2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 75-7
Presentation Time: 2:35 PM

CORRELATED MG-ZN ISOTOPE FRACTIONATION IN ARCHEAN KOMATIITIC LAVA-FLOWS


MATTIELLI, Nadine1, HAENECOUR, Pierre2, HUBLET, Geneviève1, DUCHEMIN, Claire1 and DEBAILLE, Vinciane1, (1)Laboratoire G-Time, Département des Sciences de la Terre et de l'Environnement, Université Libre de Bruxelles, CP 160/02, Avenue F.D. Roosevelt 50, Brussels, 1050, Belgium, (2)Laboratory for Space Sciences, Washington University in St. Louis, St. Louis, St. Louis, MO 63130

The Mg and Zn isotopic compositions of the mantle and the mechanisms governing Mg and Zn isotope fractionation during partial melting or fractional crystallization or mantle metasomatism are not fully resolved. Komatiites representing high-degree partial melts presumably formed by plumes hotter than the modern hot spots, make them extremely useful for addressing questions relevant to (early) mantle melting and heavy stable isotope fractionation.

The present study reports high-precision MC-ICP-MS measurements of Zn and Mg isotopic compositions in whole rocks and mineral separates from Fred’s Flow and Theo’s Flow, two thick Archean differentiated lava flows (2.7 Ga) located in the Abitibi greenstone belt (Munro Township, Canada). Fred’s Flow has a komatiitic affinity, and is classified as Al-undepleted type, whereas Theo’s Flow has a Fe-rich tholeiitic affinity and is classified as Al-depleted type. Their geographical relationship and ages suggest that they are genetically related.

The entire profiles for each flow (whole rocks) display a narrow range of δ66Zn values (0.23 δ-units) with a weighted mean of +0.29‰. However, noticeable variations in δ66Zn are observed between lithological units: spinifex and gabbroic units are characterized by slightly lower δ66Zn (down to +0.28±0.04‰) when the ultrabasic (UB) cumulate units show enrichments in heavy Zn isotopes ( δ66Zn up to +0.55±0.05‰). The same trends and profiles are observed from the δ26Mg data, with even stronger variations. The δ26Mg results range from -0.36±0.05‰ (in the gabbroic unit) up to +0.21±0.02‰ (in the UB cumulate units).

Zn isotopic compositions in mineral separates vary on a large range of 1.6 δ-units. Chromites show especially strong enrichments in light Zn isotopes, relative to the silicates. Olivine, clinopyroxene and plagioclase exhibit smaller isotopic fractionations with respect to each other, but are characterized by contrasted δ66Zn values in agreement with enrichment in heavy Zn isotopes in minerals characterized by lower coordination degree of Zn.

The present study strengthens correlations between Zn and Mg isotopes both influenced by modal mineral abundances and inter-mineral fractionations.

Session No. 75

T117. Magmas and Their Sources II: A Tribute to the Distinguished Career of Fred Frey
Sunday, 19 October 2014: 1:00 PM-5:00 PM
205/206 (Vancouver Convention Centre-West)
Geological Society of America Abstracts with Programs. Vol. 46, No. 6, p.197
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