The Adirondack Mountains, New York are an outlier of the Grenville Province that stretches from Texas to Labrador. This terrain represents the deeply eroded roots of a vast mountain belt that existed 1200 to 1000 million years ago. Like the Himalaya today, these mountains were subjected to intense heating and pressure, followed by erosion, allowing rocks formed deep in the Earth?s crust to cool and rise to the surface, accounting for the topography, ore deposits, and recreational potential of the region. This study will apply newly developed techniques of geochemistry to determine the thermal and fluid history that formed these rocks.
This study will test the results of a pilot study in a range of rock types and localities in the Adirondacks. That first study proposed that surprisingly rapid cooling and uplift occurred in the NW Adirondacks along a major mylonite zone that extends over 300 km (Carthage-Colton, Bonamici et al. 2011). The new results will test this model, constrain the peak metamorphic temperatures, and assess the significance of the concentric ?bulls-eye? pattern of isotherms that has classically been interpreted to result from domical uplift. Cooling and uplift rates will be combined with on-going studies of U-Pb geochronology to test competing theories for uplift of the Adirondack Highlands at the end of the Ottawan Orogeny (~1050 Ma). This project will develop and apply a new approach to studies of oxygen isotope thermometry and speedometry in high-grade metamorphic rocks. Novel capabilities for accurate and precise in situ analysis of oxygen isotope ratios from small 1-10 μm spots will be employed to analyze mineral inclusions that are armored by refractory host minerals and to quantify intracrystalline diffusion gradients that formed during cooling. These profiles will be modeled with updated code for the Fast Grain Boundary diffusion model. Samples will be selected primarily from granulite facies rocks. Garnet-quartz thermometry will be tested in detail across the Adirondacks with a series of outcrop tests for self-consistency, and other mineral systems will be tested locally. Garnet + quartz are common in a range of orthogneisses and metasediments, allowing the same pressure independent thermometer to be applied across the terrane. Temperatures will be calculated for both quartz inclusions in garnet, and in quartz surrounded by garnet coronas, and these T?s will be compared to those from garnets in quartzite, and to other geothermometry. Oxygen isotope profiles will be measured in single crystals of titanite as well as garnet, and selected zircons and pyroxenes.
Fig. 1. Correlated U-Pb, d18O and trace element zoning measured by SIMS in type-2 titanite from the Carthage-Colton mylonite zone, Adirondack Mts, NY. Diffusion rates of Pb and O are similar at granulite facies temperature leading to correlated patterns and increasingly reset values from core to rim. Preservation of core compositions and ages requires rapid cooling at ca 50oC/Ma supporting a model of gravitational collapse at the end of the Grenville Orogeny (from Bonamici et al. 2014, 2015). Fig. 2. Peak temperatures of 700oC and cooling rates of 1 to 50oC/Ma are estimated from in situ SIMS analysis of oxygen isotope ratios in garnet and quartz in granulite facies gneisses from the Adirondack Mts. Peak metamorphic values of d18O are preserved for quartz inclusions (QI) armored within garnet and peak T is calculated from Δ18O(QI-Grt). Cooling rate is estimated by comparison of QI and matrix quartz (MQ). The retrograde exchange of d18O between MQ and other minerals is modeled by the fast grain boundary diffusion model (Eiler et al. 1992. 1993; Bonamici et al. 2014), using diffusion data from dry quartz (Sharp et al. 1991). Values of Δ18O(MQ-QI) (blue lines) are 0 if quenched and increase for slower cooling. (Ryan Quinn, PhD in progress, UW-Madison). Fig. 3. Atom-probe tomography (APT) shows positions (±0.2 nm) of single atoms of 207Pb, 206Pb, and Y that in JH4.4, a 4.4 Ga zircon from the Jack Hills, W. Australia. [A] APT shows clustering of radiogenic Pb (&Y, REEs) at 10-nm-scale, which due to diffusion into domains of α-recoil damage during 3.4 Ga reheating. [B] SIMS U-Pb data show concordant ages of 4.4 Ga in the core of JH4.4 and 3.4 Ga in its overgrowth rim. These results show that radiogenic Pb has migrated at scales of <50 nm and that 20-micron SIMS data in the core are unaffected, validating the 4.4 Ga age. These results also support published criteria to avoid metamict zircon, which have been used to establish a secular trend for oxygen isotopes in igneous zircon recording 4.4 billion years of Earth history [C]. (Valley et al. 2014, 2015) References Bonamici CE, Kozdon R, Ushikubo T, Valley JW (2014) Intragrain oxygen isotope zoning in titanite by SIMS: Cooling rates and fluid infiltration along the Carthage-Colton Mylonite Zone, Adirondack Mountains, NY, USA, J Meta Geol. 32:71-92. Bonamici CE, Fanning CM, Kozdon R, Fournelle JH, Valley JW (2015) Combined oxygen isotope and U-Pb zoning studies of titanite: New criteria for age preservation. Chem. Geol. 298: 70-84. Valley JW, Cavosie AJ, Ushikubo T, Reinhard DA, Lawrence DF, Larson DJ, Clifton PH, Kelly TF, Wilde SA, Moser DE, Spicuzza MJ (2014) Hadean age for a post-magma-ocean zircon confirmed by atom-probe tomography. Nature Geosci 7: 219-223. Valley JW, Reinhard DA, Cavosie AJ, Ushikubo T, Lawrence DF, Larson DJ, Kelly TF, Snoeyenbos D, Strickland A (2015) Nano- and Micro-geochronology in Hadean and Archean Zircons by Atom-Probe Tomography and SIMS: New Tools for Old Minerals. Am. Mineral, doi.org/10.2138/am-2014-5134. In press.
