Pressure-temperature-time constraints for UHP Tso Morari Eclogite, NW India
Kayleigh M. Harvey, Dennis Donaldson, Carrie A. Menold, A. Alexander Webb
Presented in session T23. Exploring the Development of the Himalayan-Karakorum-Tibet Orogenic System from the Mantle to Mountain Peaks (Posters)
Petrochronology on Tso Morari eclogites supports a single, protracted UHP event across the orogen from ca. 47 Ma to 43 Ma, consistent with the initiation of the Indian subduction at ca. 51Ma. Pressure-temperature data adds constraints, and the resultant P-T-t path of the eclogites can provide significant insight into the collision event and subsequent exhumation. The eclogite occur as rare boudins in felsic, Indian supracrustal rocks. The samples contain garnet, omphacite, phengite, rutile, quartz, amphiboles, zoisite, sodic augite-plagioclase symplectite, zircon, pyrite, and titanite. The rock is largely granoblastic, and approximately 25% of the garnet porphyorblasts display atoll texture. The pale-red, inclusion-rich cores are prograde, and likely underwent selective replacement with matrix minerals from metamorphic reactions at peak conditions. Colorless, inclusion-poor rims record peak conditions. While it is likely that the unit reached coesite stability field, no evidence of coesite has been observed in these samples. P-T estimations were calculated using published thermobarometers and THERMOCALC. To minimize stoichiometric error from Fe2+/Fe3+ estimations in omphacite, net-transfer equilibria were used in place of exchange equilibria by use of the garnet-omphacite-phegite thermometer instead of the traditional garnet-pyroxene thermometer. Analyses suggest a clockwise P-T path. A prograde stage is preserved in garnet cores and yields conditions of 23.2 ± 2.6 kbar and 411 ± 67°C; U/Pb zircon data suggest the eclogite facies initiated at ca. 51 Ma. Garnet rims yield peak metamorphic conditions of 27 ± 4 kbar and 743 ± 149°C which occurred from ca. 47 to 43 Ma. Temperature estimations from Ti-in-peak phengite is consistent with the P-T-t path, as is preliminary petrochronology of rutile and titanite. The P-T path is also in broad agreement with published paths from both Kaghan Valley and Tso Morari, further suggesting the synchronous suturing of India and the Kohistan Ladakh Arc in the NW Himalaya.
Analysis of phosphate in riparian sediment pore-waters: does phosphate sequestered in the stream bed contribute to the phosphate load of the Kalamazoo River?
Kayleigh M. Harvey, Kaitlyn Pospeich Havens, Timothy N. Lincoln
Presented in session T125. Sigma Gamma Epsilon Undergraduate Research (Posters)
The upper Kalamazoo River in south-central Michigan is a low-profile, primarily gaining, meandering stream with an adjacent wetland corridor. An item of regulatory concern is the dissolved phosphate levels in the river. We investigated the flux of phosphate from organic sediments to the river as a possible contributor to the dissolved phosphate load. Sediment total phosphate ranges from .05 to 1% P205. Phosphate has strong negative correlation with SiO2 in sediment, and positive correlations with Fe2O3 and organic material. This suggests that a significant fraction of the phosphate is adsorbed on ferric hydroxide in organic sediments. This fraction is susceptible to desorption as ferric hydroxides dissolve under reducing conditions. Mapping of 18 km of river bed allows us to estimate that it contains 134,000 m3 of methane-producing sediment. A conservative estimate is that this reservoir could supply the annual dissolved phosphate load of the river for 35 years.
Profiles of pore-water show highest concentrations of dissolved phosphate between 10-20 cm into the sediment. At this depth, the average concentration of orthophosphate is 128 ppb (n=11), significantly higher than the average concentrations at all depths (73ppb, n=37), and river water (20ppb, n=10).
Pore- waters entering the stream averaged 143 ppb orthophosphate (n=8), 7 times that of the stream concentration. Calculated phosphate fluxes range from 3 x 10-10 to 5 x10-12 g PO43-/cm2/sec. The higher fluxes appear to occur in locations with high methane effervescence, as inferred from pock-marked sediment surfaces, which result from repeated methane eruptions. This release of orthophosphate from sediment provides the river with a steady supply of phosphate. Calculations show that the highest flux would provide 200% of the observed phosphate load, the low flux would provide about 5% of the load. Without more data, the precise flux is not known, but these estimates show that the release of phosphate likely contributes significantly to the river’s dissolved phosphate load.
These data have implications for remediation efforts in the watershed. Because the sediment has accumulated such a high concentration of phosphate, any reduction of current sources of contamination may not have an immediate effect on the river’s phosphate levels.
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