Modeling material loss during progressive metamorphism is a highly speculative task as the composition of the input material often is not well known. Especially the behavior of the fluid-mobile elements Li and B has been subject to debate as field-based and experimental studies seem to be in conflict.
Lithologies of distinctive chemical composition provide an option to trace a particular unit within different tectonic units of an orogen. We studied the Li and B isotopic composition of such a unit exposed to different metamorphic peak conditions. Our major results are:
* The initial Li and B isotopic variability within chemically similar lithologies may be larger than the variation typically assigned to metamorphic material loss.
* Loss and gain during metamorphism depend on the stability of mineral sequestering the elements of interest. Changes in the isotopic composition in metamorphic rocks are not properly described by models for continuous loss and continuous fractionation.
Romer, R.L., Meixner, A., and Hahne, K. (2014). Lithium and boron isotopic composition of sedimentary rocks – the role of source history and depositional environment: a 250 Ma record from the Cadomian orogeny to the Variscan orogeny. Gondwana Research. (in press)
Romer, R.L. and Meixner, A. (2014) Lithium and boron isotopic fractionation in sedimentary rocks during metamorphism – the role of rock composition and protolith mineralogy. Geochim. Cosmochim. Acta, 128: 158-177. doi:10.1016/j.gca.2013.11.032
Romer, R.L., Meixner, A., and Förster, H.-J. (2014) Lithium and boron in late-orogenic granites – Isotopic fingerprints for the source of crustal melts? Geochim. Cosmochim. Acta, 131: 98-114. doi:10.1016/j.gca.2014.01.018
back to topic