Tag: ta2.1

20-EPN-038: UNDERCOOL –UNderstanding Deep Exchange Ratio of Carbon in the sOuthern Ocean during the Last deglaciation

Virtual visit by Francois Beny, Centre de Recherche et d’Enseignement de Géosciences de l’Environnement (CEREGE), Technopôle de l’Arbois-Méditerranée (France) to TA2.1 VU Geology and Geochemistry radiogenic and non-traditional stable Isotope Facility (GGIF).
Dates of visit: 10-19 February 2022

Report Summary: The ten days virtual visit permitted the successful acquisition of trace and rare earth element composition as well as Sr-Nd isotopic composition of 16 clay size terrigenous samples from core MD12-3396Q from the Indian sector of the Southern Ocean and East of the Kerguelen Plateau. Blank levels as well as MAG-1 geostandard composition are consistent with previous analyses on this sediment core carried out in 2018 by the project leader.

The new data confirm a general trend during the last glacial period: an important increase of the contribution of material from Antarctica during the Heinrich Stadial (HS) 1 and from the HS 3 to the HS 2 due to enhanced equatorward export of AntArctic Bottom Water (AABW) likely caused by increased AABW formation. During other intervals of the last glacial period, sedimentation was dominated by particles from the Kerguelen Plateau delivered by the Antarctic Circumpolar Current (ACC).

In addition, these new data highlight the occurrence of a third source of particles whose contribution starts during the deglaciation reaching a maximum during the Holocene. This source is possibly Africa, which would imply a more efficient transport of particles from Southern Africa to the Indian Ocean by the Agulhas retroflection and/or a southern migration of the Southern Ocean climatic fronts.

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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.

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