Evidence For Isotopic Exchange Research Articles

Oxygen isotopic compositions of Allende Type C CAIs: Evidence for isotopic exchange during nebular melting and asteroidal metamorphism

In situ oxygen isotopic measurements of primary and secondary minerals in Type C CAIs from the Allende CV3 chondrite reveal that the pattern of relative enrichments and depletions of 16O in the primary minerals within each individual CAI are similar to the patterns observed in Types A and B CAIs from the same meteorite. Spinel is consistently the most 16O-rich (Δ17O=−25‰ to −15‰), followed by Al,Ti-dioside (Δ17O=−20‰ to −5‰) and anorthite (Δ17O=−15‰ to 0‰). Melilite is the most 16O-depleted primary mineral (Δ17O=−5‰ to −3‰). We conclude that the original melting event that formed Type C CAIs occurred in a 16O-rich (Δ17O⩽−20‰) nebular gas and they subsequently experienced oxygen isotopic exchange in a 16O-poor reservoir. At least three of these (ABC, TS26F1 and 93) experienced remelting at the time and place where chondrules were forming, trapping and partially assimilating 16O-poor chondrule fragments. The observation that the pyroxene is 16O-rich relative to the feldspar, even though the feldspar preceded it in the igneous crystallization sequence, disproves the class of CAI isotopic exchange models in which partial melting of a 16O-rich solid in a 16O-poor gas is followed by slow crystallization in that gas. For the typical (not associated with chondrule materials) Type C CAIs as well for as the Types A and B CAIs, the exchange that produced internal isotopic heterogeneity within each CAI must have occurred largely in the solid state. The secondary phases grossular, monticellite and forsterite commonly have similar oxygen isotopic compositions to the melilite and anorthite they replace, but in one case (CAI 160) grossular is 16O-enriched (Δ17O=−10‰ to −6‰) relative to melilite (Δ17O=−5‰ to −3‰), meaning that the melilite and anorthite must have exchanged its oxygen subsequent to secondary alteration. This isotopic exchange in melilite and anorthite likely occurred on the CV parent asteroid, possibly during fluid-assisted thermal metamorphism.

Geochemical and isotopic composition of organic matter in the Kupferschiefer of the Polish Zechstein basin: relation to maturity and base metal mineralization

vs the present depth of the Kupferschiefer, soluble organic matter (SOM) yields, and relative proportions of saturated and aromatic hydrocarbons of the SOM provide evidence for an oxidative alteration of organic matter in highly mineralized Kupferschiefer samples near the Rote Faule zones. This is confirmed by differences in the composition of the saturated and aromatic hydrocarbon fractions of the soluble organic matter: Saturated hydrocarbons from Rote Faule samples are dominated by short-chain n-alkanes and higher abundances of pristane and phytane relative to heptadecane (n-C17) and octadecane (n-C18), respectively, compared with samples more distant to the Rote Faule zone. Compositional changes of the aromatic hydrocarbon fractions with decreasing distance to that zone are characterized by the occurrence of polycyclic aromatic hydrocarbons and elevated ratios of phenanthrene to methylphenanthrenes that are attributed to demethylation reactions and resulted in a decrease of the methylphenanthrene index (MPI 1). Kupferschiefer samples from the barren zone of the Polish Basin do not show these alteration patterns. The observed variations in organic matter composition with burial depth are consistent with changes due to increasing thermal maturation. Maturity assessment is achieved from MPI 1 and the methyldibenzothiophene ratio (MDR). From the relationship between the maturity of organic matter in terms of vitrinite reflectance values and depth of the Kupferschiefer strata, a continuous increase in reflectance of vitrinite is obtained within the Polish Basin. The alteration pattern of organic matter related to base metal mineralization of the Kupferschiefer corresponds to changes in the isotopic composition of organic carbon and calcite. Kerogen within, or close to, Rote Faule zone is enriched in 13C caused by the preferential release of isotopically light organic compounds through progressive degradation of organic matter. The opposite tendency towards lower δ 13C and δ 18O values of calcite provides evidence for isotopic exchange between carbonate and the oxidizing, ore-bearing solutions and for organic matter remineralization. In contrast, organic matter and calcite from the Kupferschiefer do not show regular trends in δ 13C with increasing thermal maturation.

Evidence for Isotopic Exchange during Metabolism of Stable-Isotope-Labeled Formate in a Methanogenic Sediment

The disappearance of stable-isotope-labeled formate in freshwater sediment from Lake Vechten was studied by gas chromatography-mass spectrometry. Both deuterium- and (sup13)C-labeled formate were removed from pore water at a high rate (400 (plusmn) 35 [standard deviation] and 1,700 (plusmn) 600 (mu)M h(sup-1), respectively), with concomitant formation of nonlabeled formate. The removal rate of labeled and nonlabeled formate amounted to 240 (plusmn) 15 and 400 (plusmn) 40 (mu)M h(sup-1), respectively. The formation of nonlabeled formate is explained by isotopic exchange due to the activity of formate dehydrogenase rather than by the turnover of formate.

Comparison of acetate turnover in methanogenic and sulfate-reducing sediments by radiolabeling and stable isotope labeling and by use of specific inhibitors: evidence for isotopic exchange

Acetate turnover in the methanogenic freshwater anoxic sediments of Lake Vechten, The Netherlands, and in anoxic sediments from the Tamar Estuary, United Kingdom, and the Grosser Jasmunder Bodden, Germany, the latter two dominated by sulfate reduction, was determined. Stable isotopes and radioisotopes, inhibitors (chloroform and fluoroacetate), and methane flux were used to provide independent estimates of acetate turnover. Pore water acetate pool sizes were determined by gas chromatography with a flame ionization detector, and stable isotope-labeled acetate was determined by gas chromatography-mass spectrometry. The appearance of acetates with a different isotope labeling pattern from that initially added demonstrated that isotopic exchange occurred during methanogenic acetate metabolism. The predominant exchange processes were (i) D-H exchange in the methyl group and (ii) (sup13)C-(sup12)C exchange at the carboxyl carbon. These exchanges are most probably caused by the activity of the enzyme complex carbon monoxide dehydrogenase and subsequent methyl group dehydrogenation by tetrahydromethanopterine or a related enzyme. The methyl carbon was not subject to exchange during transformation to methane, and hence acetate with the methyl carbon labeled will provide the most reliable estimate of acetate turnover to methane. Acetate turnover rate estimates with these labels were consistent with independent estimates of acetate turnover (acetate accumulation after inhibition and methane flux). Turnover rates from either radioisotope- or stable isotope-labeled methyl carbon isotopes are, however, dependent on accurate determination of the acetate pool size. The additions of large amounts of stable isotope-labeled acetate elevate the acetate pool size, stimulating acetate consumption and causing deviation from steady-state kinetics. This can, however, be overcome by the application of a non-steady-state model. Isotopic exchange in sediments dominated by sulfate reduction was minimal.

Carbonate-hosted, shallow-submarine and burial hydrothermal mineralization in the Upper Mississippian Big Cove Formation, Port au Port Peninsula, western Newfoundland

Strata-bound sulfide (Zn-Pb-Fe) and sulfate (Ba-Sr) mineralization occurs in Upper Mississippian (Arundian-Holkerian) limestone, primarily bryozoan-microbial mounds of the Big Cove Formation on the Port au Port Peninsula, western Newfoundland. Stages of host-rock petrogenesis and mineralization are compared by integrating structure, stratigraphy, petrography, and geochemistry (fluid inclusions, and C, O, S, Sr, and Pb isotopes). Strata-bound mineralization is linked to fault-controlled fluid flow through Ordovician (basement) carbonate and into porous Mississippian host rock, with subsequent fluid migration bounded by stratigraphic permeability barriers. Mineralization is preferentially juxtaposed to deposits of Mississippian gypsum, the most likely source of sulfate. The 87 S/ 86 Sr ratios in calcite associated with mineralization suggest that fluids were most likely generated from or had undergone isotopic exchange with lower Paleozoic strata, including potentially metalliferous shale. The integrated database underscores a potentially rapid mineralizing system of Late Mississippian (343-330 Ma) age. Evidence for isotopic exchange brings a Permo-Pennsylvanian (250-300 Ma) age derived from lead isotope modeling into question.Minor syndepositional mineralization is characterized by Fe oxide stained omission surfaces within well-oxidized hyposaline lithofacies, possibly documenting incipient hydrothermal activity, and subsequent marcasite mineralization related to introduction of hydrothermal (60 degrees - 120 degrees C) brines into a very restricted hyposaline or lacustrine setting during faulting. Prominent Zn-Pb sulfides (sphalerite, galena) and subsequent Ba-Sr sulfates (barite, celestite), with intergrown marcasite and calcite, have a shallow burial (30 + m) origin. Fluids were variably hot (110 degrees - 190 degrees C), Mg-poor brines (20-25 wt % NaCl equiv). Pyrite gossans mark a last stage of mineralization. The delta 34 S values increase with the general successon of sulfide (-4 to 12ppm), sulfate (20-32ppm), and then pyrite (32ppm) mineralization, similar to a Rayleigh distillation process in a closed mineralizing system.Mineralization on the Port au Port Peninsula is similar to Mississippian-hosted Irish Zn-Pb deposits; there is an association with normal faults, prominent epigenetic mineralization, and a similar age. This commonality may reflect Late Mississippian tectonic instability within the paleohemispheric fault system that linked eastern Canada and Ireland.

Deuterium-labeling experiments relevant to the mechanism of platinum-catalyzed hydrogenation of (diolefin)dialkylplatinum(II) complexes: evidence for isotopic exchange via platinum surface hydrogen. The stereochemistry of reduction

Reduction of (diolefin)dialkylplatinum(II) complexes with dihydrogen over a platinum black catalyst is accompanied by interchange of hydrogen among the organic groups and gaseous dihydrogen. Exchange of hydrogens between an alkane solvent and these organic groups also occurs during the reaction, but only relatively slowly. An examination of the stereochemistry of reduction of (norbornadiene)dimethylplatinum(II) with D/sub 2/ indicates that the deuterium atoms add predominantly to the same (endo) face of the olefins as that coordinated to the dimethylplatinum moiety. Reduction of uncomplicated norbornadiene under the same conditions yields norbornane having primarily exo C-D bonds. These experiments are compatible with a mechanism for the reduction involving adsorption of the (diolefin)dialkylplatinum(II) complex on the surface of the platinum catalyst via its platinum atom, conversion of the organic moieties of the soluble (diolefin)dialkylplatinum complex to platinum-surface alkyls, and interchange of hydrogen atoms between these surface alkyls via a mobile pool of platinum-surface hydrogen atoms. Combination of the surface alkyls with surface hydrogen yields alkanes in a final irreversible step. Comparison of the evidence from deuterium-interchange experiments conducted under mass transport limited and reaction rate limited conditions is consistent with the hypothesis that the concentrations of hydrogen on the platinum surface is lower under massmore » transport limited conditions.« less