Composition of sulfide minerals of ODP Hole 209-1268A

This paper presents sulfide mineral occurrence, abundance, and composition in samples from hydrothermally altered peridotite and gabbro recovered during Ocean Drilling Program (ODP) Leg 209 from south of the 15°20'N Fracture Zone on the Mid-Atlantic Ridge at Site 1268. Most of the sulfide minerals occur in veins and halos around veins in serpentinized peridotite. The only sulfide phases reported that occur in proximity to gabbro are those associated with a mafic intrusion into serpentinized peridotite. Sulfide mineral species change predictably downsection but are perturbed coincident with a breccia interpreted to be generated by intrusion of a gabbroic magma. The general downhole trend suggests sulfide mineral precipitation in conditions with decreasing sulfur and oxygen fugacity. Sulfide minerals that indicate precipitation at relatively higher sulfur and oxygen fugacity occur in the central core of the intrusion breccia. Sphalerite makes a fleeting appearance in the sulfide mineral assemblage in samples from the lower part of the intrusion breccia. Strongly contrasting pyrite compositions suggest at least two episodes of pyrite precipitation, but there is no clear morphological distinction between phases. Heazelwoodite, tentatively identified in shipboard examinations, could not be confirmed in this study.

Geochemical composition and atomic formulas of saponites, celadonites and clays from ODP Leg 168 sites

A drilling transect across the sedimented eastern flank of the Juan de Fuca Ridge, conducted during Leg 168 of the Ocean Drilling Program, resulted in the recovery of samples of volcanic basement rocks (pillow basalts, massive basalts, and volcanic glass breccias) that exhibit the effects of low-temperature hydrothermal alteration. Secondary clays are ubiquitous, with Mg-rich and Fe-rich saponite and celadonitic clays commonly accounting for several percent, and up to 10%-20% by volume. Present-day temperatures of the basement sites vary from 15° to 64°C, with the coolest site being about 0.8 Ma, and the warmest site being about 3.5 Ma. Whereas clays are abundant at sites that have been heated to present temperatures of 23°C and higher, the youngest site at 15°C has only a small trace of secondary clay alteration. Alteration increases as temperatures increase and as the volcanic basement ages. The chemical compositions of secondary clays were determined by electron microprobe, and additional trace element data were determined by both conventional nebulization inductively coupled plasma-mass spectroscopy (ICP-MS) and laser-ablation ICP-MS. Trioctahedral saponite and pyrite are characteristic of the interior of altered rock pieces, forming under conditions of low-oxygen fugacity. Dioctahedral celadonite-like clays along with iron oxyhydroxide and Mg-saponite are characteristic of oxidized haloes surrounding the nonoxidized rock interiors. Chemical compositions of the clays are very similar to those determined from other deep-sea basalts altered at low temperature. The variable Mg:Fe of saponite appears to be a systematic function both of the Mg:Fe of the host rock and the oxidation state during water-rock interaction.

Geochemistry and oxidation state of Lau Basin basalts

The backarc glasses recovered during Ocean Drilling Program Leg 135 are unique among submarine tholeiitic glasses with respect to their oxygen fugacity and sulfur concentrations. Unlike mid-ocean-ridge basalt glasses, fO2 in these samples (inferred from ratios Fe3+/Fe2+) is high and variable, and S variations (90-1140 ppm) are not coupled with FeO concentration. Strong correlations occur between the alkali and alkaline-earth elements and both fO2 (positive correlations) and S concentrations (negative correlations). Correlations between fO2 and various trace elements are strongest for those elements with a known affinity for hydrous fluids (perhaps produced during slab dehydration), suggesting the presence of a hydrous fluid with high fO2 and high alkali and alkaline earth element concentrations in the Lau Basin mantle. Concentrations of S and fO2 are strongly correlated; high fO2 samples are characterized by low S in addition to high alkali and alkaline earth element concentrations. The negative correlations between S and these trace elements are not consistent with incompatible behavior of S during crystallization. Mass balance considerations indicate that the S concentrations cannot result simply from mixing between low-S and high-S sources. Furthermore, there is no relationship between S and other trace elements or isotope ratios that might indicate that the S variations reflect mixing processes. The S variations more likely reflect the fact that when silicate coexists with an S-rich vapor phase the solubility of S in the silicate melt is a function of fO2 and is at a minimum at the fO2 conditions recorded by these glasses. The absence of Fe-sulfides and the high and variable vesicle contents are consistent with the idea that S concentrations reflect silicate-vapor equilibria rather than silicate-sulfide equilibria (as in MORB). The low S contents of some samples, therefore, reflect the high fO2 of the supra-subduction zone environment rather than a low-S source component.

Petrology and geochemistry of sideromelane glass shards from Pleistocene as layers north and south of Gran Canaria

Major elements, S, F, Cl concentrations and relative proportions of S6+ to total S were analyzed with electron microprobe in sideromelane glass shards from Pleistocene volcaniclastic sediments drilled during ODP Leg 157. Glasses are moderately to strongly evolved and represent a spectrum from alkali basalt, basanite and nephelinite through hawaiite, mugearite and tephrite to phonolitic tephrite. Measured S6+/SumS (0.03±0.98) and calculated Fe2+/Fe3+ (2.5±5.8) ratios in the melt yield preeruptive redox conditions ranging from NNO-1.4 to NNO+2.1. The morphology of the glass shards, variations of S and Cl concentrations (0.010±0.127 wt% S, 0.018±0.129 wt% Cl), calculated preeruptive temperatures (1030±1200 °C) and oxygen fugacities suggest that glasses deposited even within the same ash layers have diverse origin and may have resulted from both submarine and subaerial eruptions. Most vesicle-free glasses are characterized by high concentrations of S and represent undegassed or slightly degassed submarine lavas, whereas vesiculated glasses with low concentrations of S and Cl are strongly degassed and can be ascribed to the eruptions in shallow water or on land. Sideromelane glass shards at Sites 953 are thought to have resulted from submarine eruptions northeast of Gran Canaria, glasses at Site 954 represent mostly volcaniclastic material of shallow water submarine and subaerial eruptions on Gran Canaria and Tenerife, and glasses deposited at Site 956 resulted from submarine or explosive eruptions on Tenerife.

Mineral analysis and geochemistry of Muehlig-Hofmannfjella granitoids, Central Drooning Maud Land, East Antarctica

Ferrosilite-fayalite bearing charnockite and biotite-hornblende bearing granite are exposed in Mühling-Hofmannfjella, central Dronning Maud Land of East Antarctica. Both are interpreted as essentially parts of a single pluton in spite of their contrasting mineral assemblages. Based on petrologic and geochemical studies, it is proposed that H2O-undersaturated parent magma with igneous crustal component that fractionated under different oxygen fugacity conditions resulted in the Mühlig-Hofmannfjella granitoids.

Major and trace element concentration of melt inclusions from Zhupanovsky volcano, Kamchatka

The paper presents data on naturally quenched melt inclusions in olivine (Fo 69-84) from Late Pleistocene pyroclastic rocks of Zhupanovsky volcano in the frontal zone of the Eastern Volcanic Belt of Kamchatka. The composition of the melt inclusions provides insight into the latest crystallization stages (~70% crystallization) of the parental melt (~46.4 wt % SiO2, ~2.5 wt % H2O, ~0.3 wt % S), which proceeded at decompression and started at a depth of approximately 10 km from the surface. The crystallization temperature was estimated at 1100 ± 20°C at an oxygen fugacity of deltaFMQ = 0.9-1.7. The melts evolved due to the simultaneous crystallization of olivine, plagioclase, pyroxene, chromite, and magnetite (Ol: Pl: Cpx : (Crt-Mt) ~ 13 : 54 : 24 : 4) along the tholeiite evolutionary trend and became progressively enriched in FeO, SiO2, Na2O, and K2O and depleted in MgO, CaO, and Al2O3. Melt crystallization was associated with the segregation of fluid rich in S-bearing compounds and, to a lesser extent, in H2O and Cl. The primary melt of Zhupanovsky volcano (whose composition was estimated from data on the most primitive melt inclusions) had a composition of low-Si (~45 wt % SiO2) picrobasalt (~14 wt % MgO), as is typical of parental melts in Kamchatka and other island arcs, and was different from MORB. This primary melt could be derived by ~8% melting of mantle peridotite of composition close to the MORB source, under pressures of 1.5 ± 0.2 GPa and temperatures 20-30°C lower than the solidus temperature of 'dry' peridotite (1230-1240°C). Melting was induced by the interaction of the hot peridotite with a hydrous component that was brought to the mantle from the subducted slab and was also responsible for the enrichment of the Zhupanovsky magmas in LREE, LILE, B, Cl, Th, U, and Pb. The hydrous component in the magma source of Zhupanovsky volcano was produced by the partial slab melting under water-saturated conditions at temperatures of 760-810°C and pressures of ~3.5 GPa. As the depth of the subducted slab beneath Kamchatkan volcanoes varies from 100 to 125 km, the composition of the hydrous component drastically changes from relatively low-temperature H2O-rich fluid to higher temperature H2O-bearing melt. The geothermal gradient at the surface of the slab within the depth range of 100-125 km beneath Kamchatka was estimated at 4°C/km.

A refined solution to Earth's hidden niobium: implications for evolution of continental crust and mode of core formation

New high-precision niobium (Nb) and tantalum (Ta) concentration data are presented for early Archaean metabasalts, metabasaltic komatiites and their erosion products (mafic metapelites) from SW Greenland and the Acasta gneiss complex, Canada. Individual datasets consistently show sub-chondritic Nb/Ta ratios averaging 15.1+/-11.6. This finding is discussed with regard to two competing models for the solution of the Nb-deficit that characterises the accessible Earth. Firstly, we test whether Nb could have sequestered into the core due to its slightly siderophile (or chalcophile) character under very reducing conditions, as recently proposed from experimental evidence. We demonstrate that troilite inclusions of the Canyon Diablo iron meteorite have Nb and V concentrations in excess of typical chondrites but that the metal phase of the Grant, Toluca and Canyon Diablo iron meteorites do not have significant concentrations of these lithophile elements. We find that if the entire accessible Earth Nb-deficit were explained by Nb in the core, only ca. 17% of the mantle could be depleted and that by 3.7 Ga, continental crust would have already achieved ca. 50% of its present mass. Nb/Ta systematics of late Archaean metabasalts compiled from the literature would further require that by 2.5 Ga, 90% of the present mass of continental crust was already in existence. As an alternative to this explanation, we propose that the average Nb/Ta ratio (15.1+/-11.6) of Earth's oldest mafic rocks is a valid approximation for bulk silicate Earth. This would require that ca. 13% of the terrestrial Nb resided in the Ta-free core. Since the partitioning of Nb between silicate and metal melts depends largely on oxygen fugacity and pressure, this finding could mean that metal/silicate segregation did not occur at the base of a deep magma ocean or that the early mantle was slightly less reducing than generally assumed. A bulk silicate Earth Nb/Ta ratio of 15.1 allows for depletion of up to 40% of the total mantle. This could indicate that in addition to the upper mantle, a portion of the lower mantle is depleted also, or if only the upper mantle were depleted, an additional hidden high Nb/Ta reservoir must exist. Comparison of Nb/Ta systematics between early and late Archaean metabasalts supports the latter idea and indicates deeply subducted high Nb/Ta eclogite slabs could reside in the mantle transition zone or the lower mantle. Accumulation of such slabs appears to have commenced between 2.5 and 2.0 Ga. Regardless of these complexities of terrestrial Nb/Ta systematics, it is shown that the depleted mantle Nb/Th ratio is a very robust proxy for the amount of extracted continental crust, because the temporal evolution of this ratio is dominated by Th-loss to the continents and not Nb-retention in the mantle. We present a new parameterisation of the continental crust volume versus age curve that specifically explores the possibility of lithophile element loss to the core and storage of eclogite slabs in the transition zone. (C) 2003 Elsevier Science B.V. All rights reserved.

Petrologia do Plutão Bom Jardim de Goiás (PBJG): implicação na evolução neoproterozoica da Província Tocantins

The Bom Jardim de Goiás Pluton (PBJG) is a semi-circular body, located in the central portion of the Tocantins Province, intrusive into orthogneisses and metassupracrustals of the Arenópolis Magmatic Arc. These metasupracrustals present a low to moderate dipping banding or schistosity, have a low to moderate angle of banding / foliation, defined by mica, andalusite and sillimanite and cordierite, which characterize an amphibolite facies metamorphism. This structure is crosscut by the emplacement of the PBJG rocks. The abrupt nature of the contacts and the absence of ductile structures indicate that the intrusion took place in a relatively cold crust. Under petrographic grounds, the pluton consists mainly of monzodiorites, tonalite and granodiorite, following the low to medium-K calk-alkaline alkaline trend. Rocks of the PBJG have hornblende and biotite as the main mafic phases, besides subordinate clinopyroxene, titanite, epidote and opaque. Late dikes of leucogranite contain only mineral biotite as relevant accessory mineral. One U-Pb zircon dating of a monzodiorite yielded an age of 550 ± 12 Ma (MSWD = 1.06). Whole-rock and mineral chemistry suggest that the studied rocks are calc-alkaline, having evolved by fractional crystallization of Ca- and Fe-Mg minerals under high oxygen fugacity. Using the amphibole-plagioclase geothermometer and the Al-in amphibole geobarometer, we calculate temperatures and pressures of, respectively, 692-791 °C e 2.4-5.0 kbar for the intrusion of the PBJG, which is corroborated by previous metamorphic assemblages in the country rocks. The geological, geochemical and geochronological features of PBJG demonstrate their post-tectonic or post-collisional nature, with emplacement into an already uplifted and relatively cool crust at the end of brasiliano orogeny in this portion of the Tocantins Province.

Petrologia do Plutão Bom Jardim de Goiás (PBJG): implicação na evolução neoproterozoica da Província Tocantins

The Bom Jardim de Goiás Pluton (PBJG) is a semi-circular body, located in the central portion of the Tocantins Province, intrusive into orthogneisses and metassupracrustals of the Arenópolis Magmatic Arc. These metasupracrustals present a low to moderate dipping banding or schistosity, have a low to moderate angle of banding / foliation, defined by mica, andalusite and sillimanite and cordierite, which characterize an amphibolite facies metamorphism. This structure is crosscut by the emplacement of the PBJG rocks. The abrupt nature of the contacts and the absence of ductile structures indicate that the intrusion took place in a relatively cold crust. Under petrographic grounds, the pluton consists mainly of monzodiorites, tonalite and granodiorite, following the low to medium-K calk-alkaline alkaline trend. Rocks of the PBJG have hornblende and biotite as the main mafic phases, besides subordinate clinopyroxene, titanite, epidote and opaque. Late dikes of leucogranite contain only mineral biotite as relevant accessory mineral. One U-Pb zircon dating of a monzodiorite yielded an age of 550 ± 12 Ma (MSWD = 1.06). Whole-rock and mineral chemistry suggest that the studied rocks are calc-alkaline, having evolved by fractional crystallization of Ca- and Fe-Mg minerals under high oxygen fugacity. Using the amphibole-plagioclase geothermometer and the Al-in amphibole geobarometer, we calculate temperatures and pressures of, respectively, 692-791 °C e 2.4-5.0 kbar for the intrusion of the PBJG, which is corroborated by previous metamorphic assemblages in the country rocks. The geological, geochemical and geochronological features of PBJG demonstrate their post-tectonic or post-collisional nature, with emplacement into an already uplifted and relatively cool crust at the end of brasiliano orogeny in this portion of the Tocantins Province.

Assessing the Oxidative History of Miller Range Martian Meteorites

Miller Range (MIL) Martian meteorites are oxidized nakhlites. Early studies attribute their oxidation to reduction-oxidation reactions involving assimilated sulfate. I utilize the sulfur isotope and major element composition of the MIL pairs to assess their oxidative history. MIL sulfides display an average sulfur isotope composition that is different from Nakhla sulfate and sulfide. The sulfur isotope differences produce a mixing array between juvenile sulfur and mass-independent sulfur signatures, indicating assimilation of anomalous sulfur into the melt. I estimate an fO2 of QFM (+3.5 ± 0.4) and a sulfur content of 360 ppm ± 12 – 1300 ppm ± 50. With these results, I test the hypothesis of sulfate assimilation through models of charge balance, isotope mixing, and degassing of sulfur bearing compounds. I conclude that sulfate assimilation was significant in the oxidation of the MIL pairs but, additional oxidants were assimilated.

Carbonate properties and oxygen concentrations at time series station DYFAMED

Monthly measurements of pH, alkalinity and oxygen over two years (February 1998-February 2000) at the Dyfamed site in the central zone of the Ligurian-Provençal Basin of the Mediterranean made it possible to assess the vertical distributions (5-2000 m) and the seasonal variations of these properties. Alkalinity varies linearly with salinity between surface water and the Levantine Intermediate Water (marked by a maximum of temperature and salinity). In deep water, total alkalinity is also correlated linearly to salinity, but the slope of the regression line is 15% less. In surface water, the pH at 25°C varies between 7.91 and 8.06 on the total proton scale depending upon the season. The lowest values are observed in winter, the highest in spring and in summer. These variations are primarily due to biological production. The pH goes through a minimum around 150-200 m and a small maximum below the intermediate water. The total dissolved inorganic carbon content (deduced from pH and alkalinity) is variable in surface water (2205-2310 ?mol/kg) and has a maximum in intermediate water, which is related to the salinity maximum. Normalized total inorganic carbon at a constant salinity is strongly negatively correlated with pH at 25°C. The fugacity of CO2, (fCO2) varies between 320 and 430 ?atm in surface water, according to the season. Below the seasonal thermocline, the maximum fCO2 (about 410 ?atm) is located around 150-200 m. The presence of a minimum of oxygen in the intermediate water of this area has been observed for several years, but our measurements made it possible to specify the relationship between oxygen and salinity in deep water. Data from the intense vertical mixing during the winters of 1999 and 2000 were used to calculate the oxygen quantity exchanged with the atmosphere during these periods. The estimated quantity of oxygen entering the Mediterranean Sea exceeds that deduced from exchange coefficients calculated with the formula of Wanninkhof and McGillis. During the vertical mixing in the 1999 winter, fCO2 in surface water was on average below equilibrium with atmospheric fCO2, thus implying that CO2 was entering the sea. However, on this time scale, even with high exchange coefficients, the estimated CO2 uptake had no significant influence on the inorganic carbon content in the water column.

Seawater carbonate chemistry, calcification rate, oxygen production, maximum quantum yield, symbiont density, chlorophyll concentration and crystal width of Halimeda macroloba, Halimeda cylindracea and Marginopora vertebralis during experiments, 2011

The effects of elevated CO2 and temperature on photosynthesis and calcification in the calcifying algae Halimeda macroloba and Halimeda cylindracea and the symbiont-bearing benthic foraminifera Marginopora vertebralis were investigated through exposure to a combination of four temperatures (28°C, 30°C, 32°C, and 34°C) and four CO2 levels (39, 61, 101, and 203 Pa; pH 8.1, 7.9, 7.7, and 7.4, respectively). Elevated CO2 caused a profound decline in photosynthetic efficiency (FV : FM), calcification, and growth in all species. After five weeks at 34°C under all CO2 levels, all species died. Chlorophyll (Chl) a and b concentration in Halimeda spp. significantly decreased in 203 Pa, 32°C and 34°C treatments, but Chl a and Chl c2 concentration in M. vertebralis was not affected by temperature alone, with significant declines in the 61, 101, and 203 Pa treatments at 28°C. Significant decreases in FV : FM in all species were found after 5 weeks of exposure to elevated CO2 (203 Pa in all temperature treatments) and temperature (32°C and 34°C in all pH treatments). The rate of oxygen production declined at 61, 101, and 203 Pa in all temperature treatments for all species. The elevated CO2 and temperature treatments greatly reduced calcification (growth and crystal size) in M. vertebralis and, to a lesser extent, in Halimeda spp. These findings indicate that 32°C and 101 Pa CO2, are the upper limits for survival of these species on Heron Island reef, and we conclude that these species will be highly vulnerable to the predicted future climate change scenarios of elevated temperature and ocean acidification.

Seawater carbonate chemistry and carbon and oxygen isotopes during experiments with planktonic foraminifera Orbulina universa and Globigerina bulloides, 1997

Stable oxygen and carbon isotope measurements on biogenic calcite and aragonite have become standard tools for reconstructing past oceanographic and climatic change. In aquatic organisms, 18O/16O ratios in the shell carbonate are a function of the ratio in the sea water and the calcification temperature (Epstein et al., 1953). In contrast, 13C/12C ratios are controlled by the ratio of dissolved inorganic carbon in sea water and physiological processes such as respiration and symbiont photosynthesis (Spero et al., 1991, doi:10.1029/91PA02022). These geochemical proxies have been used with analyses of foraminifera shells to reconstruct global ice volumes (Shackleton and Opdyke, 1973, doi:10.1016/0033-5894(73)90052-5), surface and deep ocean temperatures (Broecker, 1986, doi:10.1016/0033-5894(86)90087-6; Labeyrie et al., 1987, doi:10.1038/327477a0), ocean circulation changes (Duplessy et al., 1988, doi:10.1029/PA003i003p00343) and glacial-interglacial exchange between the terrestrial and oceanic carbon pools (Sackleton, 1977). Here, we report experimental measurements on living symbiotic and non-symbiotic plankton foraminifera (Orbulina universa and Globigerina bulloides respectively) showing that the 13C/12C and 18O/16O ratios of the calcite shells decrease with increasing seawater [CO3 2-]. Because glacial-period oceans had higher pH and [CO3 2-] than today (Sanyal et al., 1995, doi:10.1038/373234a0), these new relationships confound the standard interpretation of glacial foraminiferal stable-isotope data. In particular, the hypothesis that the glacial-interglacial shift in the 13C/12C ratio was due to a transfer of terrestrial carbon into the ocean(Shackleton ,1977) can be explained alternatively by an increase in ocean alkalinity (Lea et al., 1996). A carbonate-concentration effect could also help explain some of the extreme stable-isotope variations during the Proterozoic and Phanerozoic aeons (Kaufman et al., 1993, doi:10.1016/0012-821X(93)90254-7).