Oxygen Isotope Thermometry Research Articles

Most Earth-surface calcites precipitate out of isotopic equilibrium

Oxygen-isotope thermometry played a critical role in the rise of modern geochemistry and remains extensively used in (bio-)geoscience. Its theoretical foundations rest on the assumption that 18O/16O partitioning among water and carbonate minerals primarily reflects thermodynamic equilibrium. However, after decades of research, there is no consensus on the true equilibrium 18O/16O fractionation between calcite and water (18αcc/w). Here, we constrain the equilibrium relations linking temperature, 18αcc/w, and clumped isotopes (Δ47) based on the composition of extremely slow-growing calcites from Devils Hole and Laghetto Basso (Corchia Cave). Equilibrium 18αcc/w values are systematically ~1.5‰ greater than those in biogenic and synthetic calcite traditionally considered to approach oxygen-isotope equilibrium. We further demonstrate that subtle disequilibria also affect Δ47 in biogenic calcite. These observations provide evidence that most Earth-surface calcites fail to achieve isotopic equilibrium, highlighting the need to improve our quantitative understanding of non-equilibrium isotope fractionation effects instead of relying on phenomenological calibrations.

LinT, a simplified approach to oxygen-isotope thermometry and speedometry of high-grade rocks: An example from ultrahigh-temperature gneisses of southern Madagascar

LinT, a simplified approach to oxygen-isotope thermometry and speedometry of high-grade rocks: An example from ultrahigh-temperature gneisses of southern Madagascar

Calibration and application of silica-water triple oxygen isotope thermometry to geothermal systems in Iceland and Chile

Triple oxygen isotope analyses were made on geothermal fluids and precipitates from Chile and Iceland to calibrate the silica-water isotopic fractionation for abiotic silica formation at elevated temperatures and were used to evaluate potential fractionation effects of biogenic vs. abiogenic samples and polymorphism. Coexisting water and amorphous silica precipitated inside the heat exchanger of the Hellisheiði power plant at 60 and 118 °C have triple oxygen isotope fractionations in excellent agreement with previous results from analyses of biogenic silica precipitated in cold waters.In contrast to samples from the geothermal plant, natural amorphous silica precipitates and waters formed in active hot springs (T = 63–84 °C) in the Puchuldiza geothermal area of northern Chile gave temperature estimates from the silica-water thermometer far lower (37–46 °C) than the measured water temperatures. Active silica precipitation was found to only occur at and above the air-water interface on glass slides placed in the hot spring waters for 9 days. The calculated temperatures and visual inspection suggest that precipitation occurred along channel edges when saturation was overstepped by a factor of two. In contrast to the surficial neoformed amorphous silica, subsurface silica samples (>10 cm) have recrystallized to opal-CT and quartz within a sinter mound and these samples preserve isotope temperatures of 82 °C and 89 °C, in good agreement with the ambient temperatures of the thermal spring conduit system. The δ18O values of abiogenic, low temperature silica formed in spring water far from the thermal waters with a measured temperature of 19 °C correspond to a silica-water temperature estimate of 20 °C. All samples preserved isotope data corresponding to their expected formation temperatures and appear to be in equilibrium in the triple oxygen isotope system. A best-fit θ–T relationship for silica-water using our inorganic silica-water samples is θ=0.5305-1.82(±0.02)T(K),R2=0.998whereθa-b=lnα17Oa-blnα18Oa-b. This new equation is indistinguishable from a previous empirical fit by Sharp et al. (2016) based primarily on biogenic silica samples, suggesting that the biogenic and abiogenic samples secreted silica with the same fractionation. Our results show that triple oxygen isotope measurements are robust and can be used to estimate the temperature of formation, the isotopic composition of the formation water, and discern between equilibrium and non-equilibrium processes.

Oxygen Isotope Thermometry of DaG 476 and SaU 008 Martian Meteorites: Implications for Their Origin

We report the equilibration temperatures derived from the oxygen isotope thermometry of pyroxene-olivine pair from the Dar al Ghani (DaG) 476 (1200 +105/−90 °C) and Sayh al Uhaymir (SaU) 008 (1430 +220/−155 °C) meteorites showing a difference of over 200 °C at the face values. Regardless of the large associated uncertainties, contrasting geochemical and isotopic characteristics such as oxygen fugacities, hydrogen isotopic compositions (referred to as the D/H ratios), olivine abundances, presence of merrillite and/or apatite, and their chlorine contents between the two meteorites are observed in the literature. These opposing features lend support to the idea that the relative difference observed in the estimated temperatures is probably real and significant, thus providing insights into the Martian mantle magmatism. Based on our temperature estimation and previous magmatic models, we propose that SaU 008 could have been originated from a deeper depleted mantle source. However, DaG 476 may have been produced by the partial melting of the entrained pockets of the depleted mantle similar to that of the SaU 008’s source at a relatively shallower depth. Both meteorites erupted as a relatively thick lava flow or a shallow intrusion at approximately the same time followed by a launch initiated by a single meteoritic impact 1.1 million years (Ma) ago.

Formational Conditions For the Binntal Emerald Occurrence, Valais, Switzerland: Fluid Inclusion, Chemical Composition, and Stable Isotope Studies

Abstract Emerald from the Binntal occurrence in the Canton of Valais in Switzerland has been studied to determine its chemical zonation, stable isotopic signatures, depositional-fluid characteristics, pressure-temperature emplacement conditions, and formational model. The emerald is vanadium-rich, with optical and blue cathodoluminescence zoning related to chemical variations, primarily in V 2 O 3 concentrations. The hydrogen isotope signature of the emerald channel fluids is unique and in agreement with previously identified high-altitude (deuterium-depleted) Alpine-age meteoric fluids. Field studies, fluid inclusion analyses, and oxygen isotope thermometry are consistent with a metamorphic formational model for the Binntal emerald at temperatures and hydrostatic pressures ranging from 200 to 400 °C and 100 to 250 Mpa, respectively. This corresponds to formational depths on the order of 4 to 9 km and fluids consistent with a 10–20 Ma CO 2 -dominant fluid with approximate mole percentages of 84.0, 11.9, 1.5, 1.3, 0.3, and 0.5 for CO 2 , H 2 O, CH 4 , N 2 , H 2 S, and NaCl, respectively.

Oxygen isotope thermometry using quartz inclusions in garnet

AbstractOxygen isotope ratios of quartz inclusions (QI) within garnet from granulite and amphibolite facies gneisses in the Adirondack Mountains, NY were analysed and used to determine metamorphic temperatures. Primary QI for eight of 12 samples have δ18O values significantly lower than matrix quartz (MQ). The primary QI retain δ18O values representative of thermal conditions during garnet crystallization, whereas the δ18O values of MQ were raised by diffusive exchange with other matrix minerals (e.g. mica and feldspar) during cooling. The δ18O differences between QI and MQ show that garnet (a mineral with slow diffusion of oxygen) can armour QI from isotopic exchange with surrounding matrix, even during slow cooling. These differences between δ18O in MQ and QI can further be used to test cooling rates by Fast Grain Boundary diffusion modelling. Criteria for identifying QI that preserve primary compositions and are suitable for thermometry were developed based on comparative tests. Relations between δ18O and inclusion size, distance of inclusion to host–garnet rim, core–rim zonation of individual inclusions, and presence or absence of petrological features (healed cracks in QI, inclusions in contact with garnet cracks lined by secondary minerals, and secondary minerals along the inclusion grain boundary) were investigated. In this study, 61% of QI preserve primary δ18O and 39% were associated with features that were linked to reset δ18O values. If δ18O in garnet is homogeneous and inclusions are removed, laser‐fluorination δ18O values of bulk garnet are more precise, more accurate, and best for thermometry. Intragrain δ18O(Grt) profiles measured in situ by ion microprobe show no δ18O zonation. Almandine–rich garnet (Alm60–75) from each sample was measured by laser‐fluorination mass‐spectrometry (LF‐MS) for δ18O and compared with ion microprobe measurements of δ18O in QI for thermometry. The Δ18O(Qz–Grt) values for Adirondack samples range from 2.66 to 3.24‰, corresponding to temperatures of 640–740 °C (A[Qz–Alm] = 2.71). Out of 12 samples that were used for thermometry, nine are consistent with previous estimates of peak temperature (625–800 °C) based on petrological and carbon–isotope thermometry for regional granulite and upper amphibolite facies metamorphism. The three samples that disagree with independent thermometry for peak metamorphism are from the anorthosite–mangerite–charnockite–granite suite in the central Adirondacks and yield temperatures of 640–665 °C, ~100 °C lower than previous estimates. These low temperatures could be interpreted as thermal conditions during late (post‐peak) crystallization of garnet on the retrograde path.

Primary diagenetic copper carbonate at the Malbunka copper deposit, Amadeus Basin, Northern Territory, Australia

The Malbunka copper deposit, located about 220km west of Alice Springs, in the Northern Territory of Australia, may be a rare example of primary formation of copper carbonate mineralization. This deposit consists of unusual azurite disks up to 25cm diameter, and lesser amounts of secondary azurite crystals and malachite. Carbon isotope values of the copper carbonate minerals are consistent with formation from groundwater-dissolved inorganic carbon. Oxygen isotope thermometry formation temperature estimates are 5–16°C above ambient temperatures, suggesting the copper carbonates formed at a depth between 0.3 and 1.6km in the Amadeus Basin. Azurite fluid inclusion waters are rich in boron, chlorine, and other elements suggestive of dilute oil basin formation fluids. In addition, presence of euhedral tourmaline with strong chemical zonation suggest that this was a low temperature diagenetic setting. The strong correlation of structures associated with hydraulic fracturing and rich copper carbonate mineralization suggest a strongly compartmentalized overpressure environment. It is proposed that copper carbonates of the Malbunka deposit formed when deep, copper-rich formation fluids were released upward by overpressure-induced failure of basin sediments, permitting mixing with carbonate-rich fluids above. This work bears directly upon exploration for a new type of primary copper deposit, through understanding of the conditions of genesis.

Petrology, geochemistry and fluid inclusion analysis of altered komatiites of the Mendon Formation in the BARB4 drill core, Barberton greenstone belt, South Africa

The 3.33 to 3.26 Ga Mendon Formation in the Palaeoarchaean Barberton greenstone belt, South Africa, forms the uppermost unit of the Onverwacht Group. It is dominated by ultramafic volcanic rocks interbedded with thin layers of cherty sediments that show pervasive alteration, including widespread serpentinisation, silicification and chert and quartz veining. The BARB4 drill core of the ICDP Barberton drilling project exposes a unique section through the Mendon Formation in the Manzimnyama Syncline. The komatiites are pervasively altered to an assemblage comprising quartz, chlorite, carbonate, talc, biotite, and, locally, plagioclase, K-feldspar, muscovite, amphibole, stilpnomelane and ankerite. The overlying sediments are made up of banded iron formations and rare beds of siliciclastic rocks. Though the altered komatiites are pervasively silicified, SiO 2 contents do not exceed 58 wt.%, and their major and trace element geochemistry is similar to other komatiitic rocks of the Mendon Formation, particularly those of the M2v-member. Quartz veins and, less commonly, quartz-carbonate and quartz-carbonate-plagioclase veins are found throughout the core. Overall, composition and texture of the veins differ from primary and early diagenetic veins found in silicified komatiites elsewhere in the Barberton greenstone belt. In the BARB4 drill core, the veins are generally coarse-grained, and the immediate wall rocks are locally foliated along the vein margins. In addition, the δ 18 O values of vein quartz range from 14.1 to 15.3‰, significantly lower than the values typically found in veins on the modern seafloor that formed during low temperature hydrothermal seafloor alteration (~22 to 32‰). Fluid inclusions in vein quartz are homogeneous two-phase (L+V) aqueous inclusions that occur in trans- and intragranular trails and clusters. Intragranular and isolated fluid inclusions have a similar homogenisation temperature (T h ) of 130 to 200°C, with most data ranging between 145 and 175°C. Salinities cluster in three different groups of high (20 to 27wt.% NaCl equiv.), medium (10 to 15wt.% NaCl equiv.) and low salinity (0.3 to 1.5wt.% NaCl equiv.). The composition and microthermometric characteristics of the fluid inclusions analysed within the drill core show similarities to those found in quartz veins in silicified komatiites of the Mendon Formation, which are interpreted to have been entrapped during metamorphism. P-T calculations based on fluid inclusion microthermometry reveal conditions of 230 to 400 MPa and 250 to 400°C. Similar conditions of 240 to 270°C have been obtained using oxygen isotope thermometry, assuming a metamorphic fluid with a δ 18 O value of 6‰. Collectively, the δ 18 O values, together with the texture and composition of the veins, are interpreted to indicate a metamorphic origin of the veins. The presence of high salinity inclusions indicates the occurrence of a highly saline fluid that locally mixed with the dominant lower salinity fluids. The high salinity might have been derived from fluid circulation through evaporites.

DNA thermometry: A universal biothermometer in the 18O/16O ratio of PO4 in DNA

Oxygen isotope thermometry has been traditionally based on the ratio of 18O/16O in oxyanions of minerals such as carbonate (CaCO3) in shells/tests, and phosphate in bioapatites (Ca5(PO4)3OH) of marine invertebrates/vertebrates (for example, fish) and mammals. The requirement of mineral biomass, however, has restricted the application of oxygen isotope thermometry to only those organisms possessing biomineral hardparts. This has completely omitted from study, not only organisms lacking hard mineral tissues, but two entire Domains of life: Bacteria and Archaea. Prokaryotic organisms in the domains Bacteria and Archaea comprise the majority of earth9s biodiversity and also inhabit the most extreme environments on earth. This calls for a thermometry based on a more rudimentary component of biomass that is present in all organisms such as DNA. Our previous studies of the ubiquitous intracellular enzyme inorganic pyrophosphatase (PPase), which catalyzes oxygen isotope exchange between dissolved inorganic PO4 (Pi) and water inside of cells, suggest that DNA may contain even more information than the blueprint for life. Here we show that PO4 moieties in DNA record the temperature at which life forms and evolves. Our results demonstrate that the 18O/16O ratio of PO4 (δ18OP) in DNA as well as in bulk biomass, reflects the temperature-dependent exchange of oxygen isotopes between PO4 and intracellular water. Thus, δ18OP values of DNA may serve as both a new soft-tissue bio-thermometer and probe of intracellular PO4 or water during DNA synthesis and cellular growth. Results presented here of the first direct measurements of the oxygen isotopic composition of PO4 in microbial DNA, demonstrate systematic variation in δ18OP of DNA from several different strains of microorganisms as a function of temperature, and also extend the PO4-water O-isotope thermometer to >70 °C. A composite calibration curve based on several strains of bacteria is presented for DNA-PO4–water –temperature relations between 12 and 75 °C and suggests a “universal” DNA-based oxygen isotope thermometry for microorganisms that may further extend to all organisms. Our results open the possibility of connecting temperature with taxonomy and also expand the range of investigations of habitat temperatures and limits to life in extreme and diverse environments, from Earth9s subsurface deep biosphere to Antarctic ice sheets and extraterrestrial systems where life may have originated at extreme temperatures.

Clumped-isotope thermometry of magnesium carbonates in ultramafic rocks

Magnesium carbonate minerals produced by reaction of H2O–CO2 with ultramafic rocks occur in a wide range of paragenetic and tectonic settings and can thus provide insights into a variety of geologic processes, including (1) deposition of ore-grade, massive-vein cryptocrystalline magnesite; (2) formation of hydrous magnesium carbonates in weathering environments; and (3) metamorphic carbonate alteration of ultramafic rocks. However, the application of traditional geochemical and isotopic methods to infer temperatures of mineralization, the nature of mineralizing fluids, and the mechanisms controlling the transformation of dissolved CO2 into magnesium carbonates in these settings is difficult because the fluids are usually not preserved. Clumped-isotope compositions of magnesium carbonates provide a means to determine primary mineralization or (re)equilibration temperature, which permits the reconstruction of geologic processes that govern magnesium carbonate formation. We first provide an evaluation of the acid fractionation correction for magnesium carbonates using synthetic magnesite and hydromagnesite, along with natural metamorphic magnesite and low-temperature hydromagnesite precipitated within a mine adit. We show that the acid fractionation correction for magnesium carbonates is virtually indistinguishable from other carbonate acid fractionation corrections given current mass spectrometer resolution and error. In addition, we employ carbonate clumped-isotope thermometry on natural magnesium carbonates from various geologic environments and tectonic settings. Cryptocrystalline magnesite vein deposits from California (Red Mountain magnesite mine), Austria (Kraubath locality), Turkey (Tutluca mine, Eskişehir district) and Iran (Derakht-Senjed deposit) exhibit broadly uniform Δ47 compositions that yield apparent clumped-isotope temperatures that average 23.7±5.0°C. Based on oxygen isotope thermometry, these clumped-isotope temperatures suggest mineralization at shallow crustal depths in the presence of meteoric water. Hydrous magnesium carbonates from a 400-km latitudinal transect along the serpentinized-peridotite bodies of the California Coast Ranges record clumped-isotope temperatures between 14.2 and 22.7±2.8°C, in agreement with historical maximum temperatures during the rainy season for California. Talc–carbonate alteration of ultramafic rocks in Greenland (Isua Supracrustal Belt) and Vermont (Ludlow) yields clumped-isotope alteration temperatures of magnesite and dolomite between 326 and 490°C, broadly consistent with paragenesis and thermodynamic analysis for CO2 metasomatism of ultramafic rocks. These metamorphic carbonates extend the applicability of clumped-isotope thermometry to high-temperature magnesium carbonate systems and indicate equilibrium blocking temperatures for magnesite of ∼490°C. Our study demonstrates the applicability of the clumped isotope approach to provide information on the formation of magnesium carbonates as ore resources, surface records of climate, and metamorphic assemblages.

Magmatic and meteoric fluid flow in the Bitterroot extensional detachment shear zone (MT, USA) from ductile to brittle conditions

The Bitterroot shear zone developed as a rolling-hinge detachment system where a syntectonic granodiorite in the footwall was progressive exhumed beneath a detachment shear zone, providing a record of deformation and fluid-rock interaction during progressive exhumation and cooling. The shear zone displays a high strain gradient over ∼1km of structural section from the relatively undeformed footwall, where the granodiorite contains a magmatic foliation and lineation, through a mylonite sequence that culminates upward in the fine interlayering of ultramylonite layers and the development of a breccia zone. We measured the stable isotope composition of quartz, muscovite, biotite, chlorite, and epidote across the shear zone and estimated equilibrium temperatures using oxygen isotope thermometry based on mineral pairs. We also measured the hydrogen isotope ratios of hydrous minerals and of quartz fluid inclusions. The main results are: (1) The relatively undeformed footwall granodiorite interacted with magmatic fluids at 500–600°C; (2) the mylonitic fabric defined by muscovite, biotite, and chlorite developed between 500 and 300°C and interacted with a fluid system that was connected to the Earth’s surface (meteoric fluids), as indicated by the low δD values of hydrous mineral phases, including muscovite; (3) the fluxes of surface fluids were not sufficient to shift the δ18O values of muscovite significantly, but were sufficient to control the δ18O composition of biotite and chlorite during deformation-induced recrystallization and chloritization, and (4) the isotopic composition of fluid inclusions in quartz as well as the δD values of late quartz veins track the mixing of fluid sources between the magmatic and meteoric reservoirs. The distribution of stable isotope compositions in the various tectonites of the granodioritic Bitterroot shear zone, from ductile to brittle, provides a rich spatial and temporal record of the interaction between deformation and fluid flow in a crustal-scale detachment.

Argon, oxygen, and boron isotopic evidence documenting 40ArE accumulation in phengite during water-rich high-pressure subduction metasomatism of continental crust

The Luliang Shan area of the North Qaidam high pressure (HP) to ultrahigh pressure (UHP) metamorphic terrane in northwestern China features thick, garnet- and phengite-rich metasomatic selvages that formed around gneiss-hosted mafic eclogite blocks during HP conditions. Here we present new 40Ar/39Ar, δ18O, and δ11B results from a previously studied 30 m, 18 sample traverse that extends from the host gneiss into a representative eclogite block. Previous thermobarometry and new mica-quartz oxygen isotope thermometry from the traverse reveal that the phengite-rich selvage formed at temperatures similar to those recorded by the eclogites at peak pressure. Quartz and white mica δ18O data from the selvage cannot be explained by simple mixing of gneiss and eclogite, and indicate a fluid/rock ratio >1 during regional-scale infiltration of high δ18O (ca. 14‰) fluids. Heavy δ18O overgrowths of metamorphic zircon over lighter δ18O detrital grains indicate that the gneiss was similarly affected. Starkly contrasting boron content and δ11B compositions for the host gneiss and the selvage also cannot be explained by local-scale devolatilization of the gneiss to form the selvage. Instead, the boron systematics are best attributed to two distinct phases of fluid infiltration: (1) low-boron selvage phengite with δ11B from −10 to −30‰ grew under HP conditions; and (2) tourmaline and boron-rich muscovite with generally positive δ11B crystallized in the host gneiss under subsequent lower pressure epidote–amphibolite facies conditions as the Luliang Shan gneiss terrane was exhumed past shallower portions of the subduction channel. Consistent with observations made worldwide, we were able to identify uptake of excess argon (40ArE) in phengite as a high pressure phenomenon. Phengite 40Ar/39Ar ages from massive eclogite exceed the ca. 490 Ma zircon U–Pb age of eclogite metamorphism by a factor of 1.5. However, phengite ages from the more permeable schistose selvage were even older, exceeding the time of eclogite formation by a factor of 1.7. In contrast, lower pressure retrograde muscovite present within the host gneiss and in discrete shear zones cutting the selvage yield 40Ar/39Ar ages that were younger than the time of HP metamorphism and consistent with regional cooling age patterns. Our observation of high 40ArE concentrations in phengite from schistose rocks infiltrated by regionally extensive fluids at HP conditions runs contrary to widely held expectations. Conventional wisdom dictates that low phengite/fluid partition coefficients for argon (Dphg/fluidAr=10−3to10−5) coupled with the dry, closed systems conditions that are widely reported to characterize HP metamorphism of continental crust explains why high concentrations of 40ArE partitions are able to accumulate within phengite. We alternatively propose that phengite/fluid partition coefficients for argon increase linearly with pressure to values as high as 10−2 to allow phengites to accumulate large amounts of 40ArE from aqueous fluids under HP to UHP conditions.

Oxygen isotope thermometry reveals high magmatic temperatures and short residence times in Yellowstone and other hot-dry rhyolites compared to cold-wet systems

The eruption and storage temperatures of rhyolitic magmas are critical factors for understanding the mechanisms of their eruption and petrogenesis. Temperatures are particularly important when comparing the magmatic histories of hot-dry rhyolites from the Yellowstone-Snake River Plain (YSRP) and Iceland to cold-wet rhyolites such as the Bishop Tuff. Here we employ mineral pair oxygen isotope fractionations for estimating rhyolite temperatures independent of pressure and other compositional factors. We report high precision oxygen isotope analyses of quartz, pyroxene, magnetite, and zircon that we use to estimate crystallization and storage temperatures. Temperatures for YSRP and Icelandic rhyolites are highest for quartz-magnetite and quartz-clinopyroxene (~950 °C), with lower quartz-zircon (850 °C) temperatures that are similar to estimates of zircon saturation. The magnitude and pattern of these temperatures is consistent with crystallization from near-liquidus rhyolites. In contrast, oxygen isotope temperatures calculated for the Bishop and other “cold-wet” type tuffs define low ~760 °C temperatures for all three mineral pairs consistent with prolonged mineral residence at near-solidus conditions. Preservation of a down-temperature crystallization sequence of hot magnetite and clinopyroxene with colder zircon in hot-dry YSRP and Icelandic rhyolites suggest <1000 yr magma residence, where magnetite does not have sufficient time to diffusively equilibrate oxygen in a lower temperature melt. This is consistent with recently determined high precision U-Pb crystallization ages zircons from the same units indicating magma generation shortly before eruption.

Crystallization kinetics of rhyolitic melts using oxygen isotope ratios

AbstractCrystals provide the means to understand igneous systems, but natural constraints on crystallization kinetics are rare because thermal conditions and crystallization timescales are typically unknown. Oxygen isotope ratios in quartz and alkali feldspar crystals in spherulites provide a natural record of the temperature interval of crystallization and crystal growth rates in rhyolitic melts. Oxygen isotope compositions in both phases change progressively with position from the spherulite core to rim. Quartz δ18O increases from 5.0 ± 0.3‰ in the core to 5.6 ± 0.3‰ at the rims, whereas alkali feldspar decreases from 3.7 ± 0.4‰ in the core to 2.7 ± 0.9‰ at the rims. Fractionation therefore increases from 1.3 ± 0.7‰ in the cores to 2.9 ± 1.1‰ at the rims. Oxygen isotope thermometry tracks crystallization temperature with position. Spherulites nucleate at 578 ± 160°C and continue to grow until 301 ± 88°C. The in situ analyses demonstrate that spherulites self‐contain a record of their thermal history and that of the host lava.

Temperature dependence of oxygen- and clumped isotope fractionation in carbonates: A study of travertines and tufas in the 6–95 °C temperature range

Conventional carbonate–water oxygen isotope thermometry and the more recently developed clumped isotope thermometer have been widely used for the reconstruction of paleotemperatures from a variety of carbonate materials. In spite of a large number of studies, however, there are still large uncertainties in both δ18O- and Δ47-based temperature calibrations. For this reason there is a need to better understand the controls on isotope fractionation especially on natural carbonates. In this study we analyzed oxygen, carbon and clumped isotopes of a unique set of modern calcitic and aragonitic travertines, tufa and cave deposits from natural springs and wells. Together these samples cover a temperature range from 6 to 95°C. Travertine samples were collected close to the vents of the springs and from pools, and tufa samples were collected from karstic creeks and a cave. The majority of our vent and pool travertines and tufa samples show a carbonate–water oxygen isotope fractionation comparable to the one of Tremaine et al. (2011) with some samples showing higher fractionations. No significant difference between the calcite–water and aragonite–water oxygen isotope fractionation could be observed. The Δ47 data from the travertines show a strong relationship with temperature and define the regression Δ47=(0.044±0.005×106)/T2+(0.205±0.047). The pH of the parent solution, mineralogy and precipitation rate do not appear to significantly affect the Δ47-signature of carbonates, compared to the temperature effect and the analytical error. The tufa samples and three biogenic calcites show an excellent fit with the travertine calibration, indicating that this regression can be used for other carbonates as well. This work extends the calibration range of the clumped isotope thermometer to travertine and tufa deposits in the temperature range from 6°C to 95°C.

Oxygen isotope thermometry, speedometry, and hygrometry: Apparent equilibrium temperature versus closure temperature

AbstractRather than indicating formation/peak temperature, oxygen isotope fractionations preserved in mineral assemblages of slowly cooled plutonic and metamorphic rocks yield apparent equilibrium temperatures (Tae). The isotopic fractionations and Tae values deliver information about cooling history, as the extent of diffusive exchange of oxygen isotopes during cooling is controlled by the cooling time scale or cooling rate. Despite that several models, such as the Fast Grain Boundary (FGB) model, have been developed to simulate oxygen isotope exchange between coexisting minerals during cooling, extraction of cooling rate remains far from straightforward. On the other hand, there is a well‐defined quantitative relationship between the Dodson closure temperature (Tc) and the cooling rate, but Tc cannot be directly measured. Based on simulation results of existing models for a variety of rock systems, including open systems (with an infinite fluid reservoir), closed systems (with negligible fluid participation) and semi‐open systems (with moderate fluid participation), this study demonstrates that Tae of the mineral pair with the largest equilibrium isotope fractionation (PLEIF) is always bounded by their Tc values, regardless of how mineral proportions vary or how significant a role fluid has played in isotopic exchange. If the two Tc values happen to be similar, Tae will serve as a good approximation of both Tc, provided that the equilibrium fractionation factor has been precisely determined as a function of temperature. One such pair is quartz‐magnetite. By contrast, a mineral pair with similar Tc but relatively small fractionation is susceptible to the disturbance from other minerals, hence does not always have Tae confined within their Tc range. The relationship of Tae‐Tc correspondence for PLEIF with similar Tc can be used to constrain either cooling rate (i.e., as a speedometry method) or oxygen isotope diffusivity if one of them has been independently determined. In the latter case, the inferred oxygen diffusivity may be an index of water fugacity (i.e., as a hygrometry method) when compared with experimental diffusivity values measured under different fluid conditions.

Infiltration of meteoric fluids in an extensional detachment shear zone (Kettle dome, WA, USA): How quartz dynamic recrystallization relates to fluid-rock interaction

We document the interplay between meteoric fluid flow and deformation processes in quartzite-dominated lithologies within a ductile shear zone in the footwall of a Cordilleran extensional fault (Kettle detachment system, Washington, USA). Across 150 m of shear zone section, hydrogen isotope ratios (δD) from synkinematic muscovite fish are constant (δD ∼ −130‰) and consistent with a meteoric fluid source. Quartz-muscovite oxygen isotope thermometry indicates equilibrium fractionation temperatures of ∼365 ± 30 °C in the lower part of the section, where grain-scale quartz deformation was dominated by grain boundary migration recrystallization. In the upper part of the section, muscovite shows increasing intragrain compositional zoning, and quartz microstructures reflect bulging recrystallization, solution-precipitation, and microcracking that developed during progressive cooling and exhumation. The preserved microstructural characteristics and hydrogen isotope fingerprints of meteoric fluids developed over a short time interval as indicated by consistent mica 40Ar/39Ar ages ranging between 51 and 50 Ma over the entire section. Pervasive fluid flow became increasingly channelized during detachment activity, leading to microstructural heterogeneity and large shifts in quartz δ18O values on a meter scale. Ductile deformation ended when brittle motion on the detachment fault rapidly exhumed the mylonitic footwall.

U-Pb zircon and K-Ar geochronology reveal the emplacement and cooling history of the Late Cretaceous Beypazarı granitoid, central Anatolia, Turkey

The Beypazarı granitoid has been studied with respect to multi-radiometric dating and oxygen isotopic geothermometry. Radiometric dating of the granitoid yields zircon U-Pb isochron ages ranging from 72.5 ± 12.6 to 78.6 ± 4.7, and K-Ar ages of 71.4 ± 2.8 to 74.9 ± 2.9 and 59.5 ± 2.2 to 75.4 ± 2.9 Ma for hornblende and biotite, respectively. Oxygen isotope thermometry for the granitoid gives temperatures of 550 ± 25°C to 605 ± 30, 390 ± 15 to 540 ± 25°C, and 481 ± 5 to 675 ± 10°C, for hornblende, biotite, and K-feldspar, respectively, when paired with quartz. The systematic differences among ages according to different techniques used on different minerals are used to reconstruct the cooling history of the granite. The results yield rapid cooling rates of 33.3°C/Ma from 800°C to 550°C, and slow cooling rates of about 15 ± 0.5°C/Ma from 550 to 300°C. Rapid subsolidus cooling between 600°C and 550°C is documented by 40Ar/39Ar ages on amphibole and biotite between 71.4 ± 2.8 and 75.4 ± 2.9 Ma. Younger ages on biotites from two samples (59.5 ± 2.2 and 64.4 ± 2.5) are probably caused by loss of Ar. The reason for this possible Ar loss can be interpreted as slower subsolidus cooling (~375°C) ages. There is an apparent spatial and temporal relationship between the intrusion-cooling of the Beypazarı granitoid and the evolution of the İzmir–Ankara–Erzincan ocean belonging to the northern Neo-Tethyan ocean domain.

RELICT AND RE-SET OXYGEN ISOTOPE RATIOS FROM THE RUSSELL LAKE ALLOCHTHON, SOUTHERN APPALACHIANS

The Russell Lake Allochthon crops out as small, numerous, metamorphosed mafic/ultramafic bodies in the southern Appalachians Piedmont in Georgia, USA. Oxygen isotope compositions obtained by laser-fluorination mass spectrometry of samples from one of these bodies, at Bakers Ferry, show average plagioclase and olivine δ 18 O values of 6.6‰ and 5.5‰, respectively, with an average Δ 18 O plagioclase-olivine value of 1.1‰, and a closure temperature of 962 °C using the fractionations of Chiba et al. (1989). Samples from two other bodies, the River Road and the Nancy Hart complex, show average plagioclase δ 18 O values of 7.2‰ and 7.5‰, olivine δ 18 O values of 3.4‰ and 4.0‰, and Δ 18 O plagioclase-olivine values of 3.8‰ and 4.5‰, respectively. Similarly, the River Road body and the Nancy Hart complex yield much lower oxygen isotope thermometry temperatures of 409 °C and 419 °C, respectively, consistent with subsolidus 18 O exchange. These three bodies are all within 6 km of each other. Such a wide range in both mineral δ 18 O values and isotope thermometry temperatures in spatially closely located bodies is consistent with values encountered in ophiolite complexes, rather than layered intrusions. The occurrence of numerous bodies with different oxygen isotope signatures within relatively short distances from one another may suggest their origin as an ophiolite, where fracturing associated with their formation in extensional environments provided numerous pathways for fluids to interact with these bodies and reset their δ 18 O values.

Magmatic garnet in the Triassic (215 Ma) Dehnow pluton of NE Iran and its petrogenetic significance

The Triassic Dehnow pluton of NE Iran is a garnet-bearing I-type calc-alkaline metaluminous diorite-tonalite-granodiorite intrusion. The parental magma formed as the result of partial melting of intermediate to felsic rocks in the lower crust. Petrological and geochemical evidence, which indicates a magmatic origin for the garnet, includes: large size (~10–20 mm) of crystals, absence of reaction rims, a distinct composition from garnet in adjacent metapelitic rocks, and similarity in the composition of mineral inclusions (biotite, hornblende) in the garnet and in the matrix. Absence of garnet-bearing enclaves in the pluton and lack of sillimanite (fibrolite) and cordierite inclusions in magmatic garnet suggests that the garnet was not produced by assimilation of meta-sedimentary country rocks. Also, the δ18O values of garnet in the pluton (8.3–8.7‰) are significantly lower than δ18O values of garnet in the metapelitic rocks (12.5–13.1‰). Amphibole-plagioclase and garnet-biotite thermometers indicate crystallization temperatures of 708°C and 790°C, respectively. A temperature of 692°C obtained by quartz-garnet oxygen isotope thermometry points to a closure temperature for oxygen diffusion in garnet. The composition of epidote (Xep) and garnet (Xadr) indicates ~800°C for the crystallization temperature of these minerals. Elevated andradite content in the rims of garnet suggests that oxygen fugacity increased during crystallization.