Post-metamorphic fluid infiltration into granulites from the Adirondack Mountains, USA

Post-metamorphic effects in the anorthosites of the Adirondacks, New York were described. Calcite-chlorite-sericite assemblages occur as veins, in disseminated form and as clots, and document retrograde fluid infiltration. These features are associated with late-state CO2-rich fluid inclusions. Stable isotope analyses of calcites indicates that the retrograde fluids interacted with meta-igneous and supracrustal lithologies, but the precise timing of the retrogression is as yet unknown

Oxygen isotope ratios in olivine from the Hawaii Scientific Drilling Project

Oxygen isotope ratios of olivine in 23 tholeiites from the Hawaii Scientific Drilling Project (HSDP) core (15 from Mauna Kea, 8 from Mauna Loa) and three samples of outcropping subaerial or dredged submarine Mauna Kea lavas have been measured by laser fluorination. The δ^(18)O values are 4.6–5.4 ‰, confirming previous observations that some Hawaiian lavas are derived from sources with δ^(18)O values lower than typical upper mantle (δ^(18)Oolivine ≈ 5.2±0.2 ‰). The Mauna Kea-Mauna Loa transition marks a shift from δ^(18)O values lower than the mantle average in Mauna Kea olivines (∼4.8) to more typical mantle values in Mauna Loa olivines. Lavas containing olivines with δ^(18)O values similar to the typical upper mantle are associated with more “primitive” or less depleted radiogenic isotope characteristics; i.e., with higher ^3He/^4He (>13 Ra), higher ^(87)Sr/^(86)Sr (>0.7036) and lower є_(Nd) (<6.5), and with ^(206)Pb/^(204)Pb ratios less than -18.3. These relationships indicate that the δ^(18)O values of the relatively enriched source components of the Hawaiian plume sampled by Mauna Loa lavas are comparable to (or greater than) the mantle average. This conclusion is supported by δ^(18)O values of olivine from other high ^3He/^4He islands, which are also comparable to the upper mantle average. The low δ^(18)O values in Hawaiian lavas are derived from a source having more MORB-like, or depleted, He, Nd, and Sr isotope ratios, but more radiogenic Pb than is seen in the Mauna Loa lavas Assimilation of ^(18)O-depleted lower oceanic crust from the underlying Pacific crust by hot, MgO-rich parental magmas or melting of older, recycled oceanic crust entrained in the Hawaiian plume are both possible sources of this ^(18)O-depleted, MORB-like component in Hawaiian magmas

Evidence from Polymict Ureilite Meteorites for a Single "Rubble-Pile" Ureilite Parent Asteroid Gardened by Several Distinct Impactors

Ureilites are ultramafic achondrite meteorites that have experienced igneous processing whilst retaining heterogeneity in mg# and oxygen isotope ratios. Polymict ureilites represent material derived from the surface of the ureilite parent asteroid(s). Electron microprobe analysis of more than 500 olivine and pyroxene clasts in six polymict ureilites reveals that they cover a statistically identical range of compositions to that shown by all known monomict ureilites. This is considered to be convincing evidence for derivation from a single parent asteroid. Many of the polymict ureilites also contain clasts that have identical compositions to the anomalously high Mn/Mg olivines and pyroxenes from the Hughes 009 monomict ureilite (here termed the Hughes cluster ). Four of the six samples also contain distinctive ferroan lithic clasts that have been derived from oxidized impactors. The presence of several common distinctive lithologies within the polymict ureilites is additional evidence that the ureilites were derived from a single parent asteroid. Olivine in a large lithic clast of augite-bearing ureilitic has an mg# of 97, extending the compositional range of known ureilite material. Our study confirms that ureilitic olivine clasts with mg#s 85, which also show more variable Mn contents, including the melt-inclusion bearing "Hughes cluster" ureilites. We interpret this to indicate that the parent ureilite asteroid was disrupted by a major impact at a time when melt was still present in regions with a bulk mg# > 85, giving rise to the two types of ureilites: common ferroan ones that were already residual after melting and less common magnesian ones that were still partially molten when disruption occurred, some of which are the result of interaction of melts with residual mantle during disruption. A single daughter asteroid re-accreted from the disrupted remnants of the mantle of the proto-ureilite asteroid, giving rise to a "rubble-pile" body that had material of a wide variety of compositions and shock states present on its surface. The analysed polymict ureilite meteorites represent regolith that subsequently formed on this asteroidal surface, including impact-derived material from at least six different meteoritic sources

Valley receives 2003 N.L. Bowen award

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94640/1/eost14650.pd

Oxygen Isotope Variability within Nautilus Shell Growth Bands

Nautilus is often used as an analogue for the ecology and behavior of extinct externally shelled cephalopods. Nautilus shell grows quickly, has internal growth banding, and is widely believed to precipitate aragonite in oxygen isotope equilibrium with seawater. Pieces of shell from a wild-caught Nautilus macromphalus from New Caledonia and from a Nautilus belauensis reared in an aquarium were cast in epoxy, polished, and then imaged. Growth bands were visible in the outer prismatic layer of both shells. The thicknesses of the bands are consistent with previously reported daily growth rates measured in aquarium reared individuals. In situ analysis of oxygen isotope ratios using secondary ion mass spectrometry (SIMS) with 10 μm beam-spot size reveals inter- and intra-band δ18O variation. In the wild-caught sample, a traverse crosscutting 45 growth bands yielded δ18O values ranging 2.5‰, from +0.9 to -1.6 ‰ (VPDB), a range that is larger than that observed in many serial sampling of entire shells by conventional methods. The maximum range within a single band (~32 μm) was 1.5‰, and 27 out of 41 bands had a range larger than instrumental precision (±2 SD = 0.6‰). The results from the wild individual suggest depth migration is recorded by the shell, but are not consistent with a simple sinusoidal, diurnal depth change pattern. To create the observed range of δ18O, however, this Nautilus must have traversed a temperature gradient of at least ~12°C, corresponding to approximately 400 m depth change. Isotopic variation was also measured in the aquarium-reared sample, but the pattern within and between bands likely reflects evaporative enrichment arising from a weekly cycle of refill and replacement of the aquarium water. Overall, this work suggests that depth migration behavior in ancient nektonic mollusks could be elucidated by SIMS analysis across individual growth bands

13C/12C exchange between calcite and graphite: A possible thermometer in Grenville marbles

The fractionation of 13C between calcite and graphite, [Delta](Cc-Gr). is consistently small (2.6-4.8 permil) in 34 assemblages from upper amphibolite- and granulite-facies marbles of the Grenville Province. In 25 samples from the Adirondack Mountains, New York, it decreases regularly with increasing metamorphic temperature. The fractionations are independent of absolute [delta]13C values of calcite (-2.9 to +5.0). For T = 600-800[deg]C, the Adirondack data are described by [Delta](Cc-Gr) = -0.00748T ([deg]C) + 8.68. This good correlation between [Delta] and T suggests that carbon isotope equilibrium was attained in these high-grade marbles and that the theoretical calculations of this fractionation by Bottinga are approximately 2 permil too large in this temperature range. Because of the relatively high temperature sensitivity suggested by these results and by Bottinga's calculations, and the pressure independence of isotope fractionation, [Delta](Cc-Gr) may provide a very good thermometer for high-grade marbles.Comparison of this field calibration for [Delta](Cc-Gr) vs temperature with results from other terranes supports the utility of [Delta](Cc-Gr) for geothermometry and suggests that graphite is much more sluggish to exchange than calcite, that exchange between calcite and graphite occurs at temperatures as low as 300[deg]C, and that equilibrium may normally be attained only when peak metamorphic temperatures are greater than 500-600[deg]C.Because 13C exchange is an unavoidable metamorphic process at temperatures above 300[deg]C, high values of [delta]13C(Gr) in moderate- to high-grade carbonate-bearing rocks do not provide a sufficient criterion to infer an abiogenic origin for the graphite.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/24452/1/0000726.pd

Low volumes of quartz cement in deeply buried Fulmar Formation sandstones explained by a low effective stress burial history

Upper Jurassic Fulmar Formation sandstones from the Fulmar Field in the Central North Sea are buried to 3.2 km and 128 °C but contain only 3.7 ± 1.7% (1σ) quartz cement, substantially less than volumes predicted by models based on temperature-related quartz precipitation kinetics. Oxygen isotope microanalysis of quartz overgrowths suggests that only limited cementation occurred at temperatures above 110 °C. We suggest that the anomalously low volumes of quartz cement are most readily explained by the effective stress history of the Fulmar Formation. Regional pore pressure analysis strongly suggests that pore fluid pressures in the Fulmar Formation decreased substantially in the last <0.5 Ma as a result of lateral seal failure, increasing effective stress from ca. 10 MPa to the current 31 MPa. A recent increase in effective stress is supported by the common occurrence of grains that are both fractured and unhealed by quartz cement. Intergranular pressure dissolution can account for around one third of the observed quartz cement, with the remainder from deep burial feldspar dissolution. We argue that the continuous history of low effective stress, until the very recent geological past, limited the rate of silica supply by intergranular pressure dissolution, and thus the rate of quartz cementation. Effective stress histories should be incorporated into predictive models of quartz cementation of sandstones

Oxygen isotope geochemistry of oceanic-arc lava

Variations of oxygen isotope ratios in arc-related lavas can constrain the contributions of subducted crustal igneous rocks, sediments, and fluids to the sub-arc mantle. We have measured oxygen isotope ratios in 72 arc and back-arc lavas from five ocean–ocean subduction zone systems using laser-fluorination analyses of olivine and other phenocrysts and glass. Eighty percent of our samples have {delta}18O values for any given phase (olivine, plagioclase, glass, or biotite) within 0·2{per thousand} of the average value for that phase in upper-mantle peridotites and mid-ocean ridge basalt (MORB); the range for each phase is <=1·0{per thousand}. This result contrasts with previous studies of whole-rock samples (which are significantly more variable even after exclusion of samples believed to be altered or fractionated by magmatic differentiation) and demonstrates that most arc-related lavas contain <=1–2% of 18O-enriched crustal oxygen from any source (i.e. assimilation or subducted contributions). Elevations in {delta}18O that do occur in these basic, arc-derived magmas relative to values most common for mantle-derived lavas are associated both with ‘enriched’ radiogenic isotope signatures and, even more strongly, with chemical indices consistent with high integrated extents of melting of their peridotite sources. We interpret these relationships as evidence that melting in the sources of the high-{delta}18O lavas we have studied was fluxed by addition of high-{delta}18O aqueous fluid (or perhaps a hydrous melt) from the subducted slab, such that sources that contain relatively large components of slab-derived fluid or melt are both relatively 18O enriched and also experienced relatively large amounts of melting. We have developed a quantitative model linking the amount of melting to the extents of 18O, radiogenic isotope, and trace-element enrichment in a mantle source being fluxed by addition of aqueous fluid. Comparison of this model with observed variations in the geochemistry of lavas from the Vanuatu–Fiji–New Caledonia region (the suite of related samples showing the greatest range in {delta}18O observed in this study) constrains the amounts and chemical and isotopic compositions of slab-derived phases in the sources of these arc-related lavas. Assuming a {delta}18O value of 20{per thousand} for the slab-derived fluid, 0·5–1·0 wt % is added to the sources of most mantle-derived arc magmas; the maximum amount of slab-derived flux in the sources of arc magmas according to our results is 2·5 wt %

Ion microprobe analysis of oxygen isotopes in garnets of complex chemistry

Accurate ion microprobe analysis of oxygen isotope ratios in garnet is possible if appropriate standards are employed to correct for instrumental bias, a component of which depends on the cation chemistry of the analyzed mineral. In this study, 26 garnet standards (including 14 new standards) that span the compositional range of pyrope, almandine, grossular, spessartine, and andradite were analyzed repeatedly by ion microprobe to develop a new method of correcting for instrumental bias in garnets. All analyses were normalized to a single master garnet standard (UWG-2) before bias from cation composition was considered. Bias due to cation composition in garnet was found to correlate with grossular content in pyralspite garnets and with andradite in ugrandite garnets. Bias is correlated with molar volume in garnets of all compositions in this study. Although this correlation is suitable as a correction scheme for bias, a more accurate correction scheme based on the grossular and andradite compositions of garnet is proposed. This method reproduces the bias of all but one standard to within a range of 0.4%%, an accuracy that is on the same order as the reproducibility (+/-0.3%%, 2S.D.) of the master garnet standard UWG-2, but that remains an independent source of error. The new correction scheme is used to successfully reproduce laser fluorination analyses along a traverse of a polymetamorphic, zoned skarn garnet from the Adirondack Mountains. While previous analyses were at the mm-scale, the new data resolve a gradient of δ^1^8O of 2.1%% over 16 m. If experimentally derived diffusion coefficients are correct, these new results show that granulite-facies metamorphism was significantly faster than previously assumed and the thermal peak was less than 5Myr

Petrology of a margarite-bearing meta-anorthosite from Seljeneset, Nordfjord, western Norway: Implications for the P-T history of the Western Gneiss Region during Caledonian uplift

The Western Gneiss Region of Norway contains relics of eclogite and high-pressure granulite that have been extensively overprinted by amphibolite-facies assemblages. A meta-anorthosite from Seljeneset. Nordfjord, contains fine-grained margarite+quartz+plagioclase+/-muscovite clots set in a zoisite+quartz+plagioclase+/-clinozoisite matrix. In some clots relict kyanite is preserved. Equilibria modeled in the CASH system indicate a retrograde P-T path that begins in the kyanite+zoisite field (P&gt;8.2 kbar), passes through reaction (1), kyanite+zoisite+quartz[right harpoon over left]anorthite+H2O, between 610[deg]C and about 800[deg]C, and finally through reaction (2), kyanite+anorthite+H2O[right harpoon over left]margarite+quartz. The sodic composition of plagioclase in reaction (1) suggests that pressures in excess of 15-17 kbar (at 750-800[deg]C) may have been experienced during Silurian eclogite-facies metamorphism. The simplified CASH reaction (2) occurs between invariant points at 4.3 kbar,520[deg]C and 8.2 kbar,610[deg]C, but polyvariance, due to variable Na solid solution, broadens the reaction into a band about 150[deg]C wide. Cooling through 500[deg]C occurred about 410 Ma ago. This study provides the first well-defined P-T path for Caledonian retrograde metamorphism at amphibolite-facies conditions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27410/1/0000444.pd