Fluid-absent metamorphism in the Adirondacks

Results on late Proterozoic metamorphism of granulite in the Adirondacks are presented. There more than 20,000 sq km of rock are at granulite facies. Low water fugacites are implied by orthopyroxene bearing assemblages and by stability of k'spar-plag-quartz assemblages. After mentioning the popular concept of infiltration of carbon dioxide into Precambrian rocks and attendent generation of granulite facies assemblages, several features of Adirondack rocks pertinent to carbon dioxide and water during their metamorphism are summarized: wollastonite occurs in the western lowlands; contact metamorphism by anorthosite preceeding granulite metamorphism is indicated by oxygen isotopes. Oxygen fugacity lies below that of the QFM buffer; total P sub water + P sub carbon dioxide determined from monticellite bearing assemblages are much less than P sub total (7 to 7.6 kb). These and other features indicate close spatial association of high- and low-P sub carbon dioxide assemblages and that a vapor phase was not present during metamorphism. Thus Adirondack rocks were not infiltrated by carbon dioxide vapor. Their metamorphism, at 625 to 775 C, occurred either when the protoliths were relatively dry or after dessication occurred by removal of a partial melt phase

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

Stable isotopic characteristics and magmatic history of meta-igneous rocks, Adirondacks, NY

The Adirondack Highlands contain dominantly meta-igneous rocks including the anorthosite-granite association and a wide variety of unrelated orthogneisses from gabbroic to granitic composition. The internal variation of oxygen isotopes within Adirondack meta-igneous suites related to the Marcy anorthosite shows that elevated δ^(18)O is a compositionally dependent characteristic of the protolith. Analysis of δ^(18)O in previously unsampled compositions in this suite has led to a stable isotopic, petrographic and major element description of a continuous range of rock types from two-pyroxene, oxide rich monzonitic gneiss to mesoperthite, quartz rich granite gneiss within the Tupper and Saranac sheets on the NW margin of the Marcy anorthosite. This expanded data set has allowed the formulation of a semi-quantitative model, based both in thermodynamics and mass balance, for the AFC magmatic evolution of this suite. A sampling traverse of the Snowy Mountain dome anorthosite and related halo of granitic gneiss in the southern Adirondack Highlands shows the concentric spatial distribution of δ^(18)O in granites around anorthosite, constrains the extent of interaction between anorthosite and granite, and provides an additional example of crustal interaction in the magmatic evolution of this suite. This evidence, combined with previous work (Morrison and Valley, 1988), documents extensive crustal interaction in region-wide anorthosite-granite plutonism prior to the peak of Grenville metamorphism

Oxygen-isotope and trace element constraints on the origins of silica-rich melts in the subarc mantle

Peridotitic xenoliths in basaltic andesites from Batan island in the Luzon arc contain silica-rich (broadly dacitic) hydrous melt inclusions that were likely trapped when these rocks were within the upper mantle wedge underlying the arc. These melt inclusions have been previously interpreted to be slab-derived melts. We tested this hypothesis by analyzing the oxygen isotope compositions of these inclusions with an ion microprobe. The melt inclusions from Batan xenoliths have δ 18OVSMOW values of 6.45 ± 0.51‰. These values are consistent with the melts having been in oxygen isotope exchange equilibrium with average mantle peridotite at temperatures of ≥875°C. We suggest the δ 18O values of Batan inclusions, as well as their major and trace element compositions, can be explained if they are low-degree melts (or differentiation products of such melts) of peridotites in the mantle wedge that had previously undergone extensive melt extraction followed by metasomatism by small amounts (several percent or less) of slab-derived components. A model based on the trace element contents of Batan inclusions suggests that this metasomatic agent was an aqueous fluid extracted from subducted basalts and had many characteristics similar to slab-derived components of the sources of arc-related basalts at Batan and elsewhere. Batan inclusions bear similarities to “adakites,” a class of arc-related lava widely considered to be slab-derived melts. Our results suggest the alternative interpretation that at least some adakite-like liquids might be generated from low-degree melting of metasomatized peridotites

Stable isotopic disequilibrium in high-T metamorphic systems

A principal use of stable isotopes in metamorphic rocks is as thermometers, or as tests for isotopic equilibrium where metamorphic temperatures are known. Applications are often complicated when apparent isotopic temperatures are discordant and disagree with petrologic temperatures, indicating a failure of isotopic systems to record and/or preserve equilibrium, peak-T fractionations. In low-T, fluid-hosted environments such features often clearly reflect open system exchange. However, in high-T metamorphic environments a slow cooling history can be sufficient to produce such features by retrograde, closed system exchange between coexisting minerals

Oxygen-isotopic composition and high-resolution secondary ion mass spectrometry imaging of Martian carbonate in Lafayette meteorite

Carbonate from SNC meteorites gives insight into a variety of processes on and/or beneath the surface of Mars. In Lafayette, carbonate occurs in unusually intimate association with hydrous phases when compared with other carbonate-bearing SNCs [1]. We have measured the ^(18)O/^(16)O ratio of carbonate in the alteration veins of Lafayette using the magnetic sector ion microprobe. In addition, we obtained isotope images of major- and minor-element cations in veinlets with the focused ion beam (FIB) quadrupole SIMS. The spot size of the FIB probe used in this study (~0.03 µm) provides an opportunity for high spatial resolution chemical imaging, which is ideal for examining features in the fine-grained alteration assemblage

Valley receives 2003 N.L. Bowen award

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

Research on nonlinear optical materials: an assessment. IV. Photorefractive and liquid crystal materials

This panel considered two separate subject areas: photorefractive materials used for nonlinear optics and liquid crystal materials used in light valves. Two related subjects were not considered due to lack of expertise on the panel: photorefractive materials used in light valves and liquid crystal materials used in nonlinear optics. Although the inclusion of a discussion of light valves by a panel on nonlinear optical materials at first seems odd, it is logical because light valves and photorefractive materials perform common functions