Oxygen isotope studies of potassic volcanic rocks of the Roman Province,Central Italy

The ¹⁸O/¹⁶O ratios of rocks and coexisting minerals were measured for 93 samples of leucite-bearing lavas, pyroclastics, and related volcanic rocks from the Quaternary Roman Co-Magmatic Province, Italy. The ¹⁸O values were found to generally increase northward in the sequence: Ischia (5.8 to 7.0); Somma-Vesuvius and Phlegrean Fields (7.3 to 8.3); Alban Hills (7.3 to 8.7); M. Sabatini (7.3 to 9.7); Vico Volcano (7.4 to 10.2); and M. Vulsini (8.1 to 11.7). The northward increase in ¹⁸O parallels a similar increase in ⁸⁷Sr/⁸⁶Sr, and these data indicate that the Roman magmas have interacted strongly with high-¹⁸O continental crust. A marked increase in ¹⁸O occurs just north of Rome where the Roman Province begins to overlap the calc-alkaline, oversaturated Tuscan Magmatic Province. Therefore, some of the observed ¹⁸O/¹⁶O and ⁸⁷Sr/⁸⁶Sr enrichments in the Roman magmas may have been facilitated by direct mixing with the high-¹⁸O Tuscan magmas or because the high-¹⁸O country rocks underwent widespread heating during a couple of million years of Tuscan igneous activity. Although many of the Roman magmas underwent fractional crystallization without appreciable change in ¹⁸O, contamination has produced a correlation between ¹⁸O and SiO₂ content at several of the volcanic centers; thus the trachytes are typically higher in ¹⁸O than the undersaturated rocks. The major features of the oxygen isotope data can be explained in terms of a simple two-component mixing model in which one end-member was a primary, strongly undersaturated magma derived from the upper mantle, with ¹⁸O+6, ⁸⁷Sr/⁸⁶Sr0.704 to 0.705, and SiO₂<44wt.%. However, none of the analyzed samples have these values, as they have all been contaminated to some extent. The closest approach is found in some of the leucitepyroxenite ejecta from the Alban Hills. The second end-member, derived from the continental crust, had a variable composition with ¹⁸O+12 to +20, ⁸⁷Sr/⁸⁶Sr0.712 to 0.720, and SiO₂65wt.%, and it mixed in much greater proportions in the volcanoes north of Rome than in those of the Alban Hills or the Naples area. The widespread interactions between the Roman magmas and the continental crust are probably due to (1) the fact that such low-SiO₂ magmas always have a very strong tendency to interact with quartz-bearing rocks of the continental crust, and (2) in Italy, these magmas were emplaced into a tectonically very active area containing poorly consolidated sedimentary rocks, and in the northern part of the belt there had been a prior history of extensive calc-alkaline igneous activity.Publication of the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, Contribution Number 2501.     

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.     

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.     

A stable isotope study of retrograde alteration in SW Connemara,Ireland

Dalradian metamorphic rocks, Lower Ordovician meta-igneous rocks (MGS) and Caledonian granites of the Connemara complex in SW Connemara all show intense retrograde alteration. Alteration primarily involves sericitization and saussuritization of plagioclase, the alteration of biotite and hornblende to chlorite and the formation of secondary epidote. The alteration is associated with sealed microcracks in all rocks and planes of secondary fluid inclusions in quartz where it occurs, and was the result of a phase of fluid influx into these rocks. In hand specimen K-feldspar becomes progressively reddened with increasing alteration. Mineralogical alteration in the MGS and Caledonian granites took place at temperatures 275±15°C and in the MGS P_fluid is estimated to be 1.5 kbar during alteration. The °D values of alteration phases are:-18 to-29 (fluid inclusions),-47 to-61 (chlorites) and-11 to-31 (epidotes). Chlorite ¹⁸O values are +0.2 to +4.3, while ¹⁸O values for quartz-K-feldspar pairs show both positively sloped (MGS) and highly unusual negatively sloped (Caledonian granites) arrays, diverging from the normal magmatic field on a - plot. The stable isotope data show that the fluid that caused retrogression continued to be present in most rocks until temperatures fell to 200–140°C. The retrograde fluid had D -20 to-30 in all lithologies, but the fluid ¹⁸O varied both spatially and temporally within the range-4 to +7. The fO₂ of the fluid that deposited the epidotes in the MGS varied with its ¹⁸O value, with the most ¹⁸O-depleted fluid being the most oxidizing. The D values, together with low (<0) ¹⁸O values for the retrograde fluid in some lithologies indicate that this fluid was of meteoric origin. This meteoric fluid was probably responsible for the alteration in all lithologies during a single phase of fluid infiltration. The variation in retrograde fluid ¹⁸O values is attributed to the effects of variable oxygen isotope shifting of this meteoric fluid by fluid-rock interaction. Infiltration of meteoric fluid into this area was most likely accomplished by convection of pore fluids around the heat anomaly of the Galway granite soon after intrusion at 400 Ma. However convective circulation of meteoric water and mineralogical alteration could possible have occurred considerably later.     

Hydrothermal alteration and oxygen and hydrogen isotope geochemistry of the D-68 zone Cu-Zn Massive Sulfide Deposit,Noranda District,Quebec, Canada

Summary Pervasive hydrothermal alteration zones in quartz-feldspar porphyry domes underly all massive sulfide lenses in the D-68 Zone Cu-Zn deposit, Noranda. Alteration pipes are mineralogically zoned and contain chloritic cores consisting of stringer sulfides, enveloped by sericitic haloes. Silicified rocks are found locally.Alteration took place at nearly constant volume. Na depletion, and K enrichment relative to the least altered rocks, are found in all alteration zones. Fe and Mg have been added to the chloritic zone and subtracted in the sericitic and silicic zones. Ca and Si are enriched mainly in the silicic zone. Al, Ti and Zr were the least mobile of the elements studied.Whole-rock ¹⁸O values vary from +5.6 to +6.2 per mil in chloritized rocks, +5.8 to + 7.3 per mil in sericitized rocks and + 7.2 to + 8.3 per mil in silicified rocks. D values for two chloritized samples are – 63 and – 70 per mil whereas in two sericitized samples they are close to –62 per mil. Quartz from the chlorite alteration zone is isotopically heavier (¹⁸O = 8.6 per mil) than that from the sericite alteration zone (¹⁸O = 6.4 per mil), suggesting equilibration with different hydrothermal fluid or different temperature of alteration. Assuming an alteration temperature of 300° + 50°C the fluid in equilibrium with quartz and chlorite had ¹⁸O and D values of about 1.5 ± 2.0 per mil and –23 ± 5 per mil, respectively. The fluid in equilibrium with quartz and sericite had ¹⁸O and D values of about –0.5 ± 2 per mil and –30 ± 5 per mil, respectively. On the basis of isotopic data, seawater was probably the major constituent of the hydrothermal fluids.

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Hydrothermale Umwandlung und Sauerstoff-Wasserstoff-Isotopengeochemie der Zone D-68 Cu-Zn Derberz Sulfidlagerstätte, Noranda District, Quebec, Canada

Zusammenfassung Hydrothermale Umwandlungszonen in porphyrischen Quarz-Feldspat Gesteinskörpern liegen unterhalb von Derberz Sulfidlinsen in der D-68 Zone Cu-Zn Lagerstätte, Noranda. Umgewandelte pipes sind mineralogisch zoniert; sie enthalten aus Sulfiden bestehende chloritische Kerne, die von sericitischen Höfen umhüllt werden. Lokal treten silicifizierte Gesteine auf.Die Umwandlung ging bei annähernd konstantem Volumen vor sich. Na-Verarmung und K-Anreicherung, bezogen auf die am wenigsten umgewandelten Gesteine, liegen in allen Umwandlungszonen vor. Fe und Mg wurden der Chloritzone zugeführt, in den Sericit- und Si-Zonen abgeführt. Ca und Si sind vor allem in der Si-Zone angereichert. Al, Ti und Zr waren von den untersuchten Elementen am wenigsten mobil.Gesamtgesteins-¹⁸O Werte variieren von +5,6 bis +6,2 in den chloritisierten Gesteinen, von +5,8 bis 7,3 in sericitisierten Gesteinen und von +7,2 bis +8,3 in den silicifizierten Gesteinen. Die D Werte für zwei chloritisierte Proben betragen –63 und –70, in zwei sericitisierten Proben liegen sie hingegen nahe bei –62. Quarz von der Chlorit-Umwandlungszone ist isotopisch schwerer (¹⁸O = 8,6) als von der Sericit-Umwandlungszone (¹⁸O = 6.4), was eine Gleichgewichtseinstellung mit verschiedenen hydrothermalen Lösungen oder eine verschiedene Umwandlungstemperatur nahelegt. Bei einer angenommenen Umwandlungstemperatur von 300 ± 50°C, hatte die im Gleichgewicht mit Quarz und Chlorit stehende Lösung ¹⁸O und D Werte von etwa 1,5 ± 2 bzw. –23 + 5. Die im Gleichgewicht mit Quarz und Sericit befindliche Lösung hatte ¹⁸O und D Werte von etwa –0,5 ± 2%o bzw. –30 ± 5. Aufgrund der Isotopendaten war wahrscheinlich Meerwasser der Hauptbestandteil der hydrothermalen Lösungen.

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With 7 Figures     

Oxygen isotope geochemistry of pyroclastic clinopyroxene monitors carbonate contributions to Roman-type ultrapotassic magmas

The oxygen isotope geochemistry and chemical composition of clinopyroxene crystals from Alban Hills pyroclastic deposits constrain the petrological evolution of ultrapotassic Roman-type rocks. Volcanic eruptions in the 560–35 ka time interval produced thick pyroclastic deposits bearing clinopyroxene phenocrysts with recurrent chemical characteristics. Clinopyroxenes vary from Si–Mg-rich to Al–Fe-rich with no compositional break, indicating that they were derived from a continuous process of crystal fractionation. Based on the ¹⁸O and trace element data no primitive samples were recovered: monomineralic clinopyroxene cumulates set the oxygen isotope composition of primary magmas in the range of uncontaminated mantle rocks (5.5), but their REE composition resulted from extensive crystal fractionation. Departing from these mantle-like ¹⁸O_Cpx values the effects of crustal contamination of clinopyroxene O-isotope composition were identified and used to monitor chemical variations in the parental magma. ¹⁸O values in Si–Mg-rich clinopyroxene are slightly higher than typical mantle values (5.9–6.2), and the low REE contents are representative of early stages of magmatic differentiation. ¹⁸O values as high as 8.2 are associated with Al–Fe³⁺-rich clinopyroxene showing high REE contents. These ¹⁸O values are characteristic of crystals formed during the late magmatic stages of each main eruptive phase. Geochemical modelling of ¹⁸O values vs. trace element contents indicates that these ultrapotassic magmas were derived from fractional crystallization plus assimilation of limited amounts of carbonate wall rocks starting from a primary melt, and from interaction with CO₂ derived from country rocks during crystal fractionation.     

Stable isotope systematics in mesozoic granites of Central and Northern California and Southwestern Oregon

¹⁸O, D, and H₂O⁺ contents were measured for whole-rock specimens of granitoid rocks from 131 localitics in California and southwestern Oregon. With 41 new determinations in the Klamath Mountains and Sierra Nevada, initial strontium isotope ratios are known for 104 of these samples. Large variations in ¹⁸O (5.5 to 12.4), D (–130 to –31), water contents (0.14 to 2.23 weight percent) and initial strontium isotope ratios (0.7028 to 0.7095) suggest a variety of source materials and identify rocks modified by secondary processes. Regular patterns of variation in each isotopic ratio exist over large geographical regions, but correlations between the ratios are generally absent except in restricted areas. For example, the regular decrease in D values from west to east in the Sierra Nevada batholith is not correlative with a quite complex pattern of ¹⁸O values, implying that different processes were responsible for the isotopic variations in these two elements. In marked contrast to a good correlation between (⁸⁷Sr/⁸⁶Sr)_o and ¹⁸O observed in the Peninsular Ranges batholith to the south, such correlations are lacking except in a few areas. D values, on the other hand, correlate well with rock types, chemistry, and (⁸⁷Sr/⁸⁶Sr)_o except in the Coast Ranges where few of the isotopic signatures are primary. The uniformly low D values of samples from the Mojave Desert indicate that meteoric water contributed much of the hydrogen to the rocks in that area. Even so, the ¹⁸O values and ¹⁸O fractionations between quartz and feldspar are normal in these same rocks.This reconnaissance study has identified regularities in geochemical parameters over enormous geographical regions. These patterns are not well understood but merit more detailed examination because they contain information critical to our understanding of the development of granitoid batholiths.     

O-Sr isotopic variations in Miocene granitoids from the Aegean: evidence for an origin by combined assimilation and fractional crystallization

Sr, O, and D/H isotopic compositions have been analyzed in Miocene metaluminous to slightly peraluminous (I-type) granitoids of the central Aegean. Individual plutonic complexes display significant variations in their ¹⁸O and initial⁸⁷Sr/⁸⁶Sr compositions.D and ¹⁸O compositions of minerals and whole-rocks are mostly in the magmatic range. Some samples from Naxos and Mykonos/Delos show low D and ¹⁸O values characteristic of meteoric-water-hydrothermal interaction, but as a whole the changes in ¹⁸O and Sr isotopic compositions as a result of hydrothermal alteration were slight, even in instances where marked alteration is petrographically observable. Consequently, the bulk-rock variations of ¹⁸O from 8.1 to 12.0 and of⁸⁷Sr/⁸⁶Sr from 0.70438 to 0.71450 may be regarded as primary and indicative of the conditions of their evolution. Heterogeneous isotopic compositions observed in the individual plutons of Serifos, Ikaria, Samos and Kos may be caused by the multiple intrusion of chemically and isotopically distinct magma pulses, with high viscosities and relatively rapid consolidation in most cases preventing complete homogenization. The granitoids of Serifos, Ikaria and Kos display weak correlations between the initial⁸⁷Sr/⁸⁶Sr and ¹⁸O and 1/Sr. The granitoid province shows a positive correlation between⁸⁷Sr/⁸⁶Sr and ¹⁸O and a non-linear relationship between⁸⁷Sr/⁸⁶Sr and 1/Sr, whereby 1/Sr increases more rapidly than the isotopic ratio as the degree of fractionation of the rocks increases. It is argued that assimilation of older continental material by mantle-derived arc magmas with combined fractionation (AFC) is the most plausible model to explain the chemical and isotopic characteristics of the granitoids and the geological situation in which rock-types trend from granodiorites in the (south)west, near the inferred Oligocene-Miocene suture, to granites in the center and monzonites in the (north)east of the province.     

Non-equilibrium,metasomatic18O/16O effects in upper mantle mineral assemblages

Kyser, O'Neil, and Carmichael (1981, 1982) measured the ¹⁸O values of coexisting minerals from peridotite nodules in alkali basalts and kimberlites, interpreting the nodules as equilibrium assemblages. Based mainly on the systematics revealed in ¹⁸O-olivinevs. ¹⁸O-pyroxene diagrams, we have re-interpreted the Kyser et al. data as non-equilibrium phenomena. On such- diagrams, the mantle nodules exhibit data arrays that cut across the ¹⁸O=zero line; these arrays strongly resemble the non-equilibrium quartz-feldspar and feldspar-pyroxene ¹⁸O arrays that we now know arediagnostic of hydrothermally altered plutonic igneous rocks. Thus, the peridotites appear to have been open systems that underwent metasomatic exchange with an external, oxygen-bearing fluid (CO₂ magma, H₂O, etc.); during this event, the relatively inert pyroxenes exchanged at a much slower rate than did the coexisting olivines and spinels. This accounts for the correlation between ¹⁸O pyroxene-olivine and the whole-rock ¹⁸O of the peridotites, which is a major difficulty with the equilibrium interpretation. The metasomatic¹⁸O-enrichments of the peridotites can be related to metasomatic enrichments in LIL elements and the development of amphibole and phlogopite. This type of precursor metasomatic activity can explain the development of alkali basalt magmas, as well as leucitites and nephelinites (all of which tend to be slightly¹⁸O-rich relative to MORB, with ¹⁸O=+6 to +7.5). Fluids with appropriate ¹⁸O values to explain the open-system metasomatic effects can be produced by exchange with ancient subducted oceanic crust (eclogite). However, fluid/rock ratios of about 0.4 to 2.5 are required, indicating that this cannot be a mantle-wide phenomenon. Also, these non-equilibrium effects are apparently transient phenomena, probably associated with the eruptive events that brought the nodules to the surface; at characteristic mantle temperatures, the effects would likely disappear in a few tens of millions of years, or less, implying that the ultramafic nodules are not typical samples of the upper mantle.Contribution No. 4156, Publications of the Division of Geological and Planetary Sciences, California Institute of Technology     

Changes in stable isotope ratios of metapelites and marbles during regional metamorphism,Mount Lofty Ranges,South Australia: implications for crustal scale fluid flow

The Mount Lofty Ranges comprises interlayered marbles, metapsammites, and metapelites that underwent regional metamorphism during the Delamarian Orogeny at 470–515 Ma. Peak metamorphic conditions increased from lowermost biotite grade (350–400°C) to migmatite grade (700°C) over 50–55 km parallel to the lithological strike of the rocks. With increasing metamorphic grade, ¹⁸O values of normal metapelites decrease from 14–16 to as low as 9.0, while ¹⁸O values of calcite in normal marbles decrease from 22–24 to as low as 13.2 These isotopic changes are far greater than can be accounted for by devolatilisation, implying widespread fluid-rock interaction. Contact metamorphism appears not to have affected the terrain, suggesting that fluid flow occurred during regional metamorphism. Down-temperature fluid flow from synmetamorphic granite plutons (¹⁸O=8.4–8.6) that occur at the highest metamorphic grades is unlikely to explain the resetting of oxygen isotopes because: (a) there is a paucity of skarns at granite-metasediment contacts; (b) the marbles generally do not contain low-XCO₂ mineral assemblages; (c) there is insufficient granite to provide the required volumes of water; (d) the marbles and metapelites retain a several permil difference in ¹⁸O values, even at high metamorphic grades. The oxygen isotope resetting may be accounted for by along-strike up-temperature fluid flow during regional metamorphism with time-integrated fluid fluxes of up to 5x10⁹ moles/m² (10⁵ m³/m²). If fluid flow occurred over 10⁵–10⁶ years, estimated intrinsic permeabilities are 10^-20 to 10^-16m². Variations in ¹⁸O at individual outcrops suggest that time-integrated fluid fluxes and intrinsic permeabilities may locally have varied by at least an order of magnitude. A general increase in XCO₂ values of marble assemblages with metamorphic grade is also consistent with the up-temperature fluid-flow model. Fluids in the metapelites may have been derived from these rocks by devolatilisation at low metamorphic grades; however, fluids in the marbles were probably derived in part from the surrounding siliceous rocks. The marble-metapelite boundaries preserve steep gradients in both ¹⁸O and XCO₂ values, suggesting that across-strike fluid fluxes were much lower than those parallel to strike. Up-temperature fluid flow may also have formed orthoamphibole rocks and caused melting of the metapelites at high grades.This paper is a contribution to IGCP Project 304 Lower Crustal Processes     

Coupled Sr-O isotope variations as an indicator of source heterogeneity for the Northern peninsular ranges batholith

Primary ¹⁸O values for tonalitic rocks from the San Jacinto Intrusive Complex range from +9.0 to +10.6; initial⁸⁷Sr/⁸⁶Sr for the same samples varies from 0.7058 to 0.7076. Rocks with low ¹⁸O tend to have higher initial⁸⁷Sr/⁸⁶Sr. The majority of samples have a limited range of ¹⁸O values (+10.0–+10.4) that is anticorrelated with colour index; this variation is compatible with crystallization of rocks with varying mineral abundances from an homogeneous (for oxygen) reservoir. More mafic rocks show a large range of ¹⁸O values, reflecting original variations in source material compositions. The combined Sr-O isotope data demonstrate that three or more distinct components were involved in the genesis of these rocks; these three components are also seen in batholithic rocks from the Mojave Block and Sierra Nevada to the north. These data are interpreted as indicating a 0–35% (oxygen atom basis) contribution to the San Jacinto rocks from an old continental lithosphere source.Division Contribution No. 4064     

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.     

Contrasting serpentinization processes in the eastern Central Alps

Stable isotope compositions have been determined for serpentinites from between Davos (Arosa-Platta nappe, Switzerland) and the Valmalenco (Italy). D and ¹⁸O values (–120 to –60 and 6–10, respectively) in the Arosa-Platta nappe indicate that serpentinization took place on the continent at relatively low temperatures in the presence of limited amounts of metamorphic fluids that contained a component of meteoric water. One sample of chrysotile has a ¹⁸O value of 13 providing evidence of high W/R ratios and low formation temperature of lizardite-chrysotile in this area. In contrast, relatively high D values (–42 to –34) and low ¹⁸O values (4.4–7.4) for serpentine in the eastern part of the Valmalenco suggest a serpentinization process that took place at moderate temperatures in fluids that were dominated by ocean water. The antigorite in the Valmalenco is the first reported example of continental antigorite with an ocean water signature. An amphibole sample from a metasomatically overprinted contact zone to metasediments (D=-36) indicates that the metasomatic event also took place in the presence of ocean water. Lower D values (–93 to –60) of serpentines in the western part of the Valmalenco suggest a different alteration history possibly influenced by fluids associated with contact metamorphism. Low water/rock ratios during regional metamorphism (and metasomatism) have to be assumed for both regions.     

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.     

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.     

Nd,Sr, and O isotopic variations in metaluminous ash-flow tuffs and related volcanic rocks at the Timber Mountain/Oasis Valley Caldera,Complex, SW Nevada: implications for the origin and evolution of large-volume silicic magma bodies

Nd, Sr and O isotopic data were obtained from silicic ash-flow tuffs and lavas at the Tertiary age (16–9 Ma) Timber (Mountain/Oasis Valley volcanic center (TMOV) in southern Nevada, to assess models for the origin and evolution of the large-volume silicic magma bodies generated in this region. The large-volume (>900 km³), chemically-zoned, Topopah Spring (TS) and Tiva Canyon (TC) members of the Paintbrush Tuff, and the Rainier Mesa (RM) and Ammonia Tanks (AT) members of the younger Timber Mountain Tuff all have internal Nd and Sr isotopic zonations. In each tuff, high-silica rhyolites have lower initial _Nd values (1 _Nd unit), higher⁸⁷Sr/⁸⁶Sr, and lower Nd and Sr contents, than cocrupted trachytes. The TS, TC, and RM members have similar _Nd values for high-silica rhyolites (-11.7 to -11.2) and trachytes (-10.5 to -10.7), but the younger AT member has a higher _Nd for both compositional types (-10.3 and -9.4). Oxygen isotope data confirm that the TC and AT members were derived from low _Nd magmas. The internal Sr and Nd isotopic variations in each tuff are interpreted to be the result of the incorporation of 20–40% (by mass) wall-rock into magmas that were injected into the upper crust. The low _Nd magmas most likely formed via the incorporation of low ¹⁸O, hydrothermally-altered, wall-rock. Small-volume rhyolite lavas and ash-flow tuffs have similar isotopic characteristics to the large-volume ash-flow tuffs, but lavas erupted from extracaldera vents may have interacted with higher ¹⁸O crustal rocks peripheral to the main magma chamber(s). Andesitic lavas from the 13–14 Ma Wahmonie/Salyer volcanic center southeast of the TMOV have low _Nd (-13.2 to -13.8) and are considered on the basis of textural evidence to be mixtures of basaltic composition magmas and large proportions (70–80%) of anatectic crustal melts. A similar process may have occurred early in the magmatic history of the TMOV. The large-volume rhyolites may represent a mature stage of magmatism after repeated injection of basaltic magmas, crustal melting, and volcanism cleared sufficient space in the upper crust for large magma bodies to accumulate and differentiate. The TMOV rhyolites and 0–10 Ma old basalts that erupted in southern Nevada all have similar Nd and Sr isotopic compositions, which suggests that silicic and mafic magmatism at the TMOV were genetically related. The distinctive isotopic compositions of the AT member may reflect temporal changes in the isotopic compositions of basaltic magmas entering the upper crust, possibly as a result of increasing basification of a lower crustal magma source by repeated injection of mantle-derived mafic magmas.     

Oxygen isotope geochemistry of the Mesozoic volcanics of the Etendeka Formation,Namibia

The Etendeka Formation volcanics consist of a bimodal association of basalts and quartz latites. Forty three new whole rock oxygen isotope analyses are reported for all the major magma types. All the rocks except a minor suite of dolerites have higher ¹⁸O values than normal mantle. The basic rocks (average of 29=8.8) have significantly different ¹⁸O to the acid rocks (average of 10=14.4) These data are apparently consistent with previously published petrogenetic models, which propose that the basalts were affected by crustal contamination and that the quartz latites are crustally derived. However, mineral oxygen data show that there is significant oxygen isotopic disequilibrium between phenocryst and whole rock, the latter being significantly higher in most cases. One of the basic magma types (the Tafelberg basalts) shows mutual positive correlations between ¹⁸O, SiO₂ and _Sr. If these correlations are due to crustal contamination, then as much as 45% contamination is required by material having a ¹⁸O value of 15 which is the maximum observed value in the Damaran basement rocks. In the absence of pyroxene phenocryst ¹⁸O data for the high _Sr Tafelberg basalts (they are aphyric), it is not possible to confirm that contamination has taken place. An alternative explanation is that the correlation between _Sr and SiO₂ resulted from assimilation coupled with fractional crystallization (AFC) (before emplacement). Post-eruption alteration resulted in a correlation between SiO₂ ¹⁸O because the material with the most Si-O bonds was able to concentrate ¹⁸O more effectively. The limited mineral data for the quartz latites suggests that there is some source heterogeneity. A pyroxene ¹⁸O value of 10% for a southern Etendeka quartz latite is consistent with a crustal source.     

Isotopic composition of oxygen and hydrogen in mud-volcanic waters from Taman (Russia) and Kakhetia (Eastern Georgia)

The ¹⁸O and D values in mud-volcanic waters of the Taman Peninsula and Kakhetia vary from +0.7 to +10.0 and from –37 to –13 , respectively. These values increase as the Greater Caucasus is approached. The increase in 18O and D also positively correlates with fluid generation temperatures based on hydrochemical geothermometers. This is accompanied by changes in the chemical composition of waters, in which contents of alkali metals, HCO^– ₃ ion, and boron increase, while the content of halogen ions (Cl, Br, J) decreases. Changes in the isotopic composition of water are also accompanied by the increase of ¹³ in methane and decrease of ¹¹ B in clays. Analysis of formal models of the evolution of isotopic composition of mud-volcanic waters showed that mud volcanoes are recharged by freshened water from the Maikop paleobasin with an inferred isotopic composition of D –40 and ¹⁸ O –6. Based on this assumption, the ¹⁸O and D values observed in mud-volcanic waters can be explained not only by processes of distillation and condensation in a closed system, but also by combined processes of isotopic reequilibration in the water-illite-methane system.Translated from Litologiya i Poleznye Iskopaemye, No. 2, 2005, pp. 143–158.Original Russian Text Copyright © 2005 by Lavrushin, Dubinina, Avdeenko.     

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.     

The oxygen isotope composition of granitoid and sedimentary rocks of the southern Snake Range,Nevada

Six diverse intrusive igneous types are exposed as discrete outcrops within an area of 900 km² in the southern Snake Range, White Pine County, Nevada. The previously recognized variety among these igneous types is reflected in the wide range of ¹⁸O values (–1.1 to 13.4 permil) found in these rocks. This range of ¹⁸O values probably results from differences in source material and post-crystallization history of the different intrusive types.The Jurassic intrusive of the Snake Creek-Williams Canyon area represents the chemical equivalent of a large part of a differentiation sequence, with the entire range of composition (63–76 percent SiO₂) exposed over a horizontal distance of about five km. The rather regular increase of ¹⁸O values from the most mafic to the most felsic parts of this pluton, together with ¹⁸O values determined for constituent minerals recovered from five of the samples, supports a fractional crystallization model. The high ¹⁸O values found (10.2–12.2 permil) indicate that the magma likely was derived from or assimilated sedimentary materials.Nine samples of the Cretaceous two-mica granite of the Pole Canyon-Can Young Canyon area have ¹⁸O values in the range 10.6–12.1 permil. These high ¹⁸O values, an initial⁸⁷Sr/⁸⁶Sr ratio of 0.7165, and the presence of muscovite along with an accessory mineral suite limited to monazite, apatite, zircon, and an allanite-like mineral, characterize this intrusive mass as an S-type granite. It probably formed through anatexis of late Precambrian pelitic rocks.The granitoid rock exposed in the Young Canyon-Kious Basin area is Tertiary (32 m.y.). Most of this intrusive has been cataclastically deformed as a result of late (18 m.y.) movement on the overlying Snake Range decollement. The undeformed portion of this intrusive has ¹⁸O values of 8.7–10.0 permil. However, the deformed portion of this intrusive has ¹⁸O values as low as –1.1 permil, apparently resulting from isotopic exchange between this rock and ground water at the time of cataclasis.Although the igneous types exposed in the southern Snake Range differ petrologically and range in age from Jurassic to Tertiary, most have relatively high ¹⁸O values compared with other granitoid rocks of the Basin-Range Province.