DETERMINING GARNET GROWTH RATES FROM STRONTIUM ISOTOPE ZONING

In principle, garnet growth rates can be determined by measuring Sr isotope zoning in garnet, and whole-rock ⁸⁷Sr/⁸⁶Sr and Rb/Sr ratios: the garnet should record the decay of matrix ⁸⁷Rb as steadily increasing ⁸⁷Sr/⁸⁶Sr from core to rim (Christensen et al. 1989; Science, 244, 1465-1469). To test this method for inferring rates of temperature and pressure increases during metamorphism, we used ID-TIMS to measure Rb-Sr concentration and isotope zoning across 5 garnets from 4 different metamorphic belts: eastern Vermont (c. 400 Ma), western New Hampshire (c. 400 Ma), southernmost Chile (c. 80 Ma), and the Italian Alps (c. 35 Ma). These new data, however, evince two major interpretational problems: 1) Many garnets give ages that are too young, in one case by over 100 Myr, and 2) Some garnets exhibit inverse zoning, i.e. decreases in ⁸⁷Sr/⁸⁶Sr from core to rim. Both problems could reflect open system behavior changing the matrix Rb/Sr or ⁸⁷Sr/⁸⁶Sr ratios, and indeed syn-metamorphic fluid infiltration has been proposed for two of these rocks. Stable isotope zoning profiles across other garnets, however, are inconsistent with open system behavior. Alternatively, some of the problems could reflect variable participation of different minerals in the matrix, changing the effective bulk composition that the garnet “sees” during growth. Expected reactivities and diffusive closure temperatures imply that biotite (high Rb/Sr) will always participate, but muscovite (moderate Rb/Sr), plagioclase (low Rb/Sr), and epidote (low Rb/Sr) may not, especially at low garnet nucleation temperatures. As temperature increases, the low Rb/Sr phases may begin to participate, changing both the Rb/Sr and ⁸⁷Sr/⁸⁶Sr ratio of the reactive matrix. A changing effective bulk composition can cause inverse zoning and spuriously young ages, and can both increase and decrease calculated durations of garnet growth. For the garnets we studied, calculated durations range from -100 Myr (inversely zoned Alps) to +30 Myr (normally zoned Vermont).