Electron Microprobe Reintegration Method for Clinopyroxene Breakdown with Lamellae Exsolution

Pyroxene is an important rock-forming mineral in magmatic to metamorphic rocks, which often possesses certain breakdown texture with lamellae exsolution under temperature and pressure changes. In eclogite, the clinopyroxene, mostly omphacite may have different breakdown textures with quartz or amphibole-quartz, orthopyroxene and plagioclase lamellae exsolution during the retrogression process, and the latter is often further composed of clinopyroxene-plagioclase symplectite aggregate at lower grade metamorphism. To explore the temperature-pressure changes and the metamorphic evolution of the parent rock, it is necessary to restore the precursor pyroxene prior to breakdown from the residuary clinopyroxene and associated lamellae. Currently there are two major methods widely applied by different researchers in the pyroxene reintegration before its breakdown, an indirect mathematical fitting approach (method 1) and direct measurement (method 2), both of which are largely based on electron microprobe quantitative analysis. Method 1 largely relies on accurate microprobe analyses of host and exsolved components and appropriate estimates of their area percentages. Method 2 largely relies on the microprobe analytical conditions, involving a grid analysis option, raster mode for electron spot size, and accumulation calculation. Besides, correct standard material selection is also an essential factor. In this work, we exploited these two methods to reintegrate the precursor pyroxene prior to breakdown with (type-ii) orthopyroxene lamellae exsolution and (type-iii) clinopyroxene-plagioclase symplectite from eclogite and retrogressed eclogite samples. The results show that both types of clinopyroxene breakdown are a near-isochemical process that fits the requisite to apply the reintegration method. In type-ii, the reintegrated precursor pyroxene by method 1 tends to have analogous compositions with the unbroken relict omphacite in the sample, and thus this method is more suitable for such a scenario. In type-iii, in the opposite, the reintegrated precursor pyroxene by method 2 seems to have a better compositional consistency, and perhaps this method is more appropriate. Despite that both reintegration methods have their limitations, they are often widely utilized in metamorphic petrology for their technical accessibility and conveniency. In practice, we advise that both methods should be considered, and each outcome must be analyzed appropriately to determine which one is more suitable for the scenario.