20-EPN-014: Ancient oceanic crusts as tracers of terrestrial mantle evolution
October 7, 2022

20-EPN-014: Ancient oceanic crusts as tracers of terrestrial mantle evolution – Ages and mantle source fingerprints from centimetric mantle eclogite xenoliths

Virtual visit by Sonja Aulbach, Goethe University Frankfurt (Germany) to TA2.1 VU Geology and Geochemistry radiogenic and non-traditional stable Isotope Facility (GGIF).
Dates of visit: 2 June – 05 August 2022

Report Summary: Kimberlite-borne eclogite xenoliths with subducted oceanic crustal protoliths were typically affected by multiple processes during metamorphism and extended residence in cratonic lithosphere.

We acquired precise Nd isotope compositions (143Nd/144Nd) from minute amounts of eclogitic garnet and clinopyroxene derived from centimetric pristine and metasomatised eclogite xenoliths from Orapa, Botswana, originally for insights into their origin, ages and later evolution as part of the continental mantle. Though not yielding any pre-entrainment age constraints, the new data provide intriguing insights into the behaviour of Sm and Nd in the main eclogite constituents during mantle metasomatism.

Unradiogenic and isotopically homogeneous Nd in clinopyroxene from metasomatised eclogites correlates strongly with indicators of metasomatism by kimberlite melt, which also lowered its Sm/Nd ratios. In contrast, garnet has Sm/Nd showing no difference between metasomatised and pristine specimens. Clinopyroxene-garnet two-point isochron ages correlate positively with temperature of the eclogites’ last residence in the mantle, and several metasomatised samples give ages that are younger than the timing of entrainment to the surface and cooling.

These seemingly paradoxical observations can be explained when equilibration mechanisms and crystal-chemical controls are taken into consideration. Volume diffusion at high temperature caused slow Nd isotopic exchange, whereas melt-assisted recrystallisation at low temperature facilitated instant equilibration. Moreover, metasomatised eclogites residing at low temperature contain garnet with lower grossular content, both of which impeded the addition of Nd to garnet, which retained high Sm/Nd. Combined with its isotopic homogenisation, garnet with high Sm/Nd causes flattening of the two-point isochron slope, resulting in geologically implausible young ages.