Ultra-depleted, shallow cratonic mantle beneath West Greenland: dunitic xenoliths from Ubekendt Ejland

Dunitic xenoliths from late Palaeogene, alkaline basalt flows on Ubekendt Ejland, West Greenland contain olivine with 100 × Mg/(Mg + Fe), or Mg#, between 92.0 and 93.7. Orthopyroxene has very low Al₂O₃ and CaO contents (0.024–1.639 and 0.062–0.275 wt%, respectively). Spinel has 100 × Cr/(Cr + Al), or Cr#, between 46.98 and 95.67. Clinopyroxene is absent. The osmium isotopic composition of olivine and spinel mineral separates shows a considerable span of ¹⁸⁷Os/¹⁸⁸Os values. The most unradiogenic ¹⁸⁷Os/¹⁸⁸Os value of 0.1046 corresponds to a Re-depletion age of ca. 3.3 Gy, while the most radiogenic value of 0.1336 is higher than present-day chondrite. The Os isotopic composition of the xenoliths is consistent with their origin as restites from a melt extraction event in the Archaean, followed by one or more subsequent metasomatic event(s). The high Cr# in spinel and low modal pyroxene of the Ubekendt Ejland xenoliths are similar to values of some highly depleted mantle peridotites from arc settings. However, highly depleted, arc-related peridotites have higher Cr# in spinel for a given proportion of modal olivine, compared to cratonic xenolith suites from Greenland, which instead form coherent trends with abyssal peridotites, dredged from modern mid-ocean ridges. This suggests that depleted cratonic harzburgites and dunites from shallow lithospheric mantle represent the residue from dry melting in the Archaean.     

Carbonate Assimilation at Merapi Volcano, Java, Indonesia: Insights from Crystal Isotope Stratigraphy

Recent basaltic andesite lavas from Merapi volcano contain abundant,complexly zoned, plagioclase phenocrysts, analysed here fortheir petrographic textures, major element composition and Srisotope composition. Anorthite (An) content in individual crystalscan vary by as much as 55 mol% (An_40–95) across internalresorption surfaces with a negative correlation between highAn mol% (>70), MgO wt% and FeO wt%. In situ Sr isotope analysesof zoned plagioclase phenocrysts show that the ⁸⁷Sr/⁸⁶Sr ratiosof individual zones range from 0·70568 to 0·70627.The upper end of this range is notably more radiogenic thanthe host basaltic andesite whole-rocks (< 0·70574).Crystal zones with the highest An content have the highest ⁸⁷Sr/⁸⁶Srvalues, requiring a source or melt with elevated radiogenicSr, rich in Ca and with lower Mg and Fe. Recent Merapi eruptiverocks contain abundant xenoliths, including metamorphosed volcanoclasticsediment and carbonate country rock (calc-silicate skarns) analysedhere for petrographic textures, mineralogy, major element compositionand Sr isotope composition. The xenoliths contain extremelycalcic plagioclase (up to An₁₀₀) and have whole-rock ⁸⁷Sr/⁸⁶Srratios of 0·70584 to 0·70786. The presence ofthese xenoliths and their mineralogy and geochemistry, coupledwith the ⁸⁷Sr/⁸⁶Sr ratios observed in different zones of individualphenocrysts, indicate that magma–crust interaction atMerapi is potentially more significant than previously thought,as numerous crystal cores in the phenocrysts appear to be inheritedfrom a metamorphosed sedimentary crustal source. This has potentiallysignificant consequences for geochemical mass-balance calculations,volatile saturation and flux and eruptive behaviour at Merapiand similar island arc volcanic systems elsewhere. KEY WORDS: assimilation; isotopes; Merapi; xenolith; calc-silicate     

Petrogenesis and mantle source characteristics of the late Cenozoic Baekdusan (Changbaishan) basalts,North China Craton

Late Cenozoic intraplate basaltic rocks in northeastern China have been interpreted as being derived from a mantle source composed of DMM and EM1 components. To constrain the origin of the enriched mantle component, we have now determined the geochemical compositions of basaltic rocks from the active Baekdusan volcano on the border of China and North Korea. The samples show LREE-enriched patterns, with positive Eu and negative Ce anomalies. On a trace element distribution diagram, they show typical oceanic island basalt (OIB)-like LILE enrichments without significant Nb or Ta depletions. However, compared with OIB, they show enrichments in Ba, Rb, K, Pb, Sr, and P. The Nb/U ratios are generally within the range of OIB, but the Ce/Pb ratios are lower than those of OIB. Olivine phenocrysts are characterized by low Ca and high Ni contents. The radiogenic isotopic characteristics (⁸⁷Sr/⁸⁶Sr = 0.70449 to 0.70554; ε_Nd = −2.0 to +1.8; ε_Hf = −1.7 to +6.1; ²⁰⁶Pb/²⁰⁴Pb = 17.26 to 18.12) suggest derivation from an EM1-like source together with an Indian MORB-like depleted mantle. The Mg isotopic compositions (δ²⁶Mg = −0.39 ± 0.17‰) are generally lower than the average upper mantle, indicating carbonates in the source. The ⁸⁷Sr/⁸⁶Sr ratios decrease with decreasing δ²⁶Mg values whereas the ¹⁴³Nd/¹⁴⁴Nd and (Nb/La)_N ratios increase. These observations suggest the mantle source of the Baekdusan basalts contained at least two components that resided in the mantle transition zone (MTZ): (1) recycled subducted ancient (∼2.2–1.6 Ga) terrigenous silicate sediments, possessing EM1-like Sr–Nd–Pb–Hf isotopic signatures and relatively high values of δ²⁶Mg; and (2) carbonated eclogites with relatively MORB-like radiogenic isotopic compositions and low values of δ²⁶Mg. These components might have acted as metasomatizing agents in refertilizing the asthenosphere, eventually influencing the composition of the MTZ-derived plume that produced the Baekdusan volcanism.     

Medium pressure crystallization of a monchiquitic magma — evidence from megacrysts of Drever''s block, Ubekendt Ejland, West Greenland

Megacrysts and polymineralic fragments of extraordinary diversity from a Tertiary monchiquitic dyke of Ubekendt Ejland comprise three groups: (1) Cr-diopside-fassaitic diopside + olivine, Fo_90.5?81.5 + CrAl spinels. (II) Fassaitic salite-ferrisalite + KTi-pargasite-ferropargasite + apatite + AlTi-magnetite, (III) Scapolite + hyalophane + potassium feldspar + nepheline + analcime. By comparison with mineralogy and phase relations in the host rock and experimental data from alkaline rocks the megacrysts are related to a sequence of crystallization from primitive monchiquitic to potassic phonolitic magmas rich in H₂O and CO₂ at 5–11 kb. Group I megacrysts formed at temperatures of 1300-1150°C and group II between ? bar at the latter temperature. High P_co2 may have stabilized the scapolite in the more evolved liquid and K-feldspar and nepheline began to crystallize at ca. 800°C possibly together with the ferrisalite.     

Isotopic and geochemical characteristics of Upper Cretaceous and Cenozoic flysch sediments of the Kamchatka Peninsula and southern Koryak region

The isotopic and geochemical studies of the Upper Cretaceous-Cenozoic flysch sequences of the Kamchatka Peninsula and southern Koryak region revealed that they were formed at least from two sources: one depleted (T) with low ⁸⁷Sr/⁸⁶Sr and high positive Sr_Nd(T) values and one enriched (T) with high ⁸⁷Sr/⁸⁶Sr and negative Sr_Nd(T) values. The enriched source was likely represented by complexes of ancient upper continental crust. The subduction-related rocks and, to a lesser extent, basalts of mid-oceanic ridges or back-arc basins could serve as a juvenile source for most of the flysch sediments. The Upper Cretaceous flysch sediments differ from their Cenozoic analogues in composition. The Upper Cretaceous rocks are dominated by enriched upper crustal material. The Cenozoic sequences of the Ukelayat Trough and Paleocene-Eocene sequences of the Kumroch Range contain a substantial amount of island-arc volcanoclastic material; the Eocene flysch of Karaginskii Island is compositionally similar to the Upper Cretaceous flysch sequences.     

Strontium isotopic geochemistry of the volcanic rocks and associated megacrysts and inclusions from Ross Island and vicinity,Antarctica

Twelve whole-rock samples of volcanic rocks and a composite of 11 basanitoid samples from Ross Island and vicinity, Antarctica show a narrow range of ⁸⁷Sr/⁸⁶Sr ratios from 0.7030₅ to 0.7033₉. This range is consistent with a model of differentiation from a single parent magma, but the data allow a 30% variation in the ⁸⁷Rb/⁸⁶Sr ratio in the source region if the average ratio is less than 0.057 and if the source region has existed as a closed system for 1.5 b.y. Megacrysts of titaniferous augite, kaersutite, and anorthoclase are isotopically indistinguishable from the host volcanic rocks and therefore are probably cogenetic with the volcanic sequence. A single trachyte sample is isotopically distinct from the rest of the volcanic rocks and probably was contaminated with crustal strontium.Ultramafic and mafic nodules found in association with basanitoids and trachybasalts have ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7027₅ to 0.7057₅. Several of these nodules exhibit evidence of reaction with the melt and are isotopically indistinguishable from their hosts, but data for seven granulite-facies nodules show an apparent isochronal relationship. Although this isochron may be fortuitous, the resulting age of 158±22 m.y. is similar to ages reported for the voluminous Ferrar Dolerites, and suggests isotopic re-equilibration within the lower crust and upper mantle. These nodules are not genetically related to the Ferrar Dolerites, as evidenced by their lower initial ⁸⁷Sr/⁸⁶Sr ratios.Three ultramafic nodules are texturally and isotopically distinct from the rest of the analyzed nodules. These are friable, have larger ⁸⁷Sr/⁸⁶Sr ratios, and may represent a deeper sampling of mantle rock than the granulite-facies nodules. They were, however, derived at a shallower depth than the alkalic magma. Thus they are not genetically related to either the magma or the granulite-facies nodules.     

Genesis of the post-caldera eastern Upper Basin Member rhyolites, Yellowstone, WY: from volcanic stratigraphy, geochemistry, and radiogenic isotope modeling

An array of samples from the eastern Upper Basin Member of the Plateau Rhyolite (EUBM) in the Yellowstone Plateau, Wyoming, were collected and analyzed to evaluate styles of deposition, geochemical variation, and plausible sources for low δ¹⁸O rhyolites. Similar depositional styles and geochemistry suggest that the Tuff of Sulphur Creek and Tuff of Uncle Tom’s Trail were both deposited from pyroclastic density currents and are most likely part of the same unit. The middle unit of the EUBM, the Canyon flow, may be composed of multiple flows based on a wide range of Pb isotopic ratios (e.g., ²⁰⁶Pb/²⁰⁴Pb ranges from 17.54 to 17.86). The youngest EUBM, the Dunraven Road flow, appears to be a ring fracture dome and contains isotopic ratios and sparse phenocrysts that are similar to extra-caldera rhyolites of the younger Roaring Mountain Member. Petrologic textures, more radiogenic ⁸⁷Sr/⁸⁶Sr in plagioclase phenocrysts (0.7134–0.7185) than groundmass and whole-rock ratios (0.7099–0.7161), and δ¹⁸O depletions on the order of 5‰ found in the Tuff of Sulphur Creek and Canyon flow indicate at least a two-stage petrogenesis involving an initial source rock formed by assimilation and fractional crystallization processes, which cooled and was hydrothermally altered. The source rock was then lowered to melting depth by caldera collapse and remelted and erupted. The presence of a low δ¹⁸O extra-caldera rhyolite indicates that country rock may have been hydrothermally altered at depth and then assimilated to form the Dunraven Road flow.     

Pb and Sr systematics of ultrapotassic and basaltic rocks from the central Sierra Nevada,California

This study presents Sr and Pb isotopic ratios and Rb, Sr, U, Th, and Pb concentrations of an ultrapotassic basaltic suite and related rocks from the central Sierra Nevada, California. The ultrapotassic suite yields a narrow range of Sr and Pb isotopic compositions (⁸⁷Sr/⁸⁶Sr=0.70597–0.70653; ²⁰⁶Pb/ ²⁰⁴Pb=18.862–19.018; ²⁰⁷Pb/²⁰⁴Pb=15.640–15.686; ²⁰⁸Pb/ ²⁰⁴Pb=38.833–38.950). Associated basalts containing ultramafic nodules have less radiogenic Sr (⁸⁷Sr/⁸⁶=0.70430–0.70521) and generally higher Rb/Sr ratios than the ultrapotassic suite. Leucitites from Deep Springs Valley, California, contain high ⁸⁷Sr/⁸⁶Sr (71141–0.71240) and low ²⁰⁶Pb/²⁰⁴Pb (17.169–17.234) ratios, reflecting contamination by crustal granulite.The isotopic relationships support an origin of the ultrapotassic basaltic suite by partial melting of an enriched upper mantle source. Dehydration of a gently inclined oceanic slab beneath the Sierra Nevada may have provided Ba, K, Rb, Sr, and H₂O, which migrated into the overlying upper mantle lithosphere. The end of subduction 10 m.y. ago allowed increased asthenospheric heat flow into the upper mantle lithosphere. The increased heat flow enhanced fluid movement in the upper mantle and contributed towards isotopic homogenization of the upper mantle source areas. Continued heating of the enriched upper mantle caused partial melting and subsequent eruption of the ultrapotassic lavas.     

Evidence for crustal assimilation,mixing of magmas,and a87Sr-rich upper mantle

¹⁸O/¹⁶O,⁸⁷Sr/⁸⁶Sr and chemical analyses were made on 39 lavas and ignimbrites from M. Vulsini, the most northerly district of the K-rich Quaternary Roman Province of Italy. These rocks belong mainly to the undersaturated, leucite-bearing (High-K) series, but also included are samples from the less abundant, SiO₂-saturated, hypersthene-(quartz)-normative (Low-K) series. The effects of post-eruption alteration on the ¹⁸O of these lavas were taken into account by analyzing phenocrysts or by using the extrapolation procedure developed for the nearby Alban Hills center. Because of the high Sr contents (500–2400 ppm), the⁸⁷Sr/⁸⁶Sr ratios of these rocks were little affected by such alteration processes. The M. Vulsini volcanics have Sr- and O-isotopic ratios much less uniform, and on the average much higher, than at any of the other volcanic centers of the province:⁸⁷Sr/⁸⁶Sr=0.7097 to 0.7168; ¹⁸O=6.5 to 13.8. This is attributable to the fact that M. Vulsini is one of the sites of greatest crustal assimilation and hybridism between K-rich Roman magmas and SiO₂-rich Tuscan anatectic magmas. The High-K series parent magmas at M. Vulsini had a very high and uniform⁸⁷Sr/⁸⁶Sr=0.7102 to 0.7104, and a somewhat more variable ¹⁸O=+5.5 to +7.5; they must have come from an upper mantle source region previously metasomatically enriched in⁸⁷Sr and LIL elements. These¹⁸O/¹⁶O and⁸⁷Sr/⁸⁶Sr ratios are identical to the parent magma at the Alban Hills, 120 km to the south, where Low-K lavas are absent. Low-K series magmas at M. Vulsini originated from a lower-⁸⁷Sr source region than the High-K series (<0.7097); a similar relationship is observed in all of the other localities in Italy where the two magma series coexist.Contribution No. 4167, Division of Geological and Planetary Sciences, California Institute of Technology     

Fluid flow through the sedimentary cover in northern Switzerland recorded by calcite–celestite veins (Oftringen borehole,Olten)

Abundant veins filled by calcite, celestite and pyrite were found in the core of a 719 m deep borehole drilled in Oftringen near Olten, located in the north-western Molasse basin, close to the thrust of the Folded Jura. Host rocks are calcareous marl, argillaceous limestone and limestone of the Dogger and Malm. The δ¹⁸O values of vein calcite are lower than in host rock carbonate and, together with microthermometric data from fluid inclusions in vein calcite, indicate precipitation from a seawater-dominated fluid at average temperatures of 56–68°C. Such temperatures were reached at the time of maximum burial of the sedimentary pile in the late Miocene. The depth profile of δ¹³C and ⁸⁷Sr/⁸⁶Sr values and Sr content of both whole-rock carbonate and vein calcite show marked trends towards negative δ¹³C, high ⁸⁷Sr/⁸⁶Sr, and low Sr content in the uppermost 50–150 m of the Jurassic profile (upper Oxfordian). The ⁸⁷Sr/⁸⁶Sr of vein minerals is generally higher than that of host rock carbonate, up to very high values corresponding to Burdigalian seawater (Upper Marine Molasse, Miocene), which represents the last marine incursion in the region. No evidence for internally derived radiogenic Sr (clay minerals) has been found and so an external source is required. S and O isotope composition of vein celestite and pyrite can be explained by bacterial reduction of Miocene seawater sulphate. The available data set suggests the vein mineralization precipitated from descending Burdigalian seawater and not from a fluid originating in the underlying Triassic evaporites.     

Isotopic signatures of strontium,neodymium, and lead in metamorphic rocks of the Khavyven highland in eastern Kamchatka

Metamorphic units of the Khavyven Highland that crop out in the northern portion of the Khavyven Uplift of the basement structures of the Central Kamchatka Trough are formed by rocks of the Khavyven Formation, which are metamorphosed in the green-schist facies. The formation comprises two strata: the lower part that consists of amphibole-micaceous ± garnet, epidote-micaceous ± garnet crystalline schists, and micaceous ± garnet quartzite schists has a total thickness of some 500 m, and the upper part, which is formed by epidote-amphibole and phengite-amphibole green schists and overlying epidote-amphibole-micaceous quartzites, with a visible thickness of some 750 m. The isotopic ratios of Sr, Nd, and Pb were determined in the examined rocks of the Khavyven Formation for the first time. The high ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd ratios and the high K/La, Ba/Th, Th/Ta, and La/Nb ratios in combination with a deep Ta-Nb minimum indicate that the original volcanites of the crystalline schists of the lower rock mass had a subduction nature. The green schist of the upper rock mass, whose composition corresponds to that of spilitic basalts, have elevated ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios, thus combining indications of depleted melts of the N-MORB and E-MORB types and those of subduction melts, which explains the deep Ta-Nb minimum and the low (La/Yb) _N ratios. The isotopic signatures of lead in rocks of the lower and the upper strata are similar. The composition points of the crystalline schists and the green schists are located near the trend of isotopic evolution of lead in the depleted mantle, which indicates that the rocks are closely related to this mantle source.     

Geochemistry and geochronology of Early Mesozoic tholeiites from Central Morocco

From Central Morocco (Central High Atlas, Middle Atlas, Haute Moulouya) continental tholeiites were investigated geochemically and geochronologically. These tholeiites are intercalated within continental redbeds of the Early Mesozoic (Triassic-Liassic). The major, trace and rare earth element contents classify these volcanic rocks as basaltic to andesitic-basaltic, quartz-normative tholeiites. Some trace element ratios (e.g. Zr/Nb, Zr/ Y, Y/Nb, Ti/V) suffer a heterogeneous source with a composition similar to MORB (P- to N-type). The enriched LILE contents, the negative Nb anomaly and the inital⁸⁷Sr/⁸⁶Sr ratios (0.7064–0.7069) reveal the presence of a crustal component up to 13–17 wt%.⁸⁷Sr/⁸⁶Sr ratios of carbonate mineral separates from different lava flows show different cycles of alteration; however, the major and trace element chemistry together with Sr isotope evidence, indicate that the alteration phases are not submarine in origin.⁴⁰Ar/³⁹Ar age determinations on translucent plagioclase phenocrysts yield extrusion ages which range between 210.4 ± 2.1 Ma and 196.3 ± 1.2 Ma. These ages correspond to a stratigraphic period between the Norian (Rhaetian?) and the Upper Sinemurian.     

Compositional gradients in large reservoirs of silicic magma as evidenced by ignimbrites versus Taylor Creek Rhyolite lava domes

The Taylor Creek Rhyolite of southwest New Mexico consists of 20 lava domes and flows that were emplaced during a period of a few thousand years or less in late Oligocene time. Including genetically associated pyroclastic deposits, which are about as voluminous as the lava domes and flows, the Taylor Creek Rhyolite represents roughly 100 km³ of magma erupted from vents distributed throughout an area of several hundred square kilometers. Major-element composition is metaluminous to weakly peraluminous high-silica rhyolite and is nearly constant throughout the lava field. The magma reservoir for the Taylor Creek Rhyolite was vertically zoned in trace elements, ⁸⁷Sr/⁸⁶Sr, and phenocryst abundance and size. Mean trace-element concentrations, ranges in concentrations, and element-pair correlations are similar to many subalkaline silicic ignimbrites. However, the polarity of the zonation was opposite that in reservoirs for ignimbrites, for most constituents. For example, compared to the Bishop Tuff, only ⁸⁷Sr/⁸⁶Sr and Sc increased upward in both reservoirs. Quite likely, a dominant but nonerupted volume of the magma reservoir for the Taylor Creek Rhyolite was zoned like that for the Bishop Tuff, whereas an erupted, few-hundred-meter-thick cap on the magma body was variably contaminated by roof rocks whose contribution to this part of the magma system moderated relatively extreme trace-element concentrations of uncontaminated Taylor Creek Rhyolite but did not change the sense of correlation for most element pairs. The contaminant probably was a Precambrian rock of broadly granitic composition and with very high ⁸⁷Sr/⁸⁶Sr. Although examples apparently are not yet reported in the literature, evidence for a similar thin contaminated cap on reservoirs for large-volume silicic ignimbrites may exist in the bottom few meters of ignimbrites or perhaps only in the pumice fallout that normally immediately precedes ignimbrite emplacement. ⁸⁷Sr/⁸⁶Sr in sanidine phenocrysts of the Taylor Creek Rhyolite is higher than that of their host whole rocks. Covariation of this isotope ratio with sanidine abundance and size indicates positive correlations for all three features with decreasing distance to the roof of the magma reservoir. The sanidine probably is more radiogenic than host whole rock because growing phenocrysts partly incorporated Sr from the first partial melt of roof rocks, which contained the highly radiogenic Sr of Precambrian biotite ± hornblende, whereas diffusion was too slow for sanidine to incorporate much of the Sr from subsequently produced less radiogenic partial melt of roof rocks, before eruption quenched the magma system. Disequilibrium between feldspar phenocrysts and host groundmass is fairly common for ignimbrites, and a process of contamination similar to that for the Taylor Creek Rhyolite may help explain some of these situations.     

Time scales of large volume silicic magma systems: Sr isotopic systematics of phenocrysts and glass from the Bishop Tuff,Long Valley,California

The initial Sr isotopic compositions of glass and mineral separates from the 0.74 Ma Bishop Tuff ashflow in eastern California were determined to investigate the time scales of magmatic processes in a large silicic system. It was found that there is substantial isotopic heterogeneity, both between eruptive units and between glass and phenocryst phases of individual units. The frist-erupted, lower temperature units generally have higher initial ⁸⁷Sr/⁸⁶Sr than later crupted, higher temperature units. Within each unit, feldspar phenocrysts have the lowest ⁸⁷Sr/⁸⁶Sr, associated glass has higher ⁸⁷Sr/⁸⁶Sr, and biotite phenocrysts have the highest ⁸⁷Sr/⁸⁶Sr. These isotopic differences were produced by processes in the magma chamber and not by post-eruptive alteration. Two samples were similar Fe–Ti oxide temperatures but from widely separated localities have nearly identifical Sr isotopic characteristics, indicating the existence of compositionally uniform layers of substantial volume within the chamber. Trace element data indicate that the feldspars crystallized from a liquid represented by the associated glass, and that the feldspar-glass pairs are not accidental. The rhyolitic liquids of the Bishoptuff magma chamber apparently experienced increasing ⁸⁷Sr/⁸⁶Sr at a rate too fast for feldspar phenocrysts to remain in isotopic equilibrium. The increasing ⁸⁷Sr/⁸⁶Sr is caused primarily by radioactive decay of ⁸⁷Rb in the high-Rb/Sr liquids and not primarily by assimilation of radiogenic wall-rock material. A self-consistent model can be constructed to account for all of the isotopic data except for those on biotite phenocrysts. The time scale for evolution of the system is bounded on the high side at about 500 ky by observations made on precaldera lavas, and on the low side at approximately 300 ky by the time necessary to establish homogeneous layers in an actively differentiating chamber. The deduced time scale is consistent with model Rb–Sr ages, which date the differentiation of low temperature liquids from higher temperature liquids, and is compatible with the observed isotopic disequilibrium between feldspars and glass because of the low diffusivity of Sr in fieldspars (<10^-16 cm²/s). The prolonged (about 500 ky) evolution of the Bishop Tuff system was facilitated by a large influx of basaltic material (about 10^-2 km³/y) to the base of the system, which compensated for diffusive heat loss from the top and allowed large volumes of magma to maintain low crystal contents for >3x10⁵ years. The silicic-magma production rate within the Bishop Tuff magma chamber is estimated to be 10^-3km³/y. The growth rate of alkali feldspar is estimated to be about 10^-14 cm/s based on the Sr isotopic difference between sanidine and glass of the lower Bishop Tuff. The biotite population is inferred to be partially (>50 ppm) xenocrystic, the xenocrysts being introduced to the chamber less than one year prior to eruption.     

Nd–Sr isotopic constraint to the formation of metatexite and diatexite migmatites,Higo metamorphic terrane,central Kyushu,Japan

ABSTRACT

Metatexite and diatexite migmatites are widely distributed within the upper amphibolite and granulite-facies zones of the Higo low-P/high-T metamorphic terrane. Here we report Nd–Sr isotopic and whole rock composition data from an outcrop in the highest-grade part of the granulite-facies zone, in which diatexite occurs as a 3 m-thick layer between 2 m-thick layers of stromatic-structured metatexite within pelitic gneiss. The metatexite has Nd–Sr isotopes and whole rock compositions similar to those of the gneiss, but the diatexite shows the reverse. The diatexite has a higher ε_Nd(t) and ¹⁴⁷Sm/¹⁴⁴Nd ratio (ε_Nd(t) = ?0.5; ¹⁴⁷Sm/¹⁴⁴Nd = 0.1636) than the gneiss (ε_Nd(t) = ?2.1; ¹⁴⁷Sm/¹⁴⁴Nd = 0.1287) and metatexite (ε_Nd(t) = ?3.1; ¹⁴⁷Sm/¹⁴⁴Nd = 0.1188). The (⁸⁷Sr/⁸⁶Sr)_initial and ⁸⁷Rb/⁸⁶Sr of the diatexite ((⁸⁷Sr/⁸⁶Sr)_initial = 0.70568; ⁸⁷Rb/⁸⁶Sr = 0.416) are lower than those of the gneiss ((⁸⁷Sr/⁸⁶Sr)_initial = 0.70857; ⁸⁷Rb/⁸⁶Sr = 1.13) and metatexite ((⁸⁷Sr/⁸⁶Sr)_initial = 0.70792; ⁸⁷Rb/⁸⁶Sr = 1.11). The metatexite and gneiss show enrichment of Th and depletion of P and Eu and have a similar chondrite-normalized REE pattern, which shows steep LREE–MREE-enriched and gently declining HREE patterns and negative Eu anomalies, whereas the diatexite shows enrichment of Sr and depletion of Th and Y, and exhibits gently declining LREE and steeply declining HREE pattern and weak Eu depletion. The metatexite migmatite is interpreted to have formed by in situ partial melting in which the melt did not migrate from the source, whereas the diatexite migmatite included an externally derived melt with a juvenile component. The Cretaceous high-temperature metamorphism of the Higo metamorphic terrane is interpreted to reflect emplacement of mantle-derived basalts under a volcanic arc along the eastern margin of the Eurasian continent, and mass transfer and advection of heat via hybrid silicic melts from the lower crust.     

Petrology and geochemistry of Rodrigues Island,Indian Ocean

Rodrigues Island is composed of a differentiated series of transitional-mildly alkaline olivine basalts. The lavas contain phenocrysts of olivine (Fo_88–68)±plagioclase (An_73–50), together with a megacryst suite involving olivine, plagioclase, kaersutite, clinopyroxene, apatite, magnetite and hercynite-rich spinels. Troctolitic-anorthositic gabbro xenoliths are widely dispersed throughout the lavas and are probably derived from the upper parts of an underlying layered complex: the megacrysts may originate from coarse, easily disaggregated differentiates near the top of this body.Modelling of major and trace element data suggests that the majority of chemical variation in the lavas results from up to 45% fractionation of olivine, clinopyroxene, plagioclase and magnetite at low pressures, in the ratio 2035396. The clinopyroxene-rich nature of this extract assemblage is significantly different to that of the xenoliths, and suggests that clinopyroxene-rich gabbros and/or ultrabasic rocks may lie at greater depth.Sr and Nd isotopic data (⁸⁷Sr/⁸⁶Sr 0.70357–070406,¹⁴³Nd/¹⁴⁴Nd 0.51283–0.51289) indicate a mantle source with relative LREE depletion, and emphasise an unusual degree of uniformity in Indian Ocean island sources. A small group of lavas with strong HREE enrichment suggest a garnet-poor source for these, while high overall Al₂O₃/ CaO ratios imply high clinopyroxene/garnet ratios in refractory residua.     

Meta-igneous granulite xenoliths from Mount Ruapehu,New Zealand: Fragments of altered oceanic crust?

Meta-igneous granulite (MIG) xenoliths in lavas from Mount Ruapehu, Taupo Volcanic Zone, New Zealand, have variable but relatively high ¹⁸O (+8.2 to +11.7) and ⁸⁷Sr/⁸⁶Sr (0.70506 to 0.70872), and _Nd ranging from +1.5 to +6.2. They show a strong positive correlation between ⁸⁷Sr/⁸⁶Sr and ¹⁸O, both of which are also broadly correlated with Mg number, but lack any correlation between ⁸⁷Sr/⁸⁶Sr and _Nd. The xenoliths have been mineralogically re-equilibrated at lower-crustal temperatures (800–930°C) and pressures (7–10 kbar). Geochemical and isotopic evidence suggests they are fragments of oceanic crust that have been altered previously in an ocean floor hydrothermal system. Alternatively, they may be igneous rocks of unknown origin hydrothermally altered in the lower crust. Irrespective of this uncertainty, the xenoliths provide rare samples of the lower crustal basement beneath Taupo Volcanic Zone and represent a potential source rock for the voluminous rhyolites and ignimbrites that dominate the zone.     

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.     

Garnet-bearing tonalitic porphyry from East Kunlun,Northeast Tibetan Plateau: implications for adakite and magmas from the MASH Zone

A garnet-bearing tonalitic porphyry from the Achiq Kol area, northeast Tibetan Plateau has been dated by SHRIMP U-Pb zircon techniques and gives a Late Triassic age of 213 ± 3 Ma. The porphyry contains phenocrysts of Ca-rich, Mn-poor garnet (CaO > 5 wt%; MnO < 3 wt%), Al-rich hornblende (Al₂O₃ ~ 15.9 wt%), plagioclase and quartz, and pressure estimates for hornblende enclosing the garnet phenocrysts yield values of 8–10 kbar, indicating a minimum pressure for the garnet. The rock has SiO₂ of 60–63 wt%, low MgO (<2.0 wt%), K₂O (<1.3 wt%), but high Al₂O₃ (>17 wt%) contents, and is metaluminous to slightly peraluminous (ACNK = 0.89–1.05). The rock samples are enriched in LILE and LREE but depleted in Nb and Ti, showing typical features of subduction-related magmas. The relatively high Sr/Y (~38) ratios and low HREE (Yb < 0.8 ppm) contents suggest that garnet is a residual phase, while suppressed crystallization of plagioclase and lack of negative Eu anomalies indicate a high water fugacity in the magma. Nd–Sr isotope compositions of the rock (εNd_T = −1.38 to −2.33; ⁸⁷Sr/⁸⁶Sr_i = 0.7065–0.7067) suggest that both mantle- and crust-derived materials were involved in the petrogenesis, which is consistent with the reverse compositional zoning of plagioclase, interpreted to indicate magma mixing. Both garnet phenocrysts and their ilmenite inclusions contain low MgO contents which, in combination with the oxygen isotope composition of garnet separates (+6.23‰), suggests that these minerals formed in a lower crust-derived felsic melt probably in the MASH zone. Although the rock samples are similar to adakitic rocks in many aspects, their moderate Sr contents (<260 ppm) and La/Yb ratios (mostly 16–21) are significantly lower than those of adakitic rocks. Because of high partition coefficients for Sr and LREE, fractionation of apatite at an early stage in the evolution of the magma may have effectively decreased both Sr and LREE in the residual melt. It is suggested that extensive crystallization of apatite as an early phase may prevent some arc magmas from evolving into adakitic rocks even under high water fugacity.     

Strontium isotope composition of runoff from a glaciated carbonate terrain

The relationship between subglacial chemical weathering processes and the Sr isotope composition of runoff from Robertson Glacier, Alberta, Canada, is investigated. This glacier rests on predominantly carbonate bedrock of Upper Devonian age, but silicate minerals are also present. The provenance of solute in meltwaters is found to vary systematically with solute concentration and, by inference, subglacial water residence time. In dilute waters, the principal process of solute acquisition is calcite dissolution fueled by protons derived from the dissolution of CO₂ and subsequent dissociation of carbonic acid. At higher solute concentrations, dolomite dissolution coupled to sulfide oxidation is more important. Sr concentration is found to increase with total solute concentration in two separate meltwater streams draining from the glacier, but ⁸⁷Sr/⁸⁶Sr only increases in the eastern melt stream. Carbonate and K-feldspar sources are shown to dominate the Sr content of the western stream, irrespective of concentration. They also dominate the Sr content of the eastern stream at low and intermediate concentrations, but at higher concentrations, muscovite (with high ⁸⁷Sr/⁸⁶Sr) is also an important Sr source. This reflects the outcrop of muscovite-bearing lithologies in the catchment of the eastern stream and an increase in the rate of weathering of K-silicates relative to that of carbonates as more concentrated solutions approach saturation with respect to carbonates. Nonstoichiometric release of ⁸⁷Sr/⁸⁶Sr and preferential release of Sr over K from freshly ground K-silicate surfaces may also occur. This may help to explain the radiogenic nature of runoff from distributed subglacial drainage systems, which are characterized by long water:rock contact times and water flow through environments in which crushing and grinding of bedrock are active processes.Although the exchangeable Sr in tills has higher ⁸⁷Sr/⁸⁶Sr than local carbonate bedrock, only the more concentrated meltwaters from the eastern stream display similarly high values. The most dilute waters, which probably transport the bulk of the dissolved Sr flux from the glacier, have ⁸⁷Sr/⁸⁶Sr characteristic of local carbonate bedrock. Thus, the results suggest that although enhanced weathering of silicate minerals containing radiogenic Sr (such as muscovite) does occur in glaciated carbonate terrains, it is unlikely to contribute to any enhanced flux of radiogenic Sr from glaciated continental surfaces to the oceans.