Melilitite-carbonatite tuffs in the Laetolil Beds of Tanzania

The upper unit of the Laetolil Beds, 45 to 60 m thick, is about 80% wind worked or eolian tuff and 20% air-fall tuff. The air-fall tuffs comprise a phonolitic tuff and numerous thin tuffs of original melilitite-carbonatite composition. Most of the melilitite-carbonatite tuffs consist largely of sand-sized lava globules and crystals cemented by calcite. Evidence of former carbonatite ash is provided by calcite globules, fenestral textures, and high contents of Ba and Sr in early-deposited calcite. These air-fall tuffs were produced by volatile-rich eruptions of highly fluid magma. In a typical eruptive cycle, lava droplets were followed by crystals which increased in size during the eruption. Commonly the final event was an eruption of fine ash and carbonatite globules. Particularly violent explosions ejected blocks of lava and plutonic rock 10 to 15 cm in diameter for distances of 20 km.The climate was semiarid, and melilitite-carbonatite ash layers were first cemented by soluble salts such as trona resulting from incongruent solution of the carbonatite ash by rainfall. Repeated solution and crystallization of salts resulted in a polygonal fracture pattern in the thinner tuffs. Ash layers not cemented by soluble salts were eroded and redeposited by wind to form eolian tuffs. Subsequently both the air-fall and eolian tuffs were modified by several diagenetic stages, mostly in the vadose zone, to form rocks consisting principally of montmorillonite, phillipsite, and calcite. At an early stage calcium carbonate derived from carbonatite ash was precipitated as micrite both as a cement and replacement of organic matter. Glass, nepheline, and melilite were now weathered to clay, releasing components to form phillipsite. Calcite spar was precipitated last, as a replacement, cement, and pore filling. Unaltered glass, preserved in some of the eolian tuffs, has an unusually high content of Na, K, and Fe for a melilitite composition.These beds contain a rich fauna, notable for the excellent preservation of delicate fossils such as bovid dung, land snails, and bird eggs. This preservation is attributed, at least in part, to carbonatite ash. Carbonatite ash was also responsible for the preservation of footprints in one of the tuffs.     

Petrology and genesis of natrocarbonatite

Microprobe analyses of phenocrysts and groundmass, and crystal-size distributions of phenocrysts of pahoehoe natrocarbonatite lavas of the 1963 eruption of Oldoinyo Lengai have been determined. Nyerereite phenocrysts are homogeneous, with average composition Nc₄₁Kc₉Cc₅₀ (neglecting F, Cl, P₂O₅, and SO₃) where Nc=Na₂CO₃, Kc=K₂CO₃, and Cc= (Ca,Sr)CO₃. Gregoryite phenocrysts have turbid, pale brown, oscillatorily zoned cores (average composition Nc₇₇Kc₅Cc₁₈) with 0–30% oriented inclusions of exsolved nyerereite. Overgrowths on gregoryites (30 m wide) are relatively sodic (Nc₈₁Kc₄Cc₁₅) and are free of inclusions. Cores and rims are rich in SO₃ (4%) and P₂O₅ (2%). Blebs of pyrite-alabandite mixtures (100 m) occur in the groundmass. The groundmass has the simplified composition Nc₆₅Kc₁₅Cc₂₀, less calcic than the composition of the 1-kbar nyerereite+gregoryite +liquid cotectic in the ternary system Nc-Kc-Cc. Groundmass quench growth of alkali halides + carbonate was followed by slower growth of coarse-grained and irregular gregoryite +KCl+BaCO₃. Crystal size distributions of gregoryite and nyerereite in one sample are linear, implying little loss or gain of phenocrysts by crystal settling. AverageG is 0.15 mm, compared toG=0.03 mm for combeite phenocrysts from consanguineous nephelinite. Assuming an equal residence time () for both lavas, the apparent crystal growth rate (G) in carbonate melt is 5 times greater than in peralkaline undersaturated silicate melt. Data from experiments with natrocarbonatite and related synthetic systems indicate that Na–K–Ca carbonatite magmas which crystallize calcite cannot fractionate to nyerereite+gregoryite +liquid assemblages. Natrocarbonatites plot in the liquidus field of nyerereite, and minor fractionation of nyerereite to produce the erupted lavas is indicated. The term natrocarbonatite has been inappropriately applied to other eruptive rocks with calcite phenocrysts, and the only known occurrence of gregoryite-bearing natrocarbonatite is Oldoinyo Lengai. Natrocarbonatite probably originates by liquid immiscibility from strongly peralkaline nephelinites, which have also been erupted at Oldoinyo Lengai.     

Carbon and oxygen isotopic covariations in hydrothermal calcites

Isotopic covariations of carbon and oxygen in hydrothermal calcites are quantitatively modeled in terms of the following three mixing processes: (1) mixing between two different fluids which leads to the precipitation of calcite; (2) mixing between fluid and rock: (a) calcite precipitation due to fluid/rock interaction, (b) secondary alteration of primary calcite by interaction with a subsequent fluid. The models are derived from mass balance equations. A distinction among the three mixing processes can be made on a ¹³C vs ¹⁸O diagram, which places important constraints on the genesis of hydrothermal mineralization. The variables which control the ultimate isotopic composition of hydrothermal calcites include the composition of the initial fluid and the wallrock, temperature, and dissolved carbon species. Owing to significant temperature-dependent fractionation effects during equilibrium precipitation of calcite from a hydrothermal fluid, the mixing processes may be distinguished by telltale patterns of isotopic data in ¹³C vs ¹⁸O space. In particular, caution must be exercised in postulating the fluid mixing as the cause for mineral deposition. This is demonstrated for hydrothermal Pb-Zn deposits in the western Harz Mountains, Germany. A positive correlation between ¹³C and ¹⁸O values is observed for calcites from the Bad Grund deposit in the Upper Harz. Two sample profiles through calcite veins show similar correlations with the lowest -values at the center of the veins and the highest -values at the vein margins. Because the correlation array has a greater slope than for calcite precipitation at equilibrium in a closed system and because fluid mixing may not proceed perpendicular to the vein strike, it is assumed that a fluid/rock interaction is responsible for the observed correlation and thus for the precipitation of calcite. A deep-seated fluid is inferred with a ¹³C value of — 7% and a ¹⁸O value of +10%., as well as H₂CO₃ as the dominant dissolved carbon species; precipitation temperatures of the calcites are estimated to be about 280 170°C. Quite different isotopic distributions are observed for calcites from the St. Andreasberg deposit in the Middle Harz. An alteration model is suggested based mainly on the isotopic distribution through a calcite vein. In addition to a primary fluid which has the same isotopic composition as that in the Bad Grund deposit and thus seems to be responsible for the precipitation of calcite associated with sulfides, an evolved, HCO ₃ ^- -dominant subsurface fluid with ¹³C about -20 — 15% and ¹⁸O 0% is deduced to alter the primary calcite at low temperatures of 70 40°C.     

Carbon-oxygen isotopic covariation in hydrothermal calcite during degassing of CO2

The effect of the outgassing of CO₂ from a hydrothermal fluid on the C- and O-isotopic compositions of calcite, which is precipitated from this fluid, is quantitatively modelled in terms of batch and Rayleigh distillation equations. Both CO₂ degassing and calcite precipitation are considered to be the removal mechanisms for dissolved carbon species from the fluid. Combined degassing-precipitation models are then developed by taking H₂CO₃ and HCO ₃ ^– , respectively, as the dominant dissolved carbon species. A positive correlation array between ¹³C and ¹³O values of calcite can be yielded by the precipitation of calcite from a H₂CO ₃ ^– -dominant fluid, accompanied by a progressive decrease in temperature during CO₂ degassing, whereas calcite precipitated from a HCO ₃ ^– -dominant fluid under the same conditions tends to display much smaller variation in ¹³C values than in ¹⁸O values. The combined processes of CO₂ degassing and calcite precipitation result in lowering the ¹³C value of calcites with respect to those precipitated in a closed system simply due to temperature effect. Carbon and oxygen isotopic data for calcite from the Kushikino gold-mining area in Japan illustrate the application of quantitative modelling, and degassing of CO₂ is suggested as a more likely cause for the precipitation of the calcite and quartz in this mining area.     

Carbon isotopic study on coexisting calcite and graphite in the Ryoke metamorphic rocks,northern Kiso district,central Japan

Carbon isotope fractionation between coexisting calcite and grpahite ( ¹³C_cc-gr) has been determined in metamorphosed limestones and calc-silicate rocks from the Ryoke metamorphic belt in the northern Kiso district. In this district, the Ryoke metamorphic rocks, ranging from the lower greenschist facies to the upper amphibolite facies, are widely distributed. The fractionation of ¹³C/¹²C between calcite and graphite decreases regularly with increasing metamorphic grade and is independent of absolute ¹³C values of calcite. This evidence suggests that carbon isotopic exchange equilibrium has been attained during metamorphism even in the greenschist facies and isotopic modification, possibly caused by retrogressive metamorphism, is not distinguished. For T=270–650° C, the fractionation is expressed by the following equation: ¹³C_cc-gr=8.9×10⁶T^–2–7.1 (T in °K).This equation has a slope steeper than the current results on the ¹³C_cc-gr versus 10⁶T^–2 diagram. It can be used as a potential geothermometer for almost the entire temperature range of metamorphism. ¹³C values of carbonaceous matter in unmetamorphosed limestones in this district are approximately –22, due to its biogenic origin. Graphite from metamorphosed limestones is also considered to be of biogenic origin but shows enrichment of ¹³C due to isotopic exchange with calcite. ¹³C values of graphite as well as ¹³C_cc-gr confirm that zone II represents the lowest grade zone of Ryoke metamorphism. The maximum equilibrium fractionation of ¹³C between calcite and graphite is considered to be approximately 23%, which corresponds to 270° C. Below this temperature, it seems that carbon isotopic exchange between the minerals does not occur.Calcite in marble from the higher grade zones has relatively lower ¹³C and ¹⁸O values. The depletion of heavy isotopes is considered to be caused by the loss of ¹³C and ¹⁸O enriched carbon dioxide during decarbonation reactions. For oxygen, it is considered that isotopic exchange with metamorphic fluids plays an important role in lowering the ¹⁸O value of calcite in some higher grade marbles.     

Phlogopite-biotite parageneses from the K-mafic-carbonatite effusive magmatic association of Katwe-Kikorongo, SW Uganda

Summary Ti-bearing phlogopite-biotite is dominant in Ugandan kamafugite-carbonatite effusives and their entrained alkali clinopyroxenite xenoliths. It occurs as xeno/phenocrysts, microphenocrysts and groundmass minerals and also as a major xenolith mineral. Xenocrystic micas in kamafugites and carbonatites are aluminous (> 12 wt% Al₂O₃), typically contain significant levels of Cr (up to 1.1 wt% Cr₂O₃), and are Ba-poor. Microphenocryst and groundmass micas in feldspathoidal rocks extend to Al-poor compositions, are depleted in Cr, and are generally enriched in Ba. In general, xenocrystic micas occupy the Al₂O₃ and TiO₂ compositional field of the xenolith mica, and on the basis of Mg#, and high P, T experimental evidence they probably crystallised at mantle pressures. Mica xenocryst Cr contents range from those in Cr-poor megacryst and MARID phlogopite to higher values found in primary and metasomatic phlogopites in kimberlite-hosted peridotite xenoliths. Such Cr contents in Ugandan mica xenocrysts are considered consistent with derivation from carbonate-bearing phlogopite wehrlite and phlogopite-clinopyroxenite mantle. Olivine melilitite xenocryst micas are distinguished by higher Mg# and Cr content than mica in clinopyroxenite xenoliths and mica in Katwe-Kikorongo mixed melilitite-carbonatite tephra. Higher Al₂O₃ distinguishes Fort Portal carbonatite xenocrysts and some contain high Cr. It is suggested that the genesis of Katwe-Kikorongo olivine melilitite and Fort Portal carbonatite involves a carbonate-bearing phlogopite wehrlite source while the source of the mixed carbonatite-melilitite rocks may be carbonate-bearing phlogopite clinopyroxenite. Received January 24, 2000; revised version accepted September 27, 2001     

18O/16O ratios in ash-flow tuffs and lavas erupted from the central Nevada caldera complex and the central San Juan caldera complex,Colorado

Oxygen isotope compositions were measured on 129 quartz, feldspar, and biotite phenocrysts from ash-flow tuffs and lava domes erupted from the Oligocene central Nevada and central San Juan caldera complexes. Most of the ash-flow tuffs are compositionally zoned with low-phenocryst rhyolite bases and high-phenocryst quartz-latite tops, but both within individual units and throughout each of the eruptive sequences at each locality, the ¹⁸O values are remarkably constant. ¹⁸O values of the central Nevada magmas range from +9.1 to +9.8 per mil: These values are high and indicate the involvement of high-¹⁸O geosynclinal sediments in the melting process. Magmatic ¹⁸O values decrease by only about 0.4 per mil from the initial eruption sequence to the middle eruptive, the giant Monotony Tuff (3000 km³). The initial higher ¹⁸O values are reestablished in the late eruptive sequence, but decrease again by about 0.4 per mil in the latest ring-fracture eruptions. ¹⁸O values in the central San Juan magmas range from +6.8 to +7.5: These values are relatively low and indicate involvement of lower cratonal crust and upper mantle in the melting process. Magmatic ¹⁸O values decrease by about 0.4 per mil from the early sequence (Fish Canyon, Carpenter Ridge, and Mammoth Mountain Tuffs) to the late sequence (Wason Park, Nelson Mountain, and Snowshoe Mountain Tuffs). ¹⁸O/¹⁶O fractionations among phenocrysts in both Nevada and Colorado are much smaller than among corresponding minerals in plutonic granitic rocks. These fractionations also decrease from stratigraphically lower to higher samples in each cooling unit, so the ¹⁸O/¹⁶O data agree with other evidence that these represent quenched equilibrium at magmatic temperatures, and that prior to eruption the tops of the magma chambers were cooler than the deeper portions. In striking contrast to what is observed in Iceland and in the late-Tertiary to Quaternary southwest Nevada and Yellowstone caldera complexes, we have found no evidence for any low-¹⁸O rhyolitic magmas. Thus, low-¹⁸O rhyolitic magmas must be less common than heretofor believed, and their origin must be a result of special circumstances involving the timing, depth, and intensity of meteoric-hydrothermal activity. We tentatively suggest that extensional tectonics and regional rifting may be one of the prerequisites for their development.Contribution No. 4106, Publications of the Division of Geological and Planetary Sciences, California Institute of Technology     

High-Ca,low-alkali carbonatite volcanism at Fort Portal,Uganda

A Quaternary volcanic field at Fort Portal, SW Uganda, contains approximately 50 vents that erupted only carbonatite. The vents are marked by monogenetic tuff cones defining two ENE-trending belts. Lava from a fissure at the west end of the northern belt formed a flow 0.3 km² in area and 1–5 m thick. The lava is vesicular throughout with a scoriaceous top, and probably formed by agglutination of spatter from lava fountains. Phenocrysts are olivine, clinopyroxene, phlogopite, and titanomagnetite enclosing blebs of pyrrhotite. Rims of monticellite, gehlenite, and reinhardbraunsite surround olivine, clinopyroxene and phlogopite, and magnetite is rimmed by spinel. The reaction relations suggest that these phenocryst phases are actually xenocrysts, perhaps from a source similar to that which supplied phlogopite clinopyroxenite xenoliths in the Katwe-Kikorongo volcanic field 75 km SW of Fort Portal. The groundmass of fresh carbonatite lava consists of tabular calcite, spurrite, periclase, hydroxylapatite, perovskite, spinel, pyrrhotite, and barite. The lava was readily altered; where meteoric water had access, spurrite and periclase are lacking, and some calcite is recrystallized. Vesicles in lava and rare dike rocks are partly filled with calcite, followed by jennite and thaumasite. Pyroclastic deposits cover 142 km² and are far more voluminous than lava. Carbonatite ejecta were identical to lava in primary mineralogy, but are much more contaminated by crustal rock fragments and xenocrysts. At Fort Portal, eruption of a CaO-MgO-CO₂-SiO₂-P₂O₅-SO₂-H₂O-F liquid was unaccompanied by that of a more silica-rich or alkali-rich liquid. Alkali-rich carbonatite lavas and pyroclastic deposits have been documented elsewhere in East Africa, and calcite-rich volcanic carbonatites have been attributed to replacement of magmatic alkali carbonates by calcite. However, the alkali-poor volcanic carbonatites at Fort Portal were not formed by leaching of alkalis in meteoric water; tabular calcite is not pseudomorphous after alkali carbonates such as nyerereite. The Fort Portal magma was low in alkalis at the time of eruption.     

Contrasting fluid flow patterns at the Bufa del Diente contact metamorphic aureole,north-east Mexico: evidence from stable isotopes

Impure limestones with interstratified metachert layers were contact metamorphosed and metasomatized by the Bufa del Diente alkali syenite. Massive marbles exhibit mineralogical and stable isotope evidence for limited fluid infiltration, confined to a 17 m wide zone at the contact. Influx of magmatic brines along most metacherts produced up to 4 cm thick wollastonite rims, according to calcite (Cc)+quartz (Qz)= wollastonite (Wo)+CO₂, and were observed at distances of up to 400 m from the contact. The produced CO₂ exsolved as an immiscible low density CO₂-rich fluid. Chert protolith isotope compositions were ¹⁸O (Qz)=27–30%. and ¹⁸O (Cc)=24–27%.. Many wollastonites in infiltrated metacherts have low ¹⁸O ranging from 11–17 and confirm that decarbonation occurred in presence of a magmatic-signatured fluid. Large gradients in ¹⁸O (Wo) across the rims may reach 6 The ¹⁸O of remaining quartz is often lowered to 15–20 whereas caleites largely retained their original compositions. The isotopic reversals of up to 10 between quartz and calcite along with reaction textures demonstrate non-equilibrium between infiltrating fluid in the aquifer and the assemblage calcite+quartz+wollastonite. This is compatible with the assumption of a down-temperature flow of magmatic fluids that occurred exclusively in the remaining quarzite layer. The ¹³C (Cc) and ¹⁸O (Cc) of marble calcites measured perpendicular to two metachert bands reveal significant isotopic alterations along distances of 4.5 cm and 7.5 cm from the wollastonite-marble boundary only into the hanging wall marble, suggesting an advection process caused by a fluid phase which movel upwards. Covariation trends of ¹³C (Cc) and ¹⁸O (Cc) across the alteration front indicate that this fluid was CO₂-rich. Mass balance calculations show that all CO₂-rich fluid produced by the decarbonation reaction was lost into overlying marble. The metachert aquifers did not leak with respect to water-rich fluids.     

Stable isotope and fluid inclusion studies of carbonate deposits from the Tolfa Mountains mining district (Latium,central Italy)

Carbon and oxygen isotope analyses were made of representative samples of calcite and quartz from the carbonate deposits in the Tolfa Mountains mining district. Measurements were also made of hydrogen isotope compositions, filling temperatures and salinities of fluid inclusions in these minerals. There are three stages of mineralization at Tolfa. In stage I, characterized by calc-silicate hornfels, the carbonates have relatively high ¹⁸O values of 14.5 to 21.6 suggesting a rather low water/rock ratio. ¹³C values of –0.3 to 2.1 indicate that appreciable decarbonation or introduction of deep-seated carbon did not occur. Stage II is marked by phanerocrystalline carbonates; ¹⁸O values of 13.1 to 20.0 and ¹³C values of 0.7 to 5.0 identify them as hydrothermal veins rather than marbles. D values of –56 to –50 for inclusion fluids suggest a possible magmatic component to the hydrothermal fluid. Filling temperatures of coarse-grained samples of Calcite II are 309° to 362° C with a salinity range of 5.3 to 7.1 weight percent NaCl. Calculated ¹⁸O values of 11–12 for these fluids are again indicative of low water/rock ratios. The sparry calcites of stage III have ¹⁸O and ¹³C values of 8.1 to 12.9 and –1.7 to 3.2, respectively. D values of inclusion fluids are –40 to –33, clearly heavier than in earlier stages and similar to values of modern local ground waters. A salinity measurement of <0.1 weight percent NaCl in a sample of Calcite III is compatible with a relatively unaltered ground water origin for this fluid. Precipitation of the sparry calcite took place at much lower temperatures, around 160° C. For quartz, ¹⁸O values of 9.3 to 12.4 and D values for inclusions of –53 to –28 are consistent with its late occurrence and paragenetic link with associated carbonates.     

Oxygen isotope evolution of biogenic calcite and apatite during the Middle and Late Devonian

Oxygen isotope ratios of well-preserved brachiopod calcite and conodont apatite were used to reconstruct the palaeotemperature history of the Middle and Late Devonian. By assuming an oxygen isotopic composition of –1 V-SMOW for Devonian seawater, the oxygen isotope values of Eifelian and early Givetian brachiopods and conodonts give average palaeotemperatures ranging from 22 to 25 °C. Late Givetian and Frasnian palaeotemperatures calculated from ¹⁸O values of conodont apatite are close to 25 °C in the early Frasnian and increase to 32 °C in the latest Frasnian and early Famennian. Oxygen isotope ratios of late Givetian and Frasnian brachiopods are significantly lower than equilibrium values calculated from conodont apatite ¹⁸O values and give unrealistically warm temperatures ranging from 30 to 40 °C. Diagenetic recrystallization of shell calcite, different habitats of conodonts and brachiopods, as well as non-equilibrium fractionation processes during the precipitation of brachiopod calcite cannot explain the ¹⁸O depletion of brachiopod calcite. Moreover, the ¹⁸O depletion of brachiopod calcite with respect to equilibrium ¹⁸O values calculated from conodont apatite is too large to be explained by a change in seawater pH that might have influenced the oxygen isotopic composition of brachiopod calcite. The realistic palaeotemperatures derived from ¹⁸O _apatite may suggest that biogenic apatite records the oxygen isotopic composition and palaeotemperature of Palaeozoic oceans more faithfully than brachiopod calcite, and do not support the hypothesis that the ¹⁸O/¹⁶O ratio of Devonian seawater was significantly different from that of the modern ocean.     

Preliminary investigations of 18O/16O and D/H compositions in rhyo-ignimbrites in the In Hihaou (In Zize) Magmatic Center,central Ahaggar,Algeria

The 620 M.y.-old in Hihaou (In Zize) magmatic complex located at the north-western boundary of the Archaean In Ouzzal block (western Ahaggar), is composed of massive alkaline rhyo-ignimbrites and rhyolitic domes, which are intruded by a granophyric and granitic body. The whole is preserved in a cauldron structure. Extrusive rocks are strongly ¹⁸O-depleted, with -values as low as –1.5/SMOW, while granophyres are less depleted (minimum -¹⁸O value=+2.0/SMOW. The granite has values around + 6/SMOW. D/H compositions are rather low, with D–90 to –110/SMOW. Isotopic zoning of quartz phenocrysts, ¹⁸O/¹⁶O fractionation among coexisting phases, and heterogeneity of the whole-rock -¹⁸O values, suggest that the volcanic rocks have interacted with meteoric water after the eruption. Several mechanisms of isotopic alteration are discussed. The hydrothermal alteration does not seem to have been controlled by the granitic intrusion, but rather seems to have followed the deposition of thick pyroclastic deposits on permeable arkosic sandstones and fluvio-glacial conglomerates. Pervasive circulation of water through the cooling volcanic deposits could have produced the observed ¹⁸O depletion.     

Carbon and oxygen isotopic composition of carbonates in lavas and ejectites from the Alban Hills,Italy

Carbon and oxygen isotopic analyses have been carried out on carbonates from lavas, ejectites and sedimentary formations in the region of the Alban Hills.The calcite occurring in the lavas, both in veins and cavities and dispersed in the groundmass shows within each flow a fairly uniform isotopic composition not different from that normally observed in sedimentary carbonates, except in the case of one particular flow, where unusually low ¹³C values were recorded. The latter are discussed in terms of a possible contribution of organic carbon or of isotopically light carbonates, the presence of which in the Alban Hills area had been previously recorded.The ejectites examined comprise both limestone and dolostone blocks of various degree of metamorphism and materials of uncertain origin, some of which containing carbon and oxygen of isotopic composition wholly different from that of all carbonates analysed in this work, approaching the range observed in some carbonatites. The isotopic data and the geochemical features of the latter materials are discussed in terms of thermal metamorphism of limestones and of a possible syntexis of evaporite materials.The ¹⁸O and ¹³C values of certain marine limestones from major Mesozoic sedimentary formations in the region are also reported.     

Boninite petrogenesis and alteration history: constraints from stable isotope compositions of boninites from Cape Vogel,New Caledonia and Cyprus

¹⁸O values of unaltered olivine and pyroxene phenocrysts in boninites from several areas range from 5.8 to 7.4 and indicate that the source for most boninites is more ¹⁸O-rich than MORBs and other oceanic basalts. The source for oxygen and other major elements is most likely a refractory portion of the mantle having a ¹⁸O value of up to 7.0 to which must be added a small amount of H₂O-rich fluid to induce partial melting. This fluid, which is derived from subducted crust, is the vehicle for LREEs including Nd. The variable, normally low _Nd values typical of boninites do not correlate with the ¹⁸O values.Post eruptive exchange of oxygen in the glass of boninites with that of sea water at low temperatures (<150° C) produces ¹⁸O values of >10 in optically fresh glass. Hydration of the glass has increased the water contents of most boninites from estimated magmatic values of 1–2 wt% to 2–4 wt% and produced D values of < –80, which may be lower than the original magmatic D values. In contrast to most submarine pillow basalts, the magmatic volatile composition of boninite lavas has been extensively modified as a result of post eruptive interaction with seawater.     

Contrasting fluid/rock interaction between the Notch Peak granitic intrusion and argillites and limestones in western Utah: evidence from stable isotopes and phase assemblages

The Jurassic Notch Peak granitic stock, western Utah, discordantly intrudes Cambrian interbedded pure limestones and calcareous argillites. Contact metamorphosed argillite and limestone samples, collected along traverses away from the intrusion, were analyzed for ¹⁸O, ¹³C, and D. The ¹³C and ¹⁸O values for the limestones remain constant at about 0.5 (PDB) and 20 (SMOW), respectively, with increasing metamorphic grade. The whole rock ¹⁸O values of the argillites systematically decrease from 19 to as low as 8.1, and the ¹³C values of the carbonate fraction from 0.5 to –11.8. The change in ¹³C values can be explained by Rayleigh decarbonation during calcsilicate reactions, where calculated is about 4.5 permil for the high-grade samples and less for medium and low-grade samples suggesting a range in temperatures at which most decarbonation occurred. However, the amount of CO₂ released was not anough to decrease the whole rock ¹⁸O to the values observed in the argillites. The low ¹⁸O values close to the intrusion suggest interaction with magmatic water that had a ¹⁸O value of 8.5. The extreme lowering of ¹³C by fractional devolatilization and the lowering of ¹⁸O in argillites close to the intrusion indicates oxgen-equivalent fluid/rock ratios in excess of 1.0 and X(CO₂)_F of the fluid less than 0.2. Mineral assemblages in conjunction with the isotopic data indicate a strong influence of water infiltration on the reaction relations in the argillites and separate fluid and thermal fronts moving thru the argillites. The different stable isotope relations in limestones and argillites attest to the importance of decarbonation in the enhancement of permeability. The flow of fluids was confined to the argillite beds (argillite aquifers) whereas the limestones prevented vertical fluid flow and convective cooling of the stock.     

Structural control of tertiary Au-Ag-bearing breccias in an extensional environment,Nelson area,Southern Nevada,USA

Gold-silver mineralization in the Nelson area of southern Nevada was controlled by structures associated with intrusion of an east-west oriented pluton. Flatlying breccias formed during intrusion have allowed passive flooding of highly permeable zones and deposition of mineralized quartz and calcite. Steep fractures were formed in the pluton and immediate country rock during cooling, and later reactivated by north-south extension. These fractures have channelled fluids, and some have been the sites of hydrothermal eruptions which produced further brecciation and deposition of mineralized quartzcalcite veins. The mineralizing fluid was water which was boiling at or near 100 °C. The calcite deposited by this water has ¹³C = –5.4 to –7.1, and ¹⁸O = +5.8 to +11.3, and the water was probably meteoric in origin. Mineralization had an epithermal style, with strong local structural control, rather than deep-sourced regional detachment-related hydrothermal origin.     

Hydrogen and carbon isotope studies on the graphite-bearing metapelites in the northern Kiso district of central Japan

Measurements were made of the hydrogen isotope ratios of hydrous silicates (mica and amphibole) and whole rocks, and the carbon isotope ratios of graphite and carbonaceous matter in the metamorphic rocks from the northern Kiso district in central Japan.D values of hydrous silicates in the graphite-bearing metapelites are always higher than those in graphite-free schists, even though the sample localities of the two rock-types are very close. Hydrogen isotopic equilibrium has been attained between the coexisting minerals.D/H ratios of water in the metamorphic fluids seem to depend strongly on the presence or absence of graphite and seem to be not constant throughout the district. The district is divided into three areas of low (metamorphic zones I, II), medium (zones IIIa–V) and high ¹³C_gr value (zones VIa–VII) areas. In the high ¹³C_gr values area, the carbon contents of the graphite-bearing rocks decrease slightly from zones VIa to VII, whereas the ¹³C_gr values increase sharply from the upper part of zone VIa to VIb. TheD values of biotite in these graphite-bearing rocks are higher than those in the medium ¹³C_gr area. This suggests that methane enriched inH and¹²C is produced and liberated by the devolatilization reactions between muscovite, graphite and water. The fluid produced is composed of water, methane and a subordinate amount of carbon dioxide, and its logfO₂ value is deduced to be about 1.2 lower than that defined by the FMQ buffer. In the medium ¹³C_gr area, the ¹³C values of graphite are nearly constant (–20.8), while the Fe₂O₃/(Fe₂O₃ + FeO) ratio of the graphite-bearing rocks apparently decreases with increasing metamorphic grade.D differences in hydrous silicates between graphite-bearing and graphite-free rocks are observed. These facts are interpreted to mean that methane was produced in addition to water and carbon dioxide, and that its generation ( ratio of the fluid was about 2) had practically no isotope effect on the graphite. In the low ¹³C_gr area, the carbon contents of the rocks decrease clearly from zones I to IIIa. TheD and ¹³C_gr values of the non-metamorphosed shales are much lower than those of the low grade graphite-bearing metapelites. This suggests that methane is produced and liberated from the rocks even at the incipient stage of metamorphism.     

Characteristics and chronology of fracture — fluid infiltration in the Archean,Eye Dashwa Lakes pluton,Superior Province: evidence from H,C, O-isotopes and fluid inclusions

The Archean Eye Dashwa Lakes pluton (2672±24 Ma) has domains of mineralogically fresh isotropic granite, domains that have undergone bulk hydrothermal alteration, and at least eleven sets of sequential fracture arrays, each with distinctive mineral assemblages. Fresh granite is characterized by whole rock ¹⁸O=8.1 to 8.6 and primary magmatic quartz-feldspar (+1.3), quartz-biotite (5.2 to 5.4) and quartz-magnetite (+9.8) fractionations. Magmatic fluids had a calculated isotopic composition of ¹⁸O=7.9±0.5, and D=–80±5. These isotropic volumes of the granite have not experienced significant incursion of external thermal waters. Pegmatites, quartz-molybdenite veins, and phlogopite-muscovite coated fractures are sporadically distributed in the granite, and were precipitated from high-temperature magmatic fluids where ¹⁸O=8.0 to 10.3 and D=–80±5.The most abundant variety of fracture filling assemblage is epidote-quartz-chlorite±muscovite: fractures are bounded by domains of mineralogically similar bulk hydrothermal alteration of the granite. These minerals formed at 160 to 280° C, in the presence of NaCl, and NaCl-MgCl₂ brines (up to 25 wt% NaCl equivalent) of probable evolved marine water origin ( ¹⁸O=+0.4 to +3.8, D=–10 to –35) undergoing transient boiling. Upper plateau ⁴⁰Ar/³⁹Ar ages for the muscovite are 2650±15 Ma. Sequentially in the chronology of fracture-infiltration events, calcite-fluorite veins were deposited from boiling fluids at 340 to 390° C, isotopically characterized by ¹⁸O=4.7 and ¹³C=–5; and rare prehnite-chlorite lined fractures formed at 250 to 290° C. A generation of adularia-bearing veins precipitated at 140 to 230° C, from CaCl₂-NaCl brines, where ¹⁸O=0 to –6.5 and D=–10 to –30. Incremental ⁴⁰Ar/³⁹Ar age spectra on the K-feldspar yield an upper plateau of 1100 Ma. Subsequently, hematite developed during reactivation of earlier fractures, at 140 to 210° C in the presence of fluids characterized by ¹⁸O=–0.4 to –5.4 and D=–15 to –25. Arrays of open fractures partially occupied by gypsum and goethite reflect a fluid infiltration event at temperatures <50° C. Many of the earlier generations of fracture minerals have transgranular fracture infillings which record the presence of low temperature (88–190° C), hypersaline CaCl₂-NaCl brines. Narrow fractures lined with clays±calcite are sites for seepage of modern ground-waters. The isotopic signature of clay ( ¹⁸O=12 to 20, D=–80±5) plots near the line for modern kaolinites, confirming its formation in the presence of recent surface waters. Calcites coexisting with the clay minerals, and in fractured pegmatite show a common isotopic signature ( ¹⁸O=23±0.5, ¹³C=–13.6), indicating precipitation from modern groundwaters, where reactivated fractures have acted as conduits for infiltration of surface waters to depths of 200 m. Intermittent fracture-infiltration has occurred over 2.7 Ga. The early sequences of fracture-related fluid flow are interpreted in terms of devolatilization of the granite, followed by thermal contraction fracturing, incursion of marine water and convective cooling in the Archean. Hematite and adularia fracture fillings correspond to a stage when meteoric water infiltrated the volcanicplutonic terrain during Proterozoic and later times. Episodic fracture-fluid expulsion events may have been driven by seismic pumping, in response to magmatically and tectonically induced stresses within the Shield, with surface waters penetrating to depths of 15 km in the crust.     

Non-monotonic chemical and O,Sr, Nd,and Pb isotope zonations and heterogeneity in the mafic- to silicic-composition magma chamber of the Grizzly Peak Tuff,Colorado

Strong compositional zonation of the 34 Ma Grizzly Peak Tuff in west-central Colorado is attended by non-monotonic trends in O, Sr, Nd, and Pb isotope ratios. Fiamme from the tuff cluster in chemical compositions and petrographic characteristics, indicating the magma chamber was not continuously zoned but consisted of at least seven compositional layers. The most mafic magma erupted (57 wt% SiO₂, fiamme group 7) had ¹⁸O= +8.5, initial ⁸⁷Sr/⁸⁶Sr=0.7099, _Nd, and ²⁰⁶Pb/²⁰⁴Pb=17.80, suggesting that the magma was produced by 50% fractional crystallization of basaltic magma that assimilated 20 to 40 wt% Proterozoic crust. Isotopic compositions of more evolved parts of the chamber (up to 77 wt% SiO₂, fiamme group 1) depart from the mafic base-level composition of fiamme group 7, and reflect late-stage assimilation that occurred largely after compositional layering was established. ¹⁸O values decrease by as much as 1.5 from fiamme groups 7 through 4, indicating assimilation of hydrothermally altered roof rocks. ¹⁸O values abruptly inerease by up to 1.5 between fiamme groups 4 and 3. This discontinuity is interpreted to reflect evolution in an asymmetric chamber that had a split-level roof, allowing assimilation of wall rocks that varied vertically in degree of hydrothermal alteration. This chamber geometry is also supported by collapse structures in the caldera. Late-stage assimilation of heterogeneous wall rocks is also indicated by variations in Sr, Nd, and Pb isotope ratios. Large Sr isotope disequilibrium exists between some phenocrysts and whole-rock fiamme, and initial ⁸⁷Sr/⁸⁶Sr ratios in phenocrysts are as high as 0.7170. values regularly increase from-13.0 in fiamme group 7 to-11.3 in fiamme group 3, and then decrease to-12.2 in fiamme group 1. ²⁰⁶Pb/²⁰⁴Pb ratios generally increase from 17.80 to 17.94 for fiamme groups 7 through 1. The rhyolitic parts of the Grizzly Peak Tuff have isotopic compositions that could be attributed to a purely crustal melt. It is unlikely, however, that the mafic parts of the tuff were generated by crustal melting, and the compositional and isotopic variations across the entire zonation of the tuff are best explained by fractional crystallization of mantle-derived magmas, accompanied by extensive assimilation of Proterozoic crust.     

Sulfur isotopes and origin of some sulfide deposits,New England,Australia

The S-isotopic compositions of sulfide deposits from Steinmann, granitoid and felsic volcanic associations have been examined. Ores of Steinmann association have ³⁴S values close to zero per mil (³⁴S=+0.3±3.1) it appears they are of mantle origin. Isotopically, ores of granitoid association regularly show a variable enrichment in ³²S relative to meteoritic (³⁴S=–2.7±3.3). The composition is in accord with an upper mantle/lower crustal source. Two stratiform accumulations of felsic volcanic association show a narrow spread of ³⁴S values (+0.2 to 2.4); a mantle origin for the sulfur in these deposits is favored. In contrast, vein, stockwork and cement ores are moderately enriched in ³²S relative to meteoritic (³⁴S=–4.0±6.4). These ores are polygenetic; sulfur and metals appear to have been leached from local country rocks where volcanogenic and biogenic sulfur predominate.