HOW OLD IS THE OLDEST KNOWN GARNET AND WHAT CAN WE LEARN FROM LOOKING AT THE PUBLISHED RECORDS OF GARNET GEOCHRONOLOGY?

In the thirty-five years since the first garnet geochronology was undertaken by Van Breemen and Hawkesworth (1) and Griffin and Brueckner (2), substantial strides have improved garnet geochronology methods and the published literature has expanded significantly. Garnet is useful for geochronologic study since it can be dated by multiple decay systems (Sm-Nd and Lu-Hf), grows in many rock types and tectonic settings, ages can be used to understand the rates and durations of tectonic events, and garnet can be used to link temperatures and pressures to metamorphic events through thermometry and thermodynamic modeling.

Given garnet’s potential to unlock valuable rate and condition information about ancient tectonic events, garnet could be used to fill in some of the missing pieces from Earth’s earliest history. However, to unlock these secrets using garnet, old garnet must first be located. To our knowledge only one paper has published a garnet isochron age older than 3.0 billion years (3). Perhaps we know of so few ancient garnets because we have not yet looked in the right places, indicating a possible bias in the existing record; such biases have also been a subject of debate in the zircon record (e.g. 4). Or perhaps garnet that old is simply not preserved or never grew in the first place, which would relate to a shift in tectonic conditions responsible for garnet growth and/or preservation.

As a first attempt to address the question of the completeness of the garnet record through Earth’s history, we have undertaken a compilation of published garnet ages. Much has been made of compilations of zircon ages, whose peaks have been widely analyzed and interpreted (e.g. 5,6). Our goal is to compile a similar (though much smaller) dataset for garnet, which could reveal where there are holes in the record- with special emphasis on the earliest garnet ages. Additionally, the compilation could allow for analysis of trends in the broader dataset.

1) Van Breemen, O & Hawkesworth, CJ (1980) Trans Royal Soc Edinburgh: Ear Sci 71: 97-102 2) Griffin, WL & Brueckner, HK (1980) Nature 285: 319-321 3) Cutts, K, et al. (2013) GSA Bulletin 126:251-270. 4) Hawkesworth, C, et al. (2009) Science, 323: 49-50. 5) Condie, KC; Belousova, E; Griffin, WL & Sircombe, KN (2009) Gondwana Res. 15:228–242 6) Voice, PJ; Kowalewski, M & Eriksson KA (2011) J. Geol. 119.2: 109-126