20-EPN-007: Investigating mantle heterogeneity through high spatial resolution mineral Pb and Nd isotopic analyses
Virtual visit by George Cooper, Cardiff University, Wales (UK) to TA2.1 VU Geology and Geochemistry radiogenic and non-traditional stable Isotope Facility (GGIF).
Dates of visit: 22 February – 09 December 2022 (10 days remote access)
Report Summary: The traditional approach of measuring the isotopic compositions of mid-ocean ridge basalts (MORB) is problematic because MORB is homogenised prior to eruption, and therefore does not record the full heterogeneity of the mantle source. To overcome this problem, we developed low- concentration coupled Pb-Nd isotope analysis of minerals at high spatial resolution to assess the isotopic heterogeneity of melts delivered to Earth’s oceanic crust and hence that of the depleted upper mantle. We acquired small volume Pb and Nd isotope analyses from minerals in gabbroic cumulates from fast-spreading oceanic crust at Hess Deep using the Thermo Scientific TRITON Plus at the Vrije Universiteit in Amsterdam. We measured minerals from 27 samples (Nd from 25 cpx and 19 plag, Pb from 18 plag) covering the full stratigraphic depth (4350 m to 25 m) of the Hess Deep oceanic crust. Our study reveals that Pb isotopes from primitive plagioclase domains show greater heterogeneity than Nd isotopes from plagioclase and clinopyroxene, validating the new coupled Pb-Nd isotopic approach. The Pb data do not vary systematically with depth but do show a departure in 207Pb/204Pb away from the NHRL and across the main trend of East Pacific Rise MORB that may indicate cumulate-melt mixing throughout the crust or the involvement of an exotic mantle source.
Read the full scientific report, with kind permission from George Cooper.
20-EPN-039: Deep carbon- and water-rich (C-O-H) fluids record associated geodynamic processes and impacts on planetary continental lithospheres through time
Visit by Yaakov Weiss, Hebrew University of Jerusalem (Israel) to TA2.1 VU Geology and Geochemistry radiogenic and non-traditional stable Isotope Facility (GGIF).
Dates of visits: 07-12 December 2021 and 21 August – 4 September 2022
Report Summary: ‘Fibrous’ diamonds, a fast-growing form of diamonds that often encapsulate carbon- and water-rich (C-O-H) fluid microinclusions, are a primary target for studies of C-O-H mantle fluids and how these fluids influence deep mantle processes. However, only a small amount of diamond (normally <1 mg) and even smaller amounts of C-O-H fluid microinclusions can be sampled and analyzed using conventional laser ablation approaches and mass spectrometry measurements. In the present project, we implemented a novel diamond-in-liquid laser ablation technique that was developed to overcome the sample size limitation, combined with ultra-low blank column chromatography and 1013 Ohm resistor TIMS analyses, to provide the first high-precision Sr-Nd-Pb isotopic compositions of C-O-H mantle fluids in diamonds from the Kaapvaal Craton in southern Africa. We successfully processed and analyzed 12 samples from De Beers Pool, 5 from Finsch and 6 from Koffiefontein mines, as well as standards and blanks. We finished processing the collected data which show exciting Sr-Nd-Pb relationships that vary between diamonds carrying different C-O-H fluids and micro-mineral inclusions. We still need to complete some data processing and calculations, as well as correlate the isotopic ratios with trace element compositions to fully understand the results and their geological significance. Nonetheless, we are certain that the outcome of this Europlanet project will have a major impact on our understanding of the origin and evolution of C-O-H mantle fluids, the transport of mobile components between different mantle (and crustal) reservoirs, and the role of deep C-O-H fluids in the global circulation of volatiles through Earths’ history.
Read the full scientific report, with kind permission from Yaakov Weiss.
20-EPN2-064: High-precision isotope analysis of individual melt inclusions – Reassessing the compositional variability of Earth’s mantle
Visit by Felix Genske and Misha Böhnke, Westfälische Wilhelms-Universität Münster, Institut für Mineralogie (Germany) to TA2.1 VU Geology and Geochemistry radiogenic and non-traditional stable Isotope Facility (GGIF).
Dates of visit: 14-19 November 2022
Report Summary: In a unique approach, we acquired high precision Nd isotope data on well-characterised sample sets of silicate melt inclusions (MI) from the islands of Gough and Tristan da Cunha in the South Atlantic ocean. These data were collected using a Thermo Scientific Triton Plus TIMS at the VU Amsterdam. Melt inclusions from single or multiple olivine crystals were analysed and the new data provide insights into the magnitude, origin, and mode of sampling of the isotopically diverse materials that represent Earth’s mantle. The most prominent finding of this study is the extended isotopic variability of mantle melts, indicating that the mantle itself is more heterogeneous than assumed from studies of lavas (i.e. whole rocks). Further, the new data from melt inclusions entrapped in lavas from different islands greatly extend the known isotopic variation not only from individual eruption centres but also on a global scale.
Although observed small-scale mantle heterogeneity may exist down to the meter-, perhaps even cm-scale, the former also implies that distinct isolated large-scale (100’s km) reservoirs in Earth’s mantle (and other rocky planets) may not exist. Instead, the efficiency of mantle mixing via convection with time plays a more important role than previously thought; indeed, the existence of the full spectrum of mantle heterogeneity may be captured within single eruptions. As a consequence, the significance of large-scale distinct mantle domains detected by seismic imaging is questioned by our findings.
Read the full scientific report, with kind permission from Felix Genske.
(Image: Gough Island. Credit: L Kurtze)
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.
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.