Mixing of asthenospheric and lithospheric mantle-derived basalt magmas as shown by along-arc variation in Sr and Nd isotopic compositions of Early Miocene basalts from back-arc margin of the NE Japan arc

We report trace element and Sr–Nd isotopic compositions of Early Miocene (22–18 Ma) basaltic rocks distributed along the back-arc margin of the NE Japan arc over 500 km. These rocks are divided into higher TiO₂ (> 1.5 wt.%; referred to as HT) and lower TiO₂ (< 1.5 wt.%; LT) basalts. HT basalt has higher Na₂O + K₂O, HFSE and LREE, Zr/Y, and La/Yb compared to LT basalt. Both suite rocks show a wide range in Sr and Nd isotopic compositions (initial ⁸⁷Sr/⁸⁶Sr (SrI) = 0.70389 to 0.70631, initial ¹⁴³Nd/¹⁴⁴Nd(NdI) = 0.51248 to 0.51285). There is no any systematic variation amongst the studied Early Miocene basaltic rocks in terms of Sr–Nd isotope or Na₂O + K₂O and K₂O abundances, across three volcanic zones from the eastern through transitional to western volcanic zone, but we can identify gradual increases in SrI and decreases in NdI from north to south along the back-arc margin of the NE Japan arc. Based on high field strength element, REE, and Sr–Nd isotope data, Early Miocene basaltic rocks of the NE Japan back-arc margin represent mixing of the asthenospheric mantle-derived basalt magma with two types of basaltic magmas, HT and LT basaltic magmas, derived by different degrees of partial melting of the subcontinental lithospheric mantle composed of garnet-absent lherzolite, with a gradual decrease in the proportion of asthenospheric mantle-derived magma from north to south. These mantle events might have occurred in association with rifting of the Eurasian continental arc during the pre-opening stage of the Japan Sea.     

A Cretaceous dike swarm provides evidence of a spreading axis in the back-arc basin of the Kohistan paleo-island arc,northwestern Himalaya,Pakistan

A 100–4000 m wide and 15 km long dike swarm, consisting of basalt and dolerite, occurs at the base of the Thelichi Formation in the Kohistan paleo-island arc terrane, north Pakistan. The dikes contain hornblende (altered from diopsidic-augite), diopsidic-augite (relics; ophitic to subophitic texture), chlorite, epidote, sphene, apatite, zircon, ilmenite, titanomagnetite and magnetite. The geochemistry reveals two groups of dikes: (1) Higher TiO₂ (2.74–3.50 wt%), Na₂O, Fe₂O₃ and lower Al₂O₃ (12.65–14.16 wt%) and MgO (3.73–5.04 wt%); (2) Lower TiO₂ (1.24–2.05 wt%), Na₂O, Fe₂O₃ and higher Al₂O₃ (14.02–16.52 wt%) and MgO (3.98–7.52 wt%). The MgO contents (3.73–7.52-wt%) show a variation in the dikes from relatively primitive to more evolved compositions. The dikes contain high amounts of both LILE and HFSE. The major, trace and rare-earth elements data confirm the MORB affinity and the back-arc basin origin of the dike swarm. The NW–SE orientation of the dike swarm and its 134 ± 3 Ma K–Ar age suggest the spreading axis of the back-arc basin in the Early Cretaceous.     

Contribution of slab-fluid in arc magmas beneath the Japan arcs

Identifying the amount and composition of slab-derived fluid and its spatial variation is key to quantifying fluid processes in subduction zones. Based on the isotopic systematics of arc lavas, we found regional variations over the Japan arcs in terms of the amount and composition of slab-derived fluid added to the melting source region. The average amounts of slab-derived fluid differ among the arcs: 2.6 wt.% for Central Japan, 0.69 wt.% for Ryukyu, 0.17 wt.% for NE Japan, and 0.12 wt.% for both Kurile and Izu–Bonin. These differences may be attributed to the arc setting (oceanic or continental) and the geometry of the slabs. Contribution of sediment involved in the slab-derived fluid is dominant in NE Japan compared to the Izu–Bonin and Central Japan arcs. This could be attributed to mechanical features such as fractures near the subducting plate surface, in addition to the arc setting and the slab geometry. Therefore, the amount and composition of slab-derived fluid are thought to be controlled not only by the thermal conditions, but also by the tectonic and mechanical features around the subduction zone. On top of the variability of slab-derived fluid, the mantle wedge shows the regional variation in terms of proportion of the Pacific-type and Indian-type MORB-source mantle components, which also contributes to the compositional variations of arc magmas.     

Laboratory measurement of P-wave velocity in crustal and upper mantle xenoliths from Ichino-megata, NE Japan: ultrabasic hydrous lower crust beneath the NE Honshu arc

Ultrasonic laboratory measurements of P-wave velocity (Vp) were carried out up to 1.0 GPa in a temperature range of 25–400 °C for crustal and mantle xenoliths of Ichino-megata, northeast Japan. The rocks used in the present study cover a nearly entire range of lithological variation of the Ichino-megata xenoliths and are considered as representative rock samples of the lower crust and upper mantle of the back arc side of the northeast (NE) Honshu arc. The Vp values measured at 25 °C and 1.0 GPa are 6.7–7.2 km/s for the hornblende gabbros (38.6–46.9 wt.% SiO₂), 7.2 km/s for the hornblende-pyroxene gabbro (43.8 wt.% SiO₂), 6.9–7.3 km/s for the amphibolites (36.1–44.3 wt.% SiO₂), 8.0–8.1 km/s for the spinel lherzolites (46.2–47.2 wt.% SiO₂) and 6.30 km/s for the biotite granite (72.1 wt.% SiO₂). Combining the present data with the Vp profile of the NE Honshu arc [Iwasaki, T., Kato, W., Moriya, T., Hasemi, A., Umino, N., Okada, T., Miyashita, K., Mizogami, T., Takeda, T., Sekine, S., Matsushima, T., Tashiro, K., Miyamachi, H. 2001. Extensional structure in northern Honshu Arc as inferred from seismic refraction/wide-angle reflection profiling. Geophys. Res. Lett. 28 (12), 2329–2332], we infer that the 15 km thick lower crust of the NE Honshu arc is composed of amphibolite and/or hornblende (±pyroxene) gabbro with ultrabasic composition. The present study suggests that the Vp range of the lower crustal layer (6.6–7.0 km/s) in the NE Honshu arc, which is significantly lower than that obtained from various seismic measurements (e.g. the northern Izu-Bonin-Mariana arc: 7.1–7.3 km/s), is due to the thick hydrous lower crustal layer where hornblende, plagioclase and magnetite are dominant.     

Spatial variations of Sr and Nd isotope ratios of Cretaceous-Paleogene granitoid rocks,Southwest Japan Arc

Cretaceous-Paleogene granitoid rocks and contemporaneous volcanic rocks are widely distributed in the Inner Zone of Southwest Japan. This intense intermediate to felsic magmatism is considered to have taken place on the eastern margin of the Eurasian Continent, before the Southwest Japan Arc drifted away from the continent in the middle Miocene, resulting in the opening of the Japan Sea. The granitoid rocks show regional variations in terms of their radiometric age, petrography, Sr, Nd and O isotope ratios. Based on Sr and Nd isotope ratios, granitoid rocks can be divided into three zones (South, Transitional and North) between the Median Tectonic Line and the Japan Sea. Granitoid rocks and associated gabbros of the North Zone have low initial Sr isotope ratios (0.7048 to 0.7068) and high initial Nd values (+3 to-2.2), whereas granitoid rocks and gabbros from the South Zone have high initial Sr isotope ratios (0.7070 to 0.7088) and low initial Nd values (-3.0to-8.0). Most granitoid rocks from the Transitional Zone have Sr and Nd isotope ratios that lie between those of the North and South Zones, although there is some overlap. Contamination of magmas by upper crust cannot explain this geographical variation in Sr and Nd isotopes. Instead, the regional variation is attributed to compositionally different, magma sources (probably upper mantle and lower crust), beneath the North and South Zones. This is supported by the Sr and Nd isotopic ratios of upper mantle and lower crustal xenoliths included in Cenozoic volcanic rocks in the North and South Zones. These ratios are similar to those of the granitoid rocks in the respective zones. It is suggested that a micro-continent or island arc consisting of continental crust was underthrust beneath the South Zone before or during the Cretaceous, resulting in compositionally distinct sources for granitoid rocks of the North and South Zones. The large variation observed in Sr and Nd isotope ratios for Transitional Zone granitoid rocks is explained by variable proportions of the two different crustal and upper mantle components.     

Early Carboniferous intra-oceanic arc and back-arc basin system in the West Junggar,NW China

A series of Lower Carboniferous volcanic rocks occur in the Hatu, Darbut, and Baogutu areas of Xinjiang Province. Secondary ion mass spectrometry (SIMS) zircon U–Pb isotopic data indicate that two samples of these rocks coevally erupted at 324.0 ± 2.8 Ma and 324.9 ± 3.4 Ma. Three detailed profile measurements show that the volcanics include the Hatu basalt, the Baogutu andesite and dacite, and the Darbut andesite. Whole-rock compositions suggest that the Hatu volcanics are tholeiites and have a mid-ocean ridge basalt (MORB)-like signature with a small negative Nb anomaly, suggesting formation in a back-arc basin. Their isotopic compositions (?Nd(t) = +2.2 to +4.0, (⁸⁷Sr/⁸⁶Sr)i = 0.70414 to 0.70517) suggest a mixing origin from depleted to enriched mantle sources. In contrast, the Baogutu and Darbut rocks are andesite and dacite possessing a transitional tholeiite to calc–alkaline character and have E-MORB-like and OIB signatures, with a marked negative Nb anomaly and Th/Yb-enrichment, indicating that they were generated in a subduction zone setting. Isotopically, they display consistently depleted Sr–Nd isotopic compositions [(⁸⁷Sr/⁸⁶Sr)i = 0.70377–0.70469, ?Nd(t) = 1.0–5.2], suggesting that they were derived from a depleted mantle, and that fluid and sediments were involved in their petrogenesis. These features suggest that an early Carboniferous intra-oceanic arc and back-arc basin system generated the studied volcanic units in the West Junggar.     

日本列岛下太平洋俯冲板块的精细结构

尽管许多学者对日本列岛下的太平洋俯冲板块做了大量的研究,但板块内部的结构(比如板块厚度,板块内地震波速度随深度的变化以及洋壳的俯冲情况等)仍然不太清楚。利用日本地区密集台网收集到的中深和深发地震到时数据来探讨上述问题。采用三维射线追踪正演模拟法,首先利用333个远震计算得到了日本地区太平洋板块的厚度为85km;然后利用3283个地震(震源深度大于40km)的130227条P波到时进一步研究板块内部的精细结构。结果显示,沿深度方向6个地层段(间隔100km)内的速度扰动值分别为5.5%,4.0%,3.5%,2.5%,2.0%和6.0%,在40～500km范围内速度扰动随深度的增加而减小,这与温度随深度的变化情况相一致。当深度大于500km时,速度扰动突然增大到6.0%,分析认为该异常可能由发生在东亚大陆边缘下方的深发地震无法精确定位导致的。最后利用40～500km深度范围内的近震测试得到日本东北和北海道地区下方洋壳俯冲的深度均为110km,平均厚度分别为7.5km和5km,相对于一维模型的速度扰动分别为1%和-3%。这说明洋壳在俯冲到110km以深时,由于受温度和压力的影响,逐渐脱水、变质,直至与板块融合。通过分析震源与洋壳的位置关系,本研究认为北海道地区比东北地区下方的俯冲洋壳可能含有更多的流体(比如水),导致两地区洋壳内的速度相差如此之大。此外,因为日本南部与洋壳对应的区域多为海洋,观测台站较少,所以本研究无法测试得到该区域内的洋壳俯冲情况。     

Evolution of arc crust and relations between contrasting sources: U-Pb (age), Nd and Sr isotope systematics of the ophiolitic terrain of SW Norway

The U/Pb dating of ophiolite and arc complexes in the Caledonides of SW Norway has demonstrated that these spatially associated rocks are also closely related in time. A sequence of tholeiitic island arc volcanics, and an unconformably overlying sequence of calc-alkaline volcanics have been dated as 494 ± 2 Ma (2σ) and 473 ± 2 Ma respectively. Ophiolitic crust formed both prior to, and during the first 10 Ma after the tholeiitic island arc volcanism. Boninitic and island arc tholeiitic dyke swarms intruded the ophiolites soon after they formed and represent a second phase of spreading-related magmatism in the ca 20 Ma period that separated the tholeiitic and the calc-alkaline island arc volcanism. The magmatism ended with the formation of alkaline, ocean island basalt (OIB)-like magmas. Quartz dioritic and S-type granitic plutons, dated to 479 ± 5 Ma and 474 +3/−2 Ma respectively, intruded into the base of the arc crust during and subsequent to the boninitic magmatism, and at the time when calc-alkaline volcanic centres developed. The quartz dioritic and the granitic rocks contain inherited zircons of Precambrian age which prove the involvement of a continental source. This together with the geology of the terrain and the geochemistry of these plutons suggests that the granitic magmas were partly derived from subducted clastic sediments. The Sr and Nd isotope systematics indicate that the same continental source was a component in the boninitic and the calc-alkaline magmas. While the calc-alkaline magmas may have gained this continental component at a crustal level by assimilation, both geology and isotope systematics suggest that the continental component in the boninitic rocks was introduced by source contamination – possibly by a direct interaction between the mantle source and the S-type granitic magmas. A modified mid ocean ridge basalt-like mantle source was the principal source during the earliest and the main crust forming stage. This source became replaced by an OIB-like source during the later stages in the evolution of this ancient arc. Received: 27 June 1994 / Accepted: 16 September 1996     

Combined Sr,Nd, Pb and Li isotope geochemistry of alkaline lavas from northern James Ross Island (Antarctic Peninsula) and implications for back-arc magma formation

We present a comprehensive geochemical data set for a suite of back-arc alkaline volcanic rocks from James Ross Island Volcanic Group (JRIVG), Antarctic Peninsula. The elemental and isotopic (Sr, Nd, Pb and Li) composition of these Cenozoic basalts emplaced east of the Antarctic Peninsula is different from the compositions of the fore-arc alkaline volcanic rocks in Southern Shetlands and nearby Bransfield Strait. The variability in elemental and isotopic composition is not consistent with the JRIVG derivation from a single mantle source but rather it suggests that the magma was mainly derived from a depleted mantle with subordinate OIB-like enriched mantle component (EM II). The isotopic data are consistent with mantle melting during extension and possible roll-back of the subducted lithosphere of the Antarctic plate. Magma contamination by Triassic–Early Tertiary clastic sediments deposited in the back-arc basin was only localized and affected Li isotopic composition in two of the samples, while most of the basalts show very little variation in δ⁷Li values, as anticipated for “mantle-driven” Li isotopic composition. These variations are difficult to resolve with radiogenic isotope systematics but Li isotopes may prove sensitive in tracking complex geochemical processes acting through the oceanic crust pile, including hydrothermal leaching and seawater equilibration.     

Sedimentary processes caused by felsic caldera-forming volcanism in the Late Miocene to Early Pliocene intra-arc Aizu basin, NE Japan arc

Six large Late Miocene to Quaternary calderas, > 10 km in diameter, cluster together with several medium to small calderas and stratovolcanoes in a 60 × 30 km area of the Aizu volcanic field, southern NE Japan arc. These caldera volcanoes were built on a WNW–ESE trending highland coincident with a local uplifted swell since Late Miocene. The flare-up of felsic volcanism occurred synchronously along the NE Japan arc. Pyroclastic flow sheets from the calderas spread over the surrounding intra-arc basins and are interstratified with various sediments. Geochronological data indicates that the large-caldera eruptions have occurred six times since 8 Ma, at intervals of 1 to 2 million years. Late Miocene to Early Pliocene extra-caldera successions in the basin consist of nine sedimentary facies associations: (1) primary pyroclastics, (2) lahars, (3) gravelly fluvial channels, (4) sandy fluvial channels, (5) floodplains, (6) tidal flats, (7) delta fronts, (8) pro-delta slopes, and (9) pro-delta turbidites. The distribution of facies associations show westward prograding of volcaniclastic aprons, made up of braid delta, braidplain, pyroclastic flow sheet, and incised braided river deposits. The extra-caldera successions record: 1) an increase in felsic volcanism with an associated high rate of volcaniclastic sediment supply at about 10 Ma, prior to catastrophic caldera-forming eruptions; and 2) progradation of volcaniclastic aprons toward the back-arc side in response to the succeeding caldera-forming eruptions and sea-level changes, until about 3 Ma.     

同位素古温度与海平面变化的关系初探

用稳定同位素地球化学古温度的测定方法对北京周口店石笋进行氧、碳同位素测定，获得的古温度与年龄值与同期的海平面变化进行比较，表明古温度的变化与海平面变化密切相关。     

Strontium isotope ratios in volcanic rocks from the northwestern part of the Hellenic arc

${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios have been determined in fifteen volcanic rocks from the northwestern part of the Hellenic arc. They range from 0.7041 to 0.7134. There is no apparent correlation of strontium isotope values with any major chemical component or with Rb/Sr ratios. The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios appear to increase in a general way with increasing depth to the Benioff zone. The strontium isotope ratios are higher than from most island arcs; this is believed to be due to contamination.     

Strontium isotope ratios in volcanic rocks from the south-eastern part of the Hellenic arc

Isotopic ratios of strontium in 9 volcanic rocks from the south-eastern part of the Hellenic arc range from 0.7037 to 0.7075. Within individual series of differentiation, there seems to be a correlation between Sr⁸⁷/Sr⁸⁶ and K₂O/SiO₂.All strontium isotope data for the Hellenic arc are reviewed. Comparable (but slightly smaller) ranges of Sr isotope ratios are found in other island arcs with continental basement. To explain the high values of Sr⁸⁷/Sr⁸⁶ ratio for the Hellenic arc, a selective addition of Sr⁸⁷ from the wall rock, and a process of assimilation involving water, perhaps from subducted sediments, are suggested. Since closely-spaced individual volcanic centres of similar ages have very different Sr isotope ratios, and since the range of Sr isotopic composition in individual centres is quite large, the variation is unlikely to be due to primary variation in mantle composition.     

87Sr/86Sr ratios in Permo-Carboniferous sea water from the analyses of well-preserved brachiopod shells

Sr isotopic analyses of well-preserved portions of Permo-Carboniferous brachiopods distributed globally confirm the general shape of the Sr isotope age curve established by previous workers for this time interval. There is little variation between the Sr isotopic composition of unaltered portions of brachiopods and that of portions of the same shell interpreted to be diagenetically altered (based on cathodoluminescence, elemental, and stable isotopic data). However, the Sr isotopic composition in diagenetically altered micritic matrix adjacent to the shell is more radiogenic. The Sr isotopic composition in the unaltered portions of calcitic megafossils has potential as a stratigraphie tool.     

Tracing mantle sources in the northern Lau back-arc basin by independent component analysis of basalt isotopic compositions

ABSTRACT

The northern Lau back-arc basin (NLB) lavas display a diverse geochemical nature caused by complex geological processes in this region. Independent component (IC) analysis was applied to investigate the nature of mantle sources in the NLB, based on a compiled geochemical data set from the NLB, central-southern Lau Basin (C-SLB), Pacific and Indian Ocean ridges, and Samoan islands. We identified three ICs in the five-dimensional space of Sr-Nd-Pb isotopic ratios, which can account for 96.5% of the isotopic variance. The correlations between the ICs and the incompatible trace elements ratios were further used to examine the origin of these ICs. The first IC (IC1) separates Samoan islands (IC1 < ?1) from the other groups, and shows negative correlation with (La/Sm)_N ratios. The second IC (IC2) discriminates mid-ocean ridge basalts (MORBs) from Pacific (IC2 > 0) and Indian (IC2 < 0) Ocean ridges as well as the C-SLB (IC2 > 0) and NLB (IC2 < 0) lavas. IC2 correlates positively with Ba/Th ratios. The third IC (IC3) distinguishes MORBs (IC3 > 0) and back-arc basin lavas (IC3 < 0), and displays a negative correlation with Th/Nb values. On a regional scale in the NLB, there is a broad increase in IC1, IC2, and IC3 from the north to the south. Only IC3 presents obvious decrease from the west to the east, i.e. with decreasing distance from the arc. The geochemical nature and the statistical properties of these ICs suggest that IC1 is related to an enriched mantle component most likely from the nearby Samoan plume, and the IC2 corresponds to a fluid-rich component from the subducting Pacific slab. The IC3 may represent partial melt of recycled subducted sediment or recycled continental crustal materials. The geographic distribution of three ICs supports that the relatively recent mixing of Samoan plume materials with the subduction-metasomatized back-arc mantle may be responsible for the observed geochemical diversity in NLB lavas.     

Evolution of carbon dioxide-bearing saline fluids in the mantle wedge beneath the Northeast Japan arc

Lherzolite xenoliths containing fluid inclusions from the Ichinomegata volcano, located on the rear-arc side of the Northeast Japan arc, may be considered as samples of the uppermost mantle above the melting region in the mantle wedge. Thus, these fluid inclusions provide valuable information on the nature of fluids present in the sub-arc mantle. The inclusions in the Ichinomegata amphibole-bearing spinel–plagioclase lherzolite xenoliths were found to be composed mainly of CO₂–H₂O–Cl–S fluids. At equilibrium temperature of 920 °C, the fluid inclusions preserve pressures of 0.66–0.78 GPa, which correspond to depths of 23–28 km. The molar fraction of H₂O and the salinity of fluid inclusions are 0.18–0.35 and 3.71 ± 0.78 wt% NaCl equivalent, respectively. These fluid inclusions are not believed to be fluids derived directly from the subducting slab, but rather fluids exsolved from sub-arc basaltic magmas that are formed through partial melting of mantle wedge triggered by slab-derived fluids.     

Oxygen isotope ratios of phenocrysts from alkali basalts of the Pannonian basin: Evidence for an O-isotopically homogeneous upper mantle beneath a subduction-influenced area

Oxygen isotope compositions of olivine and pyroxene phenocrysts and pyroxene and amphibole megacrysts from Neogene alkali basalts of the Pannonian basin (0.5–11 Ma) have been determined by laser fluorination. Measured δ¹⁸O values in olivine and clinopyroxene phenocrysts show rather restricted variations from 5.00 to 5.20‰ and from 5.07 to 5.34%., respectively, with cpx-ol fractionations Δ¹⁸O(cpx-ol) ranging from + 0.04 to + 0.29‰. These δ¹⁸O values are significantly lower than those of the corresponding whole rocks, suggesting that low temperature alteration has increased the ¹⁸O/¹⁶O ratios of the groundmass of host rocks, even in fresh looking samples, whereas their phenocrysts have retained original oxygen isotope compositions. The uniform oxygen isotope ratio in the phenocrysts suggests that the mantle source of the alkali basalts was also homogeneous with respect to its oxygen isotope composition, which is in contrast to the relatively wide variation of Sr, Nd and Pb isotope ratios in the source. Variations in radiogenic isotope compositions in the basalts have been explained by the interaction of subduction-related fluids with the mantle source of the basalts. If this is the case, then the fluids which caused significant changes in the Sr and Pb isotope ratios of the mantle source clearly did not noticeably modify its oxygen isotope composition. These data support the opinion that the upper mantle is more homogeneous with respect to its oxygen isotope composition than it was previously considered.     

The accumulation characteristics and exploration potential of oil and gas in the back-arc basin of Japan under the background of high heat flow

The Sea of Japan is located in the southeast margin of Eurasia, in the triangle area of the western Pacific Ocean. Due to the interaction of the Pacific plate, Eurasian plate and Philippine plate, its tectonic environment is complex, forming a typical trench-arc-basin system. At present, 148 oil and gas fields have been discovered in Japan, with an oil and gas resource of 255.78×106 t, showing a good prospect for oil and gas exploration. Based on the previous research and the recently collected ...     

Sr and Sr/Sr in the Yamuna River System in the Himalaya: sources, fluxes, and controls on sr isotope composition

Sr and ⁸⁷Sr/⁸⁶Sr have been measured in the Yamuna river headwaters and many of its tributaries (YRS) in the Himalaya. These results, with those available for major ions in YRS rivers and in various lithologies of their basin, have been used to determine their contributions to riverine Sr and its isotopic budget. Sr in the YRS ranges from 120 to 13,400 nM, and ⁸⁷Sr/⁸⁶Sr from 0.7142 to 0.7932. Streams in the upper reaches, draining predominantly silicates, have low Sr and high ⁸⁷Sr/⁸⁶Sr whereas those draining the lower reaches exhibit the opposite resulting from differences in drainage lithology. ⁸⁷Sr/⁸⁶Sr shows significant co-variation with SiO₂/TDS and (Na^* + K)/TZ⁺ (indices of silicate weathering) in YRS waters, suggesting the dominant role of silicate weathering in contributing to high radiogenic Sr. This is also consistent with the observation that streams draining largely silicate terrains have the highest ⁸⁷Sr/⁸⁶Sr, analogous to that reported for the Ganga headwaters. Evaluation of the significance of other sources such as calc-silicates and trace calcites in regulating Sr budget of these rivers and their high ⁸⁷Sr/⁸⁶Sr needs detailed work on their Sr and ⁸⁷Sr/⁸⁶Sr. Preliminary calculations, however, indicate that they can be a significant source to some of the rivers.It is estimated that on an average, ∼25% of Sr in the YRS is derived from silicate weathering. In the lower reaches, the streams receive ∼15% of their Sr from carbonate weathering whereas in the upper reaches, calc-silicates can contribute significantly (∼50%) to the Sr budget of rivers. These calculations reveal the need for additional sources for rivers in the lower reaches to balance their Sr budget. Evaporites and phosphorites are potential candidates as judged from their occurrence in the drainage basin. In general, Precambrian carbonates, evaporites, and phosphorites “dilute” the high ⁸⁷Sr/⁸⁶Sr supplied by silicates, thus making Sr isotope distribution in YRS an overall two end member mixing. Major constraints in quantifying contributions of Sr and ⁸⁷Sr/⁸⁶Sr from different sources to YRS rivers are the wide range in Sr and ⁸⁷Sr/⁸⁶Sr of major lithologies, limited data on Sr and ⁸⁷Sr/⁸⁶Sr in minor phases and on the behavior of Sr, Na, and Ca during weathering and transport.The Ganga and the Yamuna together transport ∼0.1% of the global Sr flux at the foothills of the Himalaya which is in the same proportion as their contribution to global water discharge. Dissolved Sr flux from the Yamuna and its mobilization rate in the YRS basin is higher than those in the Ganga basin in the Himalaya, a result consistent with higher physical and chemical erosion rates in the YRS.