Zachary D. Sharp

Quartz-calcite oxygen isotope thermometry: A calibration based on natural isotopic variations

An empirical calibration for the quartz-calcite thermometer was derived from measured Δ18O(qz-cc) values from greenschist-facies marbles, veins composed of cogenetic quartz and calcite and various low-grade metamorphic rocks. The Δ18O(qz-cc) values vary systematically with independently determined formation temperature and can be fit to the expression 1000 ln α(qz-cc) = 0.87(±0.06) × 106/T2. In contrast, published results from direct-exchange experiments between calcite and quartz are 1000 ln α(qz-cc) = 0.38(±0.06) × 106/T2, far smaller than in the present study. Application of the experimental mineral-water and especially the direct-exchange calibrations to natural samples, yields unreasonably low geological temperatures. It is difficult to envision a mechanism whereby the measured fractionations in greenschist-grade marbles can be reconciled with the very low temperature estimates obtained with the direct-exchange experimental calibration. Oxygen diffusion rates in quartz are too slow to explain the discrepancy. Postmetamorphic exchange could have occurred with a hydrothermal fluid, but it is unlikely that the δ18O(calcite) values of all samples would be shifted by an amount that would result in a linear relationship between 1000 ln α(qz-cc) and T−2. More likely, the discrepancy is due to a kinetic effect in the experiments. The very small fractionations observed in the direct-exchange experiments may have been caused by diffusion-related effects during recrystallization of the quartz and calcite. The problem of recrystallization is eliminated in mineral-CO2 exchange experiments. Combined CO2-calcite and CO2-quartz glass experiments yield the expression 1000 ln α(qz-cc) = 0.78 (0.08), in good agreement with the empirical calibration. The new empirical calibration yields reasonable temperature estimates for a wide range of samples and can be used for thermometry in rock types and over temperature intervals where other quantitative geothermometers are lacking.

Fossil isotope records of seasonal climate and ecology: Straight from the horse's mouth

Isotope analysis of a bulk fossil tooth gives a “snapshot” impression of paleoclimatic conditions—a single point in time. However, hypsodont teeth grow over a period of a year or more, so that stable carbon and oxygen isotope variations along their length are a “tape recorder” of short-term seasonal variations from the distant past. We have used a new in situ micro-laser sampling method to determine submillimeter carbon and oxygen isotope variations in the enamel of individual fossil horse teeth to assess ancient annual meteoric water variations and feeding patterns. The δ18O values from a 6.8 Ma fossil horse tooth ( Astrohippus ansae ) from Texas vary cyclically along the 6 cm length of the tooth with a smoothed amplitude of >4‰, similar to the monthly averaged amplitude measured in modern meteoric waters from the region. The seasonal δ18O values are ∼3‰ to 4‰ higher than those calculated from modern meteoric water data, suggesting either a higher local meteoric water value in the Miocene of Texas, or that the animal received a high proportion of its dietary water from plants or highly evaporated water. A Holocene horse tooth from the shores of Glacial Lake Agassiz, North Dakota ( Equus sp.), also has isotopic variations with the same 35 mm periodicity, but a smoothed amplitude of only 2‰. This horse most likely had a buffered drinking supply. The calculated δ18O of the water in equilibrium with this tooth is the same as the modern measured annual average. The variations within a single tooth can be as large as those generally observed in entire stratigraphic sections of fossil teeth analyzed by bulk methods. The new method provides an important technique for evaluating fossil diagenesis; conventional bulk analyses of teeth fragments may not be representative of the whole tooth, thus explaining analytical scatter that has been previously attributed to diagenesis.

Stable carbon and oxygen isotope analysis of fossil tooth enamel using laser ablation

Abstract A technique is described whereby the δ13C and δ18O values of fossil tooth enamel can be measured in situ using laser ablation techniques. The laser heats the sample and forms CO2 from structural carbonate apatite. The δ18O values obtained with this method are equal to those of the phosphate oxygen due to the high temperature of reaction during ablation. Analytical precision is approximately 0.5‰ for both δ13C and δ18O. The spatial resolution is 200 μm or less, which may be reduced as the technical modifications are made. This method allows the analysis of samples that previously could not be analyzed due to difficulty with sample preparation, size, or rare nature. The apparent oxygen isotope fractionation between the carbonate and laser derived oxygen (αCO3-laser) is 1.0071 for biogenic apatite at 38°C determined using the in situ laser. Comparison of δ18O(CO3) and δ18O(laser) analyses with δ18O(laser) and δ18O(PO4) results suggest a fractionation factor α(CO3)-PO4) of 1.0083. Variations in the δ18O and δ13C values of single teeth can be easily determined with the in situ technique. This will allow the study of changes in diets or water sources during the period of tooth growth, which for larger mammals can be several years for the tooth row. The δ13C and δ18O values are important in paleodiet and paleoecology studies. They can be used to study resource partitioning within an ecosystem and can shed light on the life history strategies within living and extinct species.

Stable isotope geochemistry and phase equilibria of coesite-bearing whiteschists, Dora Maira Massif, western Alps

Peak metamorphic temperatures for the coesite-pyrope-bearing whiteschists from the Dora Maira Massif, western Alps were determined with oxygen isotope thermometry. The δ18O(smow) values of the quartz (after coesite) (δ18O=8.1 to 8.6‰, n=6), phengite (6.2 to 6.4‰, n=3), kyanite (6.1‰, n=2), garnet (5.5 to 5.8‰, n=9), ellenbergerite (6.3‰, n=1) and rutile (3.3 to 3.6‰, n=3) reflect isotopic equilibrium. Temperature estimates based on quartz-garnet-rutile fractionation are 700–750 °C. Minimum pressures are 31–32 kb based on the pressure-sensitive reaction pyrope + coesite = kyanite + enstatite. In order to stabilize pyrope and coesite by the temperature-sensitive dehydration reaction talc+kyanite=pyrope+coesite+H2O, the a(H2O) must be reduced to 0.4–0.75 at 700–750 °C. The reduced a(H2O) cannot be due to dilution by CO2, as pyrope is not stable at X(CO2)>0.02 (T=750 °C; P=30 kb). In the absence of a more exotic fluid diluent (e.g. CH4 or N2), a melt phase is required. Granite solidus temperatures are ∼680 °C/30 kb at a(H2O)=1.0 and are calculated to be ∼70°C higher at a(H2O)=0.7, consistent with this hypothesis. Kyanite-jadeite-quartz bands may represent a relict melt phase. Peak P-T-f(H2O) estimates for the whiteschist are 34±2 kb, 700–750 °C and 0.4–0.75. The oxygen isotope fractionation between quartz (δ18O=11.6‰) and garnet (δ18O=8.7‰) in the surrounding orthognesiss is identical to that in the coesitebearing unit, suggesting that the two units shared a common, final metamorphic history. Hydrogen isotope measurements were made on primary talc and phengite (δD(SMOW)=-27 to-32‰), on secondary talc and chlorite rite after pyrope (δD=-39 to -44‰) and on the surrounding biotite (δD=-64‰) and phengite (δD=-44‰) gneiss. All phases appear to be in nearequilibrium. The very high δD values for the primary hydrous phases is consistent with an initial oceanicderived/connate fluid source. The fluid source for the retrograde talc+chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization. The similar δD, but dissimilar δ18O values of the coesite bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium, but not oxygen isotope equilibrium. The unusual hydrogen and oxygen isotope compositions of the coesite-bearing unit can be explained as the result of metasomatism from slab-derived fluids at depth.

Late Permian and Early Triassic evolution of the Northern Indian margin: carbon isotope and sequence stratigraphy

AbstractThe Northern part of Great-India underwent an early rifting phase in the late Paleozoic, just at the end of the large scale Gondwanian glaciation. The beginning of the rifting processes is marked by large hiatus and discontinuities (para- conformities) between the early or middle Paleozoic sedimentary succession and the discontinuous middle-late Permian Traps and transgressive sediments. The Northern Indian passive margin consists of the present High and Lower Himalaya and a small part of the Indian craton and their sedimentary cover. The Permian rift shoulder is located in the Higher Himalaya, with part being in the underthrusted Lower Himalaya. The rim basin (landward of the shoulder) is well developed in the Pottawar- Salt Range area. From the rifting to the beginning of the drifting stages (early late Permian to late early Triassic time), the sedimentary evolution is characterised by three transgressive- regressive (T-R) second order cycles, two in the late Permian and one in the early Triassi...

A laser GC-IRMS technique for in situ stable isotope analyses of carbonates and phosphates

Abstract A technique is described whereby in situ carbon and oxygen isotope analyses of carbonates and organic phosphates can be made with the use of a CO2 laser. The CO2 gas generated by thermal decarbonation from the laser is entrained in a helium carrier gas, passes through a chromatographic column (GC), and is admitted directly into the isotope ratio mass spectrometer (IRMS). No vacuum systems, pumps, or cryogenic traps are used. All carbonates and biogenic phosphates can be analyzed, no special sample preparation is required and analyses can be made every 3 minutes. The use of a helium carrier gas allows for extremely small samples to be analyzed and the GC column effectively separates CO2 from any other potential contaminating gases (e.g., SO2 which is a particular problem in organic apatite). The average reproducibility of calcite, dolomite, magnesite, rhodochrosite, siderite, and smithsonite (ZnCO3) is 0.29‰ for oxygen and 0.1‰ for carbon (1σ); the most “homogeneous” samples are reproducible to better than 0.1‰ for carbon and 0.2‰ for oxygen. The difference between the laser and conventional values for carbon isotope ratios [Δ 13C (laser-conv)] is 0.05 ± 0.30‰ for all carbonates (excluding siderite). The Δ 18O(laser-conv) value varies from carbonate to carbonate and may be related to the electronegativities of the cations, grain size (or crystallinity), formation of CO and O2, and reaction with included organic matter. For calcite and rhodochrosite, the Δ 18O(laser-conv) value is 0.3 ± 0.4‰; for siderite, magnesite, and dolomite, the Δ 18O(laser-conv) value is 1.7 ± 0.3‰. The δ13C values of tooth enamel are the same as those obtained by conventional acid digestion. The laser δ18O values are equal to the δ 18O values of the phosphate, and approx. 7‰ lighter than the “carbonate” oxygen. The carbonate group in the apatite (equiv. 7.6% oxygen) exchanges with the (PO4=)-bound oxygen to produce CO2 with a δ 18O equal to the phosphate oxygen. The laser technique provides a rapid alternative to the difficult phosphate extraction technique for oxygen isotope measurements in tooth enamel.

Oxygen isotope thermometry of quartz-calcite veins: Unraveling the thermal-tectonic history of the subgreenschist facies Morcles nappe (Swiss Alps)

Combined structural analysis and oxygen isotope thermometry of syntectonic quartz-calcite fibrous veins can be used to correlate the thermal history of deformed rocks with specific structural and tectonic events. Results are presented for the Morcles nappe in the western Helvetic Alps, Switzerland, where mineral parageneses, illite “crystallinity,” and fluid inclusion chemistry record an apparent peak metamorphic temperature gradient that increased across the Morcles nappe from anchizonal conditions in the foreland to epizonal conditions in its hinterland root zone. Twenty-seven quartz-calcite veins were analyzed in this study in order to determine the temperatures of veining during formation and deformation of the nappe. Peak metamorphic temperatures ranged from ≈260 to 290 °C in the shallower, foreland localities and to ≈330 to 350 °C in the deeper, more hinterland localities at the end of S1-foliation formation, related to large-scale folding. Temperatures gradually decreased throughout the nappe during subsequent development of the S2 foliation and S3 crenulation cleavage. Uplift and erosion of the overlying nappe pile resulted in slow cooling of the Morcles nappe during the waning stages of the Alpine Orogeny. The dominant foliation-forming deformation of the Morcles nappe occurred at elevated temperatures over the course of 10 to 15 Ma. Combined structure–oxygen isotope analyses of quartz-calcite veins yield better temperature and temporal constraints on the thermal histories of subgreenschist vein–bearing tectonites than do other geothermometers.

A stable and 40Ar/39Ar isotope study of a major thrust in the Helvetic nappes (Swiss Alps): Evidence for fluid flow and constraints on nappe kinematics

Stable isotope and 40 Ar/ 39 Ar measurements were made on samples associated with a major tectonic discontinuity in the Helvetic Alps, the basal thrust of the Diablerets nappe (external zone of the Alpine Belt) in order to determine both the importance of fluids in this thrust zone and the timing of thrusting. A systematic decrease in the δ 18 O values (up to 6‰) of calcite, quartz, and white mica exists within a 10- to 70-m-wide zone over a distance of 37 km along the thrust, and they become more pronounced toward the root of the nappe. A similar decrease in the δ 13 C values of calcite is observed only in the deepest sections (up to 3‰). The δD SMOW (SMOW = standard mean ocean water) values of white mica are −54‰ ± 8‰ ( n = 22) and are independent of the distance from the thrust. These variations are interpreted to reflect syntectonic solution reprecipitation during fluid passage along the thrust. The calculated δ 18 O and δD values (versus SMOW) for the fluid in equilibrium with the analyzed minerals is 12‰ to 16‰ and −30‰ to +5‰, respectively, for assumed temperatures of 250 to 450 °C. The isotopic and structural data are consistent with fluids derived from the deep-seated roots of the Helvetic nappes where large volumes of Mesozoic sediments were metamorphosed to the amphibolite facies. It is suggested that connate and metamorphic waters, overpressured by rapid tectonic burial in a subductive system escaped by upward infiltration along moderately dipping pathways until they reached the main shear zone at the base of the moving pile, where they were channeled toward the surface. This model also explains the mechanism by which large amounts of fluid were removed from the Mesozoic sediments during Alpine metamorphism. White mica 40 Ar/ 39 Ar ages vary from 27 Ma far from the Diablerets thrust to 15 Ma along the thrust. An older component is observed in micas far from the thrust, interpreted as a detrital signature, and indicates that regional metamorphic temperatures were less than about 350 °C. The plateau and near plateau ages nearest the thrust are consistent with either neocrystallization of white mica or argon loss by recrystallization during thrusting, which may have been enhanced in the zones of highest fluid flow. The 15 Ma 40 Ar/ 39 Ar age plateau measured on white mica sampled exactly on the thrust surface dates the end of both fluid flow and tectonic transport.

STABLE AND RADIOGENIC ISOTOPE STUDY OF ECLOGITE XENOLITHS FROM THE ORAPA KIMBERLITE, BOTSWANA

Abstract Eclogite xenoliths from Orapa can be accurately classified as Group I or Group II on the basis of Na 2 O in garnet and K 2 O in clinopyroxene. Group I xenoliths are commonly diamondiferous while Group II xenoliths are diamond-free. Both xenolith varieties may contain graphite. Isotopic character is to some degree correlated with major- and trace-element chemistry. Group II samples with Ca-poor garnet have clinopyroxenes with radiogenic 87 Sr 86 Sr (0.705–0.709) and the least radiogenic 143 Nd 144 Nd (0.5122–0.5125). Group I eclogites with higher Ca and Fe in garnets have less radiogenic 87 Sr 86 Sr (0.702–0.7066) and bulk-Earth or higher 143 Nd 144 Nd ratios. Group I eclogites have more radiogenic 206 Pb 204 Pb (18.6–19) than Group II xenoliths (16.5–18.6). In contrast, Group II xenoliths have more variable and, in some cases, more radiogenic 206 Pb 204 Pb (36.6–39.3) than Group I xenoliths (38.3–38.4). The Sr, Sm, Nd and Pb concentrations of minerals in Orapa Group I eclogite xenoliths are much lower than in Group II samples. All the Group II xenoliths are inferred to be enriched in light rare-earth elements while Group I xenoliths are probably characterised in many cases by light rare-earth element depletion. Constituent garnet and clinopyroxene in both Group I and II eclogite xenoliths are essentially in isotopic equilibrium at the time of pipe emplacement. Mineral as well as calculated whole-rock 143 Nd 144 Nd compositions of most of the Group I eclogites are too close to bulk-Earth and depleted-mantle estimates in order to obtain useful model age information. Depleted-mantle model ages derived from the much lower 143 Nd 144 Nd compositions of the Group II eclogite xenoliths range from 661 to 1248 My, with an average clinopyroxene model age of 908 My and an average whole-rock model age of 1016 My. On the basis of an observed covariation of O and Sr isotopic compositions the entire Orapa Group I eclogite xenolith suite can be modelled as mixtures of oceanic basalt with or without a few percent of ocean floor sediment. The Group II xenoliths might have crystallised from a melt which derives from a protolith with time-averaged LREE depletion. Their radiogenic Sr isotope character could be due to interaction of the melt with metasomatised lithosphere, or might be a superimposed metasomatic signature.

Ultra-high temperatures from oxygen isotope thermometry of a coesite-sanidine grospydite

The oxygen isotope compositions of coesite, sanidine, kyanite, clinopyroxene and garnet were measured in an ultra-high pressure-temperature grospydite from the Roberts Victor kimberlite, South Africa. The δ18O values (per mil v. SMOW) of each phase and (1 σ) are as follows: coesite, 8.62 (0.31); sanidine, 8.31 (0.02); kyanite, 7.98 (0.08); pyroxene, 7.63 (0.11); garnet, 7.53 (0.03). In situ analyses of the coesite with the laser extraction system are δ18O=9.35 (0.08), n=4, demonstrating that the coesite is homogeneous. The coesite has partially inverted to polycrystalline quartz and the pyroxene is extensively altered during uplift. The larger scatter for the mineral separate coesite and pyroxene data may be due to partial reequilibration between the decompression-related breakdown products of these two phases. The anomalously high δ18O value of the grospydite (δ18Owholerock=7.7‰) is consistent with altered oceanic crust as a source rock. Temperature estimates from a linear regression of all the data to three different published calibrations correspond to an equilibrium temperature of 1310±80°C. The calculated isotopic pressure effect is to lower these estimates by about 40°C at 40 kb. The estimated temperature based on Al−Si disorder in sanidine is 1200±100°C and that from Fe−Mg exchange thermometry between garnet and clinopyroxene is 1100±50°C. Given the large errors associated with thermometry at such high temperatures, it is concluded that the xenolith equilibrated that 1200±100°C. Pressure estimates are 45±5 kb, based on dilution of the univariant equilibria albite = jadeite + coesite and 2 kyanite + 3 diopside = grossular + pyrope + 2coesite. Zoning in the outer 20 μm of the feldspar from Ab0.8 to Ab16 indicates rapid decompression to 25 kb or less. The isotopic temperature estimates are the highest ever obtained and combined with the high degree of Al−Si disorder in sanidine require rapid cooling from ultra-high temperatures. It is inferred that the xenolith was sampled at the time of equilibration, providing a point on the upper Cretaceous geotherm in the mantle below South Africa.