Oxygen 18/oxygen 16 and D/H studies of plutonic granitic and metamorphic rocks across the Cordilleran Batholiths of southern British Columbia

Hydrogen and oxygen isotope ratios of 500 samples, mainly from granitic plutons, were measured along a 700-km, E-W traverse across the “accreted terranes” of southern British Columbia (latitudes 49°–52°N). Despite the geological complexity and range of intrusive ages (Late Triassic to Tertiary) and although there are “steps” in the isotopic values at some geologic boundaries (e.g., across the Strait of Georgia), two clear patterns emerge: (1) The ^(18)O/^(16)O and D/H ratios of the waters involved in hydrothermal interactions with the granitic rocks show a regular eastward trend of depletion in D and ^(18)O. Enormous areas were affected by the hydrothermal processes, but the most intense alteration is localized along major north trending lineaments (e.g., Okanagan Lake). (2) Independent of the hydrothermal effects, the primary δ^(18)O values of the granitic rocks also change systematically eastward, from +7.0 to +8.5 in Vancouver Island, reaching a minimum of +5.5 to +7.0 in the western and central Coast Plutonic Complex, then increasing progressively from the eastern Coast Batholith to the Okanagan Batholith, and attaining a maximum of +10.0 to +12.0 in the Nelson Batholith. Two groups of samples are unique in their high δD values. The first group is represented by two geographically isolated batholiths (Guichon and Thuya) that were not affected by the Tertiary meteoric-hydrothermal systems and that have therefore preserved their Early Jurassic to Triassic K/Ar ages. The second group is represented by the Jurassic plutons of Vancouver Island; there, the hydrothermal fluids were both D-rich and ^(18)O-rich (δ^(18)O > 0), as evidenced by the fact that feldspars in the altered granites are enriched in ^(18)O relative to coexisting quartz. Both “anomalies” can be explained if these terranes were located closer to the equator and/or in a maritime environment at the time of intrusive and hydrothermal activity, in agreement with available paleomagnetic data. Excluding these anomalous areas, two distinct ages of meteoric-hydrothermal activity can be identified, Cretaceous in the west and early to mid-Tertiary in the east. The isotopic trends in the rocks are similar to the present-day patterns of meteoric waters in the region, with one primary difference: the paleowaters are enriched in D by about 20%, compatible with a northward translation of these terranes, a climatic change, or both. The similarities of the patterns suggest a topography similar to that of the present day (a mountain chain along the coast) during the early Tertiary. The whole-rock δ^(18)O values of the granitic rocks (determined by extrapolation of the trends of δ^(18)O quartz versus δ^(18)O feldspar, to correct for hydrothermal alteration effects) suggest that the main part of the Coast Plutonic Complex formed from magmas similar to those in oceanic island arcs, derived from basaltic crust or from an upper mantle source. Toward the east, an increasingly larger component of sedimentary rocks must be present in the source regions of the granitic magmas. It may be significant that right at the edge of the North American craton there is a drop in the δ^(18)O of the plutons from +11 down to +9.5, suggesting involvement of old granitic crust instead of sedimentary rocks. These new data from southern British Columbia confirm and amplify previous studies that have documented: (1) the existence of widespread and pervasive hydrothermal effects throughout the North American Cordillera, and (2) regular west-to-east asymmetries in the amounts of different kinds of parent rocks in the source regions from which these Mesozoic and Cenozoic granitic magmas were derived

Carbon isotope and magnetic polarity evidence for nondepositional events within the Cambrian-Ordovician Boundary section near Dayangcha, Jilin Province, China

Carbon isotope and magnetic polarity stratigraphic results from the Cambrian-Ordovician Boundary section at Xiaoyangqiao, near Dayangcha, Jilin Province, China, in comparison to a contemporaneous section at Black Mountain, Australia, indicate strata equivalent to major portions of the Australian sequence are either absent or are restricted to highly condensed intervals. These intervals are correlative with regressive sea level events identified in Australia and western North America, suggesting regional or eustatic sea level changes strongly influenced deposition of the Xiaoyangqiao sequence. These results also suggest the Xiaoyangqiao section is unfavourable as the site of the Cambrian-Ordovician Boundary Global Stratotype Section and Point

Carbon isotope signatures from land snail shells: Implications for palaeovegetation reconstruction in the eastern Mediterranean

In this studywecompare carbon isotope values inmodern Helix melanostoma shell carbonate (d13Cshell) from the Gebel al-Akhdar region of Libya with carbon isotope values in H. melanostomabody tissue (d13Cbody), local vegetation (d13Cplant) and soil (d13Csoil). All vegetation in the study area followed the C3 photosynthetic pathway. However, the d13Cplant values of different species formed two distinct isotopic groups. This can be best explained by different water use efficiencies with arid adapted species having significantly more positive d13Cplant values than less water efficient species. The ranges and means of d13Cbody and d13Cplant were statistically indistinguishable from one another suggesting that d13Cbody was primarily a function of local vegetation composition. H. melanostoma d13Cshell reflected the d13Cplant of local vegetation with a positive offset between body/diet and shell of 14.5± 1.4‰. Therefore, in the Gebel al-Akhdar where only C3 plants are present, highermeand13C shell values likely reflect greater abundances ofwater-efficientC3 plants in the snails diet and therefore in the landscape, whilst lower mean d13Cshell values likely reflect the consumption of less water-efficient C3 plants. The distribution of these plants is in turn affected by environmental factors such as rainfall. These findings can be applied to archaeological and geological shell deposits to reconstruct late Pleistocene to Holocene vegetation change in the southeast Mediterranean

Chemical diffusion of fluorine in melts in the system Na2OAl2O3SiO2

The volatilization of fluorine from three melts in the system Na2OAl2O3SiO2 has been investigated at 1 atm pressure and 1200–1400°C. The melts chosen have base compositions corresponding to albite, jadeite and a peraluminous melt with 75 mole % SiO2. Melt spheres were suspended from platinum loops in a vertical tube furnace in a flow of oxygen gas, then quenched, sectioned and analysed by electron microprobe. The microprobe scans indicate that transport of fluorine to the melt-vapor interface is by binary, concentration-independent interdiffusion of fluorine and oxygen. FO interdiffusivity increases in the order albite < peraluminous < jadeite. There is no simple reciprocal relationship between FO interdiffusivity and melt viscosity. Comparison with data on high-pressure interdiffusivity of fluorine and oxygen in jadeite melt indicates that FO interdiffusivity increases with pressure from 0.001 to 10 kbar while the activation energy remains unchanged. Fluorine chemical diffusivity in albite melt is substantially lower than H2O chemical diffusivity in obsidian melts suggesting that different diffusive mechanisms are responsible for the transport of F and H2O in igneous melts. Fluorine diffuses in albite melt via an anionic exchange with oxygen whereas water probably diffuses in obsidian melt via an alkali exchange mechanism

The Carnian Humid Episode of the late Triassic: a review

From 1989 to 1994 a series of papers outlined evidence for a brief episode of climate change from arid to humid, and then back to arid, during the Carnian Stage of the late Triassic Epoch. This time of climate change was compared to marine and terrestrial biotic changes, mainly extinction and then radiation of flora and fauna. Subsequently termed, albeit incorrectly, the Carnian Pluvial Event (CPE) by successive authors, interest in this episode of climatic change has increased steadily, with new evidence being published as well as several challenges to the theory. The exact nature of this humid episode, whether reflecting widespread precipitation or more local effects, as well as its ultimate cause, remains equivocal. Bed-by-bed sampling of the Carnian in the Southern Alps (Dolomites) shows the episode began with a negative carbon isotope excursion that lasted for only part of one ammonoid zone (A. austriacum). However, that the Carnian Humid Episode represents a significantly longer period, both environmentally and biotically, is irrefutable. The evidence is strongest in the European, Middle Eastern, Himalayan, North American and Japanese successions, but not always so clear in South America, Antarctica and Australia. The eruption of the Wrangellia Large Igneous Province and global warming (causing increased evaporation in the Tethyan and Panthalassic oceans) are suggested as causes for the humid episode

Chemostratigraphy of Neoproterozoic carbonates: implications for 'blind dating'

The delta C-13(carb) and Sr-87/Sr-86 secular variations in Neoproteozoic seawater have been used for the purpose of 'isotope stratigraphy' but there are a number of problems that can preclude its routine use. In particular, it cannot be used with confidence for 'blind dating'. The compilation of isotopic data on carbonate rocks reveals a high level of inconsistency between various carbon isotope age curves constructed for Neoproteozoic seawater, caused by a relatively high frequency of both global and local delta C-13(carb) fluctuations combined with few reliable age determinations. Further complication is caused by the unresolved problem as to whether two or four glaciations, and associated negative delta C-13(carb) excursions, can be reliably documented. Carbon isotope stratigraphy cannot be used alone for geological correlation and 'blind dating'. Strontium isotope stratigraphy is a more reliable and precise tool for stratigraphic correlations and indirect age determinations. Combining strontium and carbon isotope stratigraphy, several discrete ages within the 590-544 Myr interval, and two age-groups at 660-610 and 740-690 Myr can be resolved

Organic carbon content and carbon isotope variations across the Permo-Triassic boundary in the Gartnerkofel-1 borehole, Carnic Alps, Austria

The Gartnerkofel borehole is one of the most thoroughly studied and described Permo-Triassic sections in the world. Detailed bulk organic carbon isotope studies show a negative base shift from − 24‰ to − 28‰ in the Latest Permian which latter value persists into the Earliest Triassic after which it decreases slightly to − 26‰. Two strongly negative peaks of > − 38‰ in the Latest Permian and a lesser peak of − 31‰ in the Early Triassic are too negative to be due to a greater proportion of more negative organic matter and must be due to very negative methane effects. The overall change to more negative values across the Bulla/Tesero boundary fits the relative rise in sea level for this transition based on the facies changes. A positive shift in organic carbon isotope values at the Late Permian Event Horizon may be due to an increase in land-derived organic detritus at this level—a feature shown by all Tethyan Permo-Triassic boundary sections though these other sections do not have the same values. Carbonate carbon isotope trends are similar in all sections dropping by 2–3 units across the Permo-Triassic boundary. Gartnerkofel carbonate oxygen values are surprisingly, considering the ubiquitous dolomitization, compatible with values elsewhere and indicate reasonable tropical temperatures of 60 °C in the Latest Permian sabkhas to 20–40 °C in the overlying marine transition beds. Increased land-derived input at the Late Permian Event Horizon may be due to offshore transport by tsunamis whose deposits have been recognized in India at this level

Late Cretaceous tectonic history of the Sierra‐Salinia‐Mojave arc as recorded in conglomerates of the Upper Cretaceous and Paleocene Gualala Formation, northern California

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95005/1/jgrb14012.pd