1,583 research outputs found
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
Changing the Role of the Media Specialist in Curriculum Planning at Decorah Senior High School
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
Alien Registration- Valley, Effie J. (Fort Fairfield, Aroostook County)
https://digitalmaine.com/alien_docs/35705/thumbnail.jp
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
Radiation exposure by radio-diagnostics in Switzerland: a pilot patient-oriented survey
A patient-oriented survey was conducted in Switzerland with the aim to establishing the distribution of medical X-ray examinations among the population. A sample of 1235 people aged between 15 and 74 y (589 males and 637 females) was contacted. The participants were asked to give the number of medical X-ray examinations performed during the previous 12 months. Three-fourths of the sample did not undergo any medical X-ray examination during a 12 month-period, and 10% of the population underwent more than 1 medical X-ray examination in a year. These results were independent of the geographic region. The age distributions of the participants who did not undergo any medical X-ray examination and those who underwent one medical X-ray examination were similar. On the other hand, the age distribution of the participants who underwent more than one medical X-ray examination was characterised by a higher contribution from the elderl
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
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