Preliminary FTIR spectrometry study in a cratonic log: a case study from Jagersfontein mine, South Africa

 

Cratonic peridotite xenoliths are unique specimens that allow to constrain the mineralogy of the deep mantle. They are transported at ultrasonic speeds to the surface by volatile-rich kimberlite magmas. Water is an important parameter as it may impact, depending on its concentration, physical parameters such as rheology and partial melting. This study is focused on water content of 12 Spinel and Garnet harzburgites from Jagersfontein mine located at the rim of the Kaapvaal craton. The samples were cautiously selected and are modally non-metasomatized and ultra-refractory peridotites (Fo_92-95) with the aim to represent the most preserved archean mantle. Samples originate from 80 to 165 km depth allowing to obtain a continuous monitoring of the cratonic mantle. Water content obtained from FTIR spectroscopy was measured on most representative phases: Ol (15-74 ppm), Opx (70-199 ppm), Gt (3-8 ppm). Orthopyroxene and garnet display a decreasing OH content with increasing depth whereas olivine shows an increase of water content up to 4.5 GPa and then a sudden decrease. The trend shown by olivine [OH] content agrees with olivine data compiled from the literature over decades. The different reasons for explaining such results will be discussed.