We have measured hydrogen partition coefficients between nominally anhydrous minerals (olivine, pyroxenes) and basaltic melts in 13 hydrous melting experiments performed at upper mantle P‐T ...conditions (1–2 GPa and 1230–1380°C). Resulting liquids have 3.1–6.4 wt.% H2O and average mineral/melt partition coefficients as follows: DHol/melt = 0.0017 ± 0.0005 (n = 9), DHopx/melt = 0.019 ± 0.004 (n = 8), and DHcpx/melt = 0.023 ± 0.005 (n = 2). Mineral/mineral partition coefficients are DHol/opx = 0.11 ± 0.01 (n = 4), DHol/cpx = 0.08 ± 0.01 (n = 2) and DHcpx/opx = 1.4 ± 0.3 (n = 1). These measurements confirm that water behaves similarly to Ce during mantle melting (DHperidotite/melt is ∼0.009). For mantle water concentrations of 50–200 ppm, the onset of melting is 5–20 km deeper than the dry solidus, less than previous estimates.
Comprehensive quantitative theoretical evaluation of water–rock interactions in the Earth has long been restricted to a pressure of 5.0kb – too low to address processes involving deep aqueous fluids. ...Yet such fluids are thought to play an important role in the long-term geologic cycling of many chemical elements. A reason for this restriction is the lack of information on the dielectric constant of water (εH2O) needed for the revised Helgeson–Kirkham–Flowers (HKF) equations of state for aqueous species. Equation of state coefficients are available for hundreds of aqueous species in SUPCRT92, but calculations using these species can only be made to 5.0kb (Shock et al., 1992).
In the present study, the applicability of the revised HKF equations of state for aqueous species was extended to 60kb by developing estimates of (εH2O). We used a statistical mechanically-based equation for the dielectric constant of a hard-sphere fluid applicable to water (Franck et al., 1990). The equation was calibrated with experimental data, and data from a comprehensive analysis of the literature (Fernández et al., 1997), and then used to calculate (εH2O) to a density of 1.1gcm−3. The values of ln(εH2O) were found to be linear with ln(ρH2O) which enabled empirical extrapolation of (εH2O) to 60kb. Values of ρH2O were computed with a recent comprehensive evaluation consistent with experimental data and a molecular dynamics model for water (Zhang and Duan, 2005).
The resulting dielectric constants were tested at 727°C and 58kb by comparison with the results of ab initio molecular dynamics calculations (Pan et al., 2013). Additional testing was carried out by computing standard Gibbs free energies of aqueous species using the new values of (εH2O) and ρH2O in the revised HKF equations to predict equilibrium constants which in turn enabled calculation of the solubilities of quartz and corundum for comparison with experimental measurements to 20kb and 1100°C. Our results strongly suggest that geochemically useful predictions can now be made that will facilitate analysis of water–rock interactions in the Earth at depths much greater than previously possible.
Many fields of Earth sciences benefit from the knowledge of mineral formation temperatures. For example, carbonates are extensively used for reconstruction of the Earth’s past climatic variations by ...determining ocean, lake, and soil paleotemperatures. Furthermore, diagenetic minerals and their formation or alteration temperature may provide information about the burial history of important geological units and can have practical applications, for instance, for reconstructing the geochemical and thermal histories of hydrocarbon reservoirs.
Carbonate clumped isotope thermometry is a relatively new technique that can provide the formation temperature of carbonate minerals without requiring a priori knowledge of the isotopic composition of the initial solution. It is based on the temperature-dependent abundance of the rare 13C–18O bonds in carbonate minerals, specified as a Δ47 value. The clumped isotope thermometer has been calibrated experimentally from 1°C to 70°C. However, higher temperatures that are relevant to geological processes have so far not been directly calibrated in the laboratory.
In order to close this calibration gap and to provide a robust basis for the application of clumped isotopes to high-temperature geological processes we precipitated CaCO3 (mainly calcite) in the laboratory between 23 and 250°C. We used two different precipitation techniques: first, minerals were precipitated from a CaCO3 supersaturated solution at atmospheric pressure (23–91°C), and, second, from a solution resulting from the mixing of CaCl2 and NaHCO3 in a pressurized reaction vessel at a pressure of up to 80bar (25–250°C).
The calibration lines of both experimental approaches overlap and agree in the slopes with theoretical estimates and with other calibration experiments in which carbonates were reacted with phosphoric acid at temperatures above 70°C. Our study suggests a universal Δ47-T calibration (T in K, Δ47 in ‰):Δ47=0.98(±0.01)·(-3.407·109/T4+2.365·107/T3-2.607·103/T2-5.880/T)+0.293(±0.004)This new Δ47-T calibration (given in the absolute reference frame), that extends the experimentally calibrated temperature range for clumped isotopes to 250°C, can be applied to carbonates that grew at intermediate temperatures (20–250°C).
In order to better understand the behavior of highly siderophile elements (HSEs: Os, Ir, Ru, Rh, Pt, Pd, Au, Re), Ag, Pb and chalcogens (As, Se, Sb, Te and Bi) during the solidification of sulfide ...magmas, we have conducted a series of experiments to measure partition coefficients (D values) between monosulfide solid solution (MSS) and sulfide melt, as well as MSS and intermediate solid solution (ISS), at 0.1MPa and 860–926°C, log fS2 −3.0 to −2.2 (similar to the Pt–PtS buffer), with fO2 controlled at the fayalite–magnetite–quartz (FMQ) buffer. The IPGEs (Os, Ir, Ru), Rh and Re are found to be compatible in MSS relative to sulfide melt with D values ranging from ∼20 to ∼5, and DRe/DOs of ∼0.5. Pd, Pt, Au, Ag, Pb, as well as the chalcogens, are incompatible in MSS, with D values ranging from ∼0.1 to ∼1×10−3. For the same metal/sulfur ratio, D values for the IPGEs, Rh and Re are systematically larger than most past studies, correlating with higher oxygen content in the sulfide liquid, reflecting the significant effect of oxygen on increasing the activity coefficients for these elements in the melt phase. MSS/ISS partitioning experiments reveal that Ru, Os, Ir, Rh and Re are partitioned into MSS by a factor of >50, whereas Pd, Pt, Ag, Au and the chalcogens partition from weakly (Se, As) to strongly (Ag, Au) into ISS. Uniformly low MSS- and ISS- melt partition coefficients for the chalcogens, Pt, Pd, Ag and Au will lead to enrichment in the residual sulfide liquid, but D values are generally too large to reach early saturation in Pt–Pd-chalcogen-rich accessory minerals, based on current solubility estimates. Instead, these phases likely precipitate at the last dregs of crystallization. Modeled evolution curves for the PGEs and chalcogens are in reasonably good agreement with whole-rock sulfide compositions for the McCreedy East deposit (Sudbury, Ontario), consistent with an origin by crystallization of MSS, then MSS+ISS from sulfide magma.
The application of the Ti-in-zircon thermometer to granitic rock requires consideration of
a
TiO
2
and
a
SiO
2
during zircon crystallization. Thermodynamic software programs such as rhyolite-MELTS or ...Perple_X permit the estimation of
a
TiO
2
and
a
SiO
2
values from whole-rock geochemical data as a function of pressure and temperature. Model calculations carried out on a set of 14 different granite types at 2 kbar, 5 kbar, and H
2
O = 3 wt% show
a
SiO
2
during zircon crystallization close to 1 (0.75–1) and
a
TiO
2
generally far below unity (0.1–0.6). This would suggest that Ti-in-zircon temperatures for granites must be significantly upward corrected relative to the original TiO
2
- and SiO
2
-saturated calibration of the thermometer. Both the rhyolite-MELTS and Perple_X calculations indicate that
a
TiO
2
is typically around 0.5 in ilmenite-bearing granites. Thus, for ilmenite-series granites (that is, almost all S-type and many I-type granites), it could be a reasonable first order approximation to apply a constant temperature correction of + 70 °C to the Ti-in-zircon thermometer. Granites lacking the paragenesis zircon–ilmenite, that is, some A-type granites and a few special I-type granites may have even lower
a
TiO
2
(0.1–0.5) and some of them may require a huge upward correction of Ti-in-zircon temperatures on the order of 100–200 °C. Using a set of Ti-in-zircon measurements from a Variscan granite of the Bohemian Massif, we introduce a novel
T
-dependent
a
TiO
2
and
a
SiO
2
correction of Ti-in-zircon calculated temperatures which is based on
a
TiO
2
-,
a
SiO
2
–
T
functions modelled with rhyolite-MELTS. This method takes into account that early and late zircons in granitic systems may crystallize at different
a
SiO
2
and
a
TiO
2
. Furthermore, we highlight the usefulness of comparing the corrected results of Ti-in-zircon thermometry with bulk-rock-Zr-based zircon solubility thermometry and ideal zircon crystallization temperature distributions for granites, and we present a graphical method that enables this comparison. In addition, this paper addresses the problem that Ti-in-zircon measurements are commonly collected with only moderate spatial analytical resolution, which leads to an averaging effect and to difficulties in recording accurate crystallization temperatures. Therefore, we propose that Ti-in-zircon thermometry for granites should generally rely on the more representative median-
T
(
T
med
) value of a series of zircon analyses. Peak magma temperatures will be, in general, 35–50 °C above
T
med
, as can be modelled using zircon crystallization temperature distributions.
This chapter explores the growing body of stable and radiogenic Ca isotope measurements in high temperature terrestrial materials and covers the emerging applications for Ca isotope variability in ...igneous and metamorphic rocks and minerals. Calcium isotope fractionation at high temperature has been found to lead to larger effects than initially theorized, sometimes even exceeding those found in low temperature near-surface environments, yet many of the underlying fractionation mechanisms still remain poorly understood. Igneous whole-rocks span a δ44Ca range of ~2‰ (−0.9 to +0.9‰, relative to bulk-silicate Earth) and their constituent minerals have a range of ~2.5‰ (−1.2 to +1.3‰). Metamorphic whole rocks span a larger δ44Ca range of ~6‰ (−2.5 to +3.3‰), while metamorphic mineral separates span ~8‰ (−2.2 to +5.8‰). Observed Ca isotope variations can stem from a variety of sources including: (i) isolation of isotopically-distinct mineral components, such as during melting or crystallization, (ii) mixing of isotopically-distinct reservoirs into magmatic sources, such as during mantle metasomatism or assimilation of distinct crustal components, and (iii) isotopic rate differences (kinetic effects) during melting, crystallization, transport, and metamorphism, which are often governed by Ca diffusion. The emerging picture is that Ca isotopes are sensitive to a large number of high-temperature processes and can be used to understand the evolution of crust and mantle reservoirs, along with mechanisms leading to the formation of igneous, metamorphic, and hydrothermal rocks and minerals, through geologic time.
Na estreita orla litoral entre a foz do rio Douro e o Castelo do Queijo (Forte de S. Francisco Xavier), a faixa metamórfica é representada por magníficos afloramentos de rochas metassedimentares, ...espacialmente associadas a ortognaisses de diferentes tipos e a anfibolitos, que, no seu conjunto, constituem o Complexo Metamórfico da Foz do Douro. Estes afloramentos contrastam com os presentes na zona oriental da cidade, os quais não incluem ortognaisses e anfibolitos e onde micaxistos e metagrauvaques, numa sequência relativamente monótona e menos metamorfizada, são também recortados por granitos hercínicos.O Complexo Metamórfico da Foz do Douro (CMFD) encontra-se dividido em duas unidades tectonoestratigráficas: a Unidade de Gnaisses da Foz do Douro (UGFD) e a Unidade de Lordelo do Ouro (ULO).A UGFD é constituída por vários tipos de ortognaisses (biotíticos, leucocratas granatíferos, leucocratas ocelados e de tendência ocelada, leucocratas de grão fino e leucognaisses com cordierite), a que se associam anfibolitos de afinidade N-MORB.As características geoquímicas permitiram discriminar duas séries para as rochas ortognaissicas:i) a série sódica, constituída pelos ortognaisses biotíticos, ortognaisses leucocratas granatíferos, de composição tonalítica–trondhjemítica– granodiorítica, Na2O/K2O>2 e afinidade com a série “low-Al, high-Y-HREE”TTG. Estas rochas exibem uma assinatura mantélica do tipo Manto Deprimido e formaram-se num contexto de arco magmático.ii) a série potássica, constituída pelos gnaisses leucocratas ocelados e de tendência ocelada, gnaisses leucocratas de grão fino e leucognaisses com cordierite, de composição granítico–adamelíticas e granodioríticas, carácter peraluminoso e Na2O/K2O<2.As datações realizadas através dos vários métodos isotópicos permitiram reconhecer vários eventos que afetaram os gnaisses sódicos e potássicos da UGFD:i) dois eventos pré-variscos: um documentado no gnaisse ocelado, ocorrido há cerca de 536 ± 9,5 Ma (U-Pb em zircão), correspondente ao magmatismo calco-alcalino que deu origem ao granitoide original e outro documentado no gnaisse biotítico (456 ± 7,8 Ma a 486 ± 7,2 Ma; U-Pb em zircão) e no gnaisse ocelado (467 ± 7,8 Ma; U-Pb em zircão), que testemunha a recristalização da rocha de carácter potássico, ocorrida, possivelmente, durante a fase sarda;ii) um evento varisco, documentado no gnaisse de grão fino, ocorrido há cerca de 311 ± 6,4 Ma (U-Pb em zircão) e, provavelmente, relacionado com a intrusão do granito do Porto eiii) um evento tardi-varisco, documentado no gnaisse biotítico ocorrido há cerca de 280 Ma ± 2,4 Ma (Rb-Sr na biotite), provavelmente, relacionado com a intrusão do granito do Castelo do Queijo.A ULO é constituída por micaxistos silimaníticos e paragnaisses, que correspondem a argilitos a grauvaques finos ricos em K. Atendendo ao carácter intrusivo dos ortognaisses nos metassedimentos, poder-se-á inferir uma idade Proterozoica para estas rochas.As formações do CMFD e os granitos e tonalitos variscos intrusivos foram sujeitos a uma deformação e instalação controlados por estruturas transpressivas relacionadas com a Zona de Cisalhamento Porto-Tomar-Ferreira do Alentejo. A deformação dúctil que afetou as rochas do CMFD poderá estar relacionada com as fases de deformação D1 e D3 variscas.
•A radiating dyke swarm is confirmed within the Indian subcontinent at 1.88Ga.•Paleomagnetic data from India at 1.88Ga conflict with archetypal Columbia.•We report positive baked contact tests at ...2.37, 2.18 and 1.88Ga.•A combined 2.37Ga dataset represents one of the most robust for the Paleoproterozoic.•We propose that NE directions are related to Cuddapah basin initiation at 2.1Ga.
Here we report new paleomagnetic and geochronologic results from the Dharwar craton (south India) from 2.37 to 1.88Ga. The presence of a ∼85,000km2 radiating dyke swarm with a fanning angle of 65° is confirmed within Peninsular India at 1.88Ga. North of the Cuddapah basin the dykes are oriented NW-SE and progress to an E-W orientation further south, converging at a focal point southeast of the basin. The Grand Mean dual polarity paleomagnetic pole falls at 36.5°N and 333.5°E (D=129.1°, I=4.2°, α95=4.5°, λ=2.1°) for 29 sites from the present study combined with previously published sites. Our continental reconstruction for India at ∼1.9Ga conflicts with the archetypal Columbia model, suggesting that the exact configuration needs modification. We also report two separate paleomagnetic directions from NW-SE (D=3.2°, I=56.4°, α95=17.9°, λ=37°) and N-S (D=240.1°, I=−65.5°, α95=10.9°, λ=47.7°) trending ∼2.2Ga dykes. We attribute this difference in directions to the separate magmatic pulses at 2.18 and 2.21Ga identified by French and Heaman (2010). Our results place India at intermediate latitudes from 2.21 to 2.18Ga and are supported by a positive baked contact test. New paleomagnetic results from E-W and NW-SE trending 2.37Ga dykes, combined with previous work in the Dharwar craton, yields a Grand Mean dual polarity paleomagnetic pole at 15.1°N and 62.2°E (A95=4.0°), placing India at polar latitudes (D=88.7°, I=−81.7°, α95=4.8°, λ=73.7°). Here we also report a shallow NE direction (D=52.2°, I=−1.5°, α95=6.3°) previously classified as a secondary magnetization from three dykes near the Cuddapah basin. A baked contact test and petrophysical analysis of two cross-cutting dykes supports a primary remanence. Finally we present a Paleoproterozoic Apparent Polar Wander Path (APWP) for the Dharwar craton, and examine paleogeographic relationships between India and other cratonic blocks for the 2.37–1.88Ga time interval
The application of clumped isotopes (Δ47) in carbonate minerals as a sensitive temperature proxy in paleo-environments depends on a well-constrained clumped isotope fractionation for the necessary ...step of phosphoric acid digestion of the carbonate mineral to produce CO2. Published estimates for clumped isotope fractionations vary, and the effect of different carbonate mineralogies is still under debate. Differences in the sample preparation design and sample digestion temperatures are potential sources for varying acid fractionations and could be a source for discrepant Δ47-temperature calibrations observed in different laboratories. To evaluate the clumped isotope acid fractionation at 70°C and simultaneously account for a potential cation effect we analyzed a set of eight carbonate minerals (calcite, aragonite, dolomite and magnesite) that were driven to a stochastic isotope distribution by heating them to temperatures of 1000°C. Our study reveals significant cation- and mineral-specific differences for the Δ47 acid fractionation of carbonate minerals digested at 70°C or 100°C. The Δ47 acid fractionation at 70°C for calcite is 0.197±0.002 ‰, for aragonite 0.172±0.003 ‰, whereas dolomite has a significantly larger acid fractionation of 0.226±0.002 ‰. For magnesite digested at 100°C we observed a Δ47 acid fractionation of 0.218±0.020 ‰. Projected to an acid digestion at 25°C, our acid fractionation for calcite of 0.260 ‰ is statistically indistinguishable from existing studies. We further show that the Δ47 of the calcite standards ETH-1 and ETH-2 of 0.265 ‰ and 0.267 ‰, respectively, are in the range of the determined acid fractionation projected to 25°C suggesting that they have an identical and near stochastic isotope distribution. The observed differences in the Δ47 acid fractionation between calcite and aragonite (ΔΔ47=−0.025 ‰) and between calcite and dolomite (ΔΔ47=−0.029 ‰) does not correlate with the phosphoric acid fractionation of oxygen isotopes, but rather depends on the radius of the cation as well as on the mineral structure. Our results reveal that the acid fractionation of dolomite at 70°C is significantly distinct from the one of calcite, but at 90°C the two are within error of each other due to the different acid fractionation temperature dependence of calcite and dolomite. Thus it is necessary to use a mineral-specific Δ47 acid fractionation factor for dolomite to avoid differences in the final Δ47 signal from dolomites digested at 90°C and dolomites digested at lower temperatures. Similar effects may apply also to other carbonates such as magnesite and siderite. However, their mineral specific Δ47 acid fractionation at digestion temperatures around 90°C might be also similar to the one of calcite so that potential differences could be within the range of the analytical error.
•Absolute Δ47 acid fractionation of calcite, aragonite and dolomite at 70°C•Cation effect on Δ47 acid fractionation•Dolomite specific Δ47 acid fractionation temperature dependence•Overlap of the Δ47 acid fractionations of calcite and dolomite digested at 90°C
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