OVERSTEPPING OF GARNET NUCLEATION AND IMPLICATIONS FOR TECTONIC INTERPRETATION OF GARNET AGES (Invited Presentation)

Quartz inclusion in garnet (QuiG barometry) in Barrovian rocks from New England, the Dalradian, and blueschists from the Cyclades, Greece, indicate that garnet does not nucleate in near equilibrium, but rather requires several tens of degrees and kilobars of overstepping. Hence, the use of mineral assemblage diagrams (MADs) or garnet thermobarometry may not provide correct information with which to interpret garnet ages if nucleation conditions are overstepped.

For example, several samples from central Vermont contain garnet in which ilmenite inclusions in the garnet core give way to rutile inclusions in the garnet rim. QuiG plus Zr in rutile (ZiR) thermometry provides an accurate P–T point during garnet growth. Calculation of garnet core isopleths from the maximum driving force (MDF) model reveals, however, that these isopleths do not intersect at the known P–T conditions of garnet nucleation. Adjustments to the effective bulk composition (EBC), however, reveal that the isopleths can be forced to intersect at the P–T of nucleation if Mn and Ca are lower than that those of the whole-rock, which is to be expected if dissolution of reactants is kinetically controlled. For these samples, the accurate P–T conditions for garnet nucleation and growth can be used to properly interpret ages of both garnet and rutile.

A second example from Sifnos, Greece, has published Sm/Nd ages of garnet core minus rim ages are nearly 0 to 1 Ma with an average of 0.03 Ma. Equilibrium modeling of garnet nucleation and growth imply a P–T path of nearly isobaric heating over 100 ˚C (dT/dt = 100 to over 3000 ˚C/Ma). QuiG barometry from the garnet core is consistent with the peak P–T conditions of ca 550 ˚C, 22 kb, but Zr in rutile thermometry (ZiR) indicates garnet nucleated and grew above 500 ˚C and inclusion suites in garnet also indicate that garnet nucleated from an assemblage that was not stable at 450 ˚C, 22 kbar, contradicting the isobaric heating model. Apparently, garnet grew under near isothermal, isobaric conditions and the core-rim age difference reflects kinetic, not tectonic, processes. This conclusion has major implications for subduction thermal models and suggests that models that predict an episode of isobaric heating require reevaluation.