High δ18O Values Research Articles

Reconstruction of Indian monsoon precipitation variability between 4.0 and 1.6 ka BP using speleothem δ18O records from the Central Lesser Himalaya, India

The present study documents the monsoon precipitation variability spanned ~ 2500 years, between 4.0 and 1.6 ka BP (before 1950 AD), from the Central Lesser Himalaya, India, using δ18O measurements of Tityana cave stalagmite (hereafter referred as TC1). At present, the cave receives precipitation from both Indian Summer Monsoon (ISM) and Western Disturbances (WDs). The δ18O variation between − 8.04 and − 10.46‰ through growth axis of the TC1 and five 14C AMS dates (due to large age uncertainty by 230Th/U method) have allowed us to identify the mid to late Holocene multi-decadal to centennial scale climatic oscillations. The higher δ18O values indicate the weakening of the monsoon precipitation, while the lighter values represent the stronger monsoon precipitation strength. Based on the fluctuations in δ18O values, three distinct phases of the precipitation variability are distinguished as, declined/decreased precipitation between ~ 4.0 and 3.4 ka BP with peak aridity around ~ 3.4 ka BP, followed by slightly improved conditions from ~ 3.4 to ~ 2.7 ka BP. Subsequently, the climate was reduced from ~ 2.7 ka BP onwards until the end of stalagmite growth, around ~ 1.6 ka BP with spikes of two major drought events centred at ~ 1.9 and ~ 1.6 ka BP. In general, the droughts, centred at ~ 3.4, ~ 1.9 and ~ 1.6 ka BP, are characterized by abrupt drop (from − 8.12 to − 8.04‰) in the δ18O values and point to the weakening of the monsoon. One of the major drought events at ~ 3.4 ka BP can be correlated with the collapse of the Indus valley civilization in the NW India. A close correspondence of the TC1 data set with other WDs influenced regimes likely indicates a relative impact of mid-latitude WDs after transition of the mid-late Holocene around 3.5–3.4 ka BP.

Paleoproterozoic S-type granites from the Helanshan Complex in Inner Mongolia: Constraints on the provenance and the Paleoproterozoic evolution of the Khondalite Belt, North China Craton

The Khondalite Belt is a Paleoproterozoic tectonic belt in the Western Block of the North China Craton. It consists mainly of high-grade metamorphosed Al-rich sedimentary rocks, also named as the ‘Khondalite Series’. The vast majority of detrital zircons from the Khondalite Series rocks have ages dominantly at 2.2–2.0Ga, but their provenances become an enigma due to lack of available synchronous magmatic rocks. In this paper, detailed geochronological and geochemical studies were done on Mid-Paleoproterozoic granites from the Helanshan Complex in the westernmost part of the Khondalite Belt to constrain the provenance of the Khondalite Series rocks and the tectonic setting of this belt. These granites can be divided into two types: the garnet-bearing granite and the two-mica granite. Both kinds of granite contain peraluminous minerals (i.e., garnet, muscovite), and are characterized by variable SiO2 (72.1–77.5wt%), high Al2O3 (11.79–18.93wt%) and A/CNK values (1.09–1.27) as well as high zircon δ18O values (7.30–11.49‰), indicating an origin of typical S-type granites. They are enriched in Rb, Th, U, LREE ((La/Yb)N=19.2–202) and display pronounced negative Nb-Ta-Ti and Eu (Eu/Eu∗=0.28–0.94) anomalies, similar to geochemical characteristics of magmatic rocks formed in a subduction zone. Zircon SIMS U-Pb dating yielded consistent Paleoproterozoic intrusive ages of 2045±17Ma and 2069±25Ma for the garnet-bearing granite and the two-mica granite, respectively. These magmatic zircons have variable zircon εHf(t) values mostly ranging between −1.7 and +6.7, corresponding to Hf model ages (TDMC) of 2.2–2.8Ga. The geochemical and isotopic features indicate that these S-type granites have been produced by partial melting of the ancient crust with addition of juvenile crustal or mantle-derived components. Combined with previous geological data of the Khondalite Belt, it is speculated that the ∼2.0Ga magmatism was mostly likely generated in an active continental margin, providing robust evidence for a provenance for the clastic materials of the Khondalite Belt.

Petrogenesis of Lingshan highly fractionated granites in the Southeast China: Implication for Nb-Ta mineralization

Whole rock major and trace element and Sr-, Nd- and Hf-isotope data, together with zircon U-Pb, Hf- and O-isotope data, are reported for the Nb-Ta ore bearing granites from the Lingshan pluton in the Southeastern China, in order to trace their petrogenesis and related Nb-Ta mineralization. The Lingshan pluton contains hornblende-bearing biotite granite in the core and biotite granite, albite granite and pegmatite at the rim. In addition, numerous mafic microgranular enclaves occur in the Lingshan granites. Zircon SIMS U-Pb dating gives consistent crystallization ages of ca. 132Ma for the Lingshan granitoids and enclaves, consistent with the Nb-Ta mineralization age of ∼132Ma, indicating that mafic and felsic magmatism and Nb-Ta mineralization are coeval. The biotite granites contain hornblende, and are metaluminous to weakly peraluminous, with high initial 87Sr/86Sr ratios of 0.7071–0.7219, negative εNd(t) value of −5.9 to −0.3, εHf(t) values of −3.63 to −0.32 for whole rocks, high δ18O values and negative εHf(t) values for zircons, and ancient Hf and Nd model ages of 1.41–0.95Ga and 1.23–1.04Ga, indicating that they are I-type granites and were derived from partial melting of ancient lower crustal materials. They have variable mineral components and geochemical features, corresponding extensive fractionation of hornblende, biotite and feldspar, with minor fractionation of apatite. Existence of mafic microgranular enclaves in the biotite granites suggests a magma mixing/mingling process for the origin of the Lingshan granitoids, and mantle-derived mafic magmas provided the heat for felsic magma generation. In contrast, the Nb-Ta mineralized albite granites and pegmatites have distinct mineral components and geochemical features, which show that they are highly-fractionated granites with extensive melt and F-rich fluid interaction in the generation of these rocks. The fluoride-rich fluids induce the enrichment in Nb and Ta in the highly evolved melts. Therefore, we conclude that the Nb-Ta mineralization is the result of hydrothermal process rather than crystal fractionation in the Lingshan pluton, which provides a case to identify magmatic and hydrothermal processes and evaluate their relative importance as ore-forming processes.

A Nd- and O-isotope study of the REE-rich peralkaline Strange Lake granite: implications for Mesoproterozoic A-type magmatism in the Core Zone (NE-Canada)

It is well established that A-type granites enriched in high field strength elements, such as Zr, Nb and the REE, form in anorogenic tectonic settings. The sources of these elements and the processes controlling their unusual enrichment, however, are still debated. They are addressed here using neodymium and oxygen isotope analyses of samples from the 1.24 Ga Strange Lake pluton in the Paleoproterozoic Core Zone of Quebec-Labrador, an A-type granitic body characterized by hyper-enrichment in the REE, Zr, and Nb. Age-corrected eNd values for bulk rock samples and sodic amphiboles (mainly arfvedsonite) from the pluton range from −0.6 to −5.7, and −0.3 to −5.3, respectively. The eNd values for the Napeu Kainiut quartz monzonite, which hosts the pluton, range from −4.8 to −8.1. The 147Sm/144Nd ratios of the suite and the host quartz monzonite range from 0.0967 to 0.1659, large variations that can be explained by in situ fractionation of early LREE-minerals (Strange Lake), and late hydrothermal HREE remobilization. Oxygen isotope analyses of quartz of both Strange Lake and the host yielded δ18O values between +8.2 and +9.1, which are considerably higher than the mantle value of 5.7 ± 0.2‰. Bulk rock oxygen isotope analyses of biotite-gneisses in the vicinity of the Strange Lake pluton yielded δ18O values of 6.3, 8.6 and 9.6‰. The negative eNd values and positive δ18O values of the Strange Lake and Napeu Kainiut samples indicate that both magmas experienced considerable crustal contamination. The extent of this contamination was estimated, assuming that the contaminants were sedimentary-derived rocks from the underlying Archean Mistinibi (para-) gneiss complex, which is characterized by low eNd and high δ18O values. Mixing of 5–15% of a gneiss, having an eNd value of −15 and a δ18O value of +11, with a moderately enriched mantle source (eNd = +0.9, δ18O = +6.3) would produce values similar to those obtained for the Strange Lake granites. Based on analogies between the Nain Plutonic Suite and the Gardar alkaline igneous province (SW-Greenland), we conclude that the Strange Lake pluton and associated REE-mineralized anorogenic bodies formed from a combination of subduction-induced fertilization of the sublithospheric mantle, crustal extension and in situ magma evolution.

Lake evolution and hydroclimate variation at Lake Qinghai (China) over the past 32 ka inferred from ostracods and their stable isotope composition

We used ostracod species assemblages and their δ18O values in a 32-m sediment core from Lake Qinghai, China, along with information from cores collected at other sites in the lake, to infer lake evolution and hydroclimate changes since the last glacial. Dominant ostracod species Ilyocypris bradyi and its low δ18O values showed that Lake Qinghai was small in size or even consisted of several playa lakes, and the 1F core site could have even been in a wetland setting, under cold and dry climate conditions before 15.0 ka. Presence of Limnocythere inopinata with low δ18O values, and absence of I. bradyi after 15.0 ka, indicate the lake area increased or that the playas merged. The decrease or disappearance of ostracods with high δ18O values showed that the lake shrunk under dry climate from 12.0 to 11.6 ka. After 11.6 ka, hydroclimate shifts inferred from ostracod species changes (Eucypris mareotica and L. inopinata) and their δ18O values were as follows: (1) 11.6–7.4 ka—larger, but still small lake area with greater moisture availability under primarily dry climate conditions, (2) 7.4 to 3.2 ka—increasing lake level under a warmer and wetter climate, and (3) 3.2 ka to present—stable, large, brackish lake. The low ratio of lake water volume to runoff, and close proximity of the core site to freshwater input from the river mouth would have resulted in relatively lower ostracod δ18O values when Lake Qinghai was small in area during the interval from 32.0 to 15.0 ka. Lower ostracod δ18O values during interstadials and throughout the entire Last Glacial Maximum and early deglacial (ca. 24.0–16.0 ka) were caused by a greater contribution of seasonal meltwater from ice or snow and low incoming precipitation δ18O values related to cold climate conditions in the region at that time.

Geochemistry and Stable Isotopes of Travertine from Jordan Valley and Dead Sea Areas

Travertine deposits in Deir Alla, Suwayma, and Az Zara areas were investigated. Mineralogy, geochemistry, stable isotopes and age dating indicate the presence of low-Mg calcite, with minor quartz components. The variable isotope (δ13C and δ18O) signatures indicate dependence on water temperature and water/rock isotopic exchange. In contrast, the high δ13C values in some travertine samples reflect 12CO2 degassing processes, increased input of 13C-enriched groundwater, and the presence of surface and groundwater hydrological systems. The high δ18O values may be attributed to evaporation effects and low water temperature during the formation of localized travertine. The age of travertine is the Late Pleistocene.

Late Paleozoic rift-related basalts from central Inner Mongolia, China

Late Paleozoic basaltic volcanism widely occurred in central Inner Mongolia, the southeastern part of the Central Asia Orogenic Blet (CAOB). Three volcanic pulses are identified by ion microprobe (SHRIMP II) zircon U-Pb analysis, which are Late Carboniferous (ca. 315Ma), the latest Carboniferous (ca. 303Ma) and early Permian (ca. 284Ma). Basalts in the earliest stage are characterized by flat HREE, depletion in Nb and Ta, MORB-like Th/Yb ratios and depleted mantle (DM)-like Hf isotopic ratios. These are similar to forearc basalts, that have been generated by melting of a fluxed depleted mantle in an extensional environment. The latest Carboniferous basalts (sample D1363) have no Zr-Hf anomalies, relatively radiogenic Hf isotopic ratios (εHf(t)=+12.1 to +19.7) and high δ18O values up to 8.5‰, VSMOW. These are likely to be magmas originated from decompressional melting of a depleted mantle that interacted with ocean floor pillow basalts during their ascending through the crust. The early Permian sample D1380 have variable 206Pb/238U zircon ages with an interpreted magma crystallization age of 284.4±2.5Ma. These zircons have scattered Hf isotope (εHf(t)=+5 to +14) and δ18O (1.8–11.0‰, VSMOW), which suggest the involvement of a relatively enriched mantle source. Overall the temporal evolution of the Late Paleozoic basaltic volcanism from central Inner Mongolia suggests a rift-related tectonic setting.

Crustal basement controls granitoid magmatism, and implications for generation of continental crust in subduction zones: A Sr–Nd–Hf–O isotopic study from the Paleozoic Tongbai orogen, central China

Ascertaining the petrogenesis of granitoid rocks in subduction zones holds the key for understanding the processes of how continental crust is produced. The synchronous Taoyuan and Huanggang plutons occur in two different geological units of the Paleozoic Tongbai orogen of central China. They provide an optimal opportunity for a study to address the role of the crustal basement in generating voluminous granitoid magmatism in subduction zones. The Taoyuan and Huanggang plutons have identical U–Pb zircon crystallization ages of 440–444Ma, which are temporally related to northward subduction of the Paleotethyan Ocean. The Taoyuan samples show high SiO2 (73.36–79.16%) and low Al2O3 (12.00–13.45%) contents, Mg numbers (20.6–38.2), and Sr/Y (2.04–10.1) and (La/Yb)N (2.34–7.32) ratios with negative Eu anomalies (Eu/Eu*=0.33–0.93). They yielded positive εNd(t) (+3.0 to +6.7) and εHf(t) (+11.8 to +13.2) values, elevated initial Sr isotopic ratios (0.7040–0.7057) and relatively low zircon δ18O values of 4.62–5.39‰. These suggest that they were produced through partial melting of hydrothermally altered lower crust of the accreted Erlangping oceanic arc. In contrast, the Huanggang samples exhibit variable whole-rock geochemical and isotopic compositions with SiO2 contents of 57.01–64.42wt.%, initial Sr isotopic ratios of 0.7065–0.7078, and εNd(t) values of −5.7 to −9.4. Additionally, they have high zircon δ18O values of 7.57–8.45‰ and strongly negative zircon εHf(t) values of −14.4 to −10.5. They were suggested to have been mainly derived from ancient continental crust of the Kuanping crustal unit with the addition of 20–40% juvenile, mantle-derived material. Accordingly, the granitoids in both oceanic and continental arcs are likely to be mainly derived from intracrustal melting of their crustal basement. It is revealed by the Huanggang pluton that little net continental crust growth occurs in continental arcs, and addition of new volume of continental crust may be balanced by the loss of old mafic lower continental crust. Continental growth was likely achieved through the accretion and subsequent differentiation of oceanic arcs, as manifested by the Taoyuan pluton.

Late Archean high-pressure pelitic granulites in the Yinshan Block, North China Craton

Ancient high-pressure metamorphism provides constraints for the understanding of Archean tectonic processes. Archean high-pressure metamorphic rocks, however, are rarely preserved in old cratons. Here, we report the newly discovered Archean high-pressure metapelites in the Yinshan Block of the North China Craton. The peak metamorphic stage (M2) is represented by kyanite, garnet, plagioclase, K-feldspar, quartz, rutile and melt. Kyanite was replaced by sillimanite during post-peak (M3) decompression process. Phase equilibria modelling in the MnNCKFMASHT system indicates nearly isothermal decompression with peak conditions of 840–870°C and >14–11kbar. Secondary ion mass spectrometry (SIMS) zircon U–Pb dating of leucosome (13XWL35) and monazite U–Pb dating of kyanite-garnet-bearing metapelite (13XWL22) yield similar ages of ∼2520Ma, which represent the crystallization age of granulite-facies melt. The high-pressure metapelites are enriched in Na2O (1.8–3.0wt.%), MgO (3.4–5.7wt.%), Cr (245–421ppm) and Ni (21.0–91.9ppm) contents, suggesting that volcanic rocks are probably the main sedimentary provenance. High δ18O(WR) values (up to 12.4‰) and zircon oxygen isotopes (av. 11.4‰) of the metasediment source indicate they have undergone low-temperature alteration. The pressure-temperature-time constraints of this study provide a rare record indicating that surficial sediments have been buried to a depth of >40km at ca. 2520Ma in the Yinshan Block. Such crustal thickening, combined with available synchronous magmatism suggests that high-pressure metamorphism was triggered in a late Archean deep arc setting in the Yinshan Block of the North China Craton.

Two contrasting late Paleozoic magmatic episodes in the northwestern Chinese Tianshan Belt, NW China: Implication for tectonic transition from plate convergence to intra-plate adjustment during accretionary orogenesis

Late Carboniferous to Early Permian is a critical period for the final amalgamation of the Central Asian Orogenic Belt (CAOB). However, as most of the accreted terranes of the CAOB are unclear in tectonic nature and origin, the timing and processes of their mutual amalgamation have been poorly constrained. To understand assembly of the West Junggar Terrane with the Yili Block, a suite of the late Paleozoic magmatic rocks, including ignimbrite, rhyolite and granite, in northwestern Chinese Tianshan Belt were studied for their petrogenesis and tectonic implications. Our new results of secondary ion mass spectrometry (SIMS) zircon U-Pb dating reveal two separate magmatic episodes, ca. 300Ma volcanism (ignimbrite and rhyolite) and ca. 288Ma plutonsim (biotite granite). Geochemically, for the ca. 300Ma volcanism, the ignimbrites have low SiO2 (65.8–71.5wt.%) and Mg# (6–13) values, and exhibit arc affinity with significantly enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE) such as Nb, Ta and Ti. The whole-rock εNd(t) and zircon εHf(t) values range from +6.9 to +7.0 and +9.9 to +14.1 respectively, indicating a juvenile basaltic lower crustal origin. Rhyolites have slightly high SiO2 (72.7–74.0wt.%) and K2O (3.86–4.53wt.%) contents, high zircon δ18O (11.67–13.23‰) values, and low whole-rock εNd(t) (+2.9 to +3.8) and zircon εHf(t) (+2.8 to +10.0) values, which may suggest sediment involvements during magma generation. In contrast, for the ca. 288Ma plutonism, the biotite granites have obviously higher SiO2 (74.7–75.5wt.%) contents and whole-rock εNd(t) (+7.7 to +8.8), zircon εHf(t) (+9.8 to +12.7), and lower zircon δ18O (5.99–6.84‰) values, than those of the ca. 300Ma volcanic rocks, which are consistent with signatures of juvenile magma source. According to our estimates of zircon saturation temperatures, together with their contrasting genesis, we attribute the formation of ca. 300Ma high temperature (815–938°C) volcanism to oceanic slab break-off during assembly of the West Junggar Terrane with the Yili Block, and relate the generation of ca. 288Ma low temperature (723–735°C) plutonism to subsequent strike-slipping of North Tianshan Fault that facilitated introduction of water-fluxes triggering hydrous partial melting of juvenile lower crust. The sequential magmatic episodes in the northwestern Chinese Tianshan Belt may provide a crucial clue to a tectonic transition from plate convergence to intra-plate adjustment during the formation of the Kazakhstan Orocline in the late Paleozoic.

Partial melting of the orogenic lower crust: Geochemical insights from post-collisional alkaline volcanics in the Dabie orogen

Alkaline igneous rocks are a minor component in common associations of felsic to mafic rocks. Although they are usually regarded as the product of mantle-derived magmatism, they are possibly produced by partial melting of the overthickened orogenic crust. This is illustrated by post-collisional alkaline volcanics from the Dabie orogen in China. A combined study of whole-rock major and trace elements and Sr-Nd-Hf isotopes, and zircon U-Pb ages and Hf-O isotopes for these volcanics provides geochemical constraints on the nature of their magma sources and thus insights into reworking of the lower continental crust in a collisional orogen. SIMS zircon U-Pb dating yields consistent Early Cretaceous ages of 136±2Ma and 133±2Ma for magma emplacement, contemporaneous with early-stage adakitic rocks in the Dabie orogen. Relict zircons record Neoproterozoic and Triassic U-Pb ages, respectively, in agreement with ages for protolith formation and ultrahigh-pressure metamorphism in the orogen. The alkaline volcanics are composed of phonolite and trachyte, with variable contents of SiO2 (55.94–64.10wt%), MgO (0.16–0.95wt%), Al2O3 (16.80–21.32wt%), K2O (4.65–14.55wt%) and Na2O+K2O (11.51–15.01wt%). They show arc-like trace element distribution patterns, with enrichment in LILE and LREE but depletion in HFSE. They exhibit enriched whole-rock Sr-Nd-Hf isotope compositions, with generally high (87Sr/86Sr)i ratios of 0.7074 to 0.7104, but negative εNd(t) values of −16.3 to −15.1 and negative εHf(t) values of −18.4 to −16.3. Zircon Hf-O isotope analyses yield less variable εHf(t) values of −27.8 to −23.0 and δ18O values of 4.9 to 5.9‰ for magmatic zircons, but more variable εHf(t) values of −32.7 to −6.5 and δ18O values of 3.0 to 10.2‰ for Neoproterozoic and Triassic relict zircons. Such geochemical features indicate that these alkaline volcanics were derived from partial melting of the subducted continental crust of the South China Block in the Dabie orogen. The magma sources would be dominated by the mafic igneous rocks of middle Neoproterozoic age. Additional contributions were from supracrustal rocks with high δ18O and variable εHf(t) values, which were incorporated into the overthickened orogenic crust during the Triassic continental collision. The orogenic crust underwent low-degree partial melting during its thinning at the post-collisional stage, giving rise to the alkaline volcanics in the Early Cretaceous.

Neoproterozoic granitoids from the Phan Si Pan belt, Northwest Vietnam: Implication for the tectonic linkage between Northwest Vietnam and the Yangtze Block

Several Neoproterozoic granitic intrusions, including the Phin Ngan intrusion and some smaller ones from the Sin Quyen Cu mine, have been identified in the Phan Si Pan belt, Northwest Vietnam. Whole-rock geochemical and Sm-Nd isotopic data and zircon U-Pb age and Hf-O isotopes have been determined for these intrusions in order to constrain their ages, petrogenesis and tectonic implications. The Phin Ngan intrusion, mainly composed of syenogranite, was emplaced at 824±4Ma. Rocks from this intrusion have high SiO2 (72.1–73.5wt.%), and K2O (5.14–5.52wt.%), with A/CNK values ranging from 1.02 to 1.06. They have negative whole-rock εNd(t) (−8.4 to −5.2) and zircon εHf(t) values (−8.1 to −5.5), and high zircon δ18O values (9.7–10.9‰). These geochemical features suggest that the Phin Ngan intrusion was derived from ancient, K-rich crustal sources. Granitic intrusions from the Sin Quyen mine occur as stocks or dykes intruding ores and ore-hosting rocks. They are composed of monzogranite and granodiorite, emplaced between 736±8 and 758±6Ma. They have high SiO2 (68.2–76.7wt.%) and (K2O+Na2O) (6.93–8.54wt.%), and low MgO (0.19–0.87), with A/CNK values ranging from 0.91 to 1.14. Their whole-rock εNd(t) values range from −6.6 to 0, mostly clustered between −6.6 and −4.3. Their zircon εHf(t) values vary from −6.7 to +11.1, mostly lower than −2.0. Such geochemical data indicate that these intrusions were produced mainly through partial melting of ancient crustal materials, coupled with subordinate involvement of mantle-/juvenile crust-derived components. The Phin Ngan intrusion and intrusions from the Sin Quyen mine are sub-alkaline in nature, and have arc-like trace-element compositions. Compiled with other broadly synchronous igneous rocks with arc-like affinities in the region, the Neoproterozoic magmatism in the Phan Si Pan belt was generated in a subduction-related setting. The Neoproterozoic intrusions in the Phan Si Pan belt are geochronologically and geochemically correlated with the Neoproterozoic subduction-related igneous rocks along the western Yangtze Block. Thus, the Phan Si Pan belt represents the southern most part of the giant arc system of the western Yangtze Block.

Post-collisional ultrapotassic rocks and mantle xenoliths in the Sailipu volcanic field of Lhasa terrane, south Tibet: Petrological and geochemical constraints on mantle source and geodynamic setting

Post-collisional ultrapotassic magmatic rocks (15.2–18.8Ma), containing mantle xenoliths, are extensively distributed in the Sailipu volcanic field of the Lhasa terrane in south Tibet. They could be subdivided into high-MgO and low-MgO subgroups based on their petrological and geochemical characteristics. The high-MgO subgroup has olivine-I (Fo87–92), phlogopite and clinopyroxene as phenocryst phases, while the low-MgO subgroup consists mainly of phlogopite, clinopyroxene and olivine-II (Fo77–89). These ultrapotassic magmatic rocks have high MgO (4.6–14.5wt%), Ni (145–346ppm), Cr (289–610ppm) contents, and display enrichment in light rare earth element (REE) over heavy REE and enriched large ion lithophile elements (LILE) relative to high field strength elements (HFSE) with strongly negative Nb-Ta-Ti anomalies in primitive mantle-normalized trace element diagrams. They have extremely radiogenic (87Sr/86Sr)i (0.7167–0.7274) and unradiogenic (143Nd/144Nd)i (0.5118–0.5120), high (207Pb/204Pb)i (15.740–15.816) and (208Pb/204Pb)i (39.661–39.827) at a given (206Pb/204Pb)i (18.363–18.790) with high δ18O values (7.3–9.7‰). Strongly linear correlations between depleted mid-ocean ridge basalt-source mantle (DMM) and the Indian continental crust (HHCS) in Sr-Nd-Pb-O isotopic diagrams indicate that the geochemical features could result from reaction between mantle peridotite and enriched components (fluids and melts) released by the eclogitized Indian continental crust (HHCS) in the mantle wedge. The high-MgO (13.7–14.5wt%) subgroup displays higher (143Nd/144Nd)i, lower (87Sr/86Sr)i and (206Pb/204Pb)i ratios and lower δ18O values compared with the low-MgO (4.6–8.8wt%) subgroup. High Ni (850–4862ppm) contents of olivine phenocrysts and high whole-rock SiO2, NiO, low CaO contents indicate that the low-MgO ultrapotassic magmatic rocks are derived from partial melting of olivine-poor mantle pyroxenite. However, lower Ni concentrations of olivine phenocryst and lower whole-rock SiO2, NiO, higher CaO contents of the high-MgO ultrapotassic rocks may indicate their peridotite mantle source. This could be attributed to different amounts of silicate-rich components added into the mantle sources of the parental magmas in the mantle wedge caused by the northward subduction of the Indian continental lithosphere. The reaction-formed websterite xenoliths, reported for the first time in this study, are made up of anhedral and interlocking clinopyroxene (45–65vol%) and orthopyroxene (30–50vol%) with minor phlogopite (<3vol%) and quartz (<2vol%) and are suggested to be formed by silicate metasomatism of the mantle peridotite. The harzburgites, another major type of mantle xenolith in south Tibet, have a mineral assemblage of olivine (60–75vol%), orthopyroxene (20–35vol%), clinopyroxene (<3vol%), phlogopite (<2vol%) and spinel (<2vol%) and may have experienced subduction-related metasomatism. Combined with two types of ultrapotassic magmas, we propose that compositions of mantle wedge beneath south Tibet may gradually evolve from harzburgite through lherzolite to websterite with strong metasomatism of silicate-rich components in their mantle source region. Partial melting of the enriched mantle sources could be triggered by rollback of Indian continental slab during 25–8Ma in south Tibet.

Magma reservoir dynamics at Toba caldera, Indonesia, recorded by oxygen isotope zoning in quartz

Quartz is a common phase in high-silica igneous rocks and is resistant to post-eruptive alteration, thus offering a reliable record of magmatic processes in silicic magma systems. Here we employ the 75 ka Toba super-eruption as a case study to show that quartz can resolve late-stage temporal changes in magmatic δ18O values. Overall, Toba quartz crystals exhibit comparatively high δ18O values, up to 10.2‰, due to magma residence within, and assimilation of, local granite basement. However, some 40% of the analysed quartz crystals display a decrease in δ18O values in outermost growth zones compared to their cores, with values as low as 6.7‰ (maximum ∆core−rim = 1.8‰). These lower values are consistent with the limited zircon record available for Toba, and the crystallisation history of Toba quartz traces an influx of a low-δ18O component into the magma reservoir just prior to eruption. Here we argue that this late-stage low-δ18O component is derived from hydrothermally-altered roof material. Our study demonstrates that quartz isotope stratigraphy can resolve magmatic events that may remain undetected by whole-rock or zircon isotope studies, and that assimilation of altered roof material may represent a viable eruption trigger in large Toba-style magmatic systems.

Sequential δ13C and δ18O analyses of early Holocene bovid tooth enamel: Resolving vertical transhumance in Neolithic domesticated sheep and goats

Vertical transhumance is an important animal husbandry strategy that provides livestock with consistent access to pasture throughout the year and contributed to the intensification of sheep and goat husbandry in the Near East over 10,000years ago. Sequential carbon (δ13C) and oxygen (δ18O) isotope analyses of teeth from domesticated sheep and goats dating to the early Neolithic (9200 to 8700calyrB.P.) from a region of strong local topographic relief in southern Jordan exhibit inverse cyclical isotopic variation characterized by the coincidence of high δ18O values with low δ13C values indicating ingestion of 13C-depleted plants during the summer season. This pattern is consistent with vertical transhumance of caprines moving from low-elevation C3/C4 Irano-Turanian pastures to higher-elevation Mediterranean C3 pastures during the summer, but other seasonally directed animal husbandry strategies involving amendment of livestock diets generate a similar isotopic outcome. Caprine δ18O values referenced against the oxygen isotope ratios of contemporaneous obligate drinking cattle and non-obligate drinking mountain gazelle, bovids with limited home ranges, help distinguish the influence of meteoric water, 18O-enriched leaf water, and movement on the oxygen isotopic composition of sheep and goat tooth enamel. This approach assists in independent validation of vertical transhumance hypothesized for inverse cyclical variation in sequential δ13C and δ18O values and, also, decouples seasonal foddering from mobility in the carbon isotopic dietary record. The isotopic data presented here reveal that complex sheep and goat husbandry systems involving vertical transhumance, stationary flock-keeping, and winter foddering were in place by the late tenth millennium cal y.r. B.P. east of the Jordan Valley.

Across-arc geochemical variation in the Jurassic magmatic zone, Southern Tibet: Implication for continental arc-related porphyry Cu[sbnd]Au mineralization

The Jurassic Gangdese arc hosts the giant Xietongmen (Xiongcun) porphyry CuAu magmatic-hydrothermal centre. In order to understand the tectonomagmatic framework of the arc and its controls on the origin and temporal-spatial distribution of porphyry mineralization we conducted extensive major and trace, and isotopic analysis on a suite of samples collected regionally. Geochemical variations were observed across the Jurassic arc from south to north, controlled by northward subduction of the Neo-Tethyan oceanic plate and overlying Lhasa lithosphere. In the frontal arc (latitude 29°–29.55°N), the magmas had high Ba/La and Ba/Th ratios, high εNd(t) (+3 to +7) values, and their zircons have high εHf(t) (+10 to +18) values, and heterogeneous O isotopic compositions (δ18O=+3.6 to +6.6‰). The low titanium-in-zircon temperature and zircon saturation temperature (both average=~700°C) suggest that a greater flux of dehydration fluids from the subducting slab triggered partial melting of the mantle wedge in the southern arc. High zircon Eu/Eu* and Ce/Nd ratios (>0.4 and >20, respectively) indicate highly oxidized, fertile magmas, from which the Xietongmen porphyry CuAu deposit formed. In contrast, the interior arc (north of latitude 29.55°), close to the ancient Tibetan basement, shows low Ba/La and Ba/Th ratios, low εNd(t) (−0.1 to +2.5) and zircon εHf(t) (+2 to +12) values, and less heterogeneous zircon O isotopic compositions with higher δ18O values. Higher titanium-in-zircon and zircon saturation temperatures (mostly >750°C) suggest that less water was involved during the partial melting of mantle wedge in this region. Low zircon Eu/Eu* and Ce/Nd ratios (mostly <0.4 and <20, respectively) point towards less oxidized magmatic conditions. We conclude that across-arc geochemical and fluid-flux variations controlled the formation and spatial distribution of Jurassic porphyry deposits in the Gangdese belt.

Carbon and oxygen isotopic evidence for diets, environments and niche differentiation of early Pleistocene pandas and associated mammals in South China

To better understand the dietary evolution and habitat change of pandas, this study analyzed the stable carbon and oxygen isotope compositions of tooth enamel samples from the early Pleistocene pandas Ailuropoda microta and Ailuropoda wulingshanensis and their associated mammals recovered from Yanliang Cave and Longgu Cave in South China. The enamel δ13C values indicate that mammals living in these cave areas during the early Pleistocene had C3–based diets but some individuals consumed small amounts of C4 grasses. This suggests that C4 grasses were present in the region during the early Pleistocene, likely in patches of open areas (or savanna-woodlands) in a predominantly forested landscape. The δ13C and δ18O data clearly show niche partitioning among panda, pig, and other animals. The pandas had the lowest enamel-δ13C values among all mammals examined, indicating that these early Pleistocene pandas had pure C3 diets composed of plants growing in the understories of closed-canopy forests. A pure C3 diet for these ancient pandas is consistent with an already established dietary niche of bamboo (a C3 plant) in the early Pleistocene. These ancient pandas also had higher δ18O values than all other contemporaneous mammals (except the deer at Longgu Cave), suggesting that they did not drink as much as other large mammals did. The available isotope data suggest that pygmy panda (A. microta) was less ecologically flexible compared to other mammals such as pig, rhino, deer and bovid. The limited ecological flexibility of pygmy panda may have contributed to its demise during climatic fluctuations in the Pleistocene. The reconstructed δ18O values of meteoric water in the study areas during the early Pleistocene are lower than the present-day mean annual precipitation δ18O values in the region, suggesting a wetter climate or stronger summer monsoon in the early Pleistocene than today.

The δ18O of UST Quartz in Two Porphyry Deposits, China

AbstractThe unidirectional solidification textures (UST) quartz is generally thought to form from fluids exsolved from shallow intrusions and/or magma chambers, but such an idea is still poorly constrained from the evidence of stable isotopes. In this study, we report for the first time the δ18O of quartz that shows UST from the Qulong Cu–Mo and the Yechangping Mo porphyry deposits in China. The analysis results show that the UST quartz samples from the Qulong deposit have δ18O values ranging from +6.2 ‰ to +7.6 ‰, which are similar to that of quartz phenocrysts (+6.7 ‰ to +7.8 ‰). In contrast, the UST quartz samples from the Yechangping porphyry Mo deposit yield a high δ18O value (+10.0 ‰). The δ18Owater value of Yechangping UST quartz (+8.5 ‰) is also higher than that of Qulong (+4.6 ‰ to +5.8 ‰). Hydrothermal biotite from potassic alteration and sericite from early phyllic alteration at Qulong have similar δ18O values to UST quartz, suggesting the involvement of magmatic fluids during this stage of deposit evolution.

Global deepwater circulation between 2.4 and 1.7 Ma and its connection to the onset of Northern Hemisphere Glaciation

We have generated an early Pleistocene benthic isotopic record for the Ocean Drilling Program (ODP) Site 807 (2804 m) from the western equatorial Pacific. Between 2.4 and 1.7 Ma, the benthic δ13C of this site and a few other deep Pacific sites was consistently higher than the Southern Ocean Site MV0502-4JC (4286 m), pointing to a reversal relative to the current gradient and hence implying a different circulation regime. We reconstructed the deep water mass distribution of this interval using a collection of benthic isotope records from 15 Pacific and 10 Atlantic sites and a δ13C-δ18O mixing model. A two-end-member mixing regime between the North Atlantic Deep Water (NADW) and the Antarctic Bottom Water (AABW), with properties very different from today, was identified. The Southern Ocean showed strong signs of stratification and AABW with low benthic δ13C but high δ18O values reached out to other basins only below ~4000 m. In contrast, NADW ventilated most of the ocean interior, contributing ~70% to the Pacific Deep Water (PDW) volumetrically. Our model results also reveal a strong remineralization effect at the bottom sites of the Pacific and the Atlantic, suggesting significant accumulation of respired carbon in the bottom water between 2.4 and 1.7 Ma. We propose that such a circulation pattern was initiated by the reversal of salinity gradient between AABW and NADW from 3.0 to 2.4 Ma, possibly linked to Antarctic sea ice expansion and reduced southward heat transport during the onset of Northern Hemisphere Glaciation (NHG).

Origin of temporal compositional trends in monogenetic vent eruptions: Insights from the crystal cargo in the Papoose Canyon sequence, Big Pine Volcanic Field, CA

Many monogenetic vents display systematic temporal–compositional variations over the course of eruption. Previous studies have proposed that these trends may reflect variable degrees of crustal assimilation, or melting and mixing of heterogeneous mantle source(s). Discrimination between these two endmember hypotheses is critical for understanding the plumbing systems of monogenetic volcanoes, which pose a significant volcanic hazard in many areas. In this study, we examine the Papoose Canyon (PC) monogenetic vent in the Big Pine Volcanic Field (BPVF), which had been well characterized for temporal–compositional variations in erupted basalts. We present new major and trace element and Sr–Nd–Pb-O isotopic data from the PC “crystal cargo” (phenocrysts and xenoliths). Comparison of “crystal cargo” and host basalt provides new constraints on the history of magma storage, fractionation, and crustal contamination that are obscured in the bulk basalts due to pre- and syn-eruptive magma mixing processes.The abundances of phenocrysts and ultramafic xenoliths in the PC sequence decrease up-section. Olivine and clinopyroxene phenocrysts span a wide range of Mg# (77–89). The majority of phenocrysts are more evolved than olivine or clinopyroxene in equilibrium with their host basalts (Mg#=68–71, equilibrium Fo≈85–89). In addition, the ultramafic xenoliths display cumulate textures. Olivine and clinopyroxene from ultramafic xenoliths have Mg# (73–87) similar to the phenocrysts, and lower than typical mantle peridotites. Sr–Nd–Pb isotope compositions of the xenoliths are similar to early PC basalts. Finally, many clinopyroxene phenocrysts and clinopyroxene in xenoliths have trace element abundances in equilibrium with melts that are more enriched than the erupted basalts. These features suggest that the phenocrysts and xenoliths derive from melt that is more fractionated and enriched than erupted PC basalts. Pressure constraints suggest phenocrysts and ultramafic xenoliths crystallized at ∼5–7 kbar, corresponding to mid-crust depths. Correlations between HFSE depletion and Sr–Nd–Pb isotopic compositions, high δ18O values in olivines, and radiogenic Os isotopic compositions in whole rocks also suggest incorporation of a crustally contaminated component.We propose that the phenocrysts and ultramafic xenoliths derive from melts that ponded and fractionated and assimilated continental crust, possibly in mid-crustal sills. These melts were drained and mixed with more primitive melts as the eruption began, and the temporal–compositional trends and decreasing crystal phase abundances reflect gradual deflation and exhaustion of these sills as the eruption progressed. The isotopic variations in the PC sequence span much of the compositional range observed in the BPVF. Evidence for variable crustal contamination of PC basalts suggests that much of the isotopic variation observed in the BPVF may also reflect crustal contamination rather than mantle source heterogeneity as previously proposed. In addition, evidence of pre-eruptive magma ponding and fractionation, if applicable to other monogenetic vents, may have significant implications for monitoring and hazard assessment of monogenetic volcano fields.