sol18O16O of Archean clastic metasedimentary rocks: a petrogenetic indicator for Archean gneisses?

The oxygen isotopic compositions of over 100 Archean clastic metasedimentary, felsic metavolcanic and gneissic rocks from selected areas within the Superior Province have been determined. δ¹⁸O values of low grade, immature clastic metasediments range from 8.0 to 13.3%. and hence are somewhat depleted in ¹⁸O relative to other clastic metasediments. The lower ¹⁸O content is attributed to the large proportion of unaltered rock fragments in the Archean metasediments. Felsic metavolcanics have a similar range of δ¹⁸O values (7.3 to 13.0%..δ¹⁸O values for the middle amphibolite facies Pakwash paragneiss (8.8–11.7%.) are higher than those determined for the amphibolite to granulite facies Twilight paragneiss (7.3–9.2%.). Archean orthogneisses such as the Clay Lake gneiss (7.0–9.0%.), the Cedar Lake gneiss (7.4–8.7%.) and the Footprint gneiss (5.9-8.8%.) have δ¹⁸O values similar to those of the Twilight paragneiss. Therefore, at least at metamorphic grades higher than middle amphibolite facies, Archean metasedimentary gneisses may have δ¹⁸O values that are indistinguishable from those of Archean orthogneisses, and hence are no longer isotopically recognizable as metasedimentary rocks.     

Oxygen isotope study of metamorphic and granitic rocks of the Yanai district in the Ryoke belt,Japan

¹⁸O/¹⁶O ratios have been obtained for 134 whole-rocks and minerals from metamorphic and granitic rocks of the Yanai district in the Ryoke belt, Southwest Japan. The ¹⁸O/¹⁶O ratios of pelitic rocks of the marginal metamorphic zone decrease progressively with increasing metamorphic grade. In the gneiss-granite complex (zone of migmatite [1]), the most characteristic feature of the rocks is that oxygen isotopic homogenization proceeds on both local and regional scales in parallel with “granitization” or chemical homogenization. Granitic rocks of various origin are fairly uniform in isotopic composition with δ ¹⁸O of quartz of 12 to 14‰ (SMOW) and δ ¹⁸O of biotite of 7 to 9‰ and are about 3 to 4‰ enriched in ¹⁸O compared to other Cretaceous granites of non-metamorphic terranes in Japan. The high ¹⁸O/¹⁶O ratios of granitic rocks of this district were discussed in relation to the ¹⁸O-depletion in metasediments. Oxygen isotopic fractionations among coexisting minerals from various rock-types of the gneiss-granite complex indicate that these minerals were formed under near isotopic equilibrium at a temperature of about 600 to 700° C. Some abnormal fractionations of quartz-biotite pairs also were obtained for rocks which had undergone a progressive ¹⁸O-depletion or ¹⁸O-enrichment. This is due to high resistivity of quartz and contrastive susceptibility of biotite to isotopic exchange during metamorphism and “granitization”.     

Fractionation of carbon and oxygen isotopes and magnesium between coexisting metamorphic calcite and dolomite

Fractionations of carbon and oxygen isotopes and magnesium between coexisting dolomite and calcite have been determined for marbles and calcareous schists of a wide variety of metamorphic environments from Vermont and the Grenville Province of Ontario. Concordant equilibrium fractionations are given by 83% of the samples. Calibration of the isotopic thermometers using the Mg-calcite solvus thermometer gave in the temperature range: 650°>T°>100°C $$ \begin{gathered} 1,000\ln \alpha _{D - Ct}^{O^{18} } = 0.45 (10^6 T^{ - 2} ) - 0.40 \hfill \\ 1,000\ln \alpha _{D - Ct}^{O^{18} } = 0.18 (10^6 T^{ - 2} ) + 0.17. \hfill \\ \end{gathered} $$ These isotopic fractionation expressions differ significantly from the experimentally derived relations, including the dolomite-Mg-calcite C¹³ partial exchange experiments of this study. Temperature ranges obtained for the metamorphic zones of Vermont are: chlorite zone, 210° to 295° C; biotite zone, 255° to 400° C; staurolite-kyanite zone, 110° to 550° C. In amphibolite-facies rocks the quenched partition relations can be complex. The temperature of quench or recrystallization may be as large as 400° C below the inferred metamorphic maximum. Oxygen isotope disequilibrium in high grade rocks, particularly from the Chester dome area, Vermont, is characterized by large negative δO _D ¹⁸ –δO _Ct ¹⁸ values. The size of the equilibrium exchange system for carbon and oxygen isotopes and magnesium is small, less than a few inches across the inferred relict bedding. This is attributed to the lack of a mobile pore fluid except in systems undergoing decarbonation. C¹³/C¹² ratios in Grenville and Vermont marbles and O¹⁸/O¹⁶ ratios in Grenville and greenschist-facies Vermont carbonates span the range of ancient limestones. Staurolite-kyanite zone calcareous schists and marbles from the Chester dome area, Vermont are depleted in O¹⁸(δO¹⁸=12 to 20‰) due to equilibrium or disequilibrium decarbonation and some partial exchange. Extrapolation of the dolomite-calcite fractionation expressions to 20° C indicates that dolomite is enriched in O¹⁸ by about 4.9‰ and in C¹³ by about 2.4‰.     

Genesis of metapelitic migmatites in the Adirondack Mountains, New York

Oxygen isotope ratios and rare earth element (REE) concentrations provide independent tests of competing models of injection v. anatexis for the origin of migmatites from amphibolite and granulite facies metasedimentary rocks of the Adirondack Mountains, New York. Values of δ¹⁸O and REE profiles were measured by ion microprobe in garnet–zircon pairs from 10 sample localities. Prior U–Pb SIMS dating of zircon grains indicates that inherited cores (1.7–1.2 Ga) are surrounded by overgrowths crystallized during the Grenville orogenic cycle (～1.2–1.0 Ga). Cathodoluminescence imaging records three populations of zircon: (i) featureless rounded ‘whole grains’ (interpreted as metamorphic or anatectic), and rhythmically zoned (igneous) cores truncated by rims that are either (ii) discordant rhythmically zoned (igneous) or (iii) unzoned (metamorphic or anatectic). These textural interpretations are supported by geochronology and oxygen isotope analysis. In both the amphibolite facies NW Adirondacks and the granulite facies SE Adirondacks, δ¹⁸O_(Zrc) values in overgrowths and whole zircon are highly variable for metamorphic zircon (6.1–13.4‰; n = 95, 10 μm spot). In contrast, garnet is typically unzoned and δ¹⁸O_(Grt) values are constant at each locality, differing only between leucosomes and corresponding melanosomes. None of the analysed metamorphic zircon–garnet pairs attained oxygen isotope equilibrium, indicating that zircon rims and garnet are not coeval. Furthermore, REE profiles from zircon rims indicate zircon growth in all regions was prior to significant garnet growth. Thus, petrological estimates from garnet equilibria (e.g. P–T) cannot be associated uncritically with ages determined from zircon. The unusually high δ¹⁸O values (>10‰) in zircon overgrowths from leucocratic layers are distinctly different from associated metaigneous rocks (δ¹⁸O_(Zrc) < 10‰) indicating that these leucosomes are not injected magmas derived from known igneous rocks. Surrounding melanosomes have similarly high δ¹⁸O_(Zrc) values, suggesting that leucosomes are related to surrounding melanosomes, and that these migmatites formed by anatexis of high δ¹⁸O metasedimentary rocks.     

Oxygen isotope compositions of phosphate from arvicoline teeth and Quaternary climatic changes, Gigny, French Jura

Oxygen isotope compositions of biogenic phosphates from mammals are widely used as proxies of the isotopic compositions of meteoric waters that are roughly linearly related to the air temperature at high- and mid-latitudes. An oxygen isotope fractionation equation was determined by using present-day European arvicoline (rodents) tooth phosphate: δ¹⁸O_p = 20.98(±0.59) + 0.572(±0.065) δ¹⁸O_w. This fractionation equation was applied to the Late Pleistocene karstic sequence of Gigny, French Jura. Comparison between the oxygen isotope compositions of arvicoline tooth phosphate and Greenland ice core records suggests to reconsider the previously established hypothetical chronology of the sequence. According to the δ¹⁸O value of meteoric water-mean air temperature relationships, the δ¹⁸O value of arvicoline teeth records variations in mean air temperatures that range from 0° to 15°C.     

The oxygen isotope geochemistry of igneous rocks

Oxygen isotope analyses have been obtained for 443 igneous rock and mineral samples from various localities throughout the world. Detailed studies were made on the Medicine Lake, Newberry, Lassen, Clear Lake, S. E. Guatemala, Hawaii and Easter I. volcanic complexes and on the Bushveld, Muskox, Kiglapait, Guadalupe, Duluth, Nain, Egersund, Lac St. Jean, Laramie, Skaergaard, Mull, Skye, Ardnamurchan and Alta, Utah plutonic complexes, as well as upon several of the zoned ultramafic intrusions of S. E. Alaska. Basalts, gabbros, syenites and andesites are very uniform in O¹⁸/O¹⁶, commonly with δ-values of 5.5 to 7.0 per mil. Many rhyolite obsidians, particularly those from oceanic areas and the Pacific Coast of the United States, also lie in this range; this indicates that such obsidians are differentiates of basaltic or andesitic magma at high temperatures (about 1,000° C). They cannot represent melted sialic crust. The only plutonic granites with such low δ-values are some of the hypersolvus variety, suggesting that these also might form by fractional crystallization. Obsidians from the continental interior, east of the quartz-diorite line, have higher δ-values. This is compatible with their having assimilated O¹⁸-rich sialic crust. A correlation generally exists between the O¹⁸/O¹⁶ ratios of SiO₂-rich differentiates and the chemical trends in volcanic complexes. High O¹⁸/O¹⁶ ratios accompany those trends having the lower Fe/Mg ratios, while ferrogabbro trends are associated with depletion in O¹⁸. Variations in oxygen fugacity may be responsible for these effects, as abundant early precipitation of magnetite should lead to both O¹⁸-enrichment and Fe-depletion in later differentiates. Plutonic granites have higher O¹⁸/O¹⁶ ratios than their volcanic equivalents, because (a) their differentiation occurred at much lower temperatures, or (b) they are in large part derived from O¹⁸-rich sialic crust by partial melting or assimilation. Also, the oxygen isotope fractionations among coexisting minerals are distinctly larger in plutonic rocks than in volcanic rocks. This is in keeping with their lower crystallization temperatures and their longer cooling history, which promotes post-crystallization oxygen isotope exchange. Hydrated obsidians and perlites have δO¹⁸-values that are much different from their primary, magmatic values. A correlation exists between D/H and O¹⁸/O¹⁶ ratios in hydrated volcanic glass from the western U.S.A., proving that the isotopic compositions are a result of exchange with meteoric waters. The O¹⁸ contents of the glasses appear to be about 25 per mil higher than their associated waters; hence, these hydrated glasses have not simply absorbed H₂O, but they have exchanged with large quantities of it. The igneous rocks from Mull, Skye, Ardnamurchan and the Skaergaard intrusion are all abnormally depleted in O¹⁸ relative to “normal” igneous rocks. This is a result of their having exchanged at high temperatures with meteoric water that was apparently abundant in the highly jointed plateau lavas into which these igneous rocks were intruded. In part, this exchange occurred with liquid magma and in part with the crystalline rock; in the latter case the feldspar was more easily exchanged and has become much more depleted in O¹⁸ than has coexisting quartz or pyroxene. The later differentiates of the Muskox intrusion are markedly O¹⁸-rich, but this is not a result of fractional crystallization. It is in large part a result of deuteric exchange between feldspars and an oxygen-bearing fluid (H₂O ?) that was either O¹⁸-rich or had a relatively low temperature. This phenomenon was also observed in a number of granophyres from other localities, particularly those containing brick-red alkali feldspar. The exchanged feldspars in all these examples are turbid or cloudy, and may be filled with hematite dust. It is concluded that most such feldspar in nature is the result of deuteric exchange and is probably drastically out of oxygen isotopic equilibrium with its coexisting quartz.     

The oxygen isotope composition of Hercynian granites and pre-Hercynian gneisses from the Schwarzwald,SW Germany

The oxygen isotope composition of 56 Hercynian granites and 42 pre-Hercynian gneisses has been investigated. In addition some mineral δ ¹⁸O data and 5 δD values of whole rocks have been obtained. The granites from the N-Schwarzwald show, in general, relatively uniform δ ¹⁸O values between 11.5 and 13.5‰, those from the S-Schwarzwald are less uniform and lighter in ¹⁸O and range from 2.3 to 11.5‰. The gneisses from the pre-Hercynian basement exhibit more or less the same variation and range from 1.7 to 10.4‰. δ ¹⁸O values <6‰ only occur in the S-Schwarzwald and obviously indicate hydrothermal interactions of meteoric waters, which probably took place after the emplacement and solidification of the granites and which equally affected granites and gneisses. Due to the nearly identical ¹⁸O/¹⁶O ratios of S-Schwarzwald granites and gneisses, it is proposed that such gneisses in the pre-Hercynian basement qualify as the precursor rocks of the S-Schwarzwald, granites whereas for the N-Schwarzwald granites crustal rocks with heavier δ ¹⁸O values, unknown from the present surface, have to be postulated. This distribution is also reflected on a ⁸⁷Sr/⁸⁶Sr-¹⁸O/¹⁶O diagram.     

Reconstruction of seasonal temperature variability in the tropical Pacific Ocean from the shell of the scallop, Comptopallium radula

We investigated the oxygen isotope composition (δ¹⁸O) of shell striae from juvenile Comptopallium radula (Mollusca; Pectinidae) specimens collected live in New Caledonia. Bottom-water temperature and salinity were monitored in-situ throughout the study period. External shell striae form with a 2-day periodicity in this scallop, making it possible to estimate the date of precipitation for each calcite sample collected along a growth transect. The oxygen isotope composition of shell calcite (δ¹⁸O_{shell calcite}) measured at almost weekly resolution on calcite accreted between August 2002 and July 2003 accurately tracks bottom-water temperatures. A new empirical paleotemperature equation for this scallop species relates temperature and δ¹⁸O_{shell calcite}:

t(°C)=20.00(±0.61)-3.66(±0.39)×(δ¹⁸O_{shell calcite VPDB}-δ¹⁸O_{water VSMOW})     

Oxygen isotope geochemistry of the Omeo Metamorphic Complex,Victoria: Implications for metamorphic fluid flow,mineralisation and anatexis

Metamorphosed turbidites from the Omeo Metamorphic Complex show only minor changes in δ¹⁸O values with increasing metamorphic grade from 13.4 ± 1.7% in the chlorite and biotite zones to 12.3 ± 1.0% in the sillimanite + K‐feldspar zone. Rocks within 5 km of the S‐type granite at Hume Dam have δ¹⁸O values of 6.8–8.1% that probably reflect interaction with heated meteoric‐igneous fluids. Interaction with igneous fluids has also occurred close to other I‐ and S‐type granites in this region. However, pervasive metamorphic fluid‐rock interaction in this terrain did not occur, which limits the region's potential for hydrothermal mineralisation. Anatexis at high grades was probably via dehydration‐melting reactions that consumed muscovite and biotite, which is consistent with there being little fluid present during metamorphism. Small (kilometre scale or less) S‐type granites in the sillimanite + K‐feldspar zone have δ¹⁸O values similar to those of the surrounding metasediments and probably formed by melting of those rocks. By contrast, larger (tens of kilometres scale) Ca‐rich, peraluminous, S‐type granites have lower δ¹⁸O values than the surrounding metasediments, and may represent melts of underlying middle to lower crust.     

Transfer and early diagenesis of biogenic silica oxygen isotope signals during settling and sedimentation of diatoms in a temperate freshwater lake (Lake Holzmaar, Germany)

We have investigated the transfer of oxygen isotope signals of diatomaceous silica (δ¹⁸O_diatom) from the epilimnion (0-7 m) through the hypolimnion to the lake bottom (∼20 m) in freshwater Lake Holzmaar, Germany. Sediment-traps were deployed in 2001 at depths of 7 and 16 m to harvest fresh diatoms every 28 days. The 7 m trap collected diatoms from the epilimnion being the main zone of primary production, while the 16 m trap collected material already settled through the hypolimnion. Also a bottom sediment sample was taken containing diatom frustules from approximately the last 25 years. The δ¹⁸O_diatom values of the 7 m trap varied from 29.4‰ in spring/autumn to 26.2‰ in summer according to the temperature dependence of oxygen isotope fractionation and represent the initial isotope signal in this study. Remarkably, despite the short settling distance δ¹⁸O_diatom values of the 7 and the 16 m trap were identical only during spring and autumn seasons while from April to September δ¹⁸O_diatom values of the 16 m trap were roughly ∼1.5‰ enriched in ¹⁸O compared to those of the 7 m trap. Isotopic exchange with the isotopically lighter water of the hypolimnion would shift the δ¹⁸O_diatom value to lower values during settling from 7 to 16 m excluding this process as a cause for the deviation. Dissolution of opal during settling with intact organic coatings of the diatom cells and near neutral pH of the water should only cause a minor enrichment of the 16 m values. Nevertheless, opal from the bottom sediment was found to be 2.5‰ enriched in ¹⁸O compared to the weighted average of the opal from the 7 m trap. Thus, resuspension of bottom material must have contributed to the intermediate δ¹⁸O_diatom signal of the 16 m trap during summer. Dissolution experiments allowed further investigation of the cause for the remarkably enriched δ¹⁸O_diatom value of the bottom sediment. Experiments with different fresh diatomaceous materials show an increase of opaline ¹⁸O at high pH values which is remarkably reduced when organic coatings of the cells still exist or at near neutral pH. In contrast, high pH conditions do not affect the δ¹⁸O_diatom values of sub-fossil and even fossil opal. IR analyses show that the ¹⁸O enrichment of the sedimentary silica is associated with a decrease in Si-OH groups and the formation of Si-O-Si linkages. This indicates a silica dehydroxylation process as cause for the isotopic enrichment of the bottom sediment. Silica dissolution and dehydroxylation clearly induce a maturation process of the diatom oxygen isotope signal presumably following an exponential behaviour with a rapid initial phase of signal alteration. The dynamics of this process is of particular importance for the quantitative interpretation of sedimentary δ¹⁸O_diatom values in terms of palaeothermometry.     

Silicate oxygen isotope geochemistry: History,principles, techniques,and application to petrological problems

The importance of oxygen isotope geochemistry in studies of terrestrial and extra-terrestrial silicate rocks was recognized nearly sixtyeight years ago soon after the discovery of O¹⁸ and O¹⁷. As early as 1934, the significance of oxygen isotope variations in rocks and minerals was stressed by Russian geochemists who also pioneered the discipline of silicate oxygen isotope geochemistry. It is now known that processes involving isotopic interaction between rock and water, magmatic differentiation, and metamorphic recrystallization fractionate oxygen isotopes in the lithosphere. δO¹⁸ (the conventional notation for reporting O¹⁸/O¹⁶ ratios in rocks and minerals) is highest in sedimentary rocks (17 to 35 ‰) and lowest in igneous rocks (4 to 12‰). Metamorphic rocks have intermediate values. δO¹⁸ in mafic minerals (1 to 8‰) is lower than in felsic minerals (8 to 16‰). In igneous and metamorphic rocks, quartz is most enriched in O¹⁸ (10 to 16‰) and magnetite the least (1 to 2‰). An important application of O¹⁸/O¹⁶ techniques is in geothermometry, where these are capable of elucidating several petrological processes.     

Holocene hydrological variability of Lake Ladoga,northwest Russia,as inferred from diatom oxygen isotopes

This article presents a new comprehensive assessment of the Holocene hydrological variability of Lake Ladoga, northwest Russia. The reconstruction is based on oxygen isotopes of lacustrine diatom silica (δ¹⁸O_diatom) preserved in sediment core Co 1309, and is complemented by a diatom assemblage analysis and a survey of modern isotope hydrology. The data indicate that Lake Ladoga has existed as a freshwater reservoir since at least 10.8 cal. ka BP. The δ¹⁸O_diatom values range from +29.8 to +35.0‰, and relatively higher δ¹⁸O_diatom values around +34.7‰ between c. 7.1 and 5.7 cal. ka BP are considered to reflect the Holocene Thermal Maximum. A continuous depletion in δ¹⁸O_diatom since c. 6.1 cal. ka BP accelerates after c. 4 cal. ka BP, indicating Middle to Late Holocene cooling that culminates during the interval 0.8–0.2 cal. ka BP, corresponding to the Little Ice Age. Lake‐level rises result in lower δ¹⁸O_diatom values, whereas lower lake levels cause higher δ¹⁸O_diatom values. The diatom isotope record gives an indication for a rather early opening of the Neva River outflow at c. 4.4–4.0 cal. ka BP. Generally, overall high δ¹⁸O_diatom values around +33.5‰ characterize a persistent evaporative lake system throughout the Holocene. As the Lake Ladoga δ¹⁸O_diatom record is roughly in line with the 60°N summer insolation, a linkage to broader‐scale climate change is likely.     

Kiglapait geochemistry VI: Oxygen isotopes

The Kiglapait layered intrusion is the first major intrusion found to have all whole rock and calculated liquid δ¹⁸O values close to a normal uncontaminated gabbroic value of 6.0. The intrusion experienced no detectable oxygen isotope exchange with its surrounding rocks and cooling of the magma was conductive. The δ¹⁸O values of average whole rocks vary smoothly from 6.0 at the base of the Lower Zone to 6.3 at the top of the Upper Zone. The calculated liquid δ¹⁸O values lie practically superimposed on the whole rock trend. The whole-rock data and the modelled δ¹⁸O of the magma and cumulates rigorously demonstrate that the effect of incoming cumulus phases such as magnetite and augite on the δ¹⁸O of the liquid and rocks during fractional crystallization is negligible. The cancelling effects of complementary modal variations among the mafic mineral phases and feldspar, keep the δ¹⁸O of the whole rocks constant to within ±0.1 %.. The minor change in δ¹⁸O that does occur with fractionation is consistent with the enrichment of residual liquids in feldspar component and the increasing fractionation factor δ Liquid-Fsp with falling temperature.The δ¹⁸O values of the country rocks bracket the estimated δ¹⁸O of the Kiglapait magma. Modelling with oxygen isotopes indicates that contamination of the intrusion, indicated by published radiogenic Sr and Nd isotopic data, was minor. The most probable contaminant had δ¹⁸O?7.7 and the contamination most likely occurred at >99% solidified. Subsolidus oxygen isotope exchange with an external source appears to have been very minor.     

Nature of serpentinization and carbonation of ophiolitic peridotites (Eastern Desert,Egypt): constrains from stable isotopes and whole-rock geochemistry

Serpentinites are widespread in the Arabian-Nubian Shield (ANS) of the Eastern Desert of Egypt and usually enclose a tremendous carbonate alteration. Combined investigation of the stable isotope compositions of both O-H in serpentines and O-C in the whole-rock and the chemistry of the fluid-mobile elements (FMEs) in whole-rock serpentinites from Wadi (W.) Alam, Gabal (G.) El-Maiyit, and W. Atalla (Eastern Desert of Egypt) allowed to better understand the subsequent fluid sources of serpentinization and carbonation, as well as impact of these processes on the geochemistry of protolith ultramafic rocks. δ ¹⁸O values of W. Alam and W. Atalla serpentine minerals are close to the unaltered mantle and propose a lower temperature serpentinization if compared with those of G. El-Maiyit rocks. Moreover, δD values of W. Alam and W. Atalla serpentines (? 94 to ? 65‰) correspond to an igneous source that might be hydrothermal solutions mixed with the seawater in the mid-ocean ridge-arc transition setting. On the other hand, G. El-Maiyit serpentine is more depleted in ¹⁸O (with lower δ ¹⁸O values = 4.08–4.85‰), and its δD values (? 73 to 56 ‰) are most probably caused by an interaction with metamorphic fluids, acquired during on-land emplacement of oceanic peridotites or during burial in fore-arc setting. In addition, the oceanic oxygen isotope composition of most studied ophiolitic serpentinites points to the preservation of the pre-obduction δ ¹⁸O signatures and thus local-scale fluid flow at low water/rock ratios. Serpentinization fluids were CO₂-poor and the carbonation of the serpentinites resulted from infiltration of externally derived fluids. δ ¹⁸O_VSMOW values of carbonates in the studied serpentinites vary between heavier oxygen isotope composition in G. El-Maiyit samples (av. = 25.32‰) to lighter composition in W. Alam samples (av. = 19.43‰). However, δ ¹³C values of all serpentinites point mantle source of carbon. This source might have been evolved in mid-ocean ridge (W. Atalla) and subduction zone (W. Alam and G. El-Maiyit) settings. The studied serpentinites are usually enriched in FMEs, particularly Pb, Sr, Cs, and U. These enrichments were most probably the result of serpentinization and/or carbonation.     

O18 enriched ophiolitic metabasic rocks from E. Liguria (Italy), Pindos (Greece), and Troodos (Cyprus)

Low grade hydrothermally metamorphosed ophiolitic basic rocks from E. Liguria (Italy), Pindos (Greece) and Troodos (Cyprus) are enriched in O¹⁸ relative to the oxygen isotope ratio of fresh basalt (6.0±0.5‰). The maximum observed δO¹⁸ value of +13.22‰ corresponds to a positive isotope shift of 7‰ Enrichments in Sr⁸⁷ relative to Sr⁸⁶ correlate with hydrothermal alteration. The δC¹³ values of secondary calcite from E. Liguria are positive, and fall in the range from +0.2% to +3.6‰ Since ophiolitic rocks are considered to be fragments of the oceanic crust and upper mantle, and since the secondary metamorphic assemblages were produced before mechanical emplacement, it is considered that the hydrothermal metamorphism which affected these rocks occurred in the sub-sea-floor environment. The isotope data are directly consistent with the hypothesis that the alteration was produced by interaction of the basaltic material with introduced sea water. Water: rock ratios were sufficiently large to produce the observed isotope shifts. In the Troodos ophiolite sequence δO¹⁸ values decrease steadily downwards and change to progressively larger depletions in the Sheeted Intrusive Complex. The trend of δO¹⁸ decrease correlates with the original direction of increasing temperature. The O¹⁸ depletions, which have also been observed for oceanic “greenstones” (Muehlenbachs and Clayton, 1972b), resulted from water/rock interaction at temperatures greater than the particular temperature range above which whole rock-water fractionations became less than the isotopic difference between fresh basalt and sea water. Since this isotope geochemistry indicates that the water responsible for hydrothermal metamorphism was of sea water origin, the data support the more general hypothesis that convection of sea water within the upper 4–5 kms of the oceanic crust is a massive and active process at oceanic ridges. This process may be completely or partially responsible for (a.i.), the local scatter and low mean value of the conductive heat flux measured near ridges, (a.ii), the transfer of considerable quantities of heat from spreading oceanic ridges, (b) hydrothermal metamorphism, metasomatism and mineralization of oceanic crust, (c), the production of metal enriched, relatively reduced brines during sea water/basalt interaction, d), the high degree of scatter and low mean value of the compressional wave velocities of oceanic basement layer 2 and (e), the low natural remanent magnetization (NRM) intensity of the lower part of layer 2 and upper part of layer 3 of oceanic crust.     

A-type granites induced by a breaking-off and delamination of the subducted Junggar oceanic plate,West Junggar,Northwest China

The A-type granites with highly positive ε_Nd(t) values in the West Junggar, Central Asian Orogenic Belt (CAOB), have long been perceived as a group formed under the same tectonic and geodynamic setting, magmatic sourceq and petrogenetic model. Geological evidence shows that these granites occurred at two different tectonic units related to the southeastern subduction of Junggar oceanic plate: the Hongshan and Karamay granites emplaced in the southeast of West Junggar in the Baogutu continental arc; whereas the Akebasitao and Miaoergou granites formed in the accretionary prism. Here the authors present new bulk-rock geochemistry and Sr-Nd isotopes, zircon U-Pb ages and Hf-O isotopes data on these granites. The granites in the Baogutu continental arc and accretionary prism contain similar zircon ε_Hf(t) values (+10.9 to +16.2) and bulk-rock geochemical characteristics (high SiO₂ and K₂O contents, enriched LILEs (except Sr), depleted Sr, Ta and Ti, and negative anomalies in Ce and Eu). The Hongshan and Karamay granites in the Baogutu continental arc have older zircon U-Pb ages (315–305 Ma) and moderate ¹⁸O enrichments (δ¹⁸O_zircon=+6.41‰–+7.96‰); whereas the Akebasitao and Miaoergou granites in the accretionary prism have younger zircon U-Pb ages (305–301 Ma) with higher ¹⁸O enrichments (δ¹⁸O_zircon=+8.72‰–+9.89‰). The authors deduce that the elevated ¹⁸O enrichments of the Akebasitao and Miaoergou granites were probably inherited from low-temperature altered oceanic crusts. The Akebasitao and Miaoergou granites were originated from partial melting of low-temperature altered oceanic crusts with juvenile oceanic sediments below the accretionary prism. The Hongshan and Karamay granites were mainly derived from partial melting of basaltic juvenile lower crust with mixtures of potentially chemical weathered ancient crustal residues and mantle basaltic melt (induced by hot intruding mantle basaltic magma at the bottom of the Baogutu continental arc). On the other hand, the Miaoergou charnockite might be sourced from a deeper partial melting reservoir under the accretionary prism, consisting of the low-temperature altered oceanic crust, juvenile oceanic sediments, and mantle basaltic melt. These granites could be related to the asthenosphere’s counterflow and upwelling, caused by the break-off and delamination of the subducted oceanic plate beneath the accretionary prism Baogutu continental arc in a post-collisional tectonic setting.©2022 China Geology Editorial Office.     

Excimer laser isotope-ratio-monitoring mass spectrometry for in situ oxygen isotope analysis

A technique using isotope-ratio-monitoring gas chromatography-mass spectrometry (irmGCMS) and excimer laser fluorination for in situ oxygen isotope analysis of silicates is described. The irmGCMS and oxygen extraction line is connected by a newly developed interface, reducing the time for a single analysis to less than 10 min. The precision obtained for δ¹⁸O is similar to what has been reported for excimer laser fluorination using dual inlet systems. δ¹⁸O values of two olivine standards had 1σ precision of ±0.14‰ (n=19 and n=10) and that of Dörentrup quartz had ±0.17‰. Eleven analyses of a large zircon crystal had a precision of ±0.12‰. However, between 300 and 600 nmol oxygen was liberated for a single analysis, equivalent to cylindrical laser holes 250 to 350 μm in diameter and depth. In the future it will be feasible to measure the isotope ratio of cylindrical volumes 150 μm in diameter simply by reducing the volume of the extraction line. While this is still significantly larger than what is possible with ion probes, the ratios obtained by excimer laser-irmGCMS are highly accurate and precise without correction. The value of this technique for in situ oxygen isotope measurements is demonstrated with two rock slabs from metamorphic rocks of the Dabie–Sulu ultra-high-pressure belt, China.     

Low-O18 basalts from Iceland

The volcanic rocks of Iceland are anomalous in their oxygen isotope content. Recent tholeiitic and transitional alkali basalts from Iceland range in (δO¹⁸ from 1·8 to 5δ7%. Most of the tholeiitic basalts and their phenocrysts are at least 1% lower in δO¹⁸ than unaltered basalts from other oceanic islands or oceanic ridges. The Icelandic basalts that resemble ridge basalts in δO¹⁸ also resemble them in major element chemistry. δO¹⁸ values of alkali olivine basalts are closest to those of other oceanic islands. Secondary alteration processes have lowered as well as raised the δO¹⁸ values of some Icelandic rocks, but such surface mechanisms cannot account for the distribution of oxygen isotopes in the Recent basalts of Iceland. Three mechanisms that could give rise to the low-O¹⁸ magmas are (1) exchange of oxygen between magma and low-O¹⁸ hydrothermally altered rock, (2) exchange with low-O¹⁸ meteoric water, or (3) an exceptional mantle under Iceland. None of the above models can satisfactorily account for all the observations.     

Carbon,hydrogen, and oxygen isotope studies of the regional metamorphic complex at Naxos,Greece

At Naxos, Greece, a migmatite dome is surrounded by schists and marbles of decreasing metamorphic grade. Sillimanite, kyanite, biotite, chlorite, and glaucophane zones are recognized at successively greater distances from the migmatite dome. Quartz-muscovite and quartz-biotite oxygen isotope and mineralogie temperatures range from 350 to 700°C.The metamorphic complex can be divided into multiple schist-rich (including migmatites) and marblerich zones. The δ¹⁸O values of silicate minerals in migmatite and schist units and quartz segregations in the schist-rich zones decrease with increase in metamorphic grades. The calculated δ¹⁸O_H2O values of the metamorphic fluids in the schist-rich zones decrease from about 15‰ in the lower grades to an average of about 8.5‰ in the migmatite.The δD values of OH-minerals (muscovite, biotite, chlorite, and glaucophane) in the schist-rich zones also decrease with increase in grade. The calculated δD_H2O values for the metamorphic fluid decrease from ?5‰ in the glaucophane zone to an average of about ?70‰ in the migmatite. The δD values of water in fluid inclusions in quartz segregations in the higher grade rocks are consistent with this trend.Theδ¹⁸O values of silicate minerals and quartz segregations in marble-rich zones are usually very large and were controlled by exchange with the adjacent marbles. The δD values of the OH minerals in some marble-rich zones may reflect the value of water contained in the rocks prior to metamorphism.Detailed data on 20 marble units show systematic variations of δ¹⁸O values which depend upon metamorphic grade. Below the 540°C isograd very steep δ¹⁸O gradients at the margins and large δ¹⁸O values in the interior of the marbles indicate that oxygen isotope exchange with the adjacent schist units was usually limited to the margins of the marbles with more exchange occurring in the stratigraphic bottom than in the top margins. Above the 540°C isograd lower δ¹⁸O values occur in the interior of the marble units reflecting a greater degree of recrystallization and the occurrence of Ca-Mg-silicates.Almost all the δ¹³C values of the marbles are in the range of unaltered marine limestones. Nevertheless, the δ¹³C values of most marble units show a general correlation with δ¹⁸O values.The $\text{CO}{}_2\text{H}{}_2\text{O}$ mole ratio of fluid inclusions in quartz segregations range from 0.01 to 2. Theδ¹³C values of the CO₂ range from ?8.0 to 3.6‰ and indicate that at some localities CO₂ in the metamorphic fluid was not in carbon isotopic equilibrium with the marbles.