Paper No. 270-6
Presentation Time: 9:30 AM
FLUID-ROCK INTERACTION VIA INTERFACE COUPLED DISSOLUTION-REPRECIPITATION RECORDED BY TRACE ELEMENTS IN GARNET
Chemical zoning in garnet can preserve an invaluable record of pressure, temperature, composition, and temporal evolution during metamorphism. For major elements, this record is commonly strongly affected by intracrystalline diffusion at high grade conditions. We show, however, that the slowly-diffusing trace elements P, Na, and Ti can preserve considerable metamorphic history. Zoned garnets from three >900 oC rocks are investigated: (1) UHP diamondiferous saidenbachite, Saxonian Erzgebirge (e.g. Massonne, 2003), (2) eclogite facies Saxony granulite (e.g. O’Brien, 2006), and (3) UHT gneiss, Central Maine Terrane, Connecticut (e.g. Axler & Ague, 2015). Phosphorous and Na are concentrically zoned and increase toward the garnet rim in the Erzgebirge example; clear boundaries separate successive “shells” which variably overlap and cross-cut each other. The Ti zoning is largely smooth except for local depletion halos around exsolved rutile precipitates. Oscillatory growth zoning in P is preserved in parts of the Saxony granulite garnet, but this is cross-cut by dramatic, sharply-bounded compositional domains that interfinger with each other and have lower P and Na, and slightly higher Ti, than the oscillatory-zoned regions. The garnet in the UHT gneiss has a high P core that was “eaten away” and replaced by a lower P rim; the core-rim contact is sharp. Ti decreases more smoothly from core to rim except around rutile precipitates. The P zoning patterns demonstrate garnet replacement by interface coupled dissolution-reprecipitation (ICDR) in the presence of a fluid (which can be melt) (e.g. Putnis and Austrheim, 2010). Na typically records this as well via coupled P-Na substitution, but Ti is more affected by diffusion. The Erzgebirge garnet records progressive fluid-rock interaction to UHP conditions during subduction, marked by core-to-rim increases in P and Na. The domains of low P and low Na in the Saxony granulite garnet formed during retrograde, exhumation-related ICDR. The Connecticut gneiss garnet records the UHT event in its high Ti and high P core, and subsequent fluid infiltration during granulite facies overprinting on the rim. Trace elements in garnet can preserve evidence for fluid-rock interaction at extreme temperatures that major elements cannot owing to diffusional relaxation.