Wed 7th September

@ 12:00 pm

Rm 312.222 

Daniel Viete (John Hopkins University)

Investigating the origins of rhythmic major-element zoning in HP/LT garnets from worldwide subduction mélanges

Rhythmic major-element zoning has been documented in garnets from high pressure/low temperature (HP/LT) lenses within a number of worldwide subduction mélanges (e.g. California, Chinese Tianshan, Cuba, Greek Cyclades, Guatemala, Japan, Venezuela). The origin of these features may have implications for the nature of subduction-zone processes.

Conditions of rhythmic zoning acquirement in HP/LT garnets of California and Venezuela were investigated by use of Raman and synchrotron Fourier transform infrared (FTIR) microspectroscopy, and thermodynamic modelling of phase equilibria.

Quartz-in-garnet Raman barometry reveals varying P—on the order of 100­–300 MPa, over radial distances of 10s of µm—in association with the high-Mn (and low-Mg) bands that define the fine-scale rhythmic zoning. Results from FTIR microspectroscopy demonstrate association between the high-Mn bands and locally depressed (structural) OH and elevated (molecular) H2O concentrations. The microspectroscopy results suggest changes in P and fluid activity attended development of the rhythmic garnet zoning.

Perple_X modelling of phase equilibria shows that, for specific rock chemistry and subduction P–T conditions, garnet modal abundance is extremely sensitive to changes in P and T (e.g. 10–30 vol.% growth/dissolution for DP = 300 MPa or DT = 25 °C). Rhythmic major-element zoning may reflect P- and/or fluid-driven cycles of garnet stability–instability during subduction. The steep compositional gradients that define the rhythmic major-element zoning limit time scales at subduction T, requiring that individual stability–instability cycles were extremely brief.

Seismic cycles and/or porosity waves represent ephemeral phenomena capable of accounting for development of rhythmic major-element zoning in HP/LT garnet, during subduction, as a result of fluctuations in both P and fluids. Metamorphic rocks may well carry detailed records of the catastrophism that punctuates longer-term tectonometamorphic processes.