Empirical Geothermometers Research Articles

Lithium-rich basement brines: Activity versus concentration geothermometry

Lithium-rich brine plays a crucial role in the energy transition. Specifically, geothermal Upper Rhine Graben brines and the fluid inclusions in the adjacent southwesternmost Black Forest (Germany) are lithium-rich due to their interactions with Variscan basement rocks (granite and gneiss). Given their high salinity, however, conventional empirical geothermometers may prove inadequate, as these rely on elemental ratios in terms of concentration (molar or weight basis) rather than the elements’ activity. This short communication compares several classical geothermometers, such as Na/K, Na/Li, Mg/Li, K2/Mg, Mg/K, with innovative activity geothermometers that use the ion activity products of Ca2+, K+, Li+, Na+ and H2O based on the equilibrium between a generic lithium-mica, quartz, and oligoclase. Activity is calculated using the PhreeqcI code and Pitzer database optimized for LiCl-H2O, CaCl-H2O and NaF-H2O systems. The lithium minerals thermodynamic database, used as a reference for calculating the solubility product, has been further supplemented with Li-tosudite and Li-donbassite. The results of the inferred temperatures highlight the new activity geothermometer's versatility, even when compared with the to activity Na/K geothermometer based on the albite-microcline equilibrium.

Temperature of the Villarrica Lava Lake from 1963 to 2015 Constrained by Phase-Equilibrium and a New Glass Geothermometer for Basaltic Andesites

AbstractMagma/lava temperature is one of the most basic and critical parameters in volcanology. Yet at many active volcanoes, including during eruptions, this parameter remains poorly constrained due to vent accessibility, with estimates from different geothermometers commonly varying by several tens to more than a hundred degrees. One such volcano is Villarrica, one of the most active and hazardous volcanoes in South America and whose crater hosts a persistent lava lake. Here, we use experimentally determined phase equilibria to constrain the temperature of the Villarrica lava lake. The experiments were all performed at ambient pressure (i.e. 0.1 MPa), ranged in temperature from 1000°C to 1250°C and in oxygen fugacity from QFM + 1 to QFM–1 (where QFM stands for the quartz-fayalite-magnetite redox buffer). Within the investigated range, fO2 was found to have little to no effect on the phase equilibria and hence the lava lake fO2 could not be constrained. The temperature range explored, however, did bracket the entire liquidus to solidus assemblage evolution. On the basis of these experiments, two empirical geothermometers calibrated strictly between 1050°C and 1200°C (i.e. not to be used outside this range) were developed for the Villarrica lava lake based on its matrix glass composition and modal proportion. The 2015 state of the natural system was experimentally reproduced at temperatures around 1141°C. In detail, the lava lake melt composition in the period 1963 to 2015 indicates a cooling trend from 1186 ± 19°C in 1963 to 1153 ± 8°C in 1999–2000 and 1149 ± 28°C in 2015. A generic glass geothermometer was also developed for any Basaltic Andesite composition from the compilation of 175 phase equilibria experiments giving the relation: $Melt\ Temperature\ \left({}^{\circ}C\right)=12.43\times {\mathrm{Al}}_2{\mathrm{O}}_3+10.73\times \mathrm{MgO}+8.51\times \mathrm{CaO}+5.91\times \left({Na}_2\mathrm{O}+{K}_2\mathrm{O}\right)+814.08$ with all oxides in wt.% normalized. The geothermometers we have developed can be used to follow future temperature evolutions of the Villarrica lava lake or to estimate the lava temperature at any basaltic andesite eruption. Their application is simple, requiring only petrographic observations (to quantify the melt proportion) or analysis of the melt composition.

Geothermometric constraints on the thermal architecture, metamorphism, and exhumation of the Northern Range, Trinidad

AbstractThe Northern Range of Trinidad is composed of Mesozoic passive margin sedimentary rocks that underwent ductile deformation and subgreenschist‐ to greenschist‐facies metamorphism in the early Miocene. Previous studies suggested a two‐stage formation of the Northern Range between the Caribbean and South American plates: an initial collision drove mountain building in the Miocene and subsequent strike‐slip plate motion preferentially exhumed the western segment, producing a westward increase in the metamorphic thermal gradient. However, these studies were not able to resolve whether this gradient was discrete or continuous so the tectonic model awaits testing. In this study we use Raman spectroscopy on carbonaceous material (RSCM), an empirical geothermometer, to constrain peak temperatures across the Northern Range with a greater resolution than was available in previous studies. The RSCM temperatures show an abrupt increase from 337°C ± 10°C in the east to 442°C ± 16°C west of Chupara Point, where a range‐cutting fault (Chupara Fault) had been inferred in previous geologic mapping campaigns. Thus, the discrete thermal discontinuity of ~100°C very likely represents the Chupara Fault. Our RSCM‐derived peak metamorphic temperatures are 50°C to 100°C higher than those from previous estimates, requiring revision of tectonic models to account for deeper burial and greater exhumation. The peak metamorphic conditions determined here, and the deduced timing of faulting from published thermochronological data, are consistent with the two‐stage tectonic model proposed in previous studies.

Experimental constraints on pre-eruption conditions of the 1631 Vesuvius eruption

We established the phase equilibria of a representative tephriphonolitic sample of the 1631 eruption of Vesuvius (Italy). Experiments were conducted at 100 MPa, in the temperature range 950–1050 °C for melt water content ranging from 1.3 to 3.2 wt%, and at an oxygen fugacity (fO2) of NNO+1 to NNO+3 (one to three log unit above the fO2 of the Ni-NiO solid redox buffer). Results show that clinopyroxene, biotite and leucite dominate the crystallizing phase assemblage, with minor proportions of plagioclase and amphibole, in agreement with the petrological attributes of the tephra. Comparison between the phase proportions and compositions obtained in experiments and those observed in the rock indicates a pre-eruptive temperature of 950 ± 30° and a melt water content H2Omelt = 2.3 ± 0.3 wt%, for an oxygen fugacity around NNO+1. These T-H2O estimates are confirmed by empirical geothermometers based on experimental clinopyroxene and melt compositional trends. As for other Vesuvius eruptions, the most felsic part of the 1631 reservoir appears to have reached pre-eruptive leucite saturation, although a large amount of leucite microcrystals in the studied samples likely grew syn-eruptively. Our results confirm that magma storage conditions beneath Vesuvius became hotter, shallower, and more CO2-rich after the AD 79 Pompeii Plinian event.

Geothermometry using minor and trace elements in igneous and hydrothermal magnetite

The temperature (T) and oxygen fugacity (fO2) dependence of minor and trace elements (Mg, Al, Mn, Cr, Ni) in magnetite (Mag) in equilibrium with water-bearing silicate liquids varying in composition from basalt to rhyolite (49–77 wt% SiO2) is examined in a compilation of experimental data (n = 299) from 700 to 1050 °C, 50–900 MPa and five order of magnitude range in fO2. The XMg (=Mg/(Mg + Fetot)) of Mag is strongly dependent on T, and can be formulated into an empirical geothermometer: TMg−mag (°C) = − 8344(±320)/[lnXMg − 4.1(±0.28)] – 273 r2 = 0.83 that reproduces 72% of the experimentas to ±50 °C, and conventional FeTi oxide thermometry in tuffs and vitrophyres to better than ±60 °C for >85% of the samples. The TMg Mag thermometer may have potential application in igneous or hydrothermally altered rocks where conventional thermometry or oxybarometry is not possible. Application to natural Mag (n = 476) compiled from several settings shows igneous rocks mostly record >700 °C but with some lower temperatures in plutonic rocks due to slow cooling. Hydrothermal samples (skarns, porphyry deposits) show consistently subsolidus conditions (TMg-mag of 700–350 °C) consistent with other mineral and O isotopic equilibria. The element correlations in natural Mag from various settings are explored using multivariate statistical analysis, to identify which minor or trace elements most critically depend on T, fO2 or fluid composition.

Recent, deep-sourced methane/mud discharge at the most active mud volcano in the western Mediterranean

Active mud volcanism in the West Alboran Basin (WAB) is closely associated with tectonically mobilized, overpressurized shales and shale-diapirism. This appears to control mud expulsion at Carmen mud volcano, a cone-shaped structure 65 m high and 1 km in basal diameter.The presence of gas-rich mud breccia, living chemosynthetic fauna, the absence of hemipelagic draping and the abrupt transition that occurs between high dissolved sulfate in the uppermost interval and low sulfate together with high methane concentrations in the lowermost sediment interval all point to a recent expulsion of mud breccia at the summit of Carmen MV.For the lowermost interval, the depletion of major elements (i.e., Ca2+ and Mg2+) and the enrichment of trace species (i.e., Li and B) in the pore water all indicate a deep fluid source. The δ18Opw (5.7‰ VSMOW) and δDpw (−10‰ VSMOW) of pore water in the lowermost interval correspond with smectite dehydration as the main pore-water freshening mechanism. Water-formation temperatures calculated with empirical geo-thermometers (K-Na, K-Mg; δ18Opw, δDpw, and dissolved B) reveal that fluids were generated at temperatures of ~140 ± 20 °C. Taking a regional geothermal gradient for the WAB of 25–27 °C/km, this points to a fluid source from ~5 ± 1 km sediment depth. This is not only consistent with the depth of overpressurized shales and megabreccia of Lower to Middle Miocene age, but it also fits nicely with the Upper/Middle-Miocene seawater value for the porewater 87Sr/86Sr derived from dissolving carbonates. The stable carbon and hydrogen isotopic composition of methane (δ13Cmethane ~ −59.4‰ VPDB and δDmethane −184‰ VSMOW) for the deepest samples of summit-core GP05PC is consistent with the mentioned deep origin.Mud breccia expulsion of overpressurized deep sedimentary units would be accompanied by rigorous degassing, leading to rapid, ‘instantaneous’ replacement of pore fluid by bottom water in the upper sediments. The absence of oxidized sediment draping, the seawater-like pore-water composition in the uppermost part of the mud breccia interval, and the abrupt methane to sulfate transition all provide evidence for a very recent mud expulsion.The distinctively kink-shaped pore-water Cl− profile in core GP05PC has been used in a numerical transport-reaction model to derive the timing for this event. This eruptive event appears to have taken place very recently, namely 12 ± 5 yrs prior to the 2012 coring, thus in the year 2000 CE.

Melting phlogopite-rich MARID: Lamproites and the role of alkalis in olivine-liquid Ni-partitioning

In this study, we show how veined lithospheric mantle is involved in the genesis of ultrapotassic magmatism in cratonic settings. We conducted high pressure experiments to simulate vein+wall rock melting within the Earth's lithospheric mantle by reacting assemblages of harzburgite and phlogopite-rich hydrous mantle xenoliths. These comprised a mica-, amphibole-, rutile-, ilmenite-, diopside (MARID) assemblage at 3–5GPa and 1325–1450°C. Melting of the MARID assemblages results in infiltration of melt through the harzburgite, leading to its chemical alteration. At 3 and 4GPa, melts are high in K2O (>9wt%) with K2O/Na2O>>2 comparable to anorogenic lamproites. Higher pressures and temperatures (5GPa/1450°C) lead to increasing MgO contents of the melt and to some extent lower K2O contents (5–7wt%) at equally high K2O/Na2O ratios. Our experiments provide insights into the role of alkalis in nickel-partitioning (DNi) between olivine and ultrapotassic melt. We observe that the high contents of Na, K, and Al are indicative of high DNi values, implying that the melt polymerization is the dominant factor influencing the olivine/melt nickel partitioning. The change of DNi as a function of melt composition results in a pressure independent, empirical geothermometer:T°C=−88.14·lnliq(Na2O+K2O·liq1+SiO2TiO2·liqAl2O3liqMgO+FeO·DNi+1906.2Element oxides represent the composition of the glass (in wt%), and DNi is the liquid/olivine Ni-partitioning coefficient. We propose that this geothermometer is applicable to all natural silicate melts that crystallized olivine in a temperature interval between 1000 and 1600°C. Application to glass-olivine pairs from calc-alkaline settings (Mexico), MORB (East Pacific Rise), and OIB (Hawaii) yielded reasonable values of 996–1199°C, 1265°C, and 1330°C, respectively.

Raman spectra of carbonaceous materials in a fault zone in the Longmenshan thrust belt, China; comparisons with those of sedimentary and metamorphic rocks

We analyzed micro-Raman spectra of carbonaceous materials (CM) in natural and experimentally deformed fault rocks from Longmenshan fault zone that caused the 2008 Wenchuan earthquake, to characterize degree of disordering of CM in a fault zone. Raman spectral parameters for 12 samples from a fault zone in Shenxigou, Sichuan, China, all show low-grade structures with no graphite. Low crystallinity and δ13C values (−24‰ to −25‰) suggest that CM in fault zone originated from host rocks (Late Triassic Xujiahe Formation). Full width at half maximum values of main spectral bands (D1 and D2), and relative intensities of two subbands (D3 and D4) of CM were variable with sample locations. However, Raman parameters of measured fault rocks fall on established trends of graphitization in sedimentary and metamorphic rocks. An empirical geothermometer gives temperatures of 160–230°C for fault rocks in Shenxigou, and these temperatures were lower for highly sheared gouge than those for less deformed fault breccia at inner parts of the fault zone. The lower temperature and less crystallinity of CM in gouge might have been caused by the mechanical destruction of CM by severe shearing deformation, or may be due to mixing of host rocks on the footwall. CM in gouge deformed in high-velocity experiments exhibits slight changes towards graphitization characterized by reduction of D3 and D4 intensities. Thus low crystallinity of CM in natural gouge cannot be explained by our experimental results. Graphite formation during seismic fault motion is extremely local or did not occur in the study area, and the CM crystallinity from shallow to deep fault zones may be predicted as a first approximation from the graphitization trend in sedimentary and metamorphic rocks. If that case, graphite may lower the friction of shear zones at temperatures above ~300°C, deeper than the lower part of seismogenic zone.

Empirical geothermometers and geothermobarometers for spinel peridotite phase assemblages

Experimental synthesis of spinel peridotite phase assemblages for a range of compositions that mimic natural samples is used to derive a set of empirical geothermometers and geothermobarometers represented by multiple linear regression best-fit surfaces that link the variables of temperature, pressure, and composition. The calibrated geothermometers use reactions that govern the solubility of Al and Cr in both pyroxenes and the Mg–Fe exchange between silicates and spinel. Geothermobarometers map the Mg–Fe exchange between coexisting olivine and clinopyroxene and pyroxenes and Ca–Mg exchange between coexisting pyroxenes. Application of the geothermometers and geothermobarometers to suites of naturally occurring samples indicates that while reactions governing the Cr and Al solubility and solvus of orthopyroxene give useful estimates of ‘original’ mantle temperatures and pressures, respectively, comparable reactions for clinopyroxene yield estimates that are variably dependent on the transport phase of the sample suites. Temperature and pressure estimates from reactions governing Mg and Fe exchange between silicates and spinel and coexisting silicates are all sensitive to the later transport stage of the samples.

A new Fe/Mn geothermometer for hydrothermal systems: Implications for high-salinity fluids at 13°N on the East Pacific Rise

Field and experimental investigations demonstrate the chemistry of mid-ocean ridge hydrothermal vent fluids reflects fluid–mineral reaction at higher temperatures than those typically measured at the seafloor. To account for this and, in turn, be able to better constrain sub-seafloor hydrothermal processes, we have developed an empirical geothermometer based on the dissolved Fe/Mn ratio in high-temperature fluids. Using data from basalt alteration experiments, the relationship; T (°C) = 331.24 + 112.41*log[Fe/Mn] has been calibrated between 350 and 450 °C. The apparent Fe–Mn equilibrium demonstrated by the experimental data is in good agreement with natural vent fluids, suggesting broad applicability. When used in conjunction with constraints imposed by quartz solubility, associated sub-seafloor pressures can be estimated for basalt-hosted systems. As an example, this methodology is used to interpret new data from 13°N on the East Pacific Rise, where high-temperature fluids both enriched and depleted in chloride (339–646 mmol/kg), relative to seawater, are actively venting within a close proximity. Accounting for these variable salinities, active phase separation is clearly taking place at 13°N, yet the fluid Fe/Mn ratios and the silica concentrations suggest equilibration at temperatures less than those coinciding with the two-phase region. These data show the chloride-enriched fluid reflects the highest temperature and pressure (∼432 °C, 400 bars) of equilibration, consistent with circulation near the top of the inferred magma chamber. This is in agreement with the elevated CO 2 concentration relative to the chloride-depleted fluids. The noted temperature derived from the Fe/Mn geothermometer is higher than the critical temperature for a fluid of equivalent salinity. This carries the important implication that, despite being chloride-enriched relative to seawater, these fluids evolved as the vapor component of even higher salinity brine.

Experimental determination of carbon isotope fractionation between CaCO 3 and graphite

Carbon isotopic exchange between graphite and three polymorphs of CaCO 3 was investigated at temperatures of 600–1400 °C and at pressures from 1.4 to 2.3 GPa. Fractionation factors at all temperatures were determined by the partial exchange treatment of Northrop and Clayton (1966). Graphite starting material for the majority of the experiments was milled in water for 20–25 h, producing aggregates of nanosheets. The sheets range in width from 50 to 1000 nm and in thickness from 20 to 30 nm, and they retain hexagonal symmetry. Isotopic exchange appears to be the sum of surface exchange and interior exchange. At 1100–1400 °C, interior exchange exceeded surface exchange, probably by a combination of grain growth, as determined by increase in crystallite size, recrystallization, as observed in FESEM images, and diffusion. In some runs at 1200 and 1400 °C with an isotopic contrast between the initial graphite and calcite of close to 50‰, equilibrium fractionation was actually overstepped due to a kinetic effect. A weighted regression of fractionation factors from the high-temperature runs yields the line of equilibrium interior exchange: 1000 ln α carbonate – graphite = 3.28 ( 0.07 ) × 10 6 / T 2 . Our calibration lies between the empirical geothermometers of Kitchen and Valley (1995) and Valley and O’Neil (1981) and, accordingly, with a substantial body of data from granulite-facies metamorphic rocks. At 600–700 °C surface exchange greatly exceeded interior exchange, with a much lower activation energy. Interior exchange was slight to nonexistent because there was no crystal growth, no recrystallization, and, probably, little diffusion. Fractionation factors are ∼1‰ higher than the interior exchange factors. Surface exchange probably occurred in the outer one or two unit cells of nanosheets. In previous experimental studies, similar surface-dominated fractionations apparently were measured, even at high temperatures. At 750–1000 °C, exchange rates and fractionation factors followed the low-temperature surface trends. But higher progress toward equilibrium indicates that exchange may also have occurred along rapidly-migrating subgrain boundaries. At those temperatures, crystallite size decreased, but the physical appearance of nanosheets did not change, indicating internal rearrangement of the grains.

Intersector element partitioning in tourmaline: a potentially powerful single crystal thermometer

Hourglass sector zoning, and related polar overgrowths, are common features of metamorphic tourmaline, developing as a result of variations in element preference on the different growth surfaces. For sector-zoned crystals, three domains are present for each growth zone (c +, c - and a), with compositional differences most distinct for Ca and Ti, and among c + and c - sectors. Intersector differences vary, commonly showing decreasing fractionation from core to rim attributed to increasing metamorphic grade. Here we show that intersector element partitioning is temperature dependent and derive empirical geothermometers based on c +-c -and c +-a partitioning of Ca and Ti. These thermometers are applicable over a range of temperatures and bulk-rock compositions. Intersector partitioning is not affected by re-equilibration and records and preserves complete T-histories of individual tourmaline grains from prograde to peak and on to retrograde growth. Information on element mobility is preserved by tourmaline composition, because intersector partitioning is independent of element concentration. These factors make intersector partitioning an ideal tool to elucidate the thermal history of tourmaline grains and thus their host environment and tourmaline's refractory nature preserves these signatures even into the sedimentary record.

Relict sapphirine + kyanite and spinel + kyanite associations in pyropic garnet from the eastern Sør Rondane Mountains, East Antarctica

Sapphirine, spinel, orthoamphibole ± quartz and kyanite are included in porphyroblastic garnet in biotitic gneiss enclosed in a lens of metamorphosed ultramafic rocks in the Cambrian granulite-facies metamorphic complex of the eastern Sør Rondane Mountains, Queen Maud Land, East Antarctica. A bulk analysis of the biotitic gneiss reveals features characteristic both of ultramafic rocks, e.g., high contents of Cr and Ni, and of metasomatism associated with fluids having a crustal source, e.g., relatively elevated contents of Li, Rb, Mo, Cs, Ba, Tl, and Pb. This trace element signature is consistent with the biotitic gneiss being a slice of blackwall skarn that developed between harzburgite and the enclosing biotite–hornblende ± garnet ± orthopyroxene ± clinopyroxene gneiss and was subsequently infolded or inserted by faulting. The matrix assemblage of the biotitic gneiss is garnet + corundum + hercynite + biotite +plagioclase +allanite + zircon. The included associations (all with biotite and rutile) are (1) sapphirine + kyanite, (2) spinel + kyanite, (3) sapphirine +spinel, (4) kyanite, and (5) orthoamphibole + plagioclase ± quartz. The garnet porphyroblasts are compositionally zoned with broad pyropic cores ( X Mg(=Mg / (total Fe + Mg)) = 0.45–0.55) surrounded by Fe-richer rims ( X Mg ≈ 0.3 at the outermost part). The garnet cores preserve compositions homogenized under peak conditions of the granulite-facies metamorphism (760–800 °C and 7–8 kb), whereas the Fe-enriched rims are attributed to an amphibolite-facies overprint at 500–600 °C. Theoretical calculations of garnet + corundum + spinel ± sapphirine + kyanite equilibria in the FMAS system constrain possible P–T conditions for a sapphirine + spinel + kyanite + garnet ( X Mg ≈ 0.55) assemblage to form near 450 °C and 4 kb on the prograde path. In contrast, a modified calibration of the Das et al. (Das, K., Fujino, K., Tomioka, N., Miura, H., 2006. Experimental data on Fe and Mg partitioning between coexisting sapphirine and spinel: an empirical geothermometer and its application. Eur. J. Mineral., 18, 49–58). sapphirine–spinel thermometer gives 860–895 °C for the included associations; pressures would have to be at least 12 kb to stabilize kyanite at these temperatures. Neither estimate is satisfactory and the stability range of kyanite + spinel–hercynite ± sapphirine assemblages remains an unresolved question. The Sør Rondane Mountains constitute the third region for kyanite, sapphirine and spinel–hercynite inclusions in garnet in granulite-facies rocks of the Neoproterozoic–Cambrian orogen extending from the Sør Rondane Mountains to Lützow–Holm Bay and onward to Sri Lanka, southern India and southern Madagascar, and thus determining the stability range of kyanite + spinel–hercynite ± sapphirine is critical for deducing the tectonic evolution of this orogen.

Experimental data on Fe and Mg partitioning between coexisting sapphirine and spinel: an empirical geothermometer and its application

Specially designed experiments were carried out in the temperature range 950-1100 °C at 800 MPa to grow coexisting sapphirine and spinel-bearing metapelitic assemblages in the KFMASH system. Equilibrium phase assemblages contain these two phases with stoichiometric compositions. Compositional data obtained from the two coexisting phases indicate a strong dependence of the Mg-Fe distribution coefficient on temperature. An empirical thermometric expression has been formulated from the experimentally constrained relationship, lnKD = -6277/T(°K) + 6.4202, where KD = (XMg/XFe)Spr.(XFe/XMg)Spl. Application of this thermometer to natural mineral pairs indicate that earlier temperature estimates based on sapphirine-spinel need to be revised upward by 120-160 °C.

Chlorite composition and geothermometry: a comparative HRTEM/AEM-EMPA-XRD study of Cambrian basic lavas from the Ossa Morena Zone, SW Spain

AbstractChlorites from Cambrian basaltic vesicular lavas, characterized by relatively highXc (0.81-0.98), gave temperatures of formation of 285 to 350°C (Cathelineau's empirical geothermometer). Both theXcand temperature calculations gave results which were too high for the diagenetic conditions indicated by the interbedded shale mineralogy. The HRTEM and XRD studies indicate the absence of smectite layers in these chlorites; i.e. according to lattice images, the actual value ofXcis 1. The chlorite composition in these basaltic lavas must, therefore, be explained in relation to their whole-rock geochemistry, with which a good correlation has been found. The basaltic lavas are characterized by relatively high FeO/M ratios (3.28±1.66) and must be considered as an Fe-rich system, with similar chemical behaviour to pelitic rocks. In these cases, direct chlorite formation, without a previous smectite precursor, is a normal occurrence and theXccalculation and empirical geothermometric calculations are inadequate.

Partitioning of transition elements between orthopyroxene and clinopyroxene in peridotitic and websteritic xenoliths: new empirical geothermometers

The partitioning of transition elements (Sc, Ti, V, Cr, Mn, Co, Ni) between orthopyroxene (opx) and clinopyroxene (cpx) in carefully selected garnet peridotite, spinel peridotite and garnet websterite xenoliths was determined by electron probe microanalyses (EPMA) and secondary ion mass spectrometry (SIMS). Xenoliths studied cover a wide compositional range and equilibrated under variable upper mantle conditions at temperatures between about 760 and 1370°C (two-pyroxene thermometer based on the enstatite-diopside solvus) and pressures between about 0.8 and 3.6 GPa (Al-in-opx and Ca-in-olivine barometers). We found that the partitioning of transition elements between opx and cpx (expressed as D M = concentration of element M in opx [cations per formula unit]/concentration of M in cpx [cations per formula unit]) is mainly controlled by temperature and to a much lesser degree by pressure. Variations in major element compositions of pyroxenes (e.g., variable X Mg, Al IV or Na) have no influence on D M . For Sc, V, Cr, Mn, and Co, our data result in good correlations between ln D M and reciprocal absolute temperature, with correlation coefficients ( r) between 0.950 and 0.981. It is therefore possible to use the partitioning of these elements between opx and cpx from peridotites and websterites as geothermometers. On the basis of our data, we suggest the following empirical thermometer equations: T Sc = [(17.64 · P + 5663)/(3.25 − ln D Sc)], T V = [(18.06 · P + 3975)/(2.27 − ln D V)], T Cr = [(11.00 · P + 2829)/(1.56 − ln D Cr)], T Mn = [(−0.20 · P − 2229)/(−1.37 − ln D Mn)], T Co = [(−4.31 · P − 2358)/(−0.98 − ln D Co)], where T is the absolute temperature in Kelvin and P the pressure in kilobars. For Ti and Ni observed correlations between ln D M and 1/ T are less well defined.

Crystal chemistry and significance of cation ordering in Mg-Al rich spinels from high-grade hornfels (Predazzo-Monzoni, NE Italy)

AbstractTwo Mg-Al spinels (FAS1 and FAS2) in high-grade hornfels from the Toal de Mason skarn (Predazzo Monzoni, NE Italy) were investigated by means of single crystal X-ray diffraction and electron microprobe analysis in order to evaluate the last intra-crystalline closure temperature.The evidence provided by crystal chemical data from the investigated spinels, compared with that for two other spinels from a websterite dyke and a chlorite schist (TS2 and SP78a, respectively) with similar chemical characteristics, yielded a new empirical geothermometer. This allowed estimation of intra-crystalline temperatures as low as c. 400°C and indicates that the closure temperature of FAS1 and FAS2 is c. 500°C i.e. 150–200°C lower than the spinel crystallisation temperature.

A41 温度計・圧力計としての磁鉄鉱の副成分組成

A41 温度計・圧力計としての磁鉄鉱の副成分組成

Calcite–graphite isotope thermometry: a test for polymetamorphism in marble, Tudor gabbro aureole, Ontario, Canada

ABSTRACTGraphitization and coarsening of organic material in carbonate‐bearing metasedimentary rocks is accompanied by carbon isotope exchange which is the basis of a refractory, pressure‐independent geothermometer. Comparison of observed isotopic fractionations between calcite and graphite (δ13CCal–Gr) with independent petrological thermometers provides the following empirical calibration over the range 400–800°C: δ13CCal–Gr= 5.81 times 106×T–2(K) ‐ 2.61. This system has its greatest potential in marbles where calcite + graphite is a common assemblage and other geothermometers are often unavailable. The temperature dependency of this empirical calibration differs from theoretical calibrations; reasons for this are unclear but the new empirical calibration yields temperature estimates in better agreement with independent thermometry from several terranes and is preferred for geological applications.Both calcite‐graphite isotopic thermometry and calcite‐dolomite solvus thermometry are applied to marble adjacent to the Tudor gabbro in the Grenville Province of Ontario, Canada. The marble has undergone two metamorphic episodes, early contact metamorphism and later regional metamorphism. Values of δ13CCal–Gr decrease regularly from c. 8‰ in samples over 2 km from the pluton to values of 3–4‰ within 200 m of the contact. These samples appear to preserve fractionations from the early thermal aureole with the empirical geothermometer, and indicate temperatures of 450–500° C away from the intrusion and 700–750°C near the gabbro. This thermal profile around the gabbro is consistent with conductive heat flow models. In contrast, the distribution of Mg between calcite and dolomite has been completely reset during later regional metamorphism and yields uniform temperatures of c. 500°C, even at the contact.Graphite textures are important for interpreting the results of the calcite–graphite thermometer. Coarsening of graphite approaching the Tudor gabbro correlates with the decrease in isotopic fractionations and provides textural evidence that graphite crystallization took place at the time of intrusion. In contrast to isotopic exchange during prograde metamorphism, which is facilitated by graphitization, retrogressive carbon isotopic exchange appears to require recrystallization of graphite which is sluggish and easily recognized texturally. Resistance of the calcite–graphite system to resetting permits thermometry in polymetamorphic settings to see through later events that have disturbed other systems.

Experimental water-rock interaction: silica geothermometers in the temperature range 100-350°C

The reliability of silica geothermometers for different natural mineral assemblages and fluid compositions has been experimentally assessed in the P, T range 185-435 bar, 100-350°C using the Barnes hydrothermal kinetic apparatus. A series of metasediments (two phyllosilicate-rich schists and one dolomitic carbonate rock) of the Latium-Tuscany geothermal area (central Italy) and fluids of pure and sea-water compositions were used at starting materials. Experiments with the carbonate rock indicate that silica geothermometry is poorly applicable, probably because of the finite character of silica in the starting material. Experiments with feldspar-free, phyllosilicate-rich schists indicate that both the rock chemistry (mainly the redox ratio) and the fluid composition affect silica solubility at any given P-T. Oxidized assemblages in general favour higher silica solubilities. The effect of rock composition is greatly diminished when saline fluids are used. The observed solubility-temperature relationships are assessed against commonly used experimental and empirical geothermometers, giving a best approximation to the chalcedony geothermometer, followed by the quartz - no steam loss geothermometer. Three new silica geothermometers were calibrated : two for oxidized and reduced silicate assemblages and pure water fluids (EST-o and EST-r, respectively) and one for a variety of silicate assemblages interacting with saline (marine) fluids (EST-m). These geothermometers are effective for the whole experimentally investigated range (100-350°C). The new geothermometers have been tested for various geothermal areas and found to give accurate results. Combined equations allow a reliable chemical geothermometry over the temperature range 25-350°C, provided the redox conditions of the reservoir and the composition of the initial fluid are approximately known