursor under reducing conditions can produce some of the reduced features observed in primitive achondrites such as magnesian olivine, pyroxene and chromite compositions typical of primitive achondrites at temperatures of 1200-$1300 \deg$C, as well as chalcophilic behavior of previously lithophillic ions (e.g., Cr in sulfide) at temperatures at $1000\deg$C. Some features of primitive achondrites (e.g. oxygen isotopic compositions and Cr$/$(Cr+Al) ratios of chromites) appear to be intrinsic to the precursor chondrite. Further, mafic silicate and chromite reduction (increased Mg$/$(Mg+Fe)) required higher temperatures than those inferred for primitive achondrite formation. We suggest that the precurser chondrite for many primitive achondrites could have been somewhat more oxidized and subsequent melting under reducing conditions (e.g. in the presence of graphite) produced the reduction of mafic silicates and chromites in addition to chalcophilic behavior in some elements. Melt migration, solid-melt reactions and removal of key elements (e.g., S, Al) during melting might be enhanced by deformation and/or open system conditions, producing more dramatic changes in the residual solid. Other features however, must have been inherited from the precursor chondrite and therefore do not reflect changes produced during melting under reducing conditions.
V41D-03 INVITED 08:30h
How the Upper Mantle Became Oxidized
Today, Earth's upper mantle has an average oxygen fugacity near the quartz-fayalite-magnetite (QFM) redox buffer (1), although significant departures from this redox state occur in different localities and at different depths (2). However, early in Earth history, following the Moon-forming impact, the upper mantle was almost certainly uniformly more reduced. The impactor that formed the Moon was probably Mars-sized or larger (3) and had already differentiated an iron core. Successful models of lunar formation must account for the fact that the Moon has only 25 percent of Earth's iron abundance (4). This can be accomplished if the iron core of the impactor was accreted by the Earth, while the Moon was formed from the mantles of the impactor and the Earth. Other large impactors would also have brought in metallic iron, and all such large impacts would have melted large portions of Earth's mantle. It is therefore inevitable that the Earth's upper mantle began its existence with an oxygen fugacity at or below iron-wüstite (IW). How the upper mantle became oxidized from IW up to QFM is an interesting question. Much of the oxidation could have taken place during brief steam atmosphere stages following impacts (5,6) when hydrogen escape to space was extremely rapid (7). Continued oxidation could have been caused by cycling of volatiles through the mantle, accompanied by outgassing of reduced gases (8) and by subduction of ferric iron that had been oxidized at the surface (9). Oxidation of the uppermost 700 km of the mantle from QFM to IW would have required the equivalent of about half an ocean of water, assuming that the hydrogen was lost to space. This could have been accomplished in less than 2 b.y. if the average H$_2$ outgassing rate was a few times the present value, 5x10$^{12}$ mol/yr (10). The timing of mantle oxidation has important consequences for the composition of Earth's atmosphere at the time when life originated because it controls the oxidation state of volcanic gases. If redox indicators (Cr and V) from ancient rocks have been correctly interpreted (11,12), the process of mantle oxidation was essentially complete by 3.5 Ga. However, mantle oxidation would have hung up somewhat below QFM by conversion of graphite (or diamond) to CO$_2$ or carbonate, before rising to QFM. This process may therefore help explain why atmospheric O$_2$ did not rise until ~2.3 Ga (13,14), nearly half a billion years after the invention of oxygenic photosynthesis (15). References: 1. Holland, H.D. The Chemical Evolution of the Atmosphere and Oceans. Princeton Univ. Press, Princeton (1984). 2. Woodland, A.B. and Koch, M. Earth Planet. Sci. Lett. 214, 295 (2003). 3. Cameron, A. G. W. In Origin of the Earth and Moon, R. M Canup and K. Righter (eds.), p. 133, Univ. of Arizona Press, Tucson (2000). 4. Wood, J.A. In Hartmann, W.K., et al. (eds.) Origin of the Moon, p. 17, Lunar and Planetary Inst., Houston, TX (1986). 5. Matsui, T. and Abe, Y. Nature 319, 303 (1986). 6. Matsui, T. and Abe, Y. Nature 322, 526 (1986). 7. Pepin, R.O. Icarus 92, 2 (1991). 8. Kasting, J.F., et al., J. Geol. 101, 245 (1993). 9. Lecuyer, C. and Ricard, Y. Earth Planet. Sci. Lett. 165, 197 (1999). 10. Holland, H.D. Geochim. Cosmochim. Acta 66, 3811 (2002). 11. Delano, J.W. Origins of Life Evol. Biosph. 31, 311 (2001). 12. Canil, D. Earth Planet. Sci. Lett. 195, 75 (2002). 13. Holland, H. D. In Early Life on Earth, S. Bengtsson, ed., p. 237. New York, Columbia Univ. Press (1994). 14. Farquhar, J., et al., Science 289, 756 (2000). 15. Brocks, J.J., et al., Science 285, 1033 (1999).
V41D-04 08:45h
Modeling the Rise of Atmospheric Oxygen
Abundant geological evidence shows that atmospheric O$_2$ rose from less than a few ppmv to at least a few parts per thousand at $\sim$2.4 Ga. This transition is important to understand because the increase in O$_2$ levels changed the course of biological evolution. Biomarkers show that the source of O$_2$, oxygenic photosynthesis, existed long before the rise of O$_2$. A theoretical understanding remains elusive for how oxygenic photosynthesis could have originated long before a detectable rise of O$_2$ and what controlled the timing of the O$_2$ increase. We describe a time-dependent biogeochemical model of redox fluxes between the atmosphere-ocean system and the solid Earth. The rate of change of the quantity of O$_2$ in the atmosphere is given by the difference in the O$_2$ source and sink fluxes. The source of O$_2$ is equivalent to burial flux of organic carbon, whereas the losses of O$_2$ are due to photochemical destruction (including reaction with reducing volcanic and metamorphic gases) and continental weathering. The oxidizing effect of the escape of hydrogen to space is also calculated. The biosphere in the model consists of a coupled photosynthetic-methanogenic system. In addition, the model includes parameterizations of changing solar luminosity and the greenhouse effect. Results show that before the rise of O$_2$, the atmosphere is redox-dominated by methane, even in the presence of photosynthetic O$_2$ fluxes comparable to those today. Hydrogen escape, associated with the decomposition of CH$_4$ in the upper atmosphere, irreversibly drives the Earth system to more oxidized conditions. We find that the oxic transition occurs when the flux of reduced species from volcanic and metamorphic gases drops below the flux of O$_2$ associated with organic carbon burial. A precipitous drop in CH$_4$ levels accompanies the transition, lowering global temperatures to potentially Snowball Earth levels. We find that the timing of the oxic transition is primarily affected by the amount of iron in the Earth's crust. If hydrogen escape is turned off in the model, the Earth remains stuck with a Titan-like methane-rich atmosphere and no atmospheric oxic transition occurs. Thus, the basic overall model behavior-an oxic transition accompanied by a decrease of methane-is a robust feature, given the overall character of the redox fluxes and unidirectional hydrogen escape.
V41D-05 09:00h
Fe$^{3+}$-in-Clinopyroxene Oxybarometry: Effects of Na on Iron Partitioning Behavior
Traditional methods for quantifying magmatic oxygen fugacity (fO2), such as coexisting Fe-Ti oxides, are not applicable to many natural systems. Therefore, to understand the fO2 variations that exist among magmatic suites, other oxybarometers must be developed to measure the fO2 of these magmas. It is well known that iron is the only major element to occur in multiple valence states, primarily as a function of fO2. We have used synchrotron microXANES methods to investigate the concentration of Fe$^{3+}$ in clinopyroxene (augite) as a potential oxybarometer for Martian magmas, for which determination of fO2 using established methods is difficult. A series of 1-atm, gas mixing experiments were run to examine the relationship between fO2 and the Fe$^{3+}$ content of clinopyroxene. The original starting composition was an SNC-type basaltic melt consisting of CMAS+Fe with liquidus augite. Experiments were conducted over a range of fO2 values from IW+4 to IW-2. Initial results suggest that at fO2 values $<$ QFM, insignificant amounts of Fe$^{3+}$ ($<$ 10%) are present in basaltic melts to be incorporated into augite at measurable levels. At higher fO2 values, the concentration of Fe$^{3+}$, both in the melt and coexisting augite, increases systematically with fO2, indicating the usefulness of this oxybarometer in high-fO2 magmas. New experiments have been run on a starting composition CMAS+Fe+Na to determine the effect of Na on the partitioning of Fe$^{3+}$ into clinopyroxene. Coupled substitution of Na and Fe$^{3+}$ for Ca and Mg or Fe$^{2+}$ is a dominant reaction in Na-bearing pyroxenes. Therefore, the availability of Na should have a significant effect on clinopyroxene Fe$^{3+}$ uptake. Our experiments have shown that indeed the addition of Na to the starting composition increases the amount of Fe$^{3+}$ incorporated into clinopyroxene. This effect, while relatively small, appears greatest under low fO2 conditions. It is possible that under high fO2 conditions, with more Fe$^{3+}$ present in the melt, the coupled substitution described above is not as important as the direct substitution of Fe$^{3+}$-Al$^{3+}$. The experiments demonstrate the potential use of the Fe$^{3+}$ content of clinopyroxene as an oxybarometer and indicate that the presence of Na has a measurable effect on Fe$^{3+}$ partitioning. Due to the chemical complexity of natural systems, the effect of Na on clinopyroxene Fe$^{3+}$ partitioning should not be dismissed when applying this oxybarometer.
V41D-06 09:15h
Oxygen Fugacity of Basalts From Earth and Mars: Implications for Oxidation States of Terrestrial Planet Interiors
The oxidation state of a planetary interior plays an important role in the partitioning of elements between the planet's core and mantle, the geophysical properties of the mantle, the phase equilibria of igneous rocks, and the speciation of gases in the planet's atmosphere. Determining the oxidation state of the interior of the Moon, Mars, and differentiated asteroids is difficult, because planetary samples are dominated by basaltic igneous rocks. Direct mantle samples, such as mantle xenoliths and diamond inclusions, as benefit studies on Earth, are lacking. The oxidation state of these planets' interiors is inferred from the oxygen fugacity recorded in the basaltic samples. Basalts from Mars (martian meteorites) record oxygen fugacity ranging from near the IW buffer to 3 log units above ($\sim$QFM), by several methods. The range of igneous rocks on Earth overlaps, but ranges up to $\sim$7 log units above IW, with the most oxidized samples derived from island arcs. Studies of the relationship between the oxidation state of a basalt and that of its mantle source on the Earth provide potentially important contributions to the interpretation of martian basalt oxygen fugacity and the inferred oxidation state of the martian interior. Thermodynamic considerations of ferrous-ferric mineral equilibria in the spinel and garnet facies of the Earth's mantle dictate that the oxygen fugacity should decrease, relative to the QFM buffer, with increasing pressure. Ballhaus (1995) calculated a decrease of 0.6 log unit per GPa increase, assuming a constant bulk composition. In contrast, C-H-O equilibria have isopleths of opposing slope, such that fluid composition will be dominated by more reduced species (e.g., methane) at greater depths. Ballhaus and Frost (1994) argue that C-H-O buffering influences upwelling asthenosphere, particularly by the presence of graphite, and that the oxygen fugacity of a basalt at the surface depends on the depth at which first melting occurs. This depth is where the melt is separated from graphite, becomes buffered by ferrous-ferric equilibria, and undergoes a concomitant increase in relative oxygen fugacity with decreasing pressure. Thus they argue that OIB have higher oxygen fugacity relative to MORB because the former experience first melting at greater depth than the latter. Although the details of this model are debated, such as the relative role of C-H-O fluids and the assumption of constant bulk mantle composition, it is interesting to consider its application to the petrogenesis of basalts on Mars. Assuming a constant depth of first melting of 90 km on the Earth and Mars, at the same relative oxygen fugacity, and ferrous-ferric buffering subsequent to melting, the oxygen fugacity of each erupted basalt will be different, because of the different pressure-depth relationships on each planet. A depth of 90 km on Mars is equal to $\sim$1 GPa; therefore the expected increase in oxygen fugacity is only 0.6 log units. On Earth, the increase would be $\sim$2 log units ($\sim$3 GPa). The dominant control on martian basalt oxygen fugacity appears to be the oxidation state of mantle sources, which may be inherited from the crystallization of a martian magma ocean at 4.5 Ga (e.g., Herd 2003; Borg and Draper 2003). This difference between basalts from the Earth and those from Mars may reflect a fundamental difference in planetary evolution; specifically, the preservation of "redox reservoirs" on Mars due to a lack of vigorous mantle convection. The corollary is that the oxidation state of the Earth's interior has been fundamentally altered from its initial state, by plate tectonics or other processes.
V41D-07 INVITED 09:30h
Oxygen Isotopes in the Terrestrial Planets
Mechanisms that may account for oxygen isotope heterogeneity in meteorites on the microscopic scale do not seem adequate for explaining the similarities and differences in isotopic composition on a planetary scale. In chondrites, most of the isotopic variability can be attributed to photochemical enrichment of the two rare heavy isotopes with respect to the 16O-rich solar composition In the CO, CM, CI, and CR chondrites, an additional low-temperature aqueous alteration leads to mass-dependent further enrichment of the heavy isotopes. If the photochemical origin of the isotopic variation in chondrites is correct, then only a small fraction, represented primarily in CAIs, has the solar oxygen isotopic composition, and all other meteoritic components must have undergone photochemical processing. In addition, since the bulk isotopic compositions of the terrestrial planets and of the achondrite parent bodies are similar to those of chondrites, they too must be made of photochemically enriched matter. The photochemical reactions produce a non-equilibrium assemblage of gases, probably leading to a non-equilibrium assemblage of solids, particularly with respect to their oxidation state. These issues emphasize the importance of the measurement of oxygen isotopes in the Genesis solar wind mission. Within the Earth, oxygen isotope variations are due almost entirely to mass-dependent fractionation effects, giving a line of slope 0.52 on the three-isotope plot. The average crustal composition is 3 to 4 permil higher in delta-18O than the upper mantle. This difference is too large to be due to igneous fractionation effects alone, and reflects the larger, low-temperature isotope fractionation associated with aqueous weathering reactions at the Earth's surface. Similar effects are not observed in the intraplanetary isotopic variations in the Moon or in the parent bodies of the HED and SNC meteorites. The bulk oxygen isotopic compositions of Earth and Mars (assumed to be the SNC parent body) cannot be accounted for by any mixture of two components, such as those proposed by Ringwood [1979] and Wänke [1981]. In principle, three-component mixtures of ordinary chondrites, CI, and CV chondrites can match the planetary isotopic compositions, but are inconsistent with chemical compositions. An additional unexplained observation is the exact coincidence in oxygen isotopic composition between Earth and Moon. The correspondence of isotopic composition between the Earth and the enstatite chondrites has been taken by some to have direct genetic significance. In all models using primitive chondrites as building blocks for the terrestrial planets, there is a necessity to remove a major fraction of the moderately volatile elements (alkalies, S, etc.), without altering their isotopic compositions Ringwood A. E. (1979) Composition and Origin of the Earth, RSES, Aust. Nat. Univ. (65 pp.). Wänke H. (1981) Phil. Trans. Roy. Soc. Lond., A303, 287-302
V41D-08 09:45h
The Oxygen Isotope Composition of Earth and the Terrestrial Fractionation Line (TFL)
The discovery of non-mass dependent oxygen isotope fractionation effects in terrestrial surface deposits and in trace gas species in the atmosphere has renewed interest in a subject that has been in the province of cosmochemistry for decades. With the discoveries has come the commissioning of new laboratories dedicated to their pursuit. Inter-laboratory comparisons of measurements of both d17O and d18O and the slope of the attendant fractionation line, are imperative to validate rapidly growing data sets from a variety of different laboratories. Because oxygen isotope fractionation is a necessary consequence of the differentiation of planetary bodies, it is not useful to speak of a single, specific oxygen isotope composition for Earth, or any other such body. Planetary processes, however, do produce a diagnostic characteristic that uniquely defines bulk oxygen isotopic composition. A plot of d17O vs. d18O for a given body gives a linear array of data points which is characteristic of the bulk composition of the body. For the Earth the array is termed the Terrestrial Fractionation Line (TFL). Thus, the slope and intercept of a planetary body's oxygen three-isotope fractionation line are definitive. Published values of the slope of the TFL on a plot of d17O vs. d18O range from 0.5164 to 0.5288. The TFL's intercept is defined as zero, relative to VSMOW. Different slopes may be associated with different fractionation mechanisms; in particular, equilibrium vs. kinetic isotope fractionation. Modern analytical techniques should be able to resolve these fractionation mechanisms but the practical capability to do so must be validated. We are conducting an inter-laboratory comparison of silicate mineral samples analyzed at both the Open University (UK) and the Geophysical Laboratory of the Carnegie Institution of Washington. The analyses were performed in both localities by heating samples with a CO2 laser in a reaction chamber filled with BrF5 gas. Two different mass spectrometers were used: a Prism III at the Open University and a Thermo MAT-252 at the Geophysical Lab. Slopes were computed by regression of linearized measured delta values (Miller 2002). Seven samples of quartz extending over a range of d18OVSMOW from +2.40 to +33.28 give a slope of 0.5248 (+/- 0.0005) at Open University. The same samples analyzed at the Geophysical Lab give 0.5281 (+/- 0.0012). Earlier high precision values of the TFL's slope, presented in the same format, include 0.5281 +/- 0.0015 for natural waters (Li and Meijer, 1998) and 0.5263 +/- 0.0008 for the Earth-Moon system (unpublished, Geophysical Lab, CIW). The causes of variations in the TFL slope will be discussed.