Hazel J. Chapman

Isotopic constraints on the structural relationships between the Lesser Himalayan Series and the High Himalayan Crystalline Series, Garhwal Himalaya

Nd and Sr isotope systematics may provide important constraints on the location of major thrust systems that separate lithologically similar sedimentary sequences. The potential of the technique is illustrated by this isotopic study of the Main Central thrust system of the Himalaya. Nd isotope data from the Garhwal Himalaya indicate that metasedimentary rocks from the Vaikrita Group (Nd = –14 to –19) correlate closely with those from the High Himalayan Crystalline Series, which constitutes the hanging-wall lithologies of the Main Central thrust. In contrast, metasedimentary rocks from the Munsiari Group (Nd = –23 to –28) show marked similarities to the Lesser Himalayan Series in the footwall of the Main Central thrust. Sr isotopes support the correlations in that the Vaikrita Group shows partial reequilibration at 500 Ma, whereas the Munsiari Group has not undergone Sr isotope homogenization since 1800 Ma. Thus, the Vaikrita thrust that juxtaposes these two formations is recognized as the Main Central thrust in Garhwal Himalaya. The thrust coincides, approximately, with the location of the kyanite isograd, confirming that inverted metamorphism is characteristic of both hanging wall and footwall of the Main Central thrust. Along the Tons thrust (known locally as the Srinagar thrust) 50 km south of the Main Central thrust, low-grade quartzarenites with Nd-Sr isotope and trace element characteristics typical of Lesser Himalayan formations have been emplaced on phyllites and siltstones with geochemical characteristics of the High Himalayan Crystalline Series. The field relationships most probably result from out-of-sequence thrusting in which Lesser Himalayan Series rocks to the north were emplaced over low-grade equivalents of the High Himalayan Crystalline Series preserved in the external part of the orogen. This study establishes the value of isotope data for lithostratigraphic correlations within orogenic belts.

Constraints on the exhumation and erosion of the High Himalayan Slab, NW India, from foreland basin deposits

Petrography, Sr–Nd isotope compositions and single-grain laser 40Ar–39Ar ages of detrital white mica from Early–Middle Miocene molasse of the Dharamsala and Lower Siwalik Formations of Northern India, dated by magnetostratigraphy, determine the sediment sources, their metamorphic grade and exhumation rates in the Himalayan palaeo-hinterland. Deposition of the Lower Dharamsala Member (21–17 Ma) occurred during the period of rapid isothermal decompression and crustal anatexis (24–18 Ma) of the metamorphic core. This episode of decompression is thought to be coeval with thrusting on the Main Central Thrust and normal faulting on the South Tibetan Detachment System. The sediment composition and detrital mica ages indicate erosion from the rapidly exhumed metamorphic slab of the High Himalayan Crystalline Series. Deposition of the Upper Dharamsala Member (17–13 Ma) and basal Siwalik Group (13–12.5 Ma) spanned the period in which thrusting transferred south from the Main Central Thrust. The sediment composition and detrital mica ages contrast strongly with those of the Lower Dharamsala, indicating erosion from sedimentary and low grade rocks. The isotopic composition indicates that these rocks were part of the High Himalayan Series unaffected by Tertiary metamorphism, i.e. from upper structural levels of the High Himalayan Slab. This suggests that a major reorganisation of the orogenic wedge occurred at 17 Ma involving forward propagation of the MCT and cessation of rapid exhumation of the metamorphic slab.

THE SIGNIFICANCE OF HIMALAYAN RIVERS FOR SILICATE WEATHERING RATES : EVIDENCE FROM THE BHOTE KOSI TRIBUTARY

The significance of weathering by Himalayan runoff for both the Sr-isotope marine record and the removal of atmospheric CO2 through silicate dissolution has been examined by systematic sampling of dissolved loads and bedloads from the Bhote Kosi, a tributary of the Ganges that rises in Tibet from Tethyan sediment bedrock and traverses the major Himalayan lithologies of eastern Nepal before debouching onto the Gangetic plains. Throughout the section, the cation geochemistry of water samples is dominated by Ca and Mg ions, suggesting that carbonates are the predominant lithology undergoing dissolution particularly within the Lesser Himalayas. As the river transects the metasedimentary and granitic lithologies of the High Himalayas the Sr-isotope ratio of the bedload rises rapidly, closely reflecting the isotope geochemistry of the bedrock. In contrast the 87Sr/86Sr ratio of the dissolved load remains roughly constant (0.719–0.723). Downstream of the Main Central Thrust, where the river transects the carbonate-bearing lithologies of the Lesser Himalayas, the 87Sr/86Sr ratio of the dissolved load rises sharply (>0.768). The relative contributions of silicate and carbonate weathering from each of the main Himalayan units has been estimated from major cation, Sr concentration and Sr isotope mass-balance equations. These calculations suggest that the high Sr and high 87Sr/86Sr characteristics of riverine analyses arise initially from a component dissolved from the Tibetan Sedimentary Series which is substantially enhanced by input weathering fluxes, particularly as the river traverses the Lesser Himalayas. Whilst mechanical erosion is maximised within the High Himalayan Crystalline Series, as confirmed by 143Nd/144Nd ratios from the bedload, at least 63% of the dissolved load is acquired by chemical weathering of bedrock lithologies and/or of transported particulates within the Lesser Himalayas, enhanced by higher ambient temperatures and slower discharge rates. This may involve continued dissolution of the High Himalayan Crystalline Series particulates in addition to Lesser Himalayan lithologies. Although Himalayan rivers collectively have a major influence on the Sr-isotope marine record, the high 87Sr/86Sr ratios of their dissolved load results from the mixing of a small component ( 90%) of carbonate-derived material some of which is characterised by a high 87Sr/86Sr ratio (up to 0.8). Elevated 87Sr/86Sr ratios in rivers are therefore not necessarily indicative of anomalously high dissolution rates of silicates.

Fluxes of Sr into the headwaters of the Ganges

Himalayan weathering is recognized as an important agent in modifying sea water chemistry, but there are significant uncertainties in our understanding of Himalayan riverine fluxes. This paper examines causes of the variability, including that of the seasons, by analysis of downstream variations in Sr, 87Sr, and major ions in the mainstream, in relation to the composition of tributary streams from subcatchments with differing geologic substrates. Water samples were collected over four periods spanning the premonsoon, monsoon, and postmonsoon seasons. Uncertainties in the relative fluxes have been estimated, using Monte Carlo techniques, from the short-term variability of mainstream chemistry and the scatter of tributary compositions. The results show marked seasonal variations in the relative inputs related to high monsoon rainfall in the High and Lesser Himalaya, contrasting with the major contribution from glacial melt waters from the Tibetan Sedimentary Series (TSS) at times of low rainfall. Much of the spread in previously published estimates of the sources of Sr in Himalayan rivers may result from these seasonal variations in Sr fluxes. The annual fluxes of Sr into the headwaters of the Ganges are derived from the three main tectonic units in the proportions 35 ± 1% from the TSS, 27 ± 3% from the High Himalayan Crystalline Series (HHCS), and 38 ± 8% from the Lesser Himalaya. The particularly elevated 87Sr/86Sr ratios characteristic of the HHCS and the Lesser Himalaya enhance their influence on seawater Sr-isotope composition. The TSS contributes 13 ± 1%, the HHCS 30 ± 3%, and the Lesser Himalaya 57 ± 11% of the 87Sr flux in excess of the seawater 87Sr/86Sr ratio of 0.709.

Controls on the 87Sr/86Sr Ratio of Carbonates in the Garhwal Himalaya, Headwaters of the Ganges

The episodic variation of the seawater 87Sr/86Sr ratio has been attributed to either variations in the Sr flux or the Sr‐isotopic composition of the riverine‐dissolved load derived from weathering of the continental crust. The discovery that Himalayan rivers are characterized by high concentrations of dissolved Sr concentrations with high 87Sr/86Sr ratios has raised the possibility that collisional orogens play a critical role in moderating the variations in seawater 87Sr/86Sr ratios. Here we describe Himalayan carbonates and calc‐silicates from Garhwal, the headwaters of the Ganges, with extreme 87Sr/86Sr ratios (>1.0). Elevated Sr‐isotope ratios result from exchange with Rb‐rich silicate material during both Himalayan and pre‐Himalayan metamorphic episodes, and the carbonates contribute a significant fraction to the Ganges 87Sr flux. Particularly elevated 87Sr/86Sr ratios are found in calc‐silicates from the Deoban Formation of the Lesser Himalaya. A detailed traverse of shales and calc‐silicates from this unit confirms that carbonate horizons have increased 87Sr/86Sr ratios as a result of isotopic exchange over length scales of 10–30 cm. We conclude that metamorphism of carbonates may cause elevation of their 87Sr/86Sr ratios and that uplift of metamorphosed carbonates may be a consequence of collisional orogens, which contributes to the elevation of seawater 87Sr/86Sr ratios.

Silicate weathering rates decoupled from the 87Sr/86Sr ratio of the dissolved load during Himalayan erosion

This study establishes that carbonate weathering dominates the supply of Sr with elevated 87Sr/86Sr ratios in the Bhote Kosi–Sun Kosi river system, central Nepal, a major Himalayan tributary to the Ganges. The dissolved load of the Bhote Kosi–Sun Kosi displays a rapid increase in the 87Sr/86Sr ratio in its lower section, downstream of the Main Central Thrust, a feature common to other Himalayan tributaries of the Ganges. Mass balance analysis of the contributing sources to the dissolved load identifies the weathering of Palaeoproterozoic impure carbonates (comprised largely of dolomite and mica) from the Lesser Himalaya as the most significant lithological control upon elevated 87Sr/86Sr ratios. In situ laser ablation analysis of the dolomite component yields 87Sr/86Sr ratios of 0.93 to 1.11, indistinguishable from bulk rock. This confirms the presence of a highly soluble source of radiogenic strontium and suggests that diffusive exchange during a protracted metamorphic history has homogenised Sr isotopes between silicate and carbonate phases within the assemblage. Weathering of the carbonate component of such rocks has yielded unusually high 87Sr/86Sr at significant Sr fluxes in the dissolved load. Approximately 60% of the impact of the Bhote Kosi–Sun Kosi on the marine 87Sr/86Sr ratio can be traced to weathering of this carbonate. Since similar lithologies outcrop within the catchments of most Himalayan tributaries that feed the Ganges and Brahmaputra rivers, the presence of such radiogenic carbonates undermines the inference that the impact of these rivers on Sr isotopes in seawater is indicative of high silicate weathering rates that result from the uplift of Tibet and the Himalaya.

Observational evidence confirms modelling of the long-term integrity of CO2-reservoir caprocks

Storage of anthropogenic CO2 in geological formations relies on a caprock as the primary seal preventing buoyant super-critical CO2 escaping. Although natural CO2 reservoirs demonstrate that CO2 may be stored safely for millions of years, uncertainty remains in predicting how caprocks will react with CO2-bearing brines. This uncertainty poses a significant challenge to the risk assessment of geological carbon storage. Here we describe mineral reaction fronts in a CO2 reservoir-caprock system exposed to CO2 over a timescale comparable with that needed for geological carbon storage. The propagation of the reaction front is retarded by redox-sensitive mineral dissolution reactions and carbonate precipitation, which reduces its penetration into the caprock to ∼7 cm in ∼105 years. This distance is an order-of-magnitude smaller than previous predictions. The results attest to the significance of transport-limited reactions to the long-term integrity of sealing behaviour in caprocks exposed to CO2.

Paleoenvironmental history of the West Baray, Angkor (Cambodia)

Angkor (Cambodia) was the seat of the Khmer Empire from the 9th to 15th century AD. The site is noted for its monumental architecture and complex hydro-engineering systems, comprised of canals, moats, embankments, and large reservoirs, known as barays. We infer a 1,000-y, 14C-dated paleoenvironmental record from study of an approximately 2-m sediment core taken in the largest Khmer reservoir, the West Baray. The baray was utilized and managed from the time of construction in the early 11th century, through the 13th century. During that time, the West Baray received relatively high rates of detrital input. In the 14th century, linear sedimentation rates diminished by an order of magnitude, yielding a condensed section that correlates temporally with episodes of regional monsoon failure during the late 14th and early 15th century, recorded in tree ring records from Vietnam. Our results demonstrate that changes in the water management system were associated with the decline of the Angkorian kingdom during that period. By the 17th century, the West Baray again functioned as a limnetic system. Ecologic and sedimentologic changes over the last millennium, detected in the baray deposits, are attributed to shifts in regional-scale Khmer water management, evolving land use practices in the catchment, and regional climate change.

On discrimination between carbonate and silicate inputs to Himalayan rivers

We review new and published analyses of river waters, bedloads and their constituent minerals from the Dhauli Ganga and Alaknanda, headwaters of the Ganges in Garhwal, and the Marsyandi in Nepal and their tributaries. These data are used to discriminate between the inputs of major cations and Sr from silicate and carbonate sources. Methods of estimating the proportion of the carbonate and silicate inputs to river waters using mixing arrays in Sr-Ca-Mg-Na-K-87Sr/86Sr space are shown to suffer from systematic correlations between the magnitude of the precipitation of secondary calcite and the fraction of the silicate component. This results in factor-of-two overestimates of the fractions of silicate-derived Ca, Mg and Sr. To correct for this the magnitude of secondary calcite precipitated and relative fractions of silicate and carbonate-derived cations are instead calculated by modeling the displacement of water compositions from the compositions of the carbonate and silicate components of the bedload in subsets of Sr-Ca-Mg-Na-K-87Sr/86Sr space. The compositions of the carbonate and silicate end-members in the bedload are determined by sequential leaching. The results of this modeling are compared with modeling of the modal mineral inputs to waters where mineral compositions are derived from electron-microprobe analyses of the minerals in the bedload. In the upper Marsyandi catchment, which drains low-grade Tethyan Sedimentary Series formations, a set of mainstem samples collected over a two-year period define tight correlations in Sr-Ca-Mg-Na-K-87Sr/86Sr space. Modeling of the magnitude of secondary carbonate precipitation and fractions of silicate-derived Ca, Mg and Sr in Sr-Ca-Mg-87Sr/86Sr space gives self-consistent results that are compatible with both the calculations of mineral modes and published Mg-isotopic compositions, if the ratio of chlorite to biotite weathering is high or if there is another silicate source of Mg. These calculations imply that between 12 and 31 percent of the Sr and 44 and 72 percent of the Mg is derived from silicate minerals where the range reflects the seasonal change in the ratio of silicate-derived to carbonate-derived cations. Modeling in Sr-Ca-Na and/or K space is inconsistent with the Sr-isotopic and Mg-isotopic constraints and we conclude that in this catchment dissolution of Na and K are incongruent relative to Sr-Ca-Mg. Potassium is preferentially retained in micas whereas the controls on Na are unclear. Modeling of the catchments underlain by High Himalayan Crystalline and Lesser Himalayan Series in Garhwal is complicated by the presence of dolomite as well as calcite in the carbonate and the results imply that dolomite dissolves faster in the acetic acid leaches than in nature. Up to 60 percent of the Sr in the catchment on High Himalayan Crystalline Series and 20 to 30 percent of Sr in the catchments on Lesser Himalayan Series are estimated to be derived from silicates. However it should be noted that the element budgets are not all self-consistent and the use of bedrock-element ratios to model the sources of chemical inputs to river waters remains subject to uncertainties.

The origin of Pyrenean Hercynian volcanic rocks (France-Spain): REE and SmNd isotope constraints

Abstract The Hercynian orogeny of the Pyrenees generated a suite of basalt to rhyolite calc-alkaline volcanic rocks that were erupted subaerially. These melts may either have been generated above a subduction zone or by lower-crustal melting after input of mantle-derived magmas in an extensional environment. SmNd garnet/whole-rock mineral isochrons date a rhyolitic ignimbrite from the Coll de Oli Ignimbrite Member at 313±14 Ma (2σ) (MSWD=0.7) and a rhyolitic lava of the Coll de Pi region at 320±2 Ma (2σ) (MSWD=0.2). These ages suggest that some of the volcanism was synchronous with high-temperature, low-pressure metamorphism and partial melting deep in the Hercynian crust as seen in rocks now exposed in the axial and northern Pyrenean zones and dated by UPb on zircon at between 309±5 and 315±5 Ma. The silicic volcanic rocks are peraluminous and have highly fractionated REE patterns with low abundances of the HREE and ( La Yb ) n ≈70 . Mineral and whole-rock REE abundances suggest derivation of the silicic rocks by partial melting of a metasedimentary source in equilibrium with garnet and/or garnet fractionation in melts derived from peraluminous sources. Nd isotope compositions of the entire volcanic rock suite (mafic-silicic compositions) are within the same range as those of sediments metamorphosed during the Hercynian orogeny and Hercynian basement rocks. They have mid-Proterozoic depleted-mantle model SmNd ages and do not exhibit mixing trends with mantle sources. Even the most mafic of the volcanic rocks appear to have interacted extensively with the crust. Analysis of the chemistry of the volcanic rocks does not unequivocally constrain the Hercynian tectonic setting of the Pyrenees.