Evolution of microstructures and fluid inclusions of naturally deformed peridotites: Implications for physico-chemical heterogeneity in the upper mantle

Abstract

The Earths uppermost layer, the lithosphere, consists of the crust and upper mantle, in particular, the lithospheric mantle is the space that physically and chemically forms the roots of crust. Mantle-derived peridotites can provide important information about the physico-chemical properties of the upper mantle (i.e., mantle metasomatisms and deformation processes), thus, it plays an important role in understanding Earth's geochemical and rheological evolutionary processes.

In this dissertation, naturally deformed peridotites (xenolith and massif type) were studied using various analytical instruments (SEM-EBSD, EPMA, FTIR-Raman spectroscopy, FIB, Heating-Cooling stage, etc.) in order to understand how deformation microstructures and fluid inclusions were evolved from Rio Grande Rift (New Mexico, USA) and Yugu area (Gyeonggi Massif, Korea).

Five spinel peridotite xenoliths, hosted in alkali basalts, were collected from Adams Diggings in the western margin of the Rio Grande Rift (RGR), New Mexico, USA. Based on fluid inclusion petrography, two distinct generations of fluid inclusion assemblages, both hosted by orthopyroxenes, namely Type-1 (earlier) and Type-2 (later) FIs, were identified. Results from fluid inclusion petrography together with various analytical results indicated that the timing of the entrapment and the composition of the trapped fluid were different between the Type-1 and Type-2 FIs. All of these results indicate that there were at least two fluid invasions with different fluid compositions at different pressures (depth) beneath the RGR mantle. The earlier fluid invasion (C–O–N–S) happened before or during formation of exsolution lamellae and was trapped as Type-1 FI in the cores of orthopyroxenes whereas the later fluid invasion (C–O–H–S) was trapped as Type-2 FI after the formation of the orthopyroxene porphyroclasts with exsolution lamellae. The two fluid invasions in the Adams Diggings peridotites indicate the complexity of mantle fluids around the RGR.

Transitional characteristics of microstructures and olivine fabrics developed in a mantle shear zone from the Yugu peridotite body (Gyeonggi Massif, Korea) were studied. The Yugu peridotite body predominantly comprises spinel harzburgite together with minor lherzolite, dunite, and clinopyroxenite. Based on microstructural characteristics of highly deformed peridotites, we classified into proto-mylonite, proto-mylonite to mylonite transition, mylonite, and ultra-mylonite. Olivine fabrics changed from A-type (proto-mylonite) via D-type (mylonite) to E-type (ultra- mylonite). Olivine fabric transition is interpreted as occurring under hydrous conditions at low temperature and high strain, because of characteristics such as Ti- clinohumite defects (and serpentine) and fluid inclusion trails in olivine, and a hydrous mineral (pargasite) in the matrix, especially in the ultra-mylonitic peridotites.