High Himalayan Crystalline Series Research Articles

Carbonate Versus Silicate Weathering Fluxes of Sr From a High Himalayan Watershed

The Himalayan uplift has been implicated as a major factor in the global climatic cooling of the past 40 m.y. because of enhanced silicate weathering and consequent atmospheric CO2 drawdown (Raymo and Ruddiman, 1992). Supporting evidence includes the dramatic increase in marine 87Sr/86Sr ratios at 40 Ma, which appears to be related to Sr inputs from major rivers draining the Himalaya (Palmer and Edmond, 1992). Regional patterns of 87Sr/86Sr ratios in tributaries of the Ganges-Brahmaputra and Indus River systems as well as bedrock 87Sr/86Sr ratios suggest that the major source of the highly radiogenic Sr in Himalayan rivers is weathering of silicate minerals in the High Himalayan Crystalline Series (HHCS) (Krishnaswami et al., 1992; Harris, 1995). Here we report on a geochemical investigation of the ~200 km 2 Raikhot watershed in the western Himalaya on the north side of the Nanga Parbat massif in northern Pakistan, with the aim of elucidating the systematics of Sr release from individual minerals in the HHCS bedrock. The bedrock of the area is predominantly highgrade quartzofeldspathic biotite gneiss and schist with minor anatectic biotite granite and a small amount (-1%) of calc-silicate schist. The bedrock is generally representative of the HHCS, which extends over 2000 km across the length of the Himalaya and dominates the geology of the steep southern slopes that are subjected to rapid erosion during the monsoon season (Harris, 1995). Samples for analysis were taken of rainwater, snow, clear streams, Raikhot River water (which has suspended glacial flour), Raikhot riverbed sand (believed to be representative of unweathered bedrock from the watershed), bedrock from outcrops, and glacial boulders (Blum et al., 1998). Rock samples containing calcite were leached for 1 h in 4 N acetic acid to dissolve carbonate, and silicate rock samples were totally digested for elemental and Sr isotope analysis. Concentrations were measured by ICP-OES and ICP-MS; 878r/86Sr ratios were measured by TIMS. The most abundant minerals in the bedrock of the watershed are quartz, plagioclase, K-feldspar, and biotite; calcite is present in only minor abundance but is important due to its high reactivity. Plagioclase and K-feldspar are assumed to weather to kaolinite, and biotite to vermiculite (or hydrobiotite). A massbalance calculation was performed to quantify the proportions of each of the bedrock minerals that weathered to yield each water composition. This calculation has the following six steps based on the stoichiometry of the weathering reactions. (1) Atmospheric correction. (2) Attributing of Na to the weathering of albite to kaolinite. (3) Attributing of Ca in proportion to the Ca/Na ratio of plagioclase to the weathering of anorthite to kaolinite. (4) Attributing any remaining Si to the weathering of orthoclase to kaolinite. (5) Attributing the remaining K to the weathering of biotite to vermiculite. (6) Attributing excess Ca and Mg to carbonate dissolution. For the Raikhot River samples, the above calculation yields an estimate of the relative amounts of weathering of minerals as follows: 14% plagioclase, 2% orthoclase, 11% biotite, and 73% carbonate. This estimate corresponds to a riverine HCO3 flux that is 18% derived from silicate and 82% derived from carbonate weathering reactions. Analyses of the carbonate and silicate fractions of the riverbed sand indicate that only 1.0 wt.% of the sediment load is carbonate. The calculated percentages of weathering of bedrock minerals for the average quartzofeldspathic gneiss and granite bedrock stream-waters were: 24% plagioclase, <1% orthoclase, 12% biotite, and 64% carbonate. High proportions of riCO;are also derived from carbonate (68-78%) in the stream waters known to drain exclusively quartzofeldspathic gneiss and granite bedrock. The Ca/(1000Sr) ratios of silicate rocks (gneiss and granite) range from 0.07 to 0.4, whereas marble layers range from 1 to 2. 87Sr/86Sr ratios of the silicate rocks (and all their constituent minerals) range between 0.82 and 0.89, whereas marble layers

Carbonate versus silicate weathering in the Raikhot watershed within the High Himalayan Crystalline Series

The major element and Sr isotope geochemistry of surface waters, bedrock, and river sands was investigated in the Raikhot watershed within the High Himalayan Crystalline Series (HHCS) in northern Pakistan. Mass-balance calculations of mineral-weathering contributions to the dissolved flux of ions from the watershed indicate that 82% of the HCO 3 − flux is derived from the weathering of carbonate minerals and only 18% is derived from silicate weathering, even though the bedrock is predominantly quartzofeldspathic gneiss and granite with only ∼1% carbonate in the watershed. This study demonstrates the importance of trace amounts of bedrock carbonate in controlling the water chemistry of glacial watersheds. We suggest that the flux of Sr with a high 87 Sr/ 86 Sr ratio in the major Himalayan rivers may be derived in large part from weathering of trace amounts of calcite within the largely silicate HHCS. Models that use the flux of radiogenic Sr from the Himalayas as a proxy for silicate weathering rates may, therefore, overestimate the amount of CO 2 consumption due to silicate weathering in the Himalaya.

Strontium Isotope Systematics of a Himalayan Glacial Chronosequence

Chemical weathering of silicate minerals consumes atmospheric CO2, and therefore, changes in global silicate weathering rates may act as a mechanism to drive major fluctuations in the Earth's climate and long-term global carbon cycle. Recent studies suggest that steep increases in the Cenozoic marine Sr isotope record correlate with periods of global cooling and enhanced silicate weathering rates, initiated by the onset of glaciation in the uplifting Himalayan mountains (Raymo and Ruddiman, 1992). However, due to the Himalaya's unique orogenic history, strontium with high 87Sr/86Sr ratios may reside in easily weathered carbonate minerals that do not act as a net sip& for atmospheric CO2 when undergoing chemical weathering (Edmond, 1992; Blum et al., 1998). Thus, the dissolved flux of Sr from the Himalayan Mountains may not be an ideal proxy for silicate weathering and atmospheric CO2 consumption. By studying the major element and Sr isotope chemistry of soils developed on a Himalayan glacial chronosequence, we are investigating how the dissolved flux of ions derived from carbonate versus silicate mineral weathering varies as a function of time following the exposure of fresh rock surfaces to the agents of chemical weathering. Geologic setting and methods. The -200 km 2 Raikhot Valley, located at an average elevation of ~4000 m on the northern slope of the Nanga ParbatHarimosh Massif (NPHM) within the Pakistani Himalayas, contains both the ~50 km 2 Raikhot Glacier as well as a clearly visible glacial chronosequence. Four moraines, thought to be roughly Little Ice Age to neoglacial in age, are nested near the modem terminus of the glacier, while a fifth moraine, believed to be last glacial maximum in age, forms a bench about 200 m from the glacial terminus. The bedrock of the watershed, considered to be representative of the High Himalayan Crystalline Series (HHCS) which stretches for 2000 km across the length of the Himalayan Mountains, primarily consists of quartzofeldspathic biotite gneisses, biotite schists, leucogranite dikes, and granitic stocks, as well as minor amounts (~1%) of interbedded amphibolite and calc-silicate schists. In order of increasing relative age, the sample numbers 96PK1, 96PK2, 96PK3, and 96PK4 refer to soil profiles collected from the four young nested moraines, while the sample numbers 96PK5 and 96PK5a refer to 2 separate soil profiles developed on the fifth moraine. This report presents results obtained from the analysis of the major element (Ca, Mg, Na, K, and Si) concentrations, minor element (Sr, Ti, and Zr) concentrations, and the Sr isotope ratios (87Sr/86Sr) contained in the bulk and exchangeable fractions of these soils. Ca to Sr ratios. The analysis of Ca/(1000*Sr) ratios provides a useful mechanism for characterizing the mineral provenance of the bulk soil and exchangeable ions. The Ca/(1000*Sr) ratios of the bulk soil and exchangeable ions represent a mixture between the average calcium carbonate and silicate endmembers for the watershed, as determined by sequentially leaching and digesting modem riverbed sediment with 4N acetic acid and HF-HC104, respectively (Blum et al., 1998). The average Ca/(1000*Sr) ratio of the exchangeable fraction in profiles 96PK1, 96PK2, 96PK3, and 96PK4 appears similar to the calcium carbonate endmember for the watershed (1.4), while the Ca/(1000*Sr) ratios contained in the entire suite of bulk soils as well as the exchangeable fraction in the C-horizons of profiles 96PK5 and 96PK5a appears more similar to the silicate endmember (0.19). These observations suggest that even though calcium carbonate represents only -1% of the rock in the watershed, calcium carbonate is the primary source of Ca and Sr provided to the dissolved groundwater flux during the early stages of weathering. In addition, the Ca/(1000*Sr) ratios in the C-horizons of profiles 96PK5 and 96PK5a demonstrate that the calcium carbonate reservoir in the soils is nearly depleted within the -15,000 to 20,000 years during which the soils have developed on the moraines. However, a reversal toward carbonate-like Ca/(1000*Sr) ratios in the uppermost horizons of the older profiles indicates that the continual addition of fresh weatherable material,

A comparative study of diffusion profiles in Himalayan and Dalradian garnets: constraints on diffusion data and the relative duration of the metamorphic events

Cation diffusion rates at 690 ± 30 °C have been calculated by inverse modelling of observed manganese (Mn) zonation profiles in 40 garnets from two kyanite-bearing metapelite samples from the High Himalayan Crystalline Series, Zanskar, northwest India. Knowledge of the initial growth profile of Mn in garnet is a pre-requisite for this technique. Following previous workers we model Mn partitioning into growing garnet in terms of a Rayleigh fractionation process, and demonstrate that the Mngarnet:whole rock partition coefficient is 60–100. Three-dimensional zonation profiles were obtained by successively grinding and polishing ∼1 cm slabs of each sample at 0.1–0.2 mm intervals and analysing the garnets at each stage, thus ensuring that core sections were measured. The diffusion model assumes that garnet has a spherical geometry and behaves as a closed system, and simulates diffusive modification of the hypothetical Mn Rayleigh growth profile for each garnet. The derived measure of the time-integrated diffusion history for each garnet is then combined with radiometric and field-relation constraints for the duration of the Himalayan metamorphic event to calculate cation diffusion rates. The average cation interdiffusion rate calculated for garnets in the two samples examined is (6 ± 3.2) × 10−23 m2s−1. This interdiffusion rate pertains to a temperature of 690 ± 30 °C, which is 0.97 × T PEAK, the peak temperature conditions experienced by the samples estimated using standard thermobarometric techniques. Garnet compositions are Py2–17Alm65–77Gro6–16Sp1–17. These new diffusion data are consistent with, and more precise than, existing high-temperature (>1000 °C) experimentally determined diffusion data, although some uncertainties remain difficult to constrain. Qualitative comparison between diffusively modified Mn growth profiles in garnets from the Scottish Dalradian and the Himalayan garnets suggests that the duration of metamorphism affecting the Dalradian garnets was 10–20 times longer than that endured by the Himalayan garnets.

REE fractionation and Nd-isotope disequilibrium during crustal anatexis: constraints from Himalayan leucogranites

The processes controlling rare earth element (REE) behaviour during crustal anatexis have profound implications not only for REE abundances of crustally derived granites but also for their model Nd ages. For typical protoliths of anatectic granites, such as are exposed in metapelites from the High Himalayan Crystalline Series (HHCS), a high proportion (90%) of bulk-rock LREE and MREE are sited in the accessory phases apatite and monazite, and of HREE in garnet and apatite. Mass-balance constraints indicate that observed REE abundances of Himalayan leucogranites cannot be generated from REE-deficient reactant phases in their metasedimentary protoliths (muscovite, biotite, plagioclase and quartz) but must result from the dissolution of accessory phases. REE and Th abundances in anatectic melts are modelled by combining empirical expressions for monazite, apatite and zircon solubilities (as a function of temperature, melt composition and melt water content) with analysed accessory-phase REE abundances. This approach, which allows the contribution to the melt from each accessory phase to be identified, successfully predicts Himalayan leucogranite trace-element abundances. The LREE and Th concentrations of a metapelite-derived melt are buffered by monazite stability, although apatite dissolution contributes up to 10% of LREE to the melt together with ∼50% of the MREE and 90% of the HREE. Zircon contributes a negligible proportion of REE to the melt. During anatexis, dissolution of monazite and apatite in the melt results in: (1) increasing REE abundances and LREE/HREE ratios with increasing temperature of melting; (2) a negative Eu anomaly in the melt; (3) high Sm/Nd ratios in the melt relative to its metasedimentary protolith; and (4) Nd-isotope disequilibrium between the melt and its protolith. All of these features are observed in the REE geochemistry of Himalayan leucogranites. The combined effects of Sm/Nd fractionation and Nd-isotope disequilibrium during anatexis imply that model Nd ages calculated for anatectic granites derived from apatite and monazite bearing protoliths should be treated with considerable caution.

Evolution of the Himalaya since Miocene time: isotopic and sedimentological evidence from the Bengal Fan

Abstract We report Sr, Nd, O, and H isotopic data and clay mineral abundances for turbidite sediments recovered in ODP Leg 116 cores from the Bengal Fan at 1°S. The samples studied cover the period between c. 17 Ma and the present. We also present new and compiled data on the isotopic compositions of potential source regions for the Bengal Fan sediments. ɛ Nd (0) values in the Bengal Fan sediments (all samples) define a narrow range about − 16.0. 87 Sr/ 86 Sr values (all samples) are also in a narrow range near 0.741. δ 18 O values in quartz separates define a narrow range at +12.8±0.5‰. Coarse biotite-chlorite separates give δ 18 O = 3.6−5.6‰. Combined δ 18 O values of quartz and biotites indicate a metamorphic source. Clay mineral abundances define two clay facies: an illite-chlorite-rich assemblage (IC) and a smectite-kaolinite-rich assemblage (SK). δ 18 O in the IC clay fractions is 11.5–15‰, while SK clays are 18.2–22.6‰. The narrow range of isotopic values throughout the deposition history implies that the source of the Bengal Fan sediments has not changed since the early Miocene, despite changes in sedimentation rate, sedimentary facies, tectonic history and climactic regime. The difference between δ 18 O in the IC and SK clay fractions represents different alteration histories of the same source material. The SK clays appear to have been altered at low T in the Indo-Gangetic Plain, while the IC clays and coarse fractions preserve metamorphic signatures. The narrow range of the Sr values, despite wide variation in Rb/Sr ratio, also argues for a source that underwent isotopic homogenization shortly before erosion and deposition of the sediment. The source that meets these criteria is the High Himalayan Crystalline series (HHC) or a close analogue, although subordinate contributions (probably &lt;20%) from the Lesser Himalaya (LH) and Tibetan Sedimentary Series (TSS) are possible. A model in which the HHC are exposed to erosion since the early Miocene on the south flank of the orogen by thrusting along the MCT, while the TSS is simultaneously removed by northward-directed normal faulting satisfies the constraints above. The results of this study require that the Himalaya have been a significant topographic feature since at least the early Miocene. Independent evidence supports this contention. Variations in the sedimentation style in the Bengal Fan since that time appear to represent a combination of factors, including tectonic activity and the coupled effects of climate and sea-level changes.