Boron isotope variations in nature: a synthesis

The large relative mass difference between the two stable isotopes of boron, ¹⁰B and ¹¹B, and the high geochemical reactivity of boron lead to significant isotope fractionation by natural processes. Published ¹¹B values (relative to the NBS SRM-951 standard) span a wide range of 90. The lowest ¹¹B values around — 30 are reported for non-marine evaporite minerals and certain tourmalines. The most ¹¹B-enriched reservoir known to date are brines from Australian salt lakes and the Dead Sea of Israel with ¹¹B values up to +59. Dissolved boron in present-day seawater has a constant world-wide ¹¹B value of + 39.5. In this paper, available ¹¹B data of a variety of natural fluid and solid samples from different geological environments are compiled and some of the most relevant aspects, including possible tracer applications of boron-isotope geochemistry, are summarized.

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Résumé La grande différence relative de masse entre les isotopes stables du bore, ¹⁰B et ¹¹B, et la grande réactivité geochimique du bore ont pour conséquence un fractionnement isotopique naturel important. Les valeurs de ¹¹B publiées (par rapport au standard NBS SRM-951) varient de 90. Les valeurs de ¹¹B les plus basses (–30) correspondent aux evaporites non-marines et à certaines tourmalines. Le réservoir le plus enrichi en ¹¹B est représenté par les saumures des lacs salés d' Australie et par la Mer Morte en Israël, qui ont des valuers de ¹¹B allent jusqu'à + 59. L'eau de mer a une valeur de ¹¹B mondialement constante de + 39.5. Des valeurs de ¹¹B des solutions naturelles ainsi que des roches et minéraux de différentes origines, publiées jusqu'à présent, sont présentées ici. En outre quelques aspects importants concernant la géochimie des isotopes du bore y compris quelques applications sont exposés.

     

Oxygen isotopes in mantle related and geothermally altered magmatites of the Transhimalayan (Gangdese) ranges

In closed magma systems SiO₂ approximately measures differentiation progress and oxygen isotopes can seem to obey Rayleigh fractionation only as a consequence of the behaviour of SiO₂. The main role of ¹⁸O is as a sensitive indicator of contamination, either at the start of differentiation ( ¹⁸O_init) or as a proportion of fractionation in AFC. Plots of ¹⁸O vs SiO₂-allow to determine initial ¹⁸O values for different sequences for source comparison. For NBS-28=9.60, the ¹⁸O at 48% SiO₂-varies between a high 6.4 for Kiglapait (Kalamarides 1984), 5.9 for Transhimalaya, 5.8 for Hachijo-Jima (Matsuhisa 1979), 5.6 for Koloula (Chivas et al. 1982) and a low 5.3 for the Darran Complex, New Zealand. The Transhimalayan batholiths (Gangdese belt) were emplaced in the Ladakh-Lhasa terrane, between the present-day Banggong-Nujiang, and Indus-Yarlung Tsangbo suture zones, after its accretion to Eurasia. The gradient of the least contaminated continuous ( ¹⁸O vs SiO₂-igneous trend line is similar to that of Koloula, and AFC calculations suggest a low secondary assimilation rate of less than 0.05 times the rate of crystallisation. Outliers enriched in ¹⁸O are frequent in the Lhasa, and apparently rare in the Ladakh transsect. Low- ¹⁸O (5.0–0) granitoids and andesites on the Lhasa-Yangbajain axis are the result of present day or recent near-surface geothermal activity; their quartzes still trace the granitoids to the Transhimalaya ¹⁸O trend line, but the distribution of low total rock or feldspar ¹⁸O values could be a guide to more recent heat flow and thermally marked tectonic lineaments. Two ignimbrites from Maqiang show hardly any ¹⁸O-contamination by crustal material.     

Hydrothermal alteration and REE-Th mineralization at the Rodeo de Los Molles deposit,Las Chacras batholith,central Argentina

REE (rare-earth-element) and Th mineralization at the Rodeo de Los Molles deposit occurs within an elliptical body of hydrothermally altered rocks (fenite) located in a biotite monzogranite of the Las Chacras batholith. Ore assemblages are found as isolated patches of intergrown britholite, allanite, apatite, bastnaesite, fluorite, sphene, quartz, and aegirine-augite, as well as nodules of uranothorite and late-stage veins of calcite, fluorite, and bastnaesite. Composition-volume computations suggest that the fenite was produced by alteration of the biotite monzogranite by addition of K and Na, and loss of Ca and Sr. Petrographic evaluations indicate that microcline and plagioclase have been replaced by perthite, and biotite was converted to aggregates of clinochlore, anatase, kaolinite, and hematite. Relict biotite is characterized by lower Fe/(Fe+Mg) and Ti values with progressive alteration. Fluorine-rich phlogopite is present in mineralized areas, but textural evidence suggests that it was not produced via biotite alteration. Mass-balance constraints also show that Ca and Mg in ore zones may result from redistribution, rather than their being a result of external derivation. The ¹⁸O values of quartz (8.6–11.1) and feldspar (7.8–10.6) suggest that feldspar continued to exchange oxygen isotopes with a fluid to lower temperatures than did quartz. Feldspars equilibrated with a fluid of ¹⁸O8 at a fluid/rock ratio less than 1. The ¹⁸O values of quartz and aegirine-augite that crystallized during REE mineralization also suggest equilibration with a fluid of ¹⁸O8. The D values of biotite (-83 to-120) are relatively low for igneous rocks and are thought to have resulted from exsolution of a D-enriched magmatic vapor. The D values of both mineralized and barren fenites are consistent with equilibration with fluid of magmatic origin. Meteoric water was involved in the production of calcite and clinochlore alteration, and late-stage calcite-fluorite-bastnaesite veins. The ¹³C values of calcite and bastnaesite (-7.8 to-13.5%) suggest that carbon was derived by leaching of carbon from igneous and/or enclosing metamorphic rock types, and that a majority of carbon ultimately was derived from sedimentary organic meterial.     

Schwefel-Isotopenuntersuchungen an der Blei-Zink-Erzlagerstätte Grund (Westharz,Bundesrepublik Deutschland)

Zusammenfassung Die S-Isotopenverteilung wurde an 67 Sulfid- und 17 Barytproben aus der Blei-Zink-Erzlagerstätte Grund untersucht. Die ³⁴S-Werte der Zinkblende der Mineralisationsphase II liegen im Westfeld-Erzmittel I und in den östlich anschließenden Erzmitteln zwischen +4 und +6, in dem am weitesten westlich liegenden Westfeld-Erzmittel II zwischen +6 und +10. Die Werte für Bleiglanz der Mineralisationsphase II sind +2 bis +4 bzw. +4 bis +7. Die Sulfide der Mineralisationsphase III haben allgemein niedrigere -Werte. Koexistierende Sulfide zeigen eine deutliche Fraktionierung, wobei stets _ZnS > _PbS ist; die Differenz beträgt in der Mineralisationsphase II im Mittel 1,8, in der Phase III 3. Dies deutet auf niedrigere Bildungstemperatur der Minerale der Phase III hin. Zur genetischen Deutung der beobachteten -Abnahme beim Übergang zur Mineralisationsphase II werden vier Modelle diskutiert. Baryte zeigen innerhalb der Lagerstätte recht einheitliche ³⁴S-Werte zwischen +11 und +14,5%. Diese Einheitlichkeit wird durch den Einfluß deszendenter Zechstein-Lösungen erklärt.

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³⁴S-values are given for 67 sulfide and 16 barite specimens from the Pb-Zn-deposit Grund (Harz mountains, W-Germany). In the central part of the deposit the sulfide 's of the first major mineralization (phase II) range from: ZnS +4 to +6 and PbS +2 to +4. The sulfides of the second major mineralization (phase III) are depleted in ³⁴S and range from: ZnS +2 to +4, PbS –1,4 to 3. The sulfides at the western end of the vein system are heavier; the phase II minerals ranging from: ZnS +6 to 10 and PbS +4 to 7. The mean -difference between co-existing ZnS and PbS in phase II is 1,8, in phase III 3. This indicates lowering of temperature of formation for the phase III ore. Four models have been set up in order to explain the observed -variation. Barites with rather uniform 's from +11 to +14,5 are probably affected by descendent solutions from overlying sulfate sediments of Permian age.

     

Oxygen,sulfur, and strontium isotope and fluid inclusion studies of barite deposits from the Iglesiente-Sulcis mining district,Southwestern Sardinia,Italy

This paper deals with barite from stratiform, karst, and vein deposits hosted within Lower Paleozoic rocks of the Iglesiente-Sulcis mining district in southwestern Sardinia. For comparison sulfates from mine waters are studied. Stratiform barite displays ³⁴S=28.8–32.1, ¹⁸O=12.7–15.6, and ⁸⁷Sr/⁸⁶Sr=0.7087, in keeping with an essentially Cambrian marine origin of both sulfate and strontium. Epigenetic barite from post-Hercynian karst and vein deposits is indistinguishable for both sulfur and oxygen isotopes with ³⁴S=15.3–26.4 and ¹⁸O=6.6–12.5; ⁸⁷Sr/⁸⁶Sr ratios vary 0.7094–0.7140. These results and the microthermometric and salinity data from fluid inclusions concur in suggesting that barite formed at the site of mineralization by oxidation of reduced sulfur from Cambrian-Ordovician sulfide ores in warm, sometimes hot solutions consisting of dilute water and saline brine with different ¹⁸O values. The relative proportion of the two types of water may have largely varied within a given deposit during the mineralization. In the karst barite Sr was essentially provided by carbonate host rocks, whereas both carbonate and Lower Paleozoic shale host rocks should have been important sources for Sr of the vein barite. Finally, ³⁴S data of dissolved sulfate provide further support for the mixed seawater-meteoric water composition of mine waters from the Iglesiente area.     

Stable isotope constraints on the origin and depth of penetration of hydrothermal fluids associated with Hercynian regional metamorphism and crustal anatexis in the Pyrenees

Late Carboniferous (Hercynian) tectonism in the Pyrenees generated extremely steep thermal gradients at 8–14 km depth in the continental crust, producing andalusite- and sillimanite-grade metamorphism and partial melting of Lower Paleozoic metasediments under water-rich conditions. At the same time, amphibolite- and granulitefacies basal gneisses were equilibrated under dryer conditions at pressures of 4 to 7 kbar (14–25 km depth), beneath these higher-level rocks. We present 95 new oxygen isotopic analyses of samples from the Agly, St. Barthelemy, Castillon and Trois Seigneurs Massifs, highlighting contrasting ¹⁸O/¹⁶O systematics at different structural levels in the Hercynian crust, here termed Zones 1, 2, and 3. The unmetamorphosed, fossiliferous, Paleozoic shales and carbonates of Zone 1 have typical sedimentary ¹⁸O values, mostly in the range +14 to +16 for the pelitic rocks and +20 to +25 for the carbonates. The metamorphosed equivalents of these rocks in Zone 2 all have strikingly uniform and much lower ¹⁸O values; the metapelites mostly have ¹⁸O=+10 to +12, and interlayered metacarbonates from the Trois Seigneurs Massif have ¹⁸O of about +12 to +14. Typically, the Zone 3 basal gneisses are isotopically heterogeneous with variable ¹⁸O values ranging from +6 in mafic lithologies to +22 in carbonate-rich lithologies. Steep gradients in ¹⁸O (as much as 10 per mil over a few cm) are preserved at the margins of some metacarbonate layers. These data indicate that the Zone 3 gneisses were infiltrated by much smaller volumes of metamorphic pore fluids than were the overlying Zone 2 rocks, and that circulation of surface-derived H₂O (either seawater or formation waters, as evidenced by high D values) was mainly confined to the Paleozoic supracrustal sedimentary pile. This is compatible with an overall reduction of interconnected porosity with increasing depth, but perhaps even more important, the extensive partial melting at the base of Zone 2 may have produced a ductile, impermeable barrier to downward fluid penetration.Contribution No. 4287, Publication of the Division of Geological and Planetary Sciences, California Institute of Technology     

Oxygen and carbon isotopic study on the genesis of the Steirischer Erzberg siderite deposit (Austria)

The carbon and oxygen isotopic composition of Fe-carbonate ore and its calcitic to dolomitic Devonian host rocks at the Steirischer Erzberg siderite deposit (Greywacke zone, Upper Austroalpine Unit) were determined in order to constrain the source and nature of the Fe-rich mineralizing fluid. The ¹⁸O-values obtained for various Fe-carbonate generations and the carbonate host lie within a similar range between + 14.6 and + 21.6 (V-SMOW). No good correlation exists between the relative ages of the carbonate phases and their O isotopic composition. The variation in ¹⁸O-values is due to metamorphic recrystallization with locally variable fluid/rock ratios. The average ¹³C-value of the carbonate host is +0.5 ± 1.2 (PDB) which corresponds well to worldwide Phanerozoic marine carbonate values. The first Fecarbonate generation has slightly lower ¹³C-values, on average -1.4 ± 0.8 (PDB). Recrystallization of both the carbonate host minerals and the ankerite/siderite led to significantly lower ¹³C-values of -4.2 ± 0.6 and-4.7 ± 0.7, respectively. Within the basal breccia of the post-Hercynian transgression series matrix calcite/ dolomite shows an average ¹³C-value of -2.9 ± 0.7, and matrix siderite/ankerite an average value of-4.1 ± 0.4. These data, together with Sr isotope data published previously, strongly support a late-diagenetic or epigenetic first Fe-mineralization from convecting formation waters. They ascended along extension faults and were driven by an increased heat flow caused by crustal thinning during a Devonian rifting phase that initiated the separation of the Noric terrane from Africa. A potential source of the Fe could have been the underlying Ordovician acid volcanics. Regional metamorphism related to collision tectonics in the Late Carboniferous (Hercynian) and later during the Alpine orogeny, caused intensive recrystallization and partial mobilization of the various carbonate phases.     

Sulfur isotope ratios of Icelandic rocks

Sulfur isotope ratios have been determined in 27 selected volcanic rocks from Iceland together with their whole rock chemistry. The ³⁴S of analyzed basalts ranges from –2.0 to +0.4 with an average value of –0.8 Tholeiitic and alkaline rocks exhibit little difference in ³⁴S values but the intermediate and acid rocks analyzed have higher ³⁴S values up to +4.2 It is suggested that the overall variation in sulfur isotope composition of the basalts is caused by degassing. The small range of the ³⁴S values and its similarity to other oceanic and continental basalts, suggest that the depleted mantle is homogeneous in its sulfur isotope composition. The ³⁴S of the depleted mantle is estimated to be within the range for undegassed oceanic basalts, –0.5 to +1.0     

Marine hdrothermal alteration at a Kuroko ore deposit,Kosaka, Japan

Isotopic compositions were determined for quartz, sericite and bulk rock samples surrounding the Uwamuki no. 4 Kuroko ore body, Kosaka, Japan. ¹⁸O values of quartz from Siliceous Ore (S.O.), main body of Black Ore B.O.) and the upper layer of B.O. are fairly uniform, +8.7 to +10.5. Formation temperatures calculated from fractionation of ¹⁸O between sericite and quartz from B.O. and upper S.O. are 250° to 300° C. The ore-forming fluids had ¹⁸O values of +1 and D values of –10, from isotope compositions of quartz and sericite.Tertiary volcanic rocks surrounding the ore deposits at Kosaka have uniform ¹⁸O values, +8.1±1.0 (n=50), although their bulk chemical compositions are widely varied because of different degrees of alteration. White Rhyolite, which is an intensely altered rhyolite occurring in close association with the Kuroko ore bodies, has also uniform ¹⁸O values, +7.9±0.9 (n=19). Temperatures of alteration are estimated to be around 300° C from the oxygen isotope fractionation between quartz and sericite. Paleozoic basement rocks phyllite and chert, have high ¹⁸O values, +18 and +19. The Sasahata formation of unknown age, which lies between Tertiary and Paleozoic formations, has highly variable ¹⁸O, +8 to +16 (n=4). High ¹⁸O values of the basement rocks and the sharp difference in ¹⁸O at their boundary suggest that the hydrothermal system causing Kuroko mineralization was mainly confined within permeable Tertiary rocks. D values of altered Tertiary volcanic rocks are highly variable ranging from –34 to –64% (n=12). The variation of D does not correlate with change of chemical composition, ¹⁸O values, nor distance from the ore deposits. The relatively high D values of the altered rocks indicate that the major constituent of the hydrothermal fluid was sea water. However, another fluid having lower D must have also participated. The fluid could be evolved sea water modified by interaction with rocks and the admixture of magmatic fluid. The variation in D may suggest that sea water mixed dispersively with the fluid.     

Mineralogical and geochemical compositions of modern bivalve shells from the Mediterranean coast of Egypt

The stable isotopic composition (¹³C and ¹⁸O) and elemental (Sr and Mg) of marine molluscs are presented for Carditacea and Solenacea shells collected off the Mediterranean coast of Egypt. Based on shell microstructures and mineralogy, the bivalve shells are preserved in their original mineralogy and chemistry.The Sr and Mg concentrations of the bivalve shells have mean values of 1960 ppm and 226 ppm respectively. The stable isotopic composition generally show high values of ¹⁸O and ¹³C. The ¹⁸O values range from +0.1 to –1.8 PDB and most shells are highly enriched in¹³C; averaging +2.5 PDB. These elemental and isotopic signatures are analogous to modern marine bivalves from other localities.The oxygen and carbon isotopes, together with the calculated temperatures, suggest that the aragonitic bivalve shells were precipitated in isotopic equilibrium from warm marine waters.     

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.     

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.     

Formation of Mud-Volcanic Fluids in Taman (Russia) and Kakhetia (Georgia): Evidence from Boron Isotopes

Temperatures of the formation of mud-volcanic waters are determined based on concentrations of some temperature-dependent components (Na–Li, Mg–Li). Estimates obtained for the Taman and Kakhetia regions are similar and range from 45 to 170°, which correspond to depths of 1–4.5 km. The calculated temperatures correlate with the chemical (Li, Rb, Cs, Sr, Ba, B, I, and HCO₃) composition of water and ¹³ (₂) and ¹³ (CH₄) values in spontaneous gases. The isotope values indicate that mechanisms of the formation of ¹³-rich gases, i.e., gases with high ¹³ values (up to +16.0 in ₂ and –23.4 in CH₄) in mud-volcanic systems of Taman and Kakhetia are governed by fluid-generation temperatures rather than the supply of abyssal gases. The ¹¹ value was determined for the first time in mud-volcanic products of the Caucasus region. This value ranges from +22.5 to +39.4 in the volcanic water of Georgia, from –1.2 to +7.4 in the clayey pulp of Georgia, and from –7.6 to +13.2 in the clayey pulp of Taman. It is shown that the ¹¹ value in clay correlates with the fluid-generation temperature and ¹¹ correlates with ¹³ in carbon-bearing gases. These correlations probably testify to the formation of different phases of mud-volcanic emanations in a single geochemical system and suggest the crucial role of temperature in the development of isotope-geochemical features.     

Chemical and isotopic variations in an iron-rich lava flow from the Kirkpatrick Basalt,north Victoria Land,Antarctica: implications for low-temperature alteration

Chemical and isotopic (Sr, O, H) variations have been examined in an iron-rich lava flow of the Kirkpatrick Basalt from the Mesa Range in north Victoria Land, Antarctica. The flow is homogeneous with respect to the less mobile elements, whereas variations observed in K, Na, Si, Fe, and Rb result largely from alteration of glassy matrix material. Whole-rock Rb–Sr isotope data fall along a poorly-defined 103 Ma array attributed to secondary mobilization of Rb during the mid-Cretaceous. Alteration at that time is suggested by paleomagnetic data and would also account for discordant K–Ar dates. Whole-rock ¹⁸O values vary from +5.8 to +8.2 and a plagioclase separate has a ¹⁸O value of +5.6, reflecting the original composition of the magma. The range of ¹⁸O values for the whole-rock samples results from low-temperature alteration occurring primarily in the Jurassic and/or mid-Cretaceous. Whole-rock D values (-201 to -243) are markedly depleted, approaching equilibrium with modern meteoric water. In light of these data, variable Sr and O isotopic ratios in the underlying sequence of flows, previously interpreted in terms of an assimilation-fractionation model, may largely reflect post-magmatic alteration.     

Modification of δD values in eastern Nevada granitoid rocks spatially related to thrust faults

Stable isotope data have been determined for 13 Mesozoic and Tertiary plutons in eastern Nevada and nearby Utah. In the southern Snake Range of eastern Nevada, where relations are best exposed and have been most intensively studied, D, ¹⁸O, and apparent K-Ar ages depend on proximity to the Snake Range decollement. Where stresses resulting from late movement on the decollement have caused cataclasis of Oligocene (37 Ma) granitoid rock, ¹⁸O, D, and K-Ar age values as low as –2.5, –155, and 18 Ma, respectively, have been determined. Where there has been no cataclasis, ¹⁸O values of Jurassic, Cretaceous, and Oligocene granitoid rocks are apparently unaffected, but both D values and K-Ar ages have been modified for distances of tens of meters below the decollement.Results similar to those in the southern Snake Range have been observed in other eastern Nevada granitoid rocks spatially related to regional thrust faults, as in the Kern Mountains, the Toana Range, and the northern Egan Range. In each of these areas cataclasis or deformation of granitoid rocks has resulted in lowered ¹⁸O, D, and K-Ar age values. Where there has been no cataclasis or deformation, ¹⁸O values are unaffected, but both D and K-Ar age values have been lowered by stresses resulting from postcrystallization movement along overlying thrust faults.Many of the plutons discussed have not been deeply eroded, and spatially related thrust faults crop out. Where thrust faults are not in evidence and the granitoid rocks give D values lower than about –130 along with spuriously low K-Ar age results, modification of the D and K-Ar age values may have been caused by stresses related to late movement along an overlying (now eroded) thrust fault.     

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     

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.     

The Candelaria silver deposit,Nevada — preliminary sulphur,oxygen and hydrogen isotope geochemistry

The pre-Cenozoic geology at Candelaria, Nevada comprises four main lithologic units: the basement consists of Ordovician cherts of the Palmetto complex; this is overlain unconformably by Permo-Triassic marine clastic sediments (Diablo and Candelaria Formations); these are structurally overlain by a serpentinitehosted tectonic mélange (Pickhandle/Golconda allochthon); all these units are cut by three Mesozoic felsic dike systems. Bulk-mineable silver-base metal ores occur as stratabound sheets of vein stockwork/disseminated sulphide mineralisation within structurally favourable zones along the base of the Pickhandle allochthon (i.e. Pickhandle thrust and overlying ultramafics/mafics) and within the fissile, calcareous and phosphatic black shales at the base of the Candelaria Formation (lower Candelaria shear). The most prominent felsic dike system — a suite of Early Jurassic granodiorite porphyries — exhibits close spatial, alteration and geochemical associations with the silver mineralisation. Disseminated pyrites from the bulk-mineable ores exhibit a ³⁴S range from — 0.3 to + 12.1 (mean ³⁴S = +6.4 ± 3.5, 1, n = 17) and two sphalerites have ³⁴S of + 5.9 and + 8.7 These data support a felsic magmatic source for sulphur in the ores, consistent with their proximal position in relation to the porphyries. However, a minor contribution of sulphur from diagenetic pyrite in the host Candelaria sediments (mean ³⁴S = — 14.0) cannot be ruled out. Sulphur in late, localised barite veins ( ³⁴S = + 17.3 and + 17.7) probably originated from a sedimentary/seawater source, in the form of bedded barite within the Palmetto basement ( ³⁴S = + 18.9). Quartz veins from the ores have mean ¹⁸O = + 15.9 ± 0.8 (1, n = 10), which is consistent, over the best estimate temperature range of the mineralisation (360°–460°C), with deposition from ¹⁸O-enriched magmatic-hydrothermal fluids (calculated ¹⁸O fluid = + 9.4 to + 13.9). Such enrichment probably occurred through isotopic exchange with the basement cherts during fluid ascent from a source pluton. Whole rock data for a propylitised porphyry ( ¹⁸O = + 14.2, D = — 65) support a magmatic fluid source. However, D results for fluid inclusions from several vein samples (mean = — 108 ± 14, 1, n = 6) and for other dike and sediment whole rocks (mean = — 110 ± 13, 1, n = 5) reveal the influence of meteoric waters. The timing of meteoric fluid incursion is unresolved, but possibilities include late-mineralisation groundwater flooding during cooling of the Early Jurassic progenitor porphyry system and/or meteoric fluid circulation driven by Late Cretaceous plutonism.     

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.     

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.