The effect of prograde metamorphism on the chlorine isotope compositions of sedimentary rocks

Chlorine stable isotope compositions of two Swiss sedimentary sequences and their metamorphic equivalents were measured in order to study fractionation effects during prograde metamorphism and devolatilization. Protoliths (n=25) were collected from a 50 m section of Triassic deltaic and lagoonal strata and Liassic marine black shales in a well-characterized quarry. Borehole samples through the same sequence (n=12) were acquired from the collection of M. Frey. Low greenschist to middle amphibolite facies equivalents (n\u3e80) were collected from the Glarner Alps, Urseren Zone, and Lukmanier region. \u03b437Cl values of silicate-bound chloride (SBC) are constant within individual sedimentary layers, but vary from \u20132.0 to +2.5\u2030 throughout the Triassic sequence and from \u20133.0 to 0\u2030 in the black shales. All dolomitic and gypsiferous samples have positive \u03b437Cl values. Colored marls and shales from the base of the Triassic sequence are isotopically negative, whereas those from the top are isotopically positive. Water-soluble \u03b437Cl values are 0.5-3.0\u2030 lower than SBC values in Triassic samples, but are 0.4-2.4\u2030 higher than SBC in black shales. Cl\u2013 contents range from 5-100 ppm in SBC fractions in both protolith series, and from 5-70 ppm in WSC fractions. Metamorphic equivalents of the Triassic and Liassic protoliths record the same overall ranges in \u03b437Cl as their protoliths. Samples with similar bulk composition but different metamorphic grades differ in \u03b437Cl by 641\u2030. These data lead to the following conclusions: (1) Both continental and marine sedimentary rocks display large heterogeneities in \u03b437Cl. (2) Negative \u039437Cl (SBC-WSC) fractionation in black shales may reflect partitioning fluid and insoluble organohalogen compounds, in addition to silicate-fluid partitioning. (3) Alpine-style metamorphism has little to no effect on rock Cl isotopic compositions up to mid-amphibolite facies, despite significant devolatilization and major changes in Cl-hosting mineralogy. (4) Cl is largely retained in the rocks during devolatilization, contrary to the normally hydrophilic behavior of chlorine. This likely reflects the fact that the fluids in all lithologies were CO2-rich. This study shows that metamorphic rocks can preserve their protolith \u03b437Cl values, with initial cm to m scale sedimentary heterogeneities, up to amphibolite-grade condition

Oxygen isotope evidence for subduction and rift-related mantle metasomatism beneath the Colorado Plateau-Rio Grande rift transition

Spinel lherzolite and pyroxenite xenoliths from the Rio Puerco Volcanic Field, New Mexico, were analyzed for oxygen isotope ratios by laser fluorination. In lherzolites, olivine d18O values are high (+5.5 per mil), whereas d18O values for pyroxenes are low (cpx=+5.1 per mil; opx=+5.4 per mil) compared to average mantle values. Pyroxenite d18O values (cpx=+5.0 per mil; opx=+5.3 per mil) are similar to those of the lherzolites and are also lower than typical mantle oxygen isotope compositions. Texturally and chemically primary calcite in pyroxenite xenoliths is far from isotopic equilibrium with other phases, with d18O values of +21 per mil. The isotopic characteristics of the pyroxenite xenoliths are consistent with a petrogenetic origin from mixing of lherzolitic mantle with slab-derived silicate and carbonatite melts. The anomalously low d18O in the pyroxenes reflects metasomatism by a silicate melt from subducted altered oceanic crust, and high d18O calcite is interpreted to have crystallized from a high d18O carbonatitic melt derived from subducted ophicarbonate. Similar isotopic signatures of metasomatism are seen throughout the Rio Puerco xenolith suite and at Kilbourne Hole in the southern Rio Grande rift. The discrete metasomatic components likely originated from the subducted Farallon slab but were not mobilized until heating associated with Rio Grande rifting occurred. Oxygen diffusion modeling requires that metasomatism leading to the isotopic disequilibrium between calcite and pyroxene in the pyroxenites occurred immediately prior to entrainment. Melt infiltration into spinel-facies mantle (xenoliths) prior to eruption was thus likely connected to garnet-facies melting that resulted in eruption of the host alkali basalt

Pressure-temperature-time constraints of metamorphism and tectonism in the Tauern Window, Eastern Alps

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1985.Microfiche copy available in Archives and Science.Bibliography: leaves 266-279.by Jane Elizabeth Selverstone.Ph.D

Diamond formation by carbon saturation in C-O-H fluids at Lago di Cignana UHPM unit (western Alps, Italy)

Microdiamonds in garnet of graphite-free ultrahigh pressure metamorphic (UHPM) rocks from Lago di Cignana (western Alps, Italy) represent the first occurrence of diamond in a low temperature subduction complex of oceanic origin (T = 600°C; P ≥ 3.2 GPa). The presence of diamonds in fluid inclusions provides evidence for carbon transport and precipitation in an oxidized H2O-rich C-O-H crustal fluid buffered by mineral equilibria at sub-arc mantle depths. The structural state of carbon in fluid-precipitated diamonds was analyzed with 514 nm excitation source confocal Raman microspectroscopy. The first order peak of sp3-bonded carbon in crystalline diamonds lies at 1331 (±2) cm-1, similar to diamonds in other UHPM terranes. The analysis of the spectra shows additional Raman features due to sp2 carbon phases indicating the presence of both hydrogenated carbon (assigned to trans-polyacetylene segments) in grain boundaries, and graphite-like amorphous carbon in the bulk, i.e. showing a structural disorder much greater than that found in graphite of other UHPM rocks. In one rock sample, defective microdiamonds are recognized inside fluid inclusions by the presence of a weaker and broader Raman band downshifted from 1332 to 1328 cm−1. The association of sp3- with sp2-bonded carbon indicates variable kinetics during diamond precipitation. We suggest that precipitation of disordered sp2-bonded carbon acted as a precursor for diamond formation outside the thermodynamic stability field of crystalline graphite. Diamond formation started when the H2O-rich fluid reached the excess concentration of C required for the spontaneous nucleation of diamond. The interplay of rock buffered fO2 and the prograde P-T path at high pressures controlled carbon saturation (aC=1) in the fluid phase. Thermodynamic modeling confirms that the C-O-H fluids from which diamond precipitated must have been water- rich(0.992 < XH2O < 0.997), assuming that fO2 is fixed by the EMOD equilibrium

An integrated geologic framework for EarthScope's US array

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95254/1/eost15515.pd

Archived Rock Samples From Jane Selverstone\u27s Research Collection

This Excel spreadsheet lists all of the rock samples from Jane Selverstone\u27s research collection that have been archived into the Earth and Planetary Sciences Collections at the University of New Mexico. All samples are listed by original (published) sample number and by an affiliated International GeoSample Number. Samples are grouped by research project, and doi information is given for all publications resulting from work on each project. Rock samples include various combinations of hand specimens, powders, thin sections, and billets from which thin sections were prepared. Information is included to identify the drawers within which each sample can be found in the collections facility housed in the basement of the UNM Earth and Planetary Sciences Department. Additional metadata, including field locations, analytical data, and photographs, are available at www.geosamples.org. Sample suites include crustal and mantle xenoliths from New Mexico, fault-zone rocks from the Whipple Mountains core complex, peridotites from the Ivrea Zone, high- and ultrahigh-pressure rocks from the Western Alps, and a prograde metamorphic sequence from Switzerland. All samples are available upon request for further research projects

Trace-element-rich brines in eclogitic veins: implications for fluid composition and transport during subduction

International audiencePrimary and pseudosecondary fluid inclusions occur in oscillatory-and sector-zoned omphacite in eclogitic veins from the Monviso ophiolitic complex in the Western Alps. The inclusions contain aqueous brines and daughter crystals of halite, sylvite, calcite, dolomite, albite, anhydrite and/or gypsum, barite, baddeleyite, rutile, sphene, Fe oxides, pyrite and monazite. This daughter mineral suite indicates high solubilites of Na, K, Ca, Mg, Fe, Zr, Ti, P, Ba, Ce, La, Th, and S species and provides direct evidence for transport of high-fieldstrenght, large-ion-lithophile, and light-rare-earth elements as dissolved species during subduction. Fluid-inclusion heterogeneities preserved within and between adjacent grains in the veins, however, suggest that the scale of fluid equilibration was small. A crack-seal geometry in some of the veins implies that fluid release in pulses rather than steady flow controlled mineral deposition and growth in the veins. From these observations, we develop a model of fluid release and entrapment in which pulses of fluid are associated in time with increments of shear and tensile failure; the rate of fluid release and the reduction in porosity both depend on the rate of plastic flow. Vein fluids may initially be derived from decreptitation of early fluid inclusions in the host eclogites, Small-scale fluid heterogeneities implied by the fluid inclusions in the veins are best interpreted in terms of limited fluid flow, and hence limited metasomatism. We conclude that element recycling into the mantle wedge during subduction will depend at least as strongly on fluid transport mechanisms as on element solubilities in the fluid phase. At Monviso, despite evidence for high trace element solubilities in saline brines, the elements were not removed from the downgoing slab prior to teaching depths of ∼40 km

Chlorine isotope evidence for syn-subduction modification of serpentinites by interaction with sediment-derived fluid

High-pressure serpentinites and rodingites and high- to ultrahigh-pressure metasedimentary rocks from the Aosta region, Italy, preserve strikingly different chlorine isotope compositions that can be used to constrain the nature of fluid-rock interactions during subduction. Serpentinites and rodingitized gabbroic dikes subducted to 70-80 km have bulk δ37Cl values between –1.6 and +0.9‰ (median= –0.5‰, n=26 plus 5 replicates; one amphibole-vein outlier at –2.9‰). Serpentinite δ37Cl values are positively correlated with Cr ± Cl contents (r2= 0.97 and 0.58) and negatively correlated with CaO (r2=0.72). BSE imaging and X-ray mapping reveal up to three generations of compositionally distinct serpentine and chlorite in single samples. The youngest generation, which is most abundant, has the lowest chlorine content. Three rodingite samples contain abundant texturally early fluid inclusions. These samples were finely crushed and leached in 18 MΩ H2O to extract water-soluble chlorides. The leachates, which are assumed to record the compositions of the fluid inclusions, have δ37Cl values that are 0.7-1.5‰ lower than the corresponding bulk rock values. Leachate from the outlier amph-magnesite vein is indistinguishable from the bulk value at –2.7‰. There is almost no overlap between the Cl isotope compositions of HP serp/rod samples and associated HP/UHP metasedimentary rocks. Calcmica schists, diamond-bearing Mn nodules, and impure marbles subducted to \u3e130 km and calcmica schists and Mn crusts transported to 70-80 km have δ37Cl values between –4.5 and –1.5‰ (median= –2.7‰, n=25 plus 7 replicates; two outlier points at –0.5‰). Primary fluid inclusions in the diamondiferous samples contain carbonate- and silicate-bearing aqueous fluids with very low chloride contents (Frezzotti et al., 2011, Nature Geosci). Taken together, these data record a history of progressive modification of serpentinites and rodingites by mixing with low-δ37Cl, low-Cl, high-Ca fluids during subduction and metamorphism. Serpentinites with the highest Cr contents have Cl isotopic compositions identical to those of modern seafloor serpentinites (δ37Cl=0.2-0.6‰), consistent with primary serpentinization by seawater (e.g., Barnes et al. 2009, Lithos). Low-Cr serpentinites record significant interaction with a Ca-rich fluid that shifted the rocks to lower δ37Cl values and diluted the original Cr and Cl contents. The fluid was likely derived from continuous devolatilization reactions in associated low-δ37Cl, calcareous metasedimentary rocks. These data have important implications for models of subduction mass transfer associated with antigorite breakdown. If serpentinites are commonly modified by interaction with metasedimentary fluids prior to antigorite dehydration, chemical signatures imparted during deserpentinization will reflect the integrated history of fluid-rock interaction in the subduction channel rather than an endmember “serpentinite signature”. The data further suggest that Cl may be hydrophobic in HP/UHP carbonate-bearing aqueous fluids, resulting in generation of low-Cl fluid during metamorphic devolatilization

Microtextural constraints on the interplay between fluidrockreactions and deformation

Schists from two mylonitic localities in the footwall of a low-angle normal fault in the eastern Alps record different degrees of embrittlement during exhumation, depending on the extent to which fluid–rock reactions proceeded. At one site, graphitic schists preserve textural evidence for two metamorphic reactions that modified XCO2XCO2 and/or fluid volume: (1) reaction between graphite and aqueous fluid that increased XCO2XCO2 without changing the molar amount of fluid, and (2) replacement of titanite by rutile, calcite, and quartz. The latter reaction involved net consumption of increasingly CO2-rich fluid. Areas where the first reaction proceeded are associated with abundant Mode I microcracks. Fluid inclusion arrays within the microcracks show that XCO2XCO2 increased from ∼0.1 to 0.6 during decompression from 4.75 to 2 kbar at a reference temperature of 500°C. Titanite consumption is most pronounced within transgranular Mode I microcracks, but microcracks do not crosscut products of this reaction; fluid consumption during reaction was coeval with the end of microcracking, at least on a local scale. At the other site, graphitic schists lack small-scale Mode I cracks as well as evidence for graphite consumption during decompression. SEM imaging shows that graphite is anhedral and pitted at the first site, but occurs in clusters of euhedral grains at the second site. Mass balance calculations demonstrate that rocks with partially consumed graphite were infiltrated by an externally derived, H2O-rich fluid that drove subsequent graphite-fluid reaction. Evidence for similar fluid infiltration is absent at the second site. Variations in the degree of reaction progress indicate that fluid pathways and deformation style were heterogeneous on the scale of millimeters to kilometers during exhumation from mid-crustal depths