TESTING FOR RAPID PULSES OF CRUSTAL-SCALE HEAT TRANSFER BY MODELING GARNET GROWTH-DIFFUSION-RESORPTION PROFILES IN METAMORPHIC "HOT SPOTS", NEW HAMPSHIRE, USA

A series of localized high-temperature (HT) granulite facies regions (hot spots) were identified within the regional amphibolite facies background of the Central Maine Terrane in central New Hampshire. The spatial coincidence of the isograds and contours of depressed δ¹⁸O values are found centered on networks of quartz-graphite and pegmatitic veins in the hot spot near Bristol, NH. Chamberlain and Rumble (1988) propose that a large quantity of hot fluids ascending in the shallow crust produced the thermal anomaly during the Acadian orogeny (~400 Ma). The hot spot can only be preserved if the timescales of the thermal event were extremely short, otherwise heat conduction would smooth the field temperature gradient.

Two-phase flow modeling demonstrates that porosity waves evolve into fluid conduits that focus fluid and heat transfer. The corresponding thermal transients may be on the timescales of 10 kyr (Tian et al 2014). Herein, we conduct a petrological test to estimate the peak temperatures and the durations across the hot spot by modelling growth-diffusion-resorption profiles of garnets.

Chemical profiles in garnet are modelled using coupled phase equilibria and garnet diffusion simulation (similar to Gaidies et al 2008; Caddick et al 2010). The phase equilibria calculation (Chu et al 2013) is implemented by the PT-boundary-finder automation on the PT curve along which the bulk-composition is modified stepwise. The garnet growth-diffusion-resorption simulation uses equilibrium boundary conditions and newly calibrated diffusion coefficients (Chu and Ague 2015).

The pseudosection analyses indicate that the background PT conditions were 5-6 kbar, 770-810 °C, much hotter than previously estimated (muscovite-sillimanite zone; Chamberlain and Rumble 1988). The granulite-facies condition lasted >6 Myr, so that the Mg# of garnet cores were homogenized. In the center of the “hot spot”, garnet exhibits distinct zoning patterns. The simulation suggests a short-lived thermal spike (>100 °C, ~75 kyr) superimposed on the granulite-facies background. The peak temperature transiently exceeded 900 °C, reaching UHT conditions. The radius of the hot spot determined here is 1.5~2 km, fully consistent with the timescale of 75 kyr, considering the relationship L = (kt)^1/2 (thermal diffusivity k ~ 10^-6 m s^-1).