Abstract
The Baffin Island picrites are highly magnesian (≤22 wt% MgO) olivine tholeiites, erupted through felsic continental crust. Plots of most major and minor element oxides against MgO for the lavas define very tight trends consistent with modification of melts parental to the erupted suite by olivine fractionation or accumulation. However, melt inclusions trapped in primitive olivine phenocrysts in these lavas have much more diverse compositions. After correction for post-entrapment modification, the inclusions are systematically slightly lower in Al2O3, and significantly higher in SiO2, K2O and P2O5 than the lavas’ fractionation trends. CaO, Na2O and TiO2 contents lie within the lavas’ fractionation trends. Similarly, most inclusions are higher in Sr/Nd, K/Nb, Rb/Ba, Rb/Sr, U/Nb and Ba/Th than the lavas. These characteristics resulted from up to ≈15% contamination of evolving picritic-basaltic liquids by locally-derived, broadly granitic partial melts of the quartz + feldspar-rich crust through which the picrites erupted. Contamination was minor in the bulk lavas (<1%), suggesting that the inclusions’ compositions partly reflect a link between wall rock reaction and precipitation of liquidus olivine. Rapid crystallisation of liquidus olivine from the picrites, along with melting of felsic crustal wall rocks of magma chambers or conduits, were likely during emplacement of hot picritic magmas into cooler felsic crust. Inclusion compositions may thus reflect mixing trends or may be constrained to phase boundaries between olivine and a phase being resorbed, for example, an olivine-plagioclase cotectic. The extent of contamination was probably a complex function of diffusion rates of components in the magmas, and phenocryst growth rates and proximity to wall rock. These results bear on the common observation that melt inclusions’ compositions are frequently more heterogeneous than those of the lavas that host them.
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Acknowledgements
We gratefully acknowledge the assistance of Dave Steel (University of Tasmania) with the electron microscopy, and Mike Shelley (ANU) with the LA-ICPMS. Hugh O’Neill is thanked for allowing access to a 1 atm gas mixing furnace. Adam Kent kindly allowed us access to his data set of Padloping picrite melt inclusions. Leonid Danyushevsky kindly supplied a computer program to correct Fe-loss in the measured inclusion compositions. The study benefited from discussions with David Green, Leonid Danyushevsky, Adam Kent, Stephen Eggins, and from constructive formal reviews by Leonid Danushevsky and an anonymous reviewer.
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Yaxley, G.M., Kamenetsky, V.S., Kamenetsky, M. et al. Origins of compositional heterogeneity in olivine-hosted melt inclusions from the Baffin Island picrites. Contrib Mineral Petrol 148, 426–442 (2004). https://doi.org/10.1007/s00410-004-0613-z
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DOI: https://doi.org/10.1007/s00410-004-0613-z