Isotopic and elemental abundances of copper and zinc in lunar samples, Zagami, Pele’s hairs, and a terrestrial basalt

We used ICP–MS to measure the elemental concentrations and isotopic abundances of Cu and Zn in: nine Ti-rich lunar basalts (10017, 10022, 10024, 10057, 70215, 71055, 74255, 75055, and 75075); size-separated samples prepared by sieving of pyroclastic black glass 74001, orange glass 74022, and the lunar soils 15021, 15231, 70181, and 79221; a basalt from the Piton des Neiges volcano, Reunion Island; two samples of Pele’s hairs from the Nyiragongo volcano, Democratic Republic of Congo, and the martian meteorite Zagami.The isotopic fractionation of zinc in lunar basalts and Zagami is mass dependent relative to a terrestrial standard (JMC 400882B). These and published results imply that lunar, terrestrial, meteoritic, and perhaps martian zinc all come from one or more reservoirs linked by mass-dependent fractionation processes. Relative to terrestrial basalts, Ti-rich lunar basalts are enriched in the heavier isotopes of Cu and Zn: we find for Ti-rich lunar basalts the following ranges and averages ±1 − σ (‰): δ⁶⁵Cu/⁶³Cu ≡ δ⁶⁵Cu, 0.1–1.4, 0.5 ± 0.1‰ (N = 7); δ⁶⁶Zn/⁶⁴Zn ≡ δ⁶⁶Zn = 0.2–1.9, 1.2 ± 0.2‰ (N = 8; 10017 excluded). For two terrestrial samples, we find δ⁶⁶Zn  +0.3‰ and δ⁶⁵Cu  0‰, which are consistent with published values. The differences between the lunar basalts and terrestrial basalts could reflect minor, planetary-scale vaporization or igneous processes on the Moon.Data for size separates of the pyroclastic glasses 74001 and 74220 confirm the well-known surface correlation of Cu and Zn, but modeling calculations reveal no sharp differences between either the elemental ratios or the isotopic composition of grain interiors and exteriors. The absence of such differences indicates that the isotopic compositions for bulk samples are dominated by a light-isotope-rich surface component.Data for size separates of lunar soils also confirm the surface correlation of Cu and Zn, but an enrichment of heavy rather than light isotopes. Averages for bulk lunar soils from this work and the literature are (‰): δ⁶⁵Cu, from 1.4 to 4.1, average 3.0 ± 0.3 (N = 9); δ⁶⁶Zn, from 2.2 to 6.4, average 4.0 ± 0.3 (N = 14). As with the glasses, in all but soil 15231 our data show no strong differences between the isotopic composition of soil sub-samples with small and large grains.The size of the isotopic fractionation inferred for the surface component in the soils is 3× smaller than predicted by a published model of sputtering primarily by solar particles. At the same time, the observed fractionation is larger than predicted by calculations based on a model of micrometeorite impact heating and hydrodynamic quenching. Because impact heating appears unable to explain the observations, we conclude that sputtering must be important even though samples with very large isotopic fractionation of Cu and Zn have not yet been found.     

Mantle sources and crustal input as recorded in high-Mg Deccan Traps basalts of Gujarat (India)

Near-primitive picritic basalts in the northwestern Deccan Traps have MgO > 10 wt.% and consist of two groups (low-Ti and high-Ti) with markedly different incompatible element and Nd–Sr–Pb isotope characteristics. Many elemental characteristics of the low-Ti picritic basalts are similar to those of transitional or normal ocean ridge basalts. However, values of ratios like Ba/Nb (13–30) and Ce/Pb (4–11), and isotopic ratios (e.g., ε_Nd(t) + 0.3 to − 6.3, (²⁰⁷Pb/²⁰⁴Pb)_t 15.63–15.75 at (²⁰⁶Pb/²⁰⁴Pb)_t 18.19–18.84, δ¹⁸O_olivine as high as + 6.2‰) are far-removed from ocean-ridge-type values, indicating a significant contribution from continental crust. The crustal signature could represent crustal contamination of ascending magmas; alternatively, it could represent a minor component within the Indian lithospheric mantle of anciently subducted sedimentary material or fluids derived from subducted material. In contrast, the high-Ti picritic basalts are chemically and isotopically rather similar to recent shield lavas of the Réunion hotspot (e.g., ε_Nd(t) + 2 to + 4) and to volcanic rocks along the postulated pre-Deccan track of this hotspot in Pakistan. Neither type of picritic basalt is parental to the voluminous flows comprising the bulk of the Deccan Traps. However, many of the Deccan primary magmas could have been derived from mixtures of a high-Ti-type, Réunion-like source component and a component more similar to, or even more incompatible-element-depleted than, average ocean-ridge mantle.     

Oxygen isotopic compositions of Allende Type C CAIs: Evidence for isotopic exchange during nebular melting and asteroidal metamorphism

In situ oxygen isotopic measurements of primary and secondary minerals in Type C CAIs from the Allende CV3 chondrite reveal that the pattern of relative enrichments and depletions of ¹⁶O in the primary minerals within each individual CAI are similar to the patterns observed in Types A and B CAIs from the same meteorite. Spinel is consistently the most ¹⁶O-rich (Δ¹⁷O = −25‰ to −15‰), followed by Al,Ti-dioside (Δ¹⁷O = −20‰ to −5‰) and anorthite (Δ¹⁷O = −15‰ to 0‰). Melilite is the most ¹⁶O-depleted primary mineral (Δ¹⁷O = −5‰ to −3‰). We conclude that the original melting event that formed Type C CAIs occurred in a ¹⁶O-rich (Δ¹⁷O  −20‰) nebular gas and they subsequently experienced oxygen isotopic exchange in a ¹⁶O-poor reservoir. At least three of these (ABC, TS26F1 and 93) experienced remelting at the time and place where chondrules were forming, trapping and partially assimilating ¹⁶O-poor chondrule fragments. The observation that the pyroxene is ¹⁶O-rich relative to the feldspar, even though the feldspar preceded it in the igneous crystallization sequence, disproves the class of CAI isotopic exchange models in which partial melting of a ¹⁶O-rich solid in a ¹⁶O-poor gas is followed by slow crystallization in that gas. For the typical (not associated with chondrule materials) Type C CAIs as well for as the Types A and B CAIs, the exchange that produced internal isotopic heterogeneity within each CAI must have occurred largely in the solid state. The secondary phases grossular, monticellite and forsterite commonly have similar oxygen isotopic compositions to the melilite and anorthite they replace, but in one case (CAI 160) grossular is ¹⁶O-enriched (Δ¹⁷O = −10‰ to −6‰) relative to melilite (Δ¹⁷O = −5‰ to −3‰), meaning that the melilite and anorthite must have exchanged its oxygen subsequent to secondary alteration. This isotopic exchange in melilite and anorthite likely occurred on the CV parent asteroid, possibly during fluid-assisted thermal metamorphism.     

Distribution of diagenetic alterations in fluvial and paralic deposits within sequence stratigraphic framework: Evidence from the Petrohan Terrigenous Group and the Svidol Formation, Lower Triassic, NW Bulgaria

Elucidation of diagenetic alterations in the Petrohan Terrigenous Group (fluvial; highstand systems tract HST) sandstones and Svidol Formation (tide-dominated deltaic and tidal flat, transgressive systems tract TST and highstand systems tract HST, respectively) sandstones and calcarenite, Lower Triassic, NW Bulgaria was constrained within a sequence stratigraphic framework. Eogenetic alterations in the fluvial HST sandstones include (i) formation of grain-coating infiltrated clays as a result of percolation of mud-rich surface waters into underlying coarse-grained and permeable channel-fills and crevasse splay sandstones; (ii) formation of pseudomatrix by mechanical compaction of mud intraclasts that were incorporated into the coarse-grained channel sandstones during their lateral avulsion; and (iii) cementation by calcite (δ¹⁸O_VPDB = − 6.5‰ to − 3‰; δ¹³C_VPDB = − 5.1‰ to + 0.6‰) and dolomite (δ¹⁸O_VPDB = − 6.1‰ to − 0.3‰; δ¹³C_VPDB = − 7.2‰ to − 5.8‰) in the crevasse splay and floodplain sediments. Mesogenetic alterations that are encountered in the fluvial HST sandstones include (i) illitization of grain-coating clays, mud intraclasts, and mica, possibly because of simultaneous albitization of feldspars; (ii) cementation by calcite (δ¹⁸O_VPDB = − 14.5‰ to − 8.4‰; δ¹³C_VPDB = − 7.7‰ to + 0.6‰) and dolomite (δ¹⁸O_VPDB = − 15.8‰ to − 5‰; δ¹³C_VPDB = − 7.9‰ to + 1.5‰); and (iii) limited amounts of quartz overgrowths in the channel sandstones owing to occurrence of thick grain-coating clays.

Conversely, the tide-dominated deltaic TST sandstones and the tidal flat HST calcarenite were pervasively cemented by calcite (δ¹⁸O_VPDB = − 6.6‰ to − 3.1‰; δ¹³C_VPDB = − 5.1‰ to + 0.6‰) and siderite (δ¹⁸O_VPDB = − 7.2‰ to − 5.7‰; δ¹³C_VPDB = + 0.3‰ to + 0.9‰) particularly below marine and maximum flooding surfaces, due to the presence of abundant bioclasts and prolonged residence time of the sediments under certain geochemical conditions along these surfaces. The remaining open pores were cemented during mesodiagenesis by calcite (δ¹⁸O_VPDB = − 6.6‰ to − 3.1‰ and δ¹³C_VPDB = − 5.1‰ to + 0.6‰) and dolomite (δ¹⁸O_VPDB = − 6.6‰ to − 3.1‰ and δ¹³C_VPDB = − 5.1‰ to + 0.6‰).

This study shows that constructing a conceptual model for the distribution of diagenetic alterations is possible by integration of diagenesis with sequence stratigraphy. The model shows that tide-dominated deltaic TST sandstones and tidal flat HST calcarenite were pervasively cemented by carbonates during near-surface eodiagenesis, owing to the presence of abundant bioclasts. Conversely, fluvial LST sandstones remained poorly cemented during near-surface eodiagenesis due to the lack of bioclasts, but were cemented by mesogenetic calcite, dolomite and quartz overgrowths instead.     

The geochemical cycle of boron: Constraints from boron isotope partitioning experiments between mica and fluid

The fractionation of boron isotopes between synthetic boromuscovite and fluid was experimentally determined at 3.0 GPa/500 °C and 3.0 GPa/700 °C. For near-neutral fluids Δ¹¹B_(mica-fluid) = δ¹¹B_(mica) − δ¹¹B_(fluid) is − 10.9 ± 1.3‰ at 500 °C, and − 6.5 ± 0.4‰ at 700 °C. This supports earlier assumptions that the main fractionation effect is due to the change from trigonal coordination of boron in neutral fluids to tetrahedrally coordinated boron in micas, clays and melts. The T-dependence of this effect is approximated by the equation Δ¹¹B_{(mica,clay,melt–neutral fluid)} = − 10.69 · (1000/T [K]) + 3.88; R² = 0.992, valid from 25 °C for fluid–clay up to about 1000 °C for fluid–silicate melt. Experiments at 0.4 GPa that used strongly basic fluids produced significantly lower fractionations with Δ¹¹B_{(mica–fluid)} of − 7.4 ± 1.0‰ at 400 °C, and − 4.8 ± 1.0‰ at 500 °C, showing the reduced fractionation effect when large amounts of boron in basic fluids are tetrahedrally coordinated. Field studies have shown that boron concentrations and ¹¹B/¹⁰B-ratios in volcanic arcs systematically decrease across the arc with increasing distance from the trench, thus reflecting the thermal structure of the subducting slab. Our experiments show that the boron isotopic signature in volcanic arcs probably results from continuous dehydration of micas along a distinct P–T range. Continuous slab dehydration and boron transport via fluid into the mantle wedge is responsible for the boron isotopic signature in volcanic arcs.     

Stable isotope studies and processes of carbonate formation in Hungarian alkali basalts and lamprophyres: evolution of magmatic fluids and magma-sediment interactions

Processes of carbonate formation have been related to C and O isotopic compositions in the Mesozoic alkali basalt (Mecsek Mts.) and lamprophyre (Transdanubian Range) suites of Hungary. In the studied magrnatic rocks, carbonates are present as ocelli, amygdales, xenoliths, veins and groundmass carbonate. C and O isotope studies of these types of carbonate have yielded information on the origin of the carbonates and indicated the following processes of formation that determined the δ¹³C and δ¹⁸O values of the carbonates:(1)Crystallization of magmatic carbonate. Textural characteristics and δ¹³C values suggest formation of magmatic carbonate in alkali basalt and lamprophyre dikes, whereas the δ¹⁸O compositions of these carbonates indicate low temperature oxygen isotope exchange with magmatic fluids.(2) Assimilation of sedimentary carbonate by silicate magmas. Even completely recrystallized amygdales and ocelli of basalts and lamprophyres have preserved their sedimentary δ¹³C values. In contrast, variations in the extent of mobilization and isotope exchange with magmatic fluids are reflected in differences in the ranges of the δ¹⁸O values of amygdales, ocelli and veins, and can be attributed to different amounts of fluids involved in the magmatic events.(3) Low temperature alteration of magmatic rocks caused only ¹⁸O-enrichment in the carbonate amygdales of basalts and the groundmass carbonates of lamprophyres, indicating that no externally-derived CO₂ was present in the alteration fluids.(4) Degassing of magma and magmatic fluid. Correlations between δ¹³C and δ¹⁸O data, magma crystallization depths and amygdale sizes in the alkali basalts suggest that CO₂ degassing has been responsible for the negative δ¹³C and positive δ¹⁸O shifts observed. A similar trend was found in the lamprophyres, but the extent of the δ¹⁸O shift indicates that in these rocks H₂O degassing also played an important role.     

Gypsum as a monitor of the paleo-limnological–hydrological conditions in Lake Lisan and the Dead Sea

The isotopic composition and mass balances of sources and sinks of sulfur are used to constrain the limnological–hydrological evolution of the last glacial Lake Lisan (70–14 ka BP) and the Holocene Dead Sea. Lake Lisan deposited large amounts of primary gypsum during discrete episodes of lake level decline. This gypsum, which appears in massive or laminated forms, displays δ³⁴S values in the range of 14–28‰. In addition, Lake Lisan’s deposits (the Lisan Formation) contain thinly laminated and disseminated gypsum as well as native sulfur which display significantly lower δ³⁴S values (−26 to 1‰ and −20 to −10‰, respectively). The calculated bulk isotopic compositions of sulfur in the sources and sinks of Lake Lisan lacustrine system are similar (δ³⁴S ≈ 10‰), indicating that freshwater sulfate was the main source of sulfur to the lake. The large range in δ³⁴S found within the Lisan Formation (−26 to +28‰) is the result of bacterial sulfate reduction (BSR) within the anoxic lower water body (the monimolimnion) and bottom sediments of the lake.

Precipitation of primary gypsum from the Ca-chloride solution of Lake Lisan is limited by sulfate concentration, which could not exceed 3000 mg/l. The Upper Gypsum Unit, deposited before ca. 17–15 ka, is the thickest gypsum unit in the section and displays the highest δ³⁴S values (25–28‰). Yet, our calculations indicate that no more than a third of this Unit could have precipitated directly from the water column. This implies that during the lake level decline that instigated the precipitation of the Upper Gypsum Unit, significant amounts of dissolved sulfate had to reach the lake from external sources. We propose a mechanism that operated during cycles of high-low stands of the lakes that occupied the Dead Sea basin during the late Pleistocene. During high-stand intervals (i.e., Marine Isotopic Stages 2 and 4), lake brine underwent BSR and infiltrated the lake’s margins and adjacent strata. As lake level dropped, these brines, carrying ³⁴S-enriched sulfate, were flushed back to the shrinking lake and replenished the water column with sulfate, thereby promoting massive gypsum precipitation.

The Holocene Dead Sea precipitated relatively small amounts of primary gypsum, mainly in the form of thin laminae. δ³⁴S values of these laminae and disseminated gypsum are relatively constant (15 ± 0.7‰) and are close to present-day lake composition. This reflects the lower supply of freshwater to the lake and the limited BSR activity during the arid Holocene time and possibly during former arid interglacials in the Levant.     

Isotopic constraints on fluid/rock ratios in carbonate rocks: Barite-sulfide mineralization in the Schwaz Dolomite, Tyrol (Eastern Alps, Austria)

The significance of isotopic data on constraining the physical conditions of fluid-rock interaction and mineralization processes in carbonate rocks is discussed, based on the example of barite-tetrahedrite mineralization in Lower Devonian platform carbonates of the Western Greywacke Zone (Tyrol, Austria). Available strontium, oxygen, carbon and sulfur isotopic data are complemented with oxygen isotopic data for barite. Barites are homogeneous in δ¹⁸O_V-SMOW and δ³⁴S_CDT with values of + 15.4 and + 23.5‰, respectively. Their ⁸⁷Sr/⁸⁶Sr ratios vary between 0.7128 and 0.7113 for the first generation and between 0.7117 and 0.7123 for younger remobilization. The dolomitic host rock shows a significant variation in Sr, O and C isotopic composition between non-mineralized and mineralized zones: ⁸⁷Sr/⁸⁶Sr ratios vary between 0.7076 and 0.7133, δ¹⁸O_V-SMOW-values between +28.11 and +20.65‰, and δ¹³C_PDB-values between −1.15 and + 3.06‰. Fluid/rock volume ratios on the order of 1.3–3.2 are calculated for open-system behaviour by modelling Sr, O and C isotopic shifting capacities. The isotope data combined with other geological evidence support the following genetic model: Subsequent to synsedimentary sulfide mineralization during an Early Devonian rifting stage, collision tectonics in Carboniferous time led to the expulsion of Ba- and Sr-rich orogenic brines, which evolved from metamorphic fluids consisting essentially of H₂O and some CH₄, into an external sedimentary fold-and-thrust belt. The brines remobilized the synsedimentary sulfides, mixed with meteoric waters in the platform carbonates, reacted with evaporitic horizons and finally caused the recrystallization of dolomite and the precipitation of Sr-rich barite in structurally weak zones at 70–130°C. During the later Alpine orogeny supergene oxidation products were formed, and sulfates, sulfides and carbonates were further remobilized into late faults and fractures.     

A study of the chemistry of pore fluids and authigenic carbonates in methane seep environments: Kodiak Trench, Hydrate Ridge, Monterey Bay, and Eel River Basin

Analyses of the chemical and isotopic composition of carbonates rocks recovered from methane seepage areas of the Kodiak Trench, Hydrate Ridge, Monterey Bay Clam Flats, and the Eel River Basin, coupled with the studies of the chemistry of the pore fluids, have shown that these carbonates have grown within the sediment column. Geochemical profiles of pore fluids show that, in deep water seeps (Kodiak Trench—4450 m; Monterey Bay—1000 m; Hydrate Ridge—650 m), δ¹³C (DIC) values are low (isotopically light), whereas in the Eel River area ( 350–500 m), δ¹³C (DIC) values are much higher (isotopically heavier). In all cases, the δ¹³C values indicate that processes of methane oxidation, associated with sulfate reduction, are dominant in the shallow sediments. Data on the isotopic composition of authigenic carbonates found at sites in Kodiak Trench, Eel River Basin South, and Eel River Basin North indicate a variable composition and origin in different geochemical environments. Some of the authigenic carbonates from the study sites show a trend in their δ¹³C values similar to those of the pore fluids obtained in their vicinity, suggesting formation at relatively shallow depths, but others indicate formation at greater sediment depths. The latter usually consist of high magnesium calcite or dolomite, which, from their high values of δ¹³C (up to 23‰;) and δ¹⁸O (up to 7.5‰), suggest formation in the deeper horizons of the sediments, in the zone of methanogenesis. These observations are in agreement with observations by other workers at Hydrate Ridge, in Monterey Bay, and in the Eel River Basin.     

Lithium isotopic composition of Central American Volcanic Arc lavas: implications for modification of subarc mantle by slab-derived fluids

Li contents and isotopic compositions were determined for a suite of well-characterized basaltic lavas from the Central American Volcanic Arc (CAVA). Variable Li/Y (0.2–0.5), Li/Sc (0.1–0.4), and δ⁶Li values (+2.6 to −7.7‰) attest to significant compositional heterogeneity in the subarc mantle. Within specific arc segments, these parameters correlate strongly with each other and with a number of other constituents (e.g., K, Rb, Ba, B/La, ¹⁰Be/⁹Be, ⁸⁷Sr/⁸⁶Sr, U/Ce, and ²³⁰Th/²³²Th, among others); these correlations are particularly strong for Nicaragua samples. Coupling of this particular set of constituents is best explained in terms of addition of ‘subduction components' to the subarc mantle. Moreover, their selective enrichment with respect to relatively fluid-immobile incompatible elements signifies the dominance of fluid vs. silicate melt transport of slab components to the subarc mantle. Several interesting nuances are revealed by the Li data. First, although Li and B are strongly correlated in both Costa Rica and Nicaragua, there are systematic along-strike variations in Li/B that are consistent with these elements having different ‘fluid release patterns' from subducted slab segments. For example, Li/B is highest in Costa Rica where auxiliary evidence indicates higher subduction zone temperatures; apparently B is preferentially depleted and Li retained in the slab under warmer conditions. The same relations are reflected in Li/¹⁰Be and other subduction tracer systematics, all of which point to larger subduction contributions below Nicaragua. Yet, even Nicaragua lavas vary widely in levels of subduction enrichment. High-Ti basalts from Nejapa are the least enriched and have the highest δ⁶Li (1.4 to 2.6‰); these values are greater than in fresh MORB (ca. −4‰) and are not easily explained by additions of subducted Li because most oceanic crustal rocks and marine sediments have lower δ⁶Li than MORB (with typical values between −8 and −20‰). Thus, it appears the Nejapa data may be representative of isotopically light mantle domains. Relatively light δ⁶Li values in an undepleted spinel lherzolite (+11.3‰) from Zabargad Is. (Red Sea) and in primitive backarc basalts (−1.6 to −0.5‰) from Lau Basin support this conclusion. Considering representative fluid and mantle endmember compositions, the CAVA data are consistent with limited (up to a few percent) additions of slab-derived fluids to a heterogeneous mantle containing variably depleted and enriched domains to form the respective magma sources. In our view, the subarc mantle is heterogeneous on a small scale, but some arc sectors clearly received greater slab inputs than others.     

The origin of basic rocks of the Korosten AMCG complex, Ukrainian shield: Implication of Nd and Sr isotope data

The Korosten complex is a Paleoproterozoic gabbro–anorthosite–rapakivi granite intrusion which was emplaced over a protracted time interval — 1800–1737 Ma. The complex occupies an area of about 12 000 km² in the north-western region of the Ukrainian shield. About 18% of this area is occupied by various mafic rocks (gabbro, leucogabbro, anorthosite) that comprise five rock suites: early anorthositic A₁ (1800–1780 Ma), main anorthositic A₂ (1760 Ma), early gabbroic G₃ (between 1760 and 1758 Ma), late gabbroic G₄ (1758 Ma), and a suite of dykes D₅ (before 1737 Ma). In order to examine the relationships between the various intrusions and to assess possible magmatic sources, Nd and Sr isotopic composition in mafic whole-rock samples were measured. New Sr and Nd isotope measurements combined with literature data for the mafic rocks of the Korosten complex are consistent and enable construction of Rb–Sr and Sm–Nd isochronous regressions that yield the following ages: 1870 ± 310 Ma (Rb–Sr) and 1721 ± 90 Ma (Sm–Nd). These ages are in agreement with those obtained by the U–Pb method on zircons and indicate that both Rb–Sr and Sm–Nd systems have remained closed since the time of crystallisation. In detail, however, measurable differences in isotopic composition of the Korosten mafic rock depending on their suite affiliation were revealed. The oldest, A₁ rocks have lower Sr (⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ = 0.70233–0.70288) and higher Nd (εNd₍₁₇₆₀₎ = 1.6–0.9) isotopic composition. The most widespread A₂ anorthosite and leucogabbro display higher Sr and lower Nd isotopic composition: ⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ = 0.70362, εNd₍₁₇₆₀₎ varies from 0.2 to − 0.7. The G₃ gabbro–norite has slightly lower εNd₍₁₇₆₀₎ varying from − 0.7 to − 0.9. Finally, G₄ gabbroic rocks show relatively high initial ⁸⁷Sr/⁸⁶Sr (0.70334–0.70336) and the lowest Nd isotopic composition (εNd₍₁₇₆₀₎ varies from − 0.8 to − 1.4) of any of the mafic rocks of the Korosten complex studied to date. On the basis of Sr and Nd isotopic composition we conclude that Korosten initial melts may have inherited their Nd and Sr isotopic characteristics from the lower crust created during the 2.05–1.95 Ga Osnitsk orogeny and 2.0 Ga continental flood basalt event. Indeed, εNd₍₁₇₆₀₎ values in Osnitsk rocks vary from 0.0 to − 1.9 and from 0.2 to 3.4 in flood basalts. We suggest that these rocks being drawn into the upper mantle might melt and give rise to the Korosten initial melts. ⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ values also support this interpretation. We suggest that the Sr and Nd isotopic data currently available on mafic rocks of the Korosten complex are consistent with an origin of its primary melts by partial melting of lower crustal material due to downthrusting of the lower crust into upper mantle forced by Paleoproterozoic amalgamation of Sarmatia and Fennoscandia.     

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.     

Fluid evolution in the W–Cu–Zn–Pb San Cristobal vein, Peru: fluid inclusion and stable isotope evidence

The Zn–Pb±Ag±Cu San Cristobal district is located 100 km east of Lima in the western cordillera of Peru. It is centred around the Chumpe intrusion and is composed of vein and carbonate replacement ore types. The main San Cristobal vein presents a paragenesis that can be divided into three stages: (a) an early wolframite–quartz–pyrite stage, (b) a quartz–base metal stage, and (c) a late quartz–carbonate–barite stage.

Fluid inclusions in quartz from the tungsten stage are biphase (LV) at room temperature and homogenise to the liquid phase between 146 and 257 °C. Their salinities range between 2.1 and 5.1 wt.% NaCl equiv. Rare inclusions contain an additional crystal of halite and have salinities of 46–54 wt.% NaCl equiv. Data of the first two stages show a decrease in homogenisation temperatures concomitant with a salinity decline. Fluid inclusions in quartz from the late stage homogenise at higher temperatures, between 252 and 323 °C, with salinities ranging between 4.6 and 6.7 wt.% NaCl equiv.

Hydrogen and oxygen isotope data indicate a two-stage evolution. Isotopic compositions of the fluid associated with the first two stages define a trend with constant δ¹⁸O values and decreasing δD values (δ¹⁸O=3.2‰ to 5.0‰ V-SMOW and δD=−60‰ to −112‰ V-SMOW), which is interpreted as mixing of a dominantly magmatic component with minor meteoric water that had equilibrated with the host rocks. This interpretation is supported by sulphur and lead isotopic data from previous studies. By contrast, the quartz–carbonate–barite stage bears isotopic characteristics defining a trend with a coupled decrease of δ¹⁸O and δD (δ¹⁸O=−8.1‰ to 2.5‰ V-SMOW and δD=−57‰ to −91‰ V-SMOW) and is explained by addition of meteoric water to the system and subsequent mixing with a less important magmatic component.

Different fluid origins are confirmed by laser ablation ICP-MS analyses of the triphase (LVH) and biphase (LV) primary inclusions. The concentrations of the major ore elements, i.e., W, Cu, Zn and Pb, decrease throughout the paragenesis; W, and to a lesser extent Cu, show significant variations, associated with a steep decrease in their concentration. The decreasing concentrations can be explained by mineral deposition and dilution by the meteoric fluid; differences in the rate of decrease indicate selective precipitation of W. Fluid inclusions of the quartz–carbonate stages show an abrupt increase in Ba and Sr concentrations. This is interpreted to reflect a higher volume of host rock silicate alteration, probably due to the increasing size of the fluid flow cell and is explained by the input of a third fluid of unknown origin. LA-ICP-MS analyses show that the fluids were already depleted in W and Cu before reaching the emplacement of carbonate replacement ore type, whereas Zn and Pb were still present in considerable amounts. This is again due to selective precipitation and is consistent with the interpretation that the economically interesting metals were dominantly introduced by magmatic fluids.     

Mineralogical and chemical transformation of Oldoinyo Lengai natrocarbonatites, Tanzania

The minerals of Oldoinyo Lengai natrocarbonatite lavas are unstable under atmospheric conditions. Subsolidus mineral assemblages in natrocarbonatites were studied in 105 samples from contemporary eruptions ranging from present day to about 100 years old. The subsolidus minerals in natrocarbonatites were formed (i) along cracks on the lava surface from hot gases escaping during cooling, (ii) as atmospheric alteration by solution of water-soluble minerals, in particular halides and gregoryite, and by hydration of nyerereite under the influence of meteoric water and (iii) by reaction with fumarole gases. After solidification, the lavas were cut by a network of thin cracks, the edges of which are covered by polymineralic encrustations. Samples collected 2–24 h after eruption contain nahcolite, trona, sylvite, and halite with accessory kalicinite and villiaumite. Atmospheric humidity results immediately (≥ 2 h after eruption) in alteration of black lavas that is marked by the appearance of white powdery thermonatrite with nahcolite on the lava surface. Subsequent reaction (weeks, months, years) of natrocarbonatite with meteoric water and the atmosphere results in the formation of pirssonite, gaylussite, shortite, trona, thermonatrite, nahcolite and calcite. Generally, the first important step is the formation of pirssonite and the end-members are calcite carbonate rocks or loose aggregates. Fumarolic activity is common for the active northern crater of the volcano. Reaction of hot (54–141 °C) fumarolic gases with natrocarbonatite leads to the formation of sulphur, gypsum, calcite, anhydrite, monohydrocalcite, barite and celestine. Changes in mineralogy of the natrocarbonatite lead to substantial chemical transformation. The most obvious chemical changes in this process are the loss of Na, K, Cl and S, combined with an increase in H₂O, Ca, Sr, Ba, F and Mn. The oxygen and carbon isotopic composition of altered natrocarbonatites shows a significant shift from the primary “Lengai Box” to high values of δ¹⁸O and δ¹³C. Calcite exhibits δ¹³C values between − 2‰ and − 4‰ PDB and δ¹⁸O values of + 23‰ to + 26‰ SMOW. The observed assemblages of secondary minerals formed by reaction with atmosphere and meteoric water, the changes in chemical composition of the natrocarbonatite and field observations suggest that alteration of natrocarbonatite is an open-system low-temperature process. It takes place at temperatures between 8 and 43 °C with the addition of H₂O to the system and the removal of Na, K, Cl and S from the carbonatites. Low-temperature thermodynamic models developed for alkali carbonate systems can be used for the interpretation of Oldoinyo Lengai subsolidus mineralization.     

Geochemical constraints on the origin of bimodal magmatism at the Okinawa Trough, an incipient back-arc basin

The magmatism at the axial zone of the middle Okinawa Trough, a young continental back-arc basin, comprises a bimodal basaltic–rhyolitic suite, accompanied by minor intermediate rocks. We report major and trace element and Sr–Nd isotopic data for the intermediate to silicic suites, to provide constraints on their petrogenesis. The rhyolites, recovered as lava and pumice, fall into three geochemical groups (type 1, 2, and 3 rhyolites). Type 1 rhyolites have ⁸⁷Sr/⁸⁶Sr (0.7040–0.7042) and ¹⁴³Nd/¹⁴⁴Nd (0.5128–0.5129) identical to those of associated basalts, and are characterized by highly fractionated REE patterns. Petrogenesis of type 1 rhyolites is explicable in terms of fractional crystallization of the associated basalt. In contrast, type 2 rhyolites and andesite have slightly higher ⁸⁷Sr/⁸⁶Sr (0.7044–0.7047) but similar ¹⁴³Nd/¹⁴⁴Nd (0.5128) compared to those of the basalts. The compositions of type 2 rhyolite and andesite can be explained by assimilation and fractional crystallization (AFC) processes of the basalt magma; quantitative analysis suggests assimilation/fractional crystallization (M_a/M_c) ratios of ≤0.05. Hybrid andesite generated by mixing of evolved basalt and type 1 rhyolite is also present. We emphasize that mechanical extension in this part of the Okinawa Trough involves gabbroic lower crust that resulted from fractionation of mantle-derived basaltic magmas. Type 3 rhyolite occurs only as pumice, which makes its derivation questionable. This rhyolite has major and trace element compositions and Sr–Nd isotopic ratios, which suggests that it may be derived from volcanic activity on the southern Ryukyu volcanic front, and arrived in the Okinawa Trough by drifting on the Kuroshio Current.     

Oxygen and carbon isotope composition of cold-water Berriedale Limestone (Lower Permian), Tasmania, Australia

The Berriedale Limestone formed at about 80°S paleolatitude and contains many glacial dropstones. It formed during a period of major Gondwana deglaciation.

The Berriedale Limestone contains mostly bryozoans, brachiopods and bivalves, with some intraclasts and rare pellets. The faunal diversity is low and the fauna are similar to the modern cold-water foramol faunal assemblage. Micrite, microspar and spar occur as equant to well developed rhombs of calcite. The coarse spar cements are bored and are ruptured by dropstones, indicating submarine origin of low-Mg calcite at water-temperatures of around 3°C. The mixing zone cementation was preceded by erosion of early formed crystals. The eroded crystals occur as inclusions in mixing zone cements.

The fauna are characterized by heavy δ¹³C and light δ¹⁸O. The whole-rock field of δ¹⁸O-δ¹³C falls at the edge of “Normal Marine Limestone” and deviates to lighter δ¹⁸O values (down to −16.7‰ PDB). Lightest δ¹⁸O values ( −22‰ PDB) of fresh-water sparry calcite cement are similar to those in the Early Permian continental tillites, suggesting that the Permian sea was diluted by isotopically light melt waters. Micrite δ¹⁸O values (−9.2 to −12.6‰ PDB) are within the range of whole-rock values. The δ¹⁸O values of calcite in shales are lighter than limestone values.

The δ¹⁸O values of the fauna give an unrealistic range of sea-water temperatures because the fauna have equilibrated with variable amounts of melt waters. However, calculated original δ¹⁸O values of the fauna indicate temperatures < 4°C. The heaviest δ¹⁸O of fauna gives cold temperatures of 9°C (with δ_w −2.8‰) and −3°C (with δ_w −6‰). The lightest values of sparry calcite cements (−22‰ PDB) indicate that the limestone reacted with cold melt waters.

The δ¹⁸O of Permian sea is estimated to be about +1.2‰ and was diluted by melt waters as light as −27‰ SMOW.     

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.     

Neo-Proterozoic rift-related syenites (Northern Damara Belt, Namibia): Geochemical and Nd–Sr–Pb–O isotope constraints for mantle sources and petrogenesis

Major element, trace element and Nd–Sr–Pb–O isotope data for a suite of Neo-Proterozic, pre-orogenic, rift-related syenites from the Northern Damara orogen (Namibia) constrain their sources and petrogenesis. New U–Pb ages obtained on euhdreal titanite of inferred magmatic origin constrain the age of intrusion of the Lofdal and Oas syenites to ca. 750 Ma compatible with previous high-precision zircon analyses from the Oas complex. Major rock types from Lofdal and Oas are mildly sodic nepheline-normative and quartz-normative syenites and were primarily generated by fractional crystallization from a mantle-derived alkaline magma. Primitive samples from Lofdal and Oas show depletion of Rb, K and Th relative to Ba and Nb together with variable negative anomalies of P and Ti on a primitive mantle-normalized diagram. Evolved samples from Oas develop significant negative Ba, Sr, P and Ti anomalies and positive U and Th anomalies mainly as a function of crystal fractionation processes. The lack of a pronounced negative Nb anomaly in samples from Lofdal suggests that involvement of a crustal component is negligible. For the nepheline-normative samples from Lofdal, the unradiogenic Sr and radiogenic Nd isotope composition and low δ¹⁸O values suggest derivation of these samples from a moderately depleted lithospheric upper mantle with crustal-like U/Pb ratios (⁸⁷Sr/⁸⁶Sr: 0.7031–0.7035, ε Nd: ca. + 1, δ¹⁸O: 7‰, ²⁰⁶Pb/²⁰⁴Pb: ca.18.00, ²⁰⁷Pb/²⁰⁴Pb: 15.58–15.60). Primitive samples of the Oas quartz-normative syenites have identical isotope characteristics (⁸⁷Sr/⁸⁶Sr: 0.7034, ε Nd: ca. + 1, δ¹⁸O: 6.5‰, ²⁰⁶Pb/²⁰⁴Pb: ca.18.00, ²⁰⁷Pb/²⁰⁴Pb: 15.59) whereas more differentiated samples have higher ⁸⁷Sr/⁸⁶Sr ratios (0.709–0.714), slightly higher δ¹⁸O values (7.0–7.1‰), less radiogenic ε Nd values (− 1.1 to − 1.4) and more radiogenic ²⁰⁶Pb/²⁰⁴Pb ratios up to 18.27. These features together with model calculations using Sr–Nd–Pb isotopes suggest modification of a primary syenite magma by combined AFC processes involving ancient continental crust. In this case, high Nb abundances of the parental syenite liquid prevent the development of significant negative Nb anomalies that may be expected due to interaction with continental crust.     

Isotopic archives of sulphate in speleothems

The hemispheric impact of industrial emissions upon atmospheric sulphur loading is reflected in the sulphur depositional history recorded in cores from ice sheets. However, these do not reveal regional variations. Recently deposited speleothems are used here as archives of regional sulphur depositional history at two locations within the United Kingdom and Ireland. δ³⁴S-so₄ and δ¹⁸O-so₄ present within speleothem carbonate are measured for the first time as part of a dual isotope approach to decode the speleothem sulphur record. The largely refractory nature of δ³⁴S-so₄ under oxidising conditions enables source provenance of atmospheric SO₂, whereas the complex cycles of isotopic exchange and fractionation during incorporation of oxygen into sulphate molecules enable δ¹⁸O-so₄ signatures to yield insights into ambient environmental conditions and biogeochemical cycling in the ecosystem above the cave. δ³⁴S-so₄ values extracted from speleothem carbonate formed within Browns Folly Mine, UK, range from +3.5 to +5.5‰ and δ¹⁸O-so₄ +10.3 to +13.7‰. Both signatures lie within the range expected from sulphate deposition in industrial locations and reflect the transfer of sulphate into speleothem calcite with little fractionation. However, δ¹⁸O-so₄ signatures at Crag Cave, western Ireland, are isotopically heavier than expected and approach isotopic equilibrium with δ¹⁸O-h₂o under reducing conditions. Dual isotope analysis of δ³⁴S-so₄ and δ¹⁸O-so₄ optimises the correct identification of sulphur sources and biogeochemical cycling prior to incorporation into the speleothem record. At carefully selected cave sites where drip water flowpaths into the cave remain oxic, speleothems hold the potential to retain records of atmospheric sulphur loading at the local and regional scale.     

Oxygen, carbon, and strontium isotope geochemistry of diamond-bearing carbonate rocks from Kumdy-Kol, Kokchetav Massif, Kazakhstan

Diamond-bearing carbonate rocks from Kumdy-Kol, Kokchetav massif, Kazakhstan, were strongly altered by fluids flowing through fractures and infiltrating along grain boundaries during exhumation. Alteration includes retrogradation of high-grade silicate assemblages by hydrous minerals, replacement of diamond by graphite and of dolomite by calcite. Diamond-bearing carbonate rocks are among the most intensely altered isotopically with δ¹⁸O_VSMOW values as low as +9‰, δ¹³C_VPDB=−9‰, and ⁸⁷Sr/⁸⁶Sr as high as 0.8050. Evidence of isotopic equilibration between coexisting dolomite and high-Mg calcite during ultrahigh-pressure metamorphism (UHPM) is preserved only rarely in samples isolated from infiltrating fluids by distance from fractures. Isotopic heterogeneity and isotopic disequilibrium are widespread on a hand-specimen scale. Because of this lack of homogeneity, bulk analyses cannot provide definitive measurements of ¹³C/¹²C fractionation between coexisting diamond and carbonate. Our study adequately documents alteration on a scale commensurate with observed vein structures. But, testing the hypothesis of metamorphic origin of microdiamonds has not fully succeeded because our analytical spatial resolution, limited to 0.5 mm, is not small enough to measure individual dolomite inclusions or individual diamond crystals.