Tag: ta2.1

22-EPN3-124: Unfolding Geochemical Evolution of the Subcontinental Lithospheric Mantle Recorded by Diamond-Forming Carbon and Water Rich (C-O-H) Mantle Fluids Throughout Time

Virtual visit by Yael Kempe and Ofir Tirosh (The Hebrew University of Jerusalem, Israel) to TA2.1 VU Geology and Geochemistry radiogenic and non-traditional stable Isotope Facility (GGIF).
Dates of visit: 9 November 2023 – 25 January 2024.

Report Summary: Fibrous diamonds from the Voorspoed, Venetia and Koffiefontein mines record deep mantle events involving C-O-H fluid types, alongside gem diamonds containing mineral inclusions that were related to modification episodes of the Kaapvaal lithospheric region. Although a connection has been made, the longstanding debate between diamond formation in the mantle and the relationship between gem diamonds and fibrous diamonds is yet to be resolved.

While we have extensive radiometric dating of mineral inclusions trapped in gem diamonds, alongside knowledge about the major and trace elements of C-O-H fluid microinclusions in diamonds, their radiogenic isotopic data is insufficient (e.g. Sr, Nd, and Pb isotopes). High-precision Sr-Nd-Pb isotope TIMS analyses of C-O-H mantle fluids in diamonds from these three prime locations in the Kaapvaal craton were preformed using a novel laser ablation diamond-in-water technique, combined with ultra-low blank column chromatography and 10¹³ Ohm resistors.

The team successfully processed and analysed 12 samples from Voorspoed, 5 from Venetia and 5 from Koffiefontein mines, as well as standards and blanks. The collected data show intriguing Sr-Nd-Pb relationships that vary between diamonds carrying different C-O-H fluids. The team has further data processing and calculations to complete, as well as correlate the isotopic ratios with trace element compositions to fully understand the results and their geological significance. The outcome of this Europlanet project is expected to provide new insights into the complex tectonic history of this lithospheric province, the fluids themselves and the connection between different diamond types and their formation mechanism.

22-EPN3-063: The Origin of Early Archean Barite – Insights from the Geochemical and Isotopic Composition of Associated Chert Deposits

Visit by Desiree Roerdink (University of Bergen, Norway) to TA2.1 VU Geology and Geochemistry radiogenic and non-traditional stable Isotope Facility (GGIF).
Dates of visit: 6-10 November 2023

Report Summary: The formation of bedded barite (BaSO₄) deposits in the low-sulfate environments of the early Earth has been a long-standing paradox despite decades of field and geochemical studies. In this project, the team investigated the Si isotope geochemistry of chert (SiO₂) dykes and beds found in association with barite to evaluate roles of hydrothermal fluids and seawater during barite formation. A total of 14 chert samples from three localities in the ~3.3 Ga Mapepe and Mendon Formations of the Barberton Greenstone Belt, South Africa were microdrilled and dissolved using sodium hydroxide digestion. Pure silicon fractions were obtained using cation exchange chromatography columns, and analysed for ²⁹Si/²⁸Si and ³⁰Si/²⁸Si isotope ratios by multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) in wet plasma mode using standard-sample bracketing for mass bias correction. Measured silicon isotope ratios (δ³⁰Si) range from 0.27 to 1.29‰. Chert dykes (n = 6) and bedded cherts (n = 3) have similar silicon isotopic compositions, with an average δ³⁰Si value of 0.88‰ for the dykes and 0.80‰ for the bedded cherts. Black chert from the Mendon formation is isotopically distinct (δ³⁰Si = 0.45‰) from the Mapepe Formation cherts. These results tentatively suggest that the chert dykes and bedded cherts associated with barite formed from isotopically-heavy seawater (δ³⁰Si > 0‰), and that the role of high-temperature hydrothermal fluids (δ³⁰Si < 0‰) was limited. 

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)