To determine the speciation and concentrations of dissolved COH volatiles in graphite-saturated martian primitive magmas, we conducted piston-cylinder experiments on graphite-encapsulated synthetic ...melt of Adirondack-class Humphrey basaltic composition. Experiments were performed over three orders of magnitude in oxygen fugacity (IW+2.3 to IW−0.8), and at pressures (1–3.2GPa) and temperatures (1340–1617°C) similar to those of possible martian source regions. Oxygen fugacities were determined from compositions of coexisting silicate melt+FePt alloy, olivine+pyroxene+FePt alloy, or melt+FeC liquid. Infrared spectra of quenched glasses all show carbonate absorptions at 1430 and 1520cm−1, with CO2 concentrations diminishing under more reduced conditions, from 0.50wt% down to 26ppm. Carbon contents of silicate glasses and FeC liquids were measured using secondary ion mass spectrometry (SIMS) yielding 36–716ppm and 6.71–7.03wt%, respectively. Fourier transform infrared (FTIR) and SIMS analysis produced similar H2O contents of 0.26–0.85 and 0.29–0.40wt%, respectively. Raman spectra of glasses reveal evidence for OH− ions, but no indication of methane-related species. FTIR-measured concentrations of dissolved carbonate diminish linearly with oxygen fugacity, but more reduced conditions yield greater dissolved carbonate concentrations than would be expected based on oxidized conditions in previous work. C contents of silicate glasses determined by SIMS are consistently higher than C as carbonate determined by FTIR. Their difference, termed non-carbonate C, correlates well with additional IR absorptions found in reduced glasses (fO2<IW+0.4) at 1615, 2205, and 3370cm−1. These absorption bands are not seen in more oxidized glasses, except B441 (IW+1.7), presumably because they represent reduced C-bearing complexes. The 2205cm−1 peak is attributed to a CO complex, possibly an Fe-carbonyl ion. The 1615cm−1 peak does not correlate with that at 2205cm−1, but does correlate with non-carbonate C and is in a region commonly associated with CO bonding. The origin of the peak at 3370cm−1 is poorly understood and could potentially be owing to a variety of COH species or to NH bonding. The intensities of the 1615 and 3370cm−1 peaks correlate with each other leading us to provisionally attribute both to an unspecified complex with both CO and NH bonds. These results suggest that dissolved species such as carbonyl or other CO-bearing species could be a significant source of C fluxes to the martian atmosphere, with minor additions of CO2 and negligible methane contributions. By assuming that degassed, reduced C ultimately becomes atmospheric CO2, reduced C outgassing may be incorporated in models of martian atmospheric evolution. At Humphrey source region conditions (1350±50°C, 1.2±0.1GPa) the total C contents are equivalent to 1200ppm CO2 at IW+1 and 475ppm CO2 at IW, which are 2 and 4 times higher than the CO2 derived from CO32− alone. For reasonable magmatic fluxes over the last 4.5Ga of martian history, such graphite-saturated magmas would produce 0.25 and 0.60bars from sources at IW and IW+1, significantly more than expected solely from consideration of dissolved CO2. The carbon contents of FeC liquids in this study are consistent with graphite-saturated carbide liquids becoming more C-rich with increasing temperature. Experiments with melt and FeC liquid have values of DCall/sil between 1.3×103 and 2.2×103, potentially allowing planetary mantles to retain significant C following core formation.
The thermodynamic properties of 254 end‐members, including 210 mineral end‐members, 18 silicate liquid end‐members and 26 aqueous fluid species are presented in a revised and updated internally ...consistent thermodynamic data set. The PVT properties of the data set phases are now based on a modified Tait equation of state (EOS) for the solids and the Pitzer & Sterner (1995) equation for gaseous components. Thermal expansion and compressibility are linked within the modified Tait EOS (TEOS) by a thermal pressure formulation using an Einstein temperature to model the temperature dependence of both the thermal expansion and bulk modulus in a consistent way. The new EOS has led to improved fitting of the phase equilibrium experiments. Many new end‐members have been added, including several deep mantle phases and, for the first time, sulphur‐bearing minerals. Silicate liquid end‐members are in good agreement with both phase equilibrium experiments and measured heat of melting. The new dataset considerably enhances the capabilities for thermodynamic calculation on rocks, melts and aqueous fluids under crustal to deep mantle conditions. Implementations are already available in thermocalc to take advantage of the new data set and its methodologies, as illustrated by example calculations on sapphirine‐bearing equilibria, sulphur‐bearing equilibria and calculations to 300 kbar and 2000 °C to extend to lower mantle conditions.
The boron to calcium ratio (B/Ca) in biogenic CaCO3 is being increasingly utilized as a proxy for past ocean carbonate chemistry. However, B/Ca of cultured and core-top foraminifers show dependence ...on multiple physicochemical seawater properties and only a few of those have been inorganically tested for their impacts. Accordingly, our understanding of the controls on foraminiferal B/Ca and thus how to interpret B/Ca in fossil shells is incomplete. To gain a clearer understanding of the B incorporation mechanism, we performed inorganic calcite precipitation experiments using a pH-stat system. As previously reported, we confirm that B/Ca in calcite increases with both fluid pH and total B concentration (denoted as BT, where BT=B(OH)3+B(OH)4−). We provide the first evidence that B/Ca also increases with the concentration of total dissolved inorganic carbon (DIC) and calcium ion. With the exception of the BT experiments, these chemical manipulations were accompanied by an increase in calcite saturation, and accordingly precipitation rate (denoted as R). But when pH and Ca2+ were jointly varied at a fixed saturation level to maintain relatively constant R at different pH and Ca2+ combinations, B/Ca was insensitive to both pH and Ca2+ changes. These experimental results unequivocally suggest kinetic effects related to R on B/Ca. Furthermore, with a suite of chemical manipulations we found that the B/Ca variability is explicable by just R and the BT/DIC ratio in the parent fluids. This observation was particularly robust for relatively rapidly precipitated samples, whereas for relatively slowly precipitated samples, it was somewhat ambiguous whether the BT/DIC or B(OH)4−/HCO3− ratio provides a better fit to the experimental data. Nonetheless, our experimental results can be considered as indirect evidence for incorporation of both B(OH)4− and B(OH)3 into calcite. We propose a simple mathematical expression to describe the mode of B incorporation into synthetic calcite that depends only on the fluid BT/DIC ratio and the precipitation rate R. This novel finding has important implications for future calibrations and applications of the B/Ca proxy as well as the δ11B paleo-pH proxy.
This paper reports the fractionation of Δ47 during the digestion of dolomite in phosphoric acid between 25°C and 90°C using five different samples, including three of Pliocene age from the Bahamas, ...one from the Jurassic in the Middle East, and one obtained from the National Institute of Standards (NIST 88b). The composition of the dolomites analyzed varied from Ca0.56Mg0.44CO3 to Ca0.50Mg0.50CO3. Fractionation values were also compared between the common acid bath and sealed vessel techniques at various temperatures. No statistically significant differences were observed either between these two methods or as a function of the dolomite’s stoichiometry.
These data produce a difference in fractionation of 0.153±0.011‰ for dolomite samples digested at 90°C compared to those reacted at 25°C, a value higher and statistically different to previously published values. Utilization of this value in a study of dolomites from a core drilled on the island of San Salvador in the Bahamas yielded temperatures and δ18Ofluid values which agree with previous interpretations on the formation of these dolomites. Application of this value to other published studies produces lower estimates of temperature and δ18Ofluid values for the dolomitization process that are more consistent with the geologic models suggested for these studies.
Basaltic magmas constitute the primary mass flux from Earth's mantle to its crust, carrying information about the conditions of mantle melting through which they were generated. As such, changes in ...the average basaltic geochemistry through time reflect changes in underlying parameters such as mantle potential temperature and the geodynamic setting of mantle melting. However, sampling bias, preservation bias, and geological heterogeneity complicate the calculation of representative average compositions. Here we use weighted bootstrap resampling to minimize sampling bias over the heterogeneous rock record and obtain maximally representative average basaltic compositions through time. Over the approximately 4 Ga of the continental rock record, the average composition of preserved continental basalts has evolved along a generally continuous trajectory, with decreasing compatible element concentrations and increasing incompatible element concentrations, punctuated by a comparatively rapid transition in some variables such as La/Yb ratios and Zr, Nb, and Ti abundances approximately 2.5 Ga ago. Geochemical modeling of mantle melting systematics and trace element partitioning suggests that these observations can be explained by discontinuous changes in the mineralogy of mantle partial melting driven by a gradual decrease in mantle potential temperature, without appealing to any change in tectonic process. This interpretation is supported by the geochemical record of slab fluid input to continental basalts, which indicates no long-term change in the global proportion of arc versus non-arc basaltic magmatism at any time in the preserved rock record.
•Mantle potential temperature and mantle melting extent have declined throughout Earth history.•Continental basalts record a constant proportion of subduction magmatism since the early Archean.•Discontinuities in the basaltic geochemistry at 2.5 Ga are a result of mantle melting systematics.
We report new experimental data on the composition of magmatic amphiboles synthesised from a variety of granite (sensu lato) bulk compositions at near-solidus temperatures and pressures of ...0.8–10 kbar. The total aluminium content (Al
tot
) of the synthetic calcic amphiboles varies systematically with pressure (
P
), although the relationship is nonlinear at low pressures (<2.5 kbar). At higher pressures, the relationship resembles that of other experimental studies, which suggests of a general relationship between Al
tot
and P that is relatively insensitive to bulk composition. We have developed a new Al-in-hornblende geobarometer that is applicable to granitic rocks with the low-variance mineral assemblage: amphibole + plagioclase (An
15–80
) + biotite + quartz + alkali feldspar + ilmenite/titanite + magnetite + apatite. Amphibole analyses should be taken from the rims of grains, in contact with plagioclase and in apparent textural equilibrium with the rest of the mineral assemblage at temperatures close to the haplogranite solidus (725 ± 75 °C), as determined from amphibole–plagioclase thermometry. Mean amphibole rim compositions that meet these criteria can then be used to calculate
P
(in kbar) from Al
tot
(in atoms per formula unit, apfu) according to the expression:
P
kbar
=
0.5
+
0.331
8
×
Al
tot
+
0.995
4
×
Al
tot
2
This expression recovers equilibration pressures of our calibrant dataset, comprising both new and published experimental and natural data, to within ±16 % relative uncertainty. An uncertainty of 10 % relative for a typical Al
tot
value of 1.5 apfu translates to an uncertainty in pressure estimate of 0.5 kbar, or 15 % relative. Thus the accuracy of the barometer expression is comparable to the precision with which near-solidus amphibole rim composition can be characterised.
The maximum-pressure PT conditions (Pmax–T) and prograde PT paths of exhumed subduction-related metamorphic rocks are compared to predictions of PT conditions from computational thermal models of ...subduction systems. While the range of proposed models encompasses most estimated Pmax–T conditions, models predict temperatures that are on average colder than those recorded by exhumed rocks. In general, discrepancies are greatest for Pmax<2 GPa, where only a few of the highest-T model paths overlap petrologic observations and model averages are 100–300 °C colder than average conditions recorded by rocks. Prograde PT paths similarly indicate warmer subduction than typical models. Both petrologic estimates and models have inherent biases. Petrologic analysis may overestimate temperatures at Pmax where overprinting occurs during exhumation, although PT paths suggest that relatively warm conditions are experienced by rocks on the prograde subduction path. Models may underestimate temperatures at depth by neglecting shear heating, hydration reactions and fluid and rock advection. Our compilation and comparison suggest that exhumed high-P rocks provide a more accurate constraint on PT conditions within subduction zones, and that those conditions may closely represent the subduction geotherm. While exhumation processes in subduction zones require closer petrologic scrutiny, the next generation of models should more comprehensively incorporate all sources of heat. Subduction-zone thermal structures from currently available models appear to be inaccurate, and this mismatch has wide-reaching implications for our understanding of global geochemical cycles, the petrologic structure of subduction zones, and fluid–rock interactions and seismicity within subduction zones.
•P–T estimates are compiled for subduction-zone metamorphic rocks.•At P<2 GPa rock PT conditions and paths are hotter by 100–300 °C than common thermal models.•Errors in PT estimates cannot explain the discrepancies between models and rocks.•Many thermal models ignore heat from shearing, fluid/rock advection and hydration.•Numerous applications presuming cold subduction geotherms must be reevaluated.
We present new experiments, combined with a re-evaluation of published data, to characterize the topology of the silicate-carbonate two-liquid solvus in the five-component system ...SiO2-Na2O-Al2O3-CaO-CO2 (SNAC + CO2). Conjugate liquid compositions have been determined for a wide range of pressures (0·1-2·5 GPa) and temperatures (1225-1700°C) as well as variable degrees of CO2 saturation. The expansion of the two-liquid field with increasing pressure and/or decreasing temperature, and the contraction of the two-liquid field for conditions where P
CO2 < P
total is accurately presented for the first time. The shape of the two-liquid solvus suggests that alkali-rich carbonatites can have a range of SiO2 + Al2O3 contents down to very low values (<1 wt %), but that low-alkali or alkali-free immiscible carbonatites will always have SiO2 + Al2O3 contents greater than 10-15 wt %. The most commonly observed carbonatite rock compositions observed at the Earth's surface all tend towards low contents of alkalis SiO2 and Al2O3 and would have fractionated silicate phases from the carbonatite parental melts, possibly associated with alkali loss to coexisting fluids. Our results also show that carbonate liquid exsolution can occur from a CO2-undersaturated (P
CO2 < P
tot) silicate melt. Although the expanded high-pressure miscibility gap appears favourable for producing natural silicate melt compositions, a low-pressure (<1·0 GPa) magma chamber in the crust or perhaps in the shallow mantle below a rift provides the most likely environment for immiscibility to arise owing to the lower CO2 demand of the silicate magma. Unusual textures in some experiments, suggestive of a deformable liquid state for the CaCO3 phase, are conclusively shown to be characteristic of a non-quenchable, high-temperature polymorph of solid calcite. Similar calcite globules with this rounded appearance, which are also observed in some nephelinite lavas and mantle xenoliths, must be solid calcite and not immiscible liquids. This is consistent with the high SiO2 + Al2O3 requirement of low-alkali or alkali-free immiscible carbonate liquids.
The objective of this research is to assess critically the experimental rate data for O2 oxidation of dissolved Mn(II) species at 25C and to interpret the rates in terms of the solution species of ...Mn(II) in natural waters. A species kinetic rate expression for parallel paths expresses the total rate of Mn(II) oxidation as Ski aij, where ki is the rate constant of species i and aij is the species concentration fraction in solution j. Among the species considered in the rate expression are Mn(II) hydrolysis products, carbonate complexes, ammonia complexes, and halide and sulfate complexes, in addition to the free aqueous ion. Experiments in three different laboratory buffers and in seawater yield an apparent rate constant for Mn(II) disappearance, kapp, j ranging from 8.6 x 10-5 to 2.5 x 10-2 (M-1s-1), between pH 8.03 and 9.30, respectively. Observed values of kapp exceed predictions based on Marcus outer-sphere electron transfer theory by more than four orders of magnitude, lending strong support to the proposal that Mn(II) + O2 electron transfer follows an inner-sphere path. A multiple linear regression analysis fit of the observed rates to the species kinetic rate expression yields the following oxidation rate constants (M-1s-1) for the most reactive species: MnOH+, 1.66 x 10-2; Mn(OH)2, 2.09 x 101; and Mn(CO3)2-, 8.13 x 10-2. The species kinetic rate expression accounts for the influence of pH and carbonate on oxidation rates of Mn(II), through complex formation and acid-base equilibria of both reactive and unreactive species. At pH not, vert, similar8, the greater fraction of the total rate is carried by MnOH+. At pH greater than not, vert, similar8.4, the species Mn(OH)2 and Mn(CO3)2- make the greater contributions to the total rate.
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