Oxygen Isotope Disequilibrium Research Articles

Use of stable oxygen isotope determinations from brachiopod shells in palaeoenvironmental reconstruction

Stable oxygen isotope analyses were obtained from fossil brachiopod shells from a shallow marine deposit of Upper Pleistocene age (approximately 80 ka) in New Zealand as part of a palaeoenvironmental investigation. Bulk analyses of entire shells of Calloria inconspicua yielded highly variable results (a range of 2.30‰), rendering these data of limited use as palaeoenvironmental proxies. Attempts to obtain consistent oxygen isotope values by removing areas of the brachiopod shell thought to be prone to oxygen isotope disequilibrium effects (‘vital effects’) revealed that the matching anterior portions of ventral and dorsal valves from the same individual (which were secreted at the same time) also yielded different oxygen isotope ratios. The dorsal valve δ 18O values were higher and less variable than those obtained from the ventral valve of the same individual. The investigation was then extended to living representatives of the same species from a similar shallow water habitat from the South Island of New Zealand. The pattern of dorsal valves yielding higher and less variable oxygen isotope values than matching areas of the ventral valves was repeated in seven out of eight individuals investigated, although there was considerable variation in the absolute values from specimen to specimen. Contrary to common practice therefore, it seems that in this species it is the much thinner and lighter dorsal valve that provides a more consistent isotopic measurement. If converted into absolute temperature estimates, the dorsal valve measurements correspond to a temperature midway between the mean and maximum annual temperature experienced by this species in its present-day habitat, which may make it acceptable as a palaeoenvironmental indicator. The high level of variation between individuals (including an inferred temperature from one ventral valve above the mean maximum temperature experienced) indicates that a range of individuals needs to be investigated in any palaeoenvironmental investigation to avoid misleading interpretations. However, intra- and inter-specimen variation in isotopic composition may provide valuable palaeobiological information on growth rates, metabolism and possibly also pathological conditions in the past.

Oxygen isotope equilibrium between eclogite minerals and its constraints on mineral Sm-Nd chronometer

Sm-Nd and oxygen isotope analyses were carried out for mineral separates of ultrahigh pressure eclogites from the Sulu terrane in eastern China. The results show a direct correspondence in equilibrium or disequilibrium state between the oxygen and Sm-Nd isotope systems of eclogite minerals. The omphacite-garnet pairs of oxygen isotope equilibrium at eclogite-facies conditions yield meaningful Triassic Sm-Nd isochron ages, whereas those of oxygen isotope disequilibrium give non-Triassic ages of geological meaninglessness. This can be reasonably interpreted by the fact that the rates of oxygen diffusion in garnet and pyroxene are lower than, or close to, those of Nd diffusion, and thus attainment of isotopic equilibrium in the omphacite-garnet O system suggests achievement of Nd isotope equilibrium in the same mineral pairs. The presence or absence of fluid in the eclogite protoliths is a major rate-controlling factor for isotopic equilibration during high-grade metamorphism. It appears that the state of oxygen isotope equilibrium between cogenetic minerals can provide a critical test for the validity of the Sm-Nd mineral chronometer. In addition, the exact timing of the ultrahigh pressure metamorphism in the Dabie-Sulu terranes is constrained at Early Triassic rather than Late Triassic.

Kinetic mechanism of oxygen isotope disequilibrium in precipitated witherite and aragonite at low temperatures: an experimental study

To study what dictates oxygen isotope equilibrium fractionation between inorganic carbonate and water during carbonate precipitation from aqueous solutions, a direct precipitation approach was used to synthesize witherite, and an overgrowth technique was used to synthesize aragonite. The experiments were conducted at 50 and 70°C by one- and two-step approaches, respectively, with a difference in the time of oxygen isotope exchange between dissolved carbonate and water before carbonate precipitation. The two-step approach involved sufficient time to achieve oxygen isotope equilibrium between dissolved carbonate and water, whereas the one-step approach did not. The measured witherite-water fractionations are systematically lower than the aragonite-water fractionations regardless of exchange time between dissolved carbonate and water, pointing to cation effect on oxygen isotope partitioning between the barium and calcium carbonates when precipitating them from the solutions. The two-step approach experiments provide the equilibrium fractionations between the precipitated carbonates and water, whereas the one-step experiments do not. The present experiments show that approaching equilibrium oxygen isotope fractionation between precipitated carbonate and water proceeds via the following two processes: 1.Oxygen isotope exchange between [CO3]2− and H2O: (1)[C16O3]2−+2H218O=[C18O216O]2−+2H216O2.A combination of divalent metal cation M2+ with the [CO3]2− to form carbonate: (2)M2++[C18O216O]2−+H218O=MC18O3+H216OReaction 1 is the rate-limiting step for equilibrium oxygen isotope fractionation between carbonate and water, and reaction 2 bears the structural effect of carbonate crystallization on oxygen isotope fractionation during carbonate precipitation. The present results show that the time of [CO3]2−-H2O isotope exchange in the precipitation experiments is of critical importance in dictating the extent of isotopic equilibration between precipitated carbonate and water. If the time of oxygen isotope exchange between [CO3]2− and H2O is long enough to attain equilibrium before carbonate precipitation, the precipitated carbonate is able to achieve isotopic equilibration with water.

Oxygen isotope evidence for two-stage water-rock interactions of the Nianzishan A-type granite in NE China

The oxygen isotope ratios of whole-rock, common rock-forming minerals and zircon from Mesozoic A-type granitic pluton at Nianzishan in northeastern China were analyzed by the conventional BrF5 method and the laser-probe technique, respectively. Both whole-rock and rock-forming minerals show large δ18O variations up to 5.5‰ with significant oxygen isotope disequilibrium between zircon and the other minerals, whereas the δ18O values of zircon are tightly clustered between 3.12‰ and 4.19‰ and thus lower than the normal-mantle δ18O values. These results indicate that the Nianzishan A-type granite experienced two-stage water-rock interactions subsequentially. The remarkably low zircon δ18O values are genetically due to seawater exchange with granite protolith in the first stage, and the oxygen isotope disequilibrium fractionations between zircon and rock-forming minerals are caused by meteoric-hydrothermal alteration in the second stage. It is inferred that the18O-depleted A-type granitic magma was derived from partial melting of subducted lower oceanic crust which was isotopically exchanged with seawater at high temperatures. In the process of granite emplacement into the upper crust, meteoric-hydrothermal circulation was triggered to overprint crystallizing granite under subsolidus conditions.

Geochemical constraints of the eclogite and granulite facies metamorphism as recognized in the Raobazhai complex from North Dabie Shan, China

AbstractA combined study of major and trace elements, fluid inclusions and oxygen isotopes has been carried out on garnet pyroxenite from the Raobazhai complex in the North Dabie Terrane (NDT). Well‐preserved compositional zoning with Na decreasing and Ca and Mg increasing from the core to rim of pyroxene in the garnet pyroxenite indicates eclogite facies metamorphism at the peak metamorphic stage and subsequent granulite facies metamorphism during uplift. A P–T path with substantial heating (from c. 750 to 900 °C) after the maximum pressure reveals a different uplift history compared with most other eclogites in the South Dabie Terrane (SDT). Fluid inclusion data can be correlated with the metamorphic grade: the fluid regime during the peak metamorphism (eclogite facies) was dominated by N2‐bearing NaCl‐rich solutions, whereas it changed into CO2‐dominated fluids during the granulite facies retrograde metamorphism. At a late retrograde metamorphic stage, probably after amphibolite facies metamorphism, some external low‐salinity fluids were involved. In situ UV‐laser oxygen isotope analysis was undertaken on a 7 mm garnet, and impure pyroxene, amphibole and plagioclase. The nearly homogeneous oxygen isotopic composition (δ18OVSMOW = c. 6.7‰) in the garnet porphyroblast indicates closed fluid system conditions during garnet growth. However, isotopic fractionations between retrograde phases (amphibole and plagioclase) and garnet show an oxygen isotopic disequilibrium, indicating retrograde fluid–rock interactions. Unusual MORB‐like rare earth element (REE) patterns for whole rock of the garnet pyroxenite contrast with most ultra‐high‐pressure (UHP) eclogites in the Dabie‐Sulu area. However, the age‐corrected initial εNd(t) is − 2.9, which indicates that the protolith of the garnet pyroxenite was derived from an enriched mantle rather than from a MORB source. Combined with the present data of oxygen isotopic compositions and the characteristic N2 content in the fluid inclusions, we suggest that the protolith of the garnet pyroxenite from Raobazhai formed in an enriched mantle fragment, which has been exposed to the surface prior to the Triassic metamorphism.

Fluid generation, vein formation and the degree of fluid-rock interaction during decompression of high-pressure terranes: the Schistes Lustrés, Alpine Corsica, France

Centimetre- to decimetre-wide quartz+calcite veins in schistes lustres from Alpine Corsica were formed during exhumation at 30–40 Ma following blueschist facies metamorphism. The δ18O and δ13C values of the veins overlap those of the host schistes lustres, and the δ18O values of the veins are much higher than those of other rocks on Corsica. These data suggest that the vein-forming fluids were derived from the schistes lustres. Fluids were probably generated by reactions that broke down carpholite, lawsonite, chlorite and white mica at 300–350 °C during decompression between c. 1400 and 800 MPa. However, the δ18O values of the veins are locally several per mil higher than expected given those of their host rocks. The magnitude of oxygen isotope disequilibrium between the veins and the host rock is inversely proportional to the δ18O value of the host rock. Additionally, calcite in some schists is in isotopic equilibrium with calcite in adjacent veins, but not with the silicate fraction of the schists. Locally, the schists are calcite bearing only within 1–20 cm of the veins. The vein-forming fluids may have been preferentially derived from calcite-bearing, high-δ18O rocks that are common within the schistes lustres and that locally contain abundant (>15%) veins. If the fluids were unable to completely isotopically equilibrate with the rocks, due to relatively rapid flow at moderate temperatures or being confined to fractures, they could form veins with higher δ18O values than those of the surrounding rocks. Alteration of the host rocks was probably inhibited by isolation of the fluid in ‘quartz-armoured’ veins. Overall, the veins represent a metre- to hectometre-scale fluid-flow system confined to within the schistes lustres unit, with little input from external sources. This fluid-flow system is one of several that operated in the western Alps during exhumation following high-pressure metamorphism.

The Geology and Isotope Geochemistry of the Talc Deposits ofPuebla de Lillo (Cantabrian Zone, Northern Spain)

The talc deposits of Puebla de Lillo (Cantabrian zone, Variscan belt of Iberia) are hosted by hydrothermal dolostones replacing Carboniferous limestones and are generally located adjacent to Ordovician quartzites. The talc orebodies occur as metasomatic replacement zones of the dolostones near faults or lithologic contacts and, to a lesser extent, as replacements of the nearby quartzites or slates. These rocks are affected by a very low grade regional metamorphism but occur in a highly deformed setting with abundant thrusts and faults. Dolostones similar to those hosting the talc deposits are widespread in the Cantabrian zone; most of them are barren, although a few host epithermal sulfide mineralization. Four hydrothermal events are recognized. The first two formed gray (I) and beige (II) hydrothermal dolostones (± quartz), very similar to the regional barren dolostones. The third, a white coarse-grained dolomite (III) is directly associated with the talc. The final episode is characterized by late-stage vuggy dolomite (IV), calcite, and quartz. Fluid inclusion studies in dolomite and quartz suggest that the dolomitizing fluids were aqueous NaCl-CaCl2 brines of variable salinities (0–23 wt % NaCl equiv) and a low CO2 content (0.003–0.01 XCO2). Thermodynamic considerations, fluid inclusion data, and comparison to regional studies suggest that dolomite (events I and II) formed at temperatures between 100° and 280°C, the talc-forming event (III) was between 280° and 405°C, and the postore carbonates and quartz (IV) precipitated between 65° and 170°C. Fluid pressure during the hydrothermal event was low, probably between 165 and 450 bars. The δ 18O value of the dolomite reflects the interaction of an infiltrating 18O-depleted fluid with the country-rock limestones having δ 18OSMOW = 22.5 to 33.2 per mil and leading to a concomitant lowering of the average δ 18O value to 19.9 ± 4.0 (dolostone I), 17.2 ± 6.2 (dolostone II), and 16.8 ± 1.9 per mil (dolomite III) during the different dolomitization events. The talc replacing the dolostone has a monotonous δ 18O signature of 10.7 to 12.7 per mil and a δ 18SMOW composition of –64 to –62 per mil. However, the talc replacing quartzites, slates, or impure dolostones has more variable and lower δ 18O values (8.1, 9.0, and 11.5‰, respectively). The late dolomite IV has a δ 18O composition of 14.7 ± 3 per mil, whereas the associated quartz has a δ 18O of 13.0 to 15.7 per mil. The oxygen isotope results are indicative of a systematic oxygen isotope disequilibrium between the hydrothermal minerals. The combined δ 18O-87Sr/86Sr data support a complex hydrothermal evolution involving the mixing of two fluids. Fluids related to the dolomitization of the limestones were dominated by deep brines equilibrated with the underlying siliciclastic rocks ( δ 18O> 5–7‰; 87Sr/86Sr > 0.7090). They gradually mixed with a meteoric fluid with incomplete equilibration with the basement. The hydrothermal fluid during talc formation had an isotopic signature of δ 18O = 5.0 to 7.9 per mil and δ 18 close to –70 per mil, whereas the postore hydrothermal fluids were mostly meteoric in origin ( δ 18O 0.7086). The δ 13C values of all hydrothermal carbonates investigated (–2 to +4.9‰) are indicative of carbon being buffered by the host limestones, in agreement with the observed low XCO2 of the fluid. The isotopic composition of sulfur in pyrite ( δ 34SCDT;– 3.8 to +10.2‰) also indicates derivation from two different sources, the host limestones containing sulfides with high δ 34S formed by the abiogenic reduction of seawater sulfates and siliciclastic rocks from the basement having isotopically light sulfur of bacteriogenic origin. The talc mineralization of Puebla de Lillo resulted from a regional-scale episodic hydrothermal activity related to early Alpine wrench faulting. The mineralization is located along tensional structures adjacent to the deep-seated Cofinal strike-slip fault, a major structure of the Cantabrian zone. These extensional faults acted as conduits for large amounts of hydrothermal fluids. Upwelling silica and Mg-rich brines reacted with the carbonate rocks, producing the dolomitization and superimposed talc mineralization. The critical factor in the formation of the talc instead of the regional dolostones (+stable quartz) was a local increase in temperature. This localized high-heat flow cannot be solely explained by normal geothermal gradients and suggests the existence of a deep igneous intrusion near the fault, which provided heat that drove the hydrothermal system. The role of the nearby quartzites was probably to saturate the hydrothermal fluids in silica as they were most likely SiO2 undersaturated due to heating after equilibration with the basement.

An oxygen and carbon isotopic study of multiple episodes of fluid flow in the Dalradian and Highland Border Complex, Stonehaven, Scotland

The carbon and oxygen systematics of rocks north of Stonehaven, Scotland were studied to provide new constraints on the nature and timing of metamorphic and post‐metamorphic fluid infiltration. Carbon and oxygen isotopic data were collected from carbonated spilites and carbonate‐bearing veins in the Highland Boundary Fault, from Dalradian metacarbonate layers and pervasively carbonated schists, and from three generations of carbonate‐bearing veins, and a quartz porphyry dyke within the Dalradian. Isotopic evidence for syn‐metamorphic fluid infiltration in the Dalradian is preserved in quartz–calcite veins in the upper chlorite zone and in staurolite zone metacarbonates. δ 18 O values for coexisting quartz (δ 18 O quartz = 13.6‰) and calcite (δ 18 O calcite = 11.6‰) in a vein from the upper chlorite zone are consistent with precipitation of both minerals from a single fluid at c. 375°C. Oxygen isotopic data from staurolite zone metacarbonate layers (δ 18 O calcite = 10.4 to 12.9‰) suggest major isotopic exchange with an external fluid equilibrated with a quartzo‐feldspathic reservoir, such as the Dalradian metasediments, at metamorphic temperatures. Low, negative carbon isotope data from these metacarbonate layers (δ 13 C calcite = −12.4 to −17.7‰) may indicate a component of oxidized organic material in the infiltrating fluid. Post‐metamorphic fluids derived from or equilibrated with a high‐temperature silicate reservoir such as felsic igneous rocks were responsible for precipitation of post‐metamorphic vein carbonate, carbonation of pelitic schists, and alteration of metamorphic feldspar porphyroblasts from Barrow’s famous spotted chloritoid schists in an area geographically associated with several carbonated quartz porphyry dykes. Some veins in the biotite and garnet zones have been conduits for multiple generations of fluid flow. Quartz from this generation of veins preserves a metamorphic isotopic signature (δ 18 O quartz = 11.2 to 13.2‰) whereas dolomite and calcite are considered to be post‐metamorphic based on textural evidence and isotopic data indicating extreme oxygen isotopic disequilibrium between vein quartz and carbonate minerals (δ 18 O dolomite = 23.7 to 24.2‰, δ 18 O calcite = 25.3‰, δ 18 O quartz = 11.2 to 13.2‰). Variably depleted carbon isotopes from carbonate minerals in these veins (δ 13 C dolomite = −8.1 to −13.4‰, δ 13 C calcite = −12.2‰) may reflect input of oxidized organic material in the mineralizing fluid. Isotopic data from carbonate veins and variably carbonated spilites from the Highland Boundary Fault are consistent with hydrothermal alteration of basalts by seawater, which had its carbon isotope systematics modified by addition of magmatic CO 2 and/or oxidation of organic carbon. Such alteration may have occurred on the seafloor or during emplacement in the Highland Boundary Fault.

Recent fluid processes in the Kaapvaal Craton, South Africa: coupled oxygen isotope and trace element disequilibrium in polymict peridotites

Oxygen-isotope mapping of thin sections of polymict peridotite xenoliths shows that significant oxygen isotope disequilibrium is preserved on a sub-millimetre scale in primary and secondary minerals. Primary porphyroblastic phases (e.g., olivine, orthopyroxene, garnet, diopside) tend to have higher δ 18O ratios than secondary minerals (e.g., mica, ilmenite, neoblastic olivine, orthopyroxene rims). Polymict minerals have a lower oxygen isotope composition than ‘average mantle’ (δ 18O=5.2±0.3‰) and show clear evidence of inter- and intra-mineral oxygen isotope disequilibrium. Disequilibrium is also evident in the elemental geochemistry of the mantle minerals and a general correlation exists between oxygen isotopes and major (Si, Mg, Ca, Fe) and trace elements (Ce, Cr, Zr, Nb, REE). The interpretation that isotopic heterogeneity may relate to melt processes is supported by δ 18O zonation in garnets, significant isotopic variation close to secondary veins, δ 18O (primary phases)>δ 18O (secondary phases) and oxygen isotope disequilibria in many minerals. In addition, a positive correlation between δ 18O and grain size indicates a role for deformation processes as a result of diffusion reactions perhaps inextricably linked to melt processes. We suggest that polymict peridotites formed as a result of movement along mantle shear zones which led to the juxtaposition of minerals of varied provenance. Contemporaneous melt transfer reacted with these mantle breccias and rapid entrainment by ‘kimberlite’ meant that any associated mineral disequilibrium was very effectively preserved.

RECENT FLUID PROCESSES IN THE KAAPVAAL CRATON, SOUTH AFRICA: OXYGEN ISOTOPE DISEQUILIBRIUM IN POLYMICT PERIDOTITE BRECCIAS

Oxygen-isotope mapping (Zhang et al., 1999) of primary and secondary phases in South African polymict peridotite xenoliths (Lawless et al., 1979) shows, for the first time, that significant oxygen isotope disequilibrium is preserved on a millimetre scale. Primary porphyroblastic phases (e.g. olivine, orthopyroxene, garnet, diopside) tend to have higher d18O ratios than secondary minerals (e.g. mica, ilmenite, neoblastic olivine, orthopyroxene rims). On the scale of a thin section, minerals have oxygen isotope heterogeneity that extends beyond that observed in most mantle rocks (Mattey and MacPherson, 1993) including diamond-, garnet- & spinel-facies peridotites (Lowry et al., 1999) regardless of whether they are wet or dry (Chazot et al., 1997), low or high temperature, sheared or granular. Polymict minerals have a lower oxygen isotope composition than “average mantle” (d18O= 5.2±0.3‰), indicative of inter- and intra- mineral oxygen isotope disequilibrium. Disequilibrium is also evident in the elemental geochemistry of the mantle minerals and a general correlation exists between oxygen isotopes and major (Si, Mg, Ca, Fe) and trace elements (Ce, Cr, Zr, Nb, REE). The interpretation that isotopic heterogeneity may relate to melt processes is supported by d18O zonation in garnets, significant isotopic variation close to secondary veins, d18O (primary phases) > d18O (secondary phases) and oxygen isotope disequilibria in many minerals . In addition a positive correlation between d18O and grain size indicates a role for deformation processes as a result of diffusion reactions perhaps inextricably linked to melt processes. The following conclusions can be reached from the oxygen isotope studies and the investigation of the correlation between the oxygen isotopes and the elemental geochemistry in polymict xenoliths, South Africa: (1) All the mineral phases in the polymict xenoliths show a large inter-mineral disequilibrium in oxygen isotope and have low d18O values relative to equivalent minerals in mantle xenoliths from elsewhere. Some minerals (i.e. garnet, ilmenite, and diopside) still show a intra-mineral d18O disequilibrium. Normally the reaction rim has a lower d18O value than the core of the mineral. (2) Fluid migration and deformation are the main processes resulting in the inter-and intra-mineral disequilibrium in the minerals and these may have been recently captured and frozen in time by the entrainment processes. (3) Fluid influx was associated with the introduction of low d18O, and high LREE, CaO, TiO2, and FeO. The positive correlation between d18O and elemental geochemistry is associated with melt ingress. (4) Some minerals have retained information about the precursory lithologies (i.e. lherzolite) that were disrupted during the deformation process. (5) Oxygen isotopes in many minerals from the polymict xenoliths straddle a large variety of mantle provenances. Together with the elemental consequences these polymict xenoliths show large mixed provenances of lithospheric peridotites. (6) No unequivocal evidence exists for the presence of an eclogite precursor. We suggest that polymict peridotites formed as a result of movement along mantle shear zones which led to the juxtaposition of minerals of varied provenance. Contemporaneous melt transfer reacted with these mantle breccias and rapid entrainment by “kimberlite” meant that any associated mineral disequilibria was very effectively frozen in time.

Oxygen Isotope Disequilibrium Between Calcite and Forsterite in the Presence of a Large Amount of Fluid

Oxygen Isotope Disequilibrium Between Calcite and Forsterite in the Presence of a Large Amount of Fluid

Fluid flow in marbles at Jervois, central Australia: oxygen isotope disequilibrium and zoning produced by decoupling of mineralogical and isotopic resetting

The Jervois region of the Arunta Inlier, central Australia, contains para- and orthogneisses that underwent low-pressure amphibolite facies metamorphism (P = 200–300 MPa, T = 520–600 °C). Marble layers cut by metre-wide quartz + garnet ± epidote veins comprise calcite, quartz, epidote, clinopyroxene, grandite garnet, and locally wollastonite. The marbles also contain locally discordant decimetre-thick garnet and epidote skarn layers. The mineral assemblages imply that the rocks were infiltrated by water-rich fluids (XCO2 = 0.1–0.3) at ∼600 °C. The fluids were probably derived from the quartz-garnet vein systems that represent conduits for fluids exsolved from crystallizing pegmatites emplaced close to the metamorphic peak. At one locality, the marble has calcite (Cc) δ18O values of 9–18‰ and garnet (Gnt) δ18O values of 10–14‰. The δ18O(Gnt) values are only poorly correlated with δ18O(Cc), and the δ18O values of some garnet cores are higher than the rims. The isotopic disequilibrium indicates that garnet grew before the δ18O values of the rock were reset. The marbles contain ≤15% garnet and, for water-rich fluids, garnet-forming reactions are predicted to propagate faster than O-isotopes are reset. The Sm-Nd and Pb-Pb ages of garnets imply that fluid flow occurred at 1750–1720 Ma. There are no significant age differences between garnet cores and rims, suggesting that fluid flow was relatively rapid. Texturally late epidote has δ18O values of 1.5–6.2‰ implying δ18O(H2O) values of 2–7‰. Waters with such low-δ18O values are probably at least partly meteoric in origin, and the epidote may be recording the late influx of meteoric water into a cooling hydrothermal system.

Stable isotope composition of spelean calcites and gaseous CO 2 from Tylicz (Polish Carpathians)

Spelean calcites discovered in fissures and caverns within flysch deposits at Tylicz (Polish Carpathians) have been examined isotopically together with CO 2-rich water from several springs occurring in this area. δ 13C-values of TDC and spelean calcites indicate that CaCO 3 has precipitated under conditions close to carbon isotope equilibrium, with participation of CO 2 of deep origin. Analyses of δ 18O suggest the existence of oxygen isotope disequilibrium between water and gaseous CO 2 which can be explained by the short time of contact between the CO 2 flux and the shallow groundwaters of meteoric origin.

Active hydrothermal systems during the recent uplift of Nanga Parbat, Pakistan Himalaya

During the last 10 m.y., the Nanga Parbat Haramosh Massif in the northwestern Himalaya has been intruded by granitic magmas, has undergone high‐grade metamorphism and anatexis, and has been rapidly uplifted and denuded. As part of an ongoing project to understand the relationship between tectonism and petrologic processes, we have undertaken an isotopic study of the massif to determine the importance of hydrothermal activity during this recent metamorphism. Our studies show that both meteoric and magmatic hydrothermal systems have been active over the last 10 m.y. We suggest that the rapid uplift of the massif created a dual hydrothermal system, consisting of a near‐surface flow system dominated by meteoric water and a flow regime at deeper levels dominated by magmatic/metamorphic volatiles. Meteoric fluids derived from glaciers near the summit of Nanga Parbat were driven deep into the massif along the transpressional faults causing δ18O and δD depletions in the gneisses and marked oxygen isotopic disequilibrium between mineral pairs from the fault zones. The discharge of these meteoric fluids occurs in active hot springs that are found along the steep faults that border the massif. At deeper levels within the massif, infiltration of low δ18O magmatic fluids caused δ18O depletions in the gneisses within the migmatite zone. These low δ18O fluids were derived from the young (<4 Ma), relatively low δ18O granites (∼8‰c) that are found within the core of the massif. Geochronological evidence in the form of fission track and 40Ar/39Ar cooling ages and U/Pb ages on accessory minerals from the granites and gneisses provide a constraint on the timing of fluid flow in the surface outcrops we examined. Fluid infiltration in the migmatite zone rocks located along the Tato traverse was coeval with metamorphism, granite emplacement, and rapid denudation, in the interval 0.8–3.3 Ma. Finally, we infer from the presence of active hot springs that significant flow systems continue to be active at depth within the central portion of the Nanga Parbat‐Haramosh Massif.

Oxygen isotope disequilibrium precipitation of calcite in Lake Arresø, Denmark

Research Article| May 01, 1995 Oxygen isotope disequilibrium precipitation of calcite in Lake Arresø, Denmark Torben Fronval; Torben Fronval 1Department of Geology, University of Bergen, Allegaten 41, N-5007 Bergen, Norway Search for other works by this author on: GSW Google Scholar Niels Bo Jensen; Niels Bo Jensen 1Department of Geology, University of Bergen, Allegaten 41, N-5007 Bergen, Norway Search for other works by this author on: GSW Google Scholar Bjørn Buchardt Bjørn Buchardt 2Department of Geology, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark Search for other works by this author on: GSW Google Scholar Author and Article Information Torben Fronval 1Department of Geology, University of Bergen, Allegaten 41, N-5007 Bergen, Norway Niels Bo Jensen 1Department of Geology, University of Bergen, Allegaten 41, N-5007 Bergen, Norway Bjørn Buchardt 2Department of Geology, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1995) 23 (5): 463–466. https://doi.org/10.1130/0091-7613(1995)023<0463:OIDPOC>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Torben Fronval, Niels Bo Jensen, Bjørn Buchardt; Oxygen isotope disequilibrium precipitation of calcite in Lake Arresø, Denmark. Geology 1995;; 23 (5): 463–466. doi: https://doi.org/10.1130/0091-7613(1995)023<0463:OIDPOC>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Lake carbonates are generally assumed to precipitate at or very close to oxygen isotopic equilibrium with the ambient water. We present results from the eutrophic Lake Arresø, northeastern Sealand, Denmark, that demonstrate disequilibrium precipitation of present-day calcite. According to the temperature equation of Epstein et al., these calcites are depleted in 18O by 2‰–3‰ when compared to the expected equilibrium values. Core studies indicate that the development of disequilibrium conditions in Lake Arresø is correlated with the onset of an enhanced eutrophication. A high precipitation rate is suggested as the mechanism responsible for the observed 18O depletion of the calcite precipitates. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Post-metamorphic low δ13 C calcite in the Cycladic complex (Greece) and their implications for modeling fluid infiltration processes using carbon isotope compositions

Calcite veins and overgrowths with δ 13 C values of -12 ‰ to -8 ‰ and δ 18 O values of 24 ‰ to 27 ‰ are shown to be characteristic features of the Cyclades metamorphic complex of Greece. Field and petrographic characteristics and the marked carbon and oxygen isotope disequilibrium that the veins and overgrowths show with host metamorphic rocks indicate that they developed during post-metamorphic evolution of the complex. The low δ 13 C values are interpreted to be either the result of the oxidation of organic carbon or solution of soil CO 2 by surface waters moving downwards along extensional fractures generated during the late stages of exhumation. Carbon and oxygen isotopic compositions reported for the carbonate fraction of many pelitic schists on Naxos overlap with the range of these late carbonates and may in part reflect a contribution of this late calcite. Uncertainties regarding the origin of low δ 13 C values in the carbonates of metamorphic rocks shows that they must be used with caution as tracers of fluid-rock exchange

Oxygen and carbon isotope variation in the otoliths of Beryx splendens and Coryphaenoides profundicolus

Oxygen and carbon isotopes were measured by mass spectrometry from gasified samples drilled out along the nucleus-to-dorsal growth axes of 11 sagittal otoliths of Beryx splendens and one otolith of Coryphaenoides profundicolus. Oxygen and carbon isotope variability was high along the growth axes of the sagittal otoliths of both B. splendens and C. profundicolus, indicating heterogeneous thermal environments over the life of the fish in both species. The correlation between oxygen and carbon isotope disequilibrium was low in B. splendens (r2 = 19.3%) but higher in C. profundicolus (r2 = 68.3%).

Evidence for oxygen isotope disequilibrium in selected geothermal and hydrothermal ore deposit systems

There is ample evidence from geothermal systems that temperatures estimated from the oxygen isotope fractionation between alteration phases and coexisting aquifer fluids agree closely (±25°C) with measured bore-hole temperatures. Conversely, many hydrothermal systems exhibit varying degrees of apparent fluid-rock oxygen isotope disequilibrium (measured α rock-water ≠ equilibrium α rock-water). A comparison of data from a variety of hydrothermal systems indicates that an increase in the degree of isotopic equilibration can be related, in a general way, to an increase in the temperature, salinity, and intensity of fracturing. This relationship is entirely consistent with calculations carried out for a simple closed-system model that estimates the fraction of isotope exchange between rock and fluid as a function of temperature, rock type, salinity of the fluid, grain size, water/rock ratio, and the rate of isotope exchange. The fact that many silicate-fluid reactions and their isotopic counterparts are fast leads us to conclude that observed variations in the degree of exchange may have resulted from local self-sealing of the fracture network prior to equilibration. The ability for fractures to remain open or to propagate, allowing continued fluid flow, may be the deciding factor in the attainment of isotopic equilibration.

The evolution of a calc-alkaline basic to silicic magma system: Geochemical and Rb-Sr, Sm-Nd, and [formula omitted] isotopic evidence from the Late Hercynian Atesina-Cima d'Asta volcano-plutonic complex, northern Italy

Geochemical and Sr-Nd-O isotopic data presented for basaltic andesitic to rhyolitic and for quartz noritic to monzogranitic rock suites from the Late Hercynian calc-alkaline Atesina volcanic complex (AVC) and the Cima d'Asta pluton (CAP), Southern Alps (northern Italy), provide information on both the primary magmatic processes and the effects of (mainly Triassic) hydrothermal overprint. Fluid infiltration led to mobilization of major and trace elements (K 2O, Na 2O, CaO, Rb, Sr, and Ba), opensystem behavior in total-rock Rb-Sr, and shift in δ 18 O to elevated values (total rock up to 16.6%. and volcanic matrix up to 17.8%.). Oxygen isotopic disequilibrium between quartz-feldspar pairs suggests water-rock interaction at medium/low temperatures. The δ 18 O values of quartz, the REE characterized by regular LREE enrichment/HREE depletion, and the Sm-Nd isotopic signatures, however, remained virtually unaffected by secondary processes. The initial ϵ Nd values (at 270 Ma) of the AVC and CAP magmatites are restricted to overlapping ranges of −3.6 to −6.5 and of −2.7 to −6.5, respectively, indicating significant crustal contribution; these values and associated T DM model ages of 1.1–1.6 Ga agree well with those of typical South Alpine lower crustal magmatites. The AVC and CAP rocks do not follow the “normal” trend of increasingly crustal Nd isotopic signatures with progressive degree of magma evolution expected for a single-stage AFC-type process, but instead display an inversion of this relationship. Geochemical and isotopic constraints favor a model of a large-scale MASH-type melting and mixing zone at or near the base of the continental crust. Distinct elemental enrichment/depletion and REE crossover patterns displayed by high-silica as compared to less silicic AVC rhyolites suggest subsequent magma evolution within a shallow-level compositionally zoned chamber.

Volume self-diffusion of oxygen in biotite, muscovite, and phlogopite micas

The kinetics of oxygen self-diffusion have been measured in natural samples of biotite, muscovite, and phlogopite micas under hydrothermal conditions using both bulk exchange and profiling analyses. All experiments were run at 1000 bars (100 MPa) water pressure except those designed to measure water pressure dependence, which ranged from 200 to 2000 bars. The Arrhenius relations obtained for layer-parallel diffusive transport in the three micas by bulk exchange experiments, fit to an infinite cylinder model, are Mineral biotite muscovite phlogopite Q (kcal mol) 34 ±2 39 ± 5 42 +- 3 D 0 (cm 2 s) 9.1 × 10 −6 7.7 × 10 −5 1.4 × 10 −4 T(°C) 500–800 512–700 600–900. The results of ion microprobe (SIMS) depth profiling perpendicular to the layers (parallel to c-axis) yield diffusion coefficients 3 to 4 orders of magnitude lower than for transport parallel to the layers. A single layer-parallel step profiling experiment yields a diffusion coefficient identical, within uncertainty, to those obtained from bulk exchange experiments. A slight dependence of diffusion rate on water pressure may obtain in phlogopite; however, the difference is approximately within the uncertainty of the measurement. Diffusion rates are similar, within about a half an order of magnitude, for a range of biotite composition from annite 4 to annite 63, as well as for muscovite. Comparison of oxygen diffusion rates in biotite from this study with Ar, K, and Rb diffusion from published data supports the idea that water is the oxygen-bearing species in hydrothermal diffusion experiments. The implication of these results for natural systems is that the micas, especially biotite, will continue to exchange oxygen isotopes via volume diffusion down to temperatures as low as 300 °C. The rapid diffusion kinetics may explain biotite oxygen isotope disequilibrium, which is commonly observed even in rocks where other mineral phases preserve equilibrium compositions.