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Re-evaluating the P-T-t History of the Metamorphic Terranes of Western New England with Novel Thermobarometry Methods.

紀錄類型:	書目-電子資源 : Monograph/item
正題名/作者:	Re-evaluating the P-T-t History of the Metamorphic Terranes of Western New England with Novel Thermobarometry Methods./
作者:	Wolfe, Oliver Maxwell.
出版者:	Ann Arbor : ProQuest Dissertations & Theses, : 2019,
面頁冊數:	230 p.
附註:	Source: Dissertations Abstracts International, Volume: 81-02, Section: B.
Contained By:	Dissertations Abstracts International81-02B.
標題:	Petrology. -
電子資源:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=13813629
ISBN:	9781085570954

Re-evaluating the P-T-t History of the Metamorphic Terranes of Western New England with Novel Thermobarometry Methods.
Wolfe, Oliver Maxwell.

Re-evaluating the P-T-t History of the Metamorphic Terranes of Western New England with Novel Thermobarometry Methods. - Ann Arbor : ProQuest Dissertations & Theses, 2019 - 230 p.

Source: Dissertations Abstracts International, Volume: 81-02, Section: B.

Thesis (Ph.D.)--Rensselaer Polytechnic Institute, 2019.

This item must not be sold to any third party vendors.

(1) The novel method of inclusion barometry coupled with calculation of the required affinity for garnet nucleation were applied to three samples from the previously well-characterized Connecticut Valley Synclinorium in central Vermont. Raman shifts for quartz inclusions record a range of maximum peak shifts of the quartz 464 cm-1 peak from 2.4-3.0 cm-1. Temperature of garnet nucleation was constrained by calculating mineral assemblage diagrams in the MnNCKFMASHT system and plotting the intersection of quartz inclusion in garnet barometry (QuiG) with Zr-in-rutile thermometry. Utilizing the intersection of Zr-in-rutile thermometry with QuiG barometry, garnet nucleation is inferred to have occurred within a P-T range of ~8.6-9.5 kbar and ~560-575 ˚C. These P-T conditions for garnet nucleation are significantly higher than calculated equilibrium garnet-in isograds for the three samples.Affinities for garnet nucleation were calculated as the difference between the free energy of a fictive garnet composition based on the matrix assemblage and the free energy of the nucleated garnet. The calculated nucleation affinity varied from 300-600 kJ/mol O for St-Ky grade samples. These results suggest that the assumption that metamorphism proceeds as a sequence of near-equilibrium conditions cannot, in general, be made for regional metamorphic terranes. This body of work agrees with numerous recent studies showing that garnet producing reactions must be overstepped in order to for garnet to nucleate.(2) Diffusion modeling of metapelite samples from the Connecticut Valley Trough (CVT), eastern Vermont, was performed to constrain the temperature-time (T-t) paths during the Acadian Orogeny. Examined samples preserve retrograde decompression textures and garnet rim back-diffusion spessartine enrichment. Notably, garnet in one of the altered samples preserves a core to rim difference in quartz inclusion Raman shifts from 2.4 cm-1 to -1.1 cm-1. Quartz in garnet Raman barometry was applied to quartz inclusions and the resulting pressure difference between the core and rim isomekes was ~5 kbars. Diffusion modeling of rim Mn back-diffusion profiles indicated ~1-2.5 Ma durations were required at near peak T to form these compositional profiles. Similar durations were derived from 2D thermal modeling of the thermobarometric and thermochronologic data. The retrograde T-t paths from both models are defined by a period of time spent at near peak T conditions followed by a period of rapid cooling that reached 100-120 °C/Ma. Both modeling approaches produce T-t paths that are consistent with existing thermochronology data on the timing of peak metamorphism and 40Ar/39Ar muscovite closure during exhumation and cooling. The slow cooling segment of the T-t path is interpreted to have occurred along a nearly isothermal decompression path from a depth of ~32 to ~13 km at near peak temperatures with a exhumation rate of ~8 mm/yr. Rapid cooling was inferred to follow decompression to preserve the garnet compositional profile after formation.(3) A comparative analysis of Raman shifts of quartz inclusions in garnet was made on metamorphic rocks from the Connecticut Valley Trough (CVT) in western New England to examine the regional trends of Raman shifts along strike, and to compare quartz inclusion in garnet (QuiG) Raman barometry results with conventional thermobarometry. Overall, Raman shifts of quartz inclusions ranged from 1.2-3.5 cm-1 over all field areas and displayed a south to north decrease, matching the overall decrease in mapped metamorphic grade. Along an E-W transect across strike of the CVT at the Strafford Dome, there were observed discontinuities in Raman shifts at the Richardson Memorial Contact (RMC) and east of the Strafford Dome, indicating a fault contact and post peak metamorphism shortening respectively. Shifts across the RMC result in a shift discontinuity of ~ 0.4-0.7 cm-1 and a possible pressure break of up to ~1 kbar between the CVT and underlying Ordovician rocks. Along an E-W traverse in the vicinity of the Goshen Dome, there was little apparent variation in Raman shifts across the RMC with a shift of 2.8 cm-1 directly west of the contact and a range of 2.5-3.0 cm-1 from samples east of the contact. Shifts broadly decrease to 2.4 cm-1 further west from the RMC and indicate an overall smooth decrease in shifts. Across both field sites, Raman shifts of quartz inclusions typically do not show systematic variation with respect to their position from the garnet core, an indication that that garnet likely grew at isothermal and isobaric P-T conditions and is a typical occurrence for garnet in CVT metamorphic rocks.QuiG barometry results were broadly consistent with garnet nucleating and growing at near peak P-T conditions. In a suite of samples from the Goshen traverse, QuiG isomekes intersected the range of thermobarometry conditions retrieved from garnet cores and garnet rims. Peak metamorphic conditions in the western Massachusetts field area fell within a range of ~8-10.5 kbar and ~540-585 °C and showed significant overlap of across the RMC. The one sample that did not provide consistent results was OW-18, where garnet core barometry results plotted ~3 kbar below QuiG barometry isomekes. In this case, it is possible that this sample has inherited garnet cores from a previous metamorphic event and experienced Acadian rim overgrowth. In general, QuiG barometry results lies between the rim and core conventional barometry results and provides tighter pressure constraints than the ~1-2 kbar pressure range inferred from the core to rim difference of conventional thermobarometry results.

ISBN: 9781085570954Subjects--Topical Terms:

535210
Petrology.
Subjects--Index Terms:

nucleation

Re-evaluating the P-T-t History of the Metamorphic Terranes of Western New England with Novel Thermobarometry Methods.
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300 $a 230 p.
500 $a Source: Dissertations Abstracts International, Volume: 81-02, Section: B.
500 $a Advisor: Spear, Frank S.
502 $a Thesis (Ph.D.)--Rensselaer Polytechnic Institute, 2019.
506 $a This item must not be sold to any third party vendors.
520 $a (1) The novel method of inclusion barometry coupled with calculation of the required affinity for garnet nucleation were applied to three samples from the previously well-characterized Connecticut Valley Synclinorium in central Vermont. Raman shifts for quartz inclusions record a range of maximum peak shifts of the quartz 464 cm-1 peak from 2.4-3.0 cm-1. Temperature of garnet nucleation was constrained by calculating mineral assemblage diagrams in the MnNCKFMASHT system and plotting the intersection of quartz inclusion in garnet barometry (QuiG) with Zr-in-rutile thermometry. Utilizing the intersection of Zr-in-rutile thermometry with QuiG barometry, garnet nucleation is inferred to have occurred within a P-T range of ~8.6-9.5 kbar and ~560-575 ˚C. These P-T conditions for garnet nucleation are significantly higher than calculated equilibrium garnet-in isograds for the three samples.Affinities for garnet nucleation were calculated as the difference between the free energy of a fictive garnet composition based on the matrix assemblage and the free energy of the nucleated garnet. The calculated nucleation affinity varied from 300-600 kJ/mol O for St-Ky grade samples. These results suggest that the assumption that metamorphism proceeds as a sequence of near-equilibrium conditions cannot, in general, be made for regional metamorphic terranes. This body of work agrees with numerous recent studies showing that garnet producing reactions must be overstepped in order to for garnet to nucleate.(2) Diffusion modeling of metapelite samples from the Connecticut Valley Trough (CVT), eastern Vermont, was performed to constrain the temperature-time (T-t) paths during the Acadian Orogeny. Examined samples preserve retrograde decompression textures and garnet rim back-diffusion spessartine enrichment. Notably, garnet in one of the altered samples preserves a core to rim difference in quartz inclusion Raman shifts from 2.4 cm-1 to -1.1 cm-1. Quartz in garnet Raman barometry was applied to quartz inclusions and the resulting pressure difference between the core and rim isomekes was ~5 kbars. Diffusion modeling of rim Mn back-diffusion profiles indicated ~1-2.5 Ma durations were required at near peak T to form these compositional profiles. Similar durations were derived from 2D thermal modeling of the thermobarometric and thermochronologic data. The retrograde T-t paths from both models are defined by a period of time spent at near peak T conditions followed by a period of rapid cooling that reached 100-120 °C/Ma. Both modeling approaches produce T-t paths that are consistent with existing thermochronology data on the timing of peak metamorphism and 40Ar/39Ar muscovite closure during exhumation and cooling. The slow cooling segment of the T-t path is interpreted to have occurred along a nearly isothermal decompression path from a depth of ~32 to ~13 km at near peak temperatures with a exhumation rate of ~8 mm/yr. Rapid cooling was inferred to follow decompression to preserve the garnet compositional profile after formation.(3) A comparative analysis of Raman shifts of quartz inclusions in garnet was made on metamorphic rocks from the Connecticut Valley Trough (CVT) in western New England to examine the regional trends of Raman shifts along strike, and to compare quartz inclusion in garnet (QuiG) Raman barometry results with conventional thermobarometry. Overall, Raman shifts of quartz inclusions ranged from 1.2-3.5 cm-1 over all field areas and displayed a south to north decrease, matching the overall decrease in mapped metamorphic grade. Along an E-W transect across strike of the CVT at the Strafford Dome, there were observed discontinuities in Raman shifts at the Richardson Memorial Contact (RMC) and east of the Strafford Dome, indicating a fault contact and post peak metamorphism shortening respectively. Shifts across the RMC result in a shift discontinuity of ~ 0.4-0.7 cm-1 and a possible pressure break of up to ~1 kbar between the CVT and underlying Ordovician rocks. Along an E-W traverse in the vicinity of the Goshen Dome, there was little apparent variation in Raman shifts across the RMC with a shift of 2.8 cm-1 directly west of the contact and a range of 2.5-3.0 cm-1 from samples east of the contact. Shifts broadly decrease to 2.4 cm-1 further west from the RMC and indicate an overall smooth decrease in shifts. Across both field sites, Raman shifts of quartz inclusions typically do not show systematic variation with respect to their position from the garnet core, an indication that that garnet likely grew at isothermal and isobaric P-T conditions and is a typical occurrence for garnet in CVT metamorphic rocks.QuiG barometry results were broadly consistent with garnet nucleating and growing at near peak P-T conditions. In a suite of samples from the Goshen traverse, QuiG isomekes intersected the range of thermobarometry conditions retrieved from garnet cores and garnet rims. Peak metamorphic conditions in the western Massachusetts field area fell within a range of ~8-10.5 kbar and ~540-585 °C and showed significant overlap of across the RMC. The one sample that did not provide consistent results was OW-18, where garnet core barometry results plotted ~3 kbar below QuiG barometry isomekes. In this case, it is possible that this sample has inherited garnet cores from a previous metamorphic event and experienced Acadian rim overgrowth. In general, QuiG barometry results lies between the rim and core conventional barometry results and provides tighter pressure constraints than the ~1-2 kbar pressure range inferred from the core to rim difference of conventional thermobarometry results.
590 $a School code: 0185.
650 4 $a Petrology. $3 535210
650 4 $a Geology. $3 516570
653 $a nucleation
653 $a Zr-in-rutile thermometry
653 $a Acadian Orogeny
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690 $a 0372
710 2 0 $a Rensselaer Polytechnic Institute. $b Geology. $3 3168474
773 0 $t Dissertations Abstracts International $g 81-02B.
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791 $a Ph.D.
792 $a 2019
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856 4 0 $u http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=13813629