Retrograde net transfer reaction insurance for pressure-temperature estimates

Retrograde net transfer reactions significantly affect compositions of meta morphic minerals, yet are rarely considered when determining pressure-tempe rature (P-T) conditions. Two natural amphibolite facies metapelites from th e central Himalaya of Nepal exhibit extremely common compositional patterns , including increases in Mn and Fe/(Fe + Mg) at the rims of garnets, which are the result of retrograde garnet dissolution and Fe-Mg exchange with bio tite. However, typical thermobarometric approaches for these rocks result i n errors of hundreds of degrees and 3-6 kbar compared with thermobarometry of nearby rocks and petrogenetic grids. These large errors result because d issolution of high-Fe garnet has strongly affected the Fe/Mg ratio of matri x biotite, X-ray maps help evaluate the extent and chemical effects of retr ograde reactions in these samples by identifying mineral regions that retai n highest-T compositions, or, through a new data-processing approach, by pe rmitting correction of mineral compositions to original high-T values. Thes e approaches ensure against retrograde net transfer reactions and should be applied routinely in thermobarometric studies-they ultimately yield P-T es timates that are more petrologically reasonable, and permit rapid screening of samples for those least affected by retrograde reactions. Reconsiderati on of thermobarometry in the central and eastern Himalaya indicates that re trograde net transfer reactions are extremely common. Therefore, previous t hermobarometric studies based on garnet major element compositions from tha t region should be reevaluated.