Matthew J. Kohn

UWG-2, a garnet standard for oxygen isotope ratios: Strategies for high precision and accuracy with laser heating

UWG-2 is a new garnet standard for oxygen isotope analysis prepared from a single large porphyroblast that was homogeneous (±0.21‰) at the millimeter-scale before grinding. The δ 18O value of UWG-2 has been determined in seven laboratories using either a laser probe system or externally heated Ni reaction vessels. The raw laser probe value is 5.74‰ at the University of Wisconsin. If all data are normalized to NBS-28 = 9.59‰, then the UW value (5.89‰) is in good agreement with the average of all labs (5.78‰). There is no significant difference between garnet analyses made with the two techniques, nor among labs using different wavelengths of IR laser. UWG-2 is available for interlaboratory comparison, and for assessing the performance of microanalytical techniques including laser probes and ion microprobes with a recommended value of δ 18O = 5.8‰ SMOW. Multiple, daily analyses of UWG-2 at the University of Wisconsin provide an accurate evaluation of all components in our laser-probe, mass-spectrometer system, and allow analytical problems to be rapidly identified. With this standardization, the accuracy of a single laser probe analysis is better than ±0.10‰. Over 1000 analyses of UWG-2 have been made. The average of all uncorrected δ 18O values is 5.74 ± 0.15‰ (1 sd). The precision on a single day averages ±0.07‰ and is frequently better than 0.05‰. The uncertainty in the mean for all analyses is ±0.005‰ (1σ). A small drift of the daily average over time results from inevitable changes in the vacuum line which require careful attention and maintenance.

Carbon isotope compositions of terrestrial C3 plants as indicators of (paleo)ecology and (paleo)climate

A broad compilation of modern carbon isotope compositions in all C3 plant types shows a monotonic increase in δ13C with decreasing mean annual precipitation (MAP) that differs from previous models. Corrections for temperature, altitude, or latitude are smaller than previously estimated. As corrected for altitude, latitude, and the δ13C of atmospheric CO2, these data permit refined interpretation of MAP, paleodiet, and paleoecology of ecosystems dominated by C3 plants, either prior to 7–8 million years ago (Ma), or more recently at mid- to high latitudes. Twenty-nine published paleontological studies suggest preservational or scientific bias toward dry ecosystems, although wet ecosystems are also represented. Unambiguous isotopic evidence for C4 plants is lacking prior to 7–8 Ma, and hominid ecosystems at 4.4 Ma show no isotopic evidence for dense forests. Consideration of global plant biomass indicates that average δ13C of C3 plants is commonly overestimated by approximately 2‰.

Predicting animal δ18O: Accounting for diet and physiological adaptation

Abstract Theoretical predictions and measured isotope variations indicate that diet and physiological adaptation have a significant impact on animals δ18O and cannot be ignored. A generalized model is therefore developed for the prediction of animal body water and phosphate δ18O to incorporate these factors quantitatively. Application of the model reproduces most published compositions and compositional trends for mammals and birds. A moderate dependence of animal δ18O on humidity is predicted for drought-tolerant animals, and the correlation between humidity and North American deer bone composition as corrected for local meteoric water is predicted within the scatter of the data. In contrast to an observed strong correlation between kangaroo δ18O and humidity (Δδ18O/Δh ∼ 2.5± 0.4‰/10%r.h.), the predicted humidity dependence is only 1.3 – 1.7‰/10% r.h., and it is inferred that drinking water in hot dry areas of Australia is enriched in 18O over rainwater. Differences in physiology and water turnover readily explain the observed differences in δ18O for several herbivore genera in East Africa, excepting antelopes. Antelope models are more sensitive to biological fractionations, and adjustments to the flux of transcutaneous water vapor within experimentally measured ranges allows their δ18O values to be matched. Models of the seasonal changes of forage composition for two regions with dissimilar climates show that significant seasonal variations in animal isotope composition are expected, and that animals with different physiologies and diets track climate differently. Analysis of different genera with disparate sensitivities to surface water and humidity will allow the most accurate quantification of past climate changes.

Altered states: effects of diagenesis on fossil tooth chemistry

Investigation of modern and fossil teeth from northern and central Kenya, using the ion micro- probe, electron microprobe, and transmission electron microscope, confirms that fossil tooth chemistry is controlled not only by the diagenetic precipitation of secondary minerals but also by the chemical alteration of the biogenic apatite. Increases in the concentrations of Fe, Mn, Si, Al, Ba, and possibly Cu in fossil vs. modern teeth reflect mixtures of apatite and secondary minerals. These secondary minerals occur in concen- trations ranging from ;0.3% in enamel to ;5% in dentine and include sub-mm, interstitial Fe-bearing manganite ((Fe 31 ,M n 31 )O(OH)), and smectite. The pervasive distribution and fine grain size of the secondary minerals indicate that mixed analyses of primary and secondary material are unavoidable in in situ methods, even in ion microprobe spots only 10 mm in diameter, and that bulk chemical analyses are severely biased. Increases in other elements, including the rare earth elements, U, F, and possibly Sr apparently reflect additional alteration of apatite in both dentine and enamel. Extreme care will be required to separate secondary minerals from original biogenic apatite for paleobiological or paleoclimate studies, and nonetheless bulk analyses of purified apatite may be suspect. Although the PO 4 component of teeth seems resistant to chemical alteration, the OH component is extensively altered. This OH alteration implies that bulk analyses of fossil tooth enamel for oxygen isotope composition may be systematically biased by 61‰, and seasonal records of oxygen isotope composition may be spuriously shifted, enhanced, or diminished. Copyright

Herbivore tooth oxygen isotope compositions: Effects of diet and physiology

Abstract The applicability of rapid and precise laser probe analysis of tooth enamel for δ18O has been verified, and the method has been applied to different modern herbivores in East Africa. Sampling and pretreatment procedures involve initial bleaching and grinding of enamel to 95% apatite) can be analyzed reliably. Different East African herbivores exhibit previously unsuspected compositional differences. Average enamel δ18O values (V-SMOW) are approximately: 25‰ (goat), 27‰ (oryx), 28‰ (dikdik and zebra), 29‰ (topi), 30‰ (gerenuk), and 32‰ (gazelle). These compositions differ from generalized theoretical models, but are broadly consistent with expected isotope effects associated with differences in how much each animal (a) drinks, (b) eats C3 vs. C4 plants, and (c) pants vs. sweats. Consideration of diet, water turnover, and animal physiology will allow the most accurate interpretation of ancient teeth and targeting of environmentally-sensitive animals in paleoclimate studies.

Geochronologic and thermobarometric constraints on the evolution of the Main Central Thrust, central Nepal Himalaya

The Main Central Thrust (MCT) juxtaposes the high-grade Greater Himalayan Crystallines over the lower-grade Lesser Himalaya Formation; an apparent inverted metamorphic sequence characterizes the shear zone that underlies the thrust. Garnet-bearing assemblages sampled along the Marysandi River and Darondi Khola in the Annapurna region of central Nepal show striking differences in garnet zoning of Mn, Ca, Mg, and Fe above and below the MCT. Thermobarometry of MCT footwall rocks yields apparent inverted temperature and pressure gradients of ∼18°C km−1 and ∼0.06 km MPa−1, respectively. Pressure-temperature (P-T) paths calculated for upper Lesser Himalaya samples that preserve prograde compositions show evidence of decompression during heating, whereas garnets from the structurally lower sequences grew during an increase in both pressure and temperature. In situ (i.e., analyzed in thin section) ion microprobe ages of monazites from rocks immediately beneath the Greater Himalayan Crystallines yield ages from 18 to 22 Ma, whereas late Miocene and Pliocene monazite ages characterize rocks within the apparent inverted metamorphic sequence. A Lesser Himalayan sample collected near the garnet isograd along the Marysandi River transect contains 3.3±0.1 Ma monazite ages (P ≈ 0.72 GPa, T ≈ 535°C). This remarkably young age suggests that this portion of the MCT shear zone accommodated a minimum of ∼30 km of slip over the last 3 Ma (i.e., a slip rate of >10 mm yr−1) and thus could account for nearly half of the convergence across the Himalaya in this period. The distribution of ages and P-T histories reported here are consistent with a thermokinematic model in which the inverted metamorphic sequences underlying the MCT formed by the transposition of right-way-up metamorphic sequences during late Miocene-Pliocene shearing.

Seasonal stable isotope evidence for a strong Asian monsoon throughout the past 10.7 m.y

O of wet-season rainfall was significantly morenegative (29.5‰ SMOW) prior to 7.5 Ma than after ( 26.5‰SMOW). If this change is attributable to a lessening of the amounteffect in rainfall, this agrees with floral and soil geochemical datathat indicate increasing aridity beginning at 7.5 Ma.Keywords: Tibetan Plateau, monsoon, stable isotopes, paleohydrology,seasonality.INTRODUCTIONThe Tibetan Plateau is the engine that drives the modern Asianmonsoon by generating a high-altitude region of low pressure in thesummer as the plateau heats, and a region of high pressure in the winteras the plateau cools (Hastenrath, 1991). During the summer, warm airrises from the plateau, pulling moist air off the ocean, across the Indiansubcontinent, and into the highlands; this results in heavy summer rain-fall on the subcontinent. The opposite occurs in the winter, resultingin cold dry air spilling off the plateau and effectively excluding rainfrom the subcontinent. Thus, the presence of a strong wet-season–dry-season alternation implies the presence of a plateau broad and highenough to drive the monsoon.The timing of the uplift of the plateau remains a matter of con-siderable debate because there are few direct indicators of paleotopog-raphy in the geologic record. Consequently, past workers in Tibet, re-lying on indirect indicators of uplift, have proposed dates ranging from40 to 3.4 Ma, on the basis of initiation of potassic volcanism (Chunget al., 1998; Turner et al., 1993) or extension on the plateau (Harrisonet al., 1995; Coleman and Hodges, 1995), changes in marine sedimen-tation rates (Burbank et al., 1993), sediment types (Rea et al., 1998),or biota (Nigrini and Caulet, 1992; Kroon et al., 1991), and changesin stable carbon isotope and palynological patterns on the Indian sub-continent (Quade et al., 1989; Chen, 1981). Although different areasof the plateau may have risen at different times, many workers haveinferred rapid simultaneous uplift of large areas of the plateau at 7–8Ma by a process such as lithospheric delamination (Molnar et al.,1993). This inference was based on the following approximately coevalphenomena: a major change in plant communities of the Indian sub-continent (Quade et al., 1989), and shifts in marine upwelling patternsthat are linked to an intense monsoon (Kroon et al., 1991). Althoughthere is strong evidence for significant climate change at 7–8 Ma, it isunclear whether this is the onset of the monsoon. The floral transitionseems to have been a global rather than local phenomenon (Cerling etal., 1997) and monsoonally driven upwelling may have already beenpresent by 10–12 Ma (Nigrini and Caulet, 1992; Kroon et al., 1991).Because there is an intimate association between the intense sea-sonality of the modern monsoon and a high Tibetan Plateau and be-cause evaporation can be unambiguously recognized in the d

Retrograde net transfer reaction insurance for pressure-temperature estimates

Retrograde net transfer reactions significantly affect compositions of metamorphic minerals, yet are rarely considered when determining pressure-temperature (P-T) conditions. Two natural amphibolite facies metapelites from the central Himalaya of Nepal exhibit extremely common compositional patterns, including increases in Mn and Fe/(Fe + Mg) at the rims of garnets, which are the result of retrograde garnet dissolution and Fe-Mg exchange with biotite. However, typical thermobarometric approaches for these rocks result in errors of hundreds of degrees and 3–6 kbar compared with thermobarometry of nearby rocks and petrogenetic grids. These large errors result because dissolution of high-Fe garnet has strongly affected the Fe/Mg ratio of matrix biotite. X-ray maps help evaluate the extent and chemical effects of retrograde reactions in these samples by identifying mineral regions that retain highest-T compositions, or, through a new data-processing approach, by permitting correction of mineral compositions to original high-T values. These approaches ensure against retrograde net transfer reactions and should be applied routinely in thermobarometric studies—they ultimately yield P-T estimates that are more petrologically reasonable, and permit rapid screening of samples for those least affected by retrograde reactions. Reconsideration of thermobarometry in the central and eastern Himalaya indicates that retrograde net transfer reactions are extremely common. Therefore, previous thermobarometric studies based on garnet major element compositions from that region should be reevaluated.

Formation of monazite via prograde metamorphic reactions among common silicates: Implications for age determinations

Three lines of evidence from schists of the Great Smoky Mountains, NC, indicate that isogradic monazite growth occurred at the staurolite-in isograd at ∼600°C: (1) Monazite is virtually absent below the staurolite-in isograd, but is ubiquitous (several hundred grains per thin section) in staurolite- and kyanite-grade rocks. (2) Many monazite grains are spatially associated with biotite coronas around garnets, formed via the reaction Garnet + Chlorite + Muscovite = Biotite + Plagioclase + Staurolite + H2O. (3) Garnets contain high-Y annuli that result from prograde dissolution of garnet via the staurolite-in reaction, followed by regrowth, and rare monazite inclusions occur immediately outside the annulus and in the matrix, but not in the garnet core. Larger monazite grains also exhibit quasi-continuous Th zoning with high Th cores and low Th rims, consistent with monazite growth via a single reaction and fractional crystallization during prograde growth. Common silicates may host sufficient P and LREEs that reactions among them can produce observable LREE phosphate. Specifically phosphorus contents of garnet and plagioclase are hundreds of parts per million, and dissolution of garnet and recrystallization of plagioclase could form thousands of phosphate grains several micrometers in diameter per thin section. LREEs may be more limiting, but sheet silicates and plagioclase can contain tens to ∼100 (?) ppm LREE, so recrystallization of these silicates to lower LREE contents could produce hundreds of grains of monazite per thin section. Monazite ages, determined via electron and ion microprobes, are ∼400 Ma, directly linking prograde Barrovian metamorphism of the Western Blue Ridge with the “Acadian” orogeny, in contrast to previous interpretations that metamorphism was “Taconian” (∼450 Ma). Interpretation of ages from metamorphic monazite grains will require prior chemical characterization and identification of relevant monazite-forming reactions, including reactions previously viewed as involving solely common silicates.

P-T-t data from central Nepal support critical taper and repudiate large-scale channel flow of the Greater Himalayan Sequence

A new synthesis of pressure-temperature conditions and pressure-temperature-time (P-T-t) paths is presented for high-grade metamorphic thrust sheets associated with the Main Central Thrust, in the Langtang and Darondi regions, central Nepal. From structurally low to structurally high, major structures include the Lesser Himalayan Duplex, Munsiari Thrust, Main Central Thrust (thrust contact between the Greater and Lesser Himalayan Sequences), and Langtang Thrust. Key P-T-t results include the following: in a transect from Lesser Himalayan Duplex to Langtang Thrust rocks, peak metamorphic P-T conditions are uniformly ∼550 °C and 8 kbar in the Lesser Himalayan Duplex, and show a strong gradient to ∼725 °C and 10–12 kbar over a structural distance of less than 2 km associated with the Munsiari Thrust and Main Central Thrust; T9s then increase and P9s decrease gradually upsection, reaching ∼825 °C and 8 kbar in the Langtang Thrust. Juxtaposition of thrust sheets occurred at moderate pressure (8–12 kbar) on thrust surfaces roughly coincident with the modern Main Himalayan Thrust. Published monazite ages demonstrate synmetamorphic thrusting, with peak metamorphic ages decreasing progressively downward: 21 ± 2 Ma (Langtang Thrust), 16 ± 1 Ma (Main Central Thrust), 10.5 ± 0.5 Ma (Munsiari Thrust), and 3.5 ± 0.5 Ma (Lesser Himalayan Duplex). Together with published thermochronologic results, these data constrain initial cooling ages and rates: 15–20 Ma and ∼40 °C/m.y. for the Langtang Thrust and Main Central Thrust, and 3–10 Ma and ≥100 °C/m.y. for the Munsiari Thrust and Lesser Himalayan Duplex. Overall, metamorphic and chronologic patterns are matched well by expectations of critical taper models, including (1) uniformly high pressures of metamorphism (8–12 kbar) for all structural levels and thrust movement along the paleo–Main Himalayan Thrust, (2) isobaric cooling from the peak of metamorphism for Greater Himalayan rocks (deep juxtaposition of thrust sheets), (3) “hairpin” P-T paths for Lesser Himalayan rocks, and (4) relatively slow cooling rates for Greater Himalayan rocks. However, observations contrast significantly with published channel flow models, which predict (1) peak P-T conditions within the sillimanite stability field for Lesser and lower Greater Himalayan rocks (versus observations of P-T conditions in the kyanite stability field), (2) peak metamorphic pressures that decrease structurally downward—7–13, 6, 5, and 5 kbar for rocks achieving temperatures recorded by Langtang Thrust, Main Central Thrust, Munsiari Thrust, and Lesser Himalayan Duplex rocks (versus observations of 8, 10–12, 10, and 8 kbar), (3) retrograde isothermal exhumation P-T trajectories for Greater Himalayan rocks (versus isobaric cooling of the Main Central Thrust and Langtang Thrust), (4) cooling of migmatitic Greater Himalayan rocks after 10 Ma (versus observations of 15–20 Ma), and (5) isobaric heating of the Lesser Himalayan rocks (versus observations of simultaneous increases in P and T for some Lesser Himalayan rocks). Neither model matches “clockwise” P-T paths observed in structurally high Lesser Himalayan rocks, or the extraordinary cooling rate of the Lesser Himalayan Duplex, which points to complications in their evolution in the context of end-member models. Most generally, although channel flow may have initiated since ca. 10 Ma due to focused erosion above the Lesser Himalayan Duplex, it does not appear responsible for past transport and exhumation of the migmatitic core of the Himalaya.