This CAREER grant focused on the development of field-based geochronologic-petrologic approaches to measure the rate and duration of metamorphic reactions within convergent margin settings.  During this funding period, we have invested heavily in the establishment of methods to date the progressive growth of garnet using the Samarium to Neodymium decay system.  Garnets are a common mineral which grows in response to broad tectonic processes such as continental collision, mountain building, and subduction of oceanic crust.  Akin to tree rings, garnet growth concentrically; thus garnets thus hold one of the longest continuous records of tectonic processes. We have now shown on numerous samples the ability to acquire growth ages with <1 million year age precision on small garnet samples from a variety of samples (e.g. Lancaster et al. 2008; Harvey and Baxter, 2009; Peterman et al. 2009; Pollington & Baxter 2010, 2011; Dragovic et al. 2012; Korhonen et al. 2012).  Three key innovations have been realized – and integrated together – paving the way for future success in Sm-Nd geochronology of zoned garnets: 1) We developed a new method permitting TIMS analysis of very small quantities of Neodymium (such as would be extracted from small concentric garnet microsamples) at high precision (Harvey & Baxter 2009); 2) We have refined our abilities to cleanse garnets of undesirable contaminating micro-mineral inclusions (Pollington & Baxter 2011); and 3) We have established routine methods for chemically contoured microdrilling of zoned garnets for geochronology (Pollington & Baxter 2010, 2011).  These methodological investments have been made available to the entire geosciences community; indeed, visitors to the lab have already published garnet ages in their respective field areas (Peterman et al. 2008; Korhonen t al. 2012). 

With these and other methods we have made several discoveries about the timescales of metamorphic and tectonic processes.  In a spectacular 6 cm diameter garnet collected from the Austrian Alps, we produced an 8 million year continuous record of tectonic processes (Pollington & Baxter 2010).  This garnet may be likened to a giant redwood recording the “tree rings” of tectonic processes.  The data consist of 12 discrete chemically contoured microsampled growth rings, each dated with the Sm/Nd isotopic system at precision better than +/- 700 Kyr.  These data reveal that garnet growth was punctuated by at least two pulses of accelerated growth from 28 to 20 million years ago during the waning stages of the Alpine Orogeny, which are related to pulses of fluid infiltration and related heating and/or deformation and exhumation driven by tectonics.  In the subduction zone setting, we have shown that garnet growth can be linked thermodynamically to water release from subducted rocks.  It is this water, liberated by metamorphic dehydration reactions, that leads to melting of the mantle and volcanism above all the worlds subdiction zones.  Direct chronology of zoned subduction zone garnets from the island of Sifnos, Greece indicates short pulse(s) of accelerated garnet growth and related dehydration.  These water pulses are not consistent with many existing models of subduction zones which predict water release should be slow and steady.  The existence of the water pulses will help test and refine new geodynamic models of subduction zones some of which may predict periods of rapid water release.  We have also documented evidence for brief pulses of metamorphic heating and mineral growth in regional mountain building events (Ague & Baxter 2007; Lancaster et al. 2008).  All of this evidence suggests that metamorphism in general may be dominated by brief events, perhaps superimposed on slower longer lasti...

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