Type of Culminating Activity
Master of Science in Geology
Matthew J. Kohn, Ph.D.
Garnet growth rates have provided valuable information for understanding the rates of tectonometamorphic processes. In theory, during its growth, garnet records the decay of 87Rb within a rock matrix as steadily increasing 87Sr/86Sr from core to rim. By measuring the Sr isotopic zoning within garnet and matrix Rb/Sr and 87Sr/86Sr, the rate of garnet growth can be determined. To test this, we used ID-TIMS to measure Rb-Sr data for five samples from three major orogenic belts: central New England, southernmost Chile, and western Italian Alps. These new data introduce two major difficulties encountered using Rb-Sr garnet chronology: 1) Model ages are much younger than previous estimates of peak metamorphism (by over 100 Myr in some cases) and 2) garnets display an inverse Sr isotopic zoning (decreasing 87Sr/86Sr from core to rim).
Based upon the observed younger model ages, it was postulated that the measured matrix data does not accurately represent the matrix reservoir from which the garnet grew. To further constrain the elemental and isotopic composition of the matrix, we measured Rb and Sr (as well as several other elements) for individual phases or mixed phase line traverses in the matrix using LA-ICPMS for three samples. The mixed phase lines indicate heterogeneities of both Rb and Sr within the matrix at scales comparable to garnet grain sizes. Measurements for individual phases within the matrix were used to calculate the matrix 87Rb/86Sr. Growth rates for those samples were calculated using the newly measured matrix 87Rb/86Sr and a 87Sr/86Sr that allowed garnet ages to reflect the published ages of metamorphism. The growth rates for three garnet crystals ranged from 0.24 – 3.0 mm/Myr and are geologically reasonable. Growth rates for two samples could not be calculated due to the inverse zoning and must have been subjected to the addition of unradiogenic Sr.
The younger apparent ages and inverse Sr zoning may be the result of post or syngarnet growth fluid infiltration. Alternatively, the composition of the reactive matrix may have been changing during garnet growth. Simple models demonstrate that if garnet was not equilibrating with all phases within the matrix, then measured matrix data will underestimate the Rb/Sr from which the garnet grew and can result in too young apparent ages and inversely zoned garnets.
Sousa, Jessica, "Measuring the Rate of Garnet Growth: Implications for Rb-Sr Garnet Chronology" (2011). Boise State University Theses and Dissertations. Paper 187.