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Volume 4

Microbial nitrogen cycle enhanced by continental input recorded in the Gunflint Formation

Abstract:
Nitrogen isotope compositions (δ15N values) of kerogen in the sedimentary rocks from the 1878 Ma Gunflint Formation were analysed to understand the relationships among microbial activities, ocean chemistry, and tectonic evolution in the Animikie Basin. In the present study, the stepwise combustion analysis, performed on 13 kerogen samples, indicates that the δ15N values of kerogen can be sub-divided into two fractions with discrete values depending on the combustion temperatures: a lower-temperature fraction (from 500 to 575 ˚C), and a higher-temperature fraction (higher than 575 ˚C). A positive correlation was observed between the δ15N values of the lower-temperature fractions and Pr/Sm ratios, which represent contributions from the continental input. In contrast, no correlation was observed between the δ15N values of the higher-temperature fractions and the Pr/Sm ratios. This relationship between the δ15N values and the continental inputs is explained by the isotopic fractionation effects of the biological nitrogen cycle associated with the enhanced microbial activity, triggered by the active tectonic settings in the Animikie basin.


A. Ishida, K. Hashizume, T. Kakegawa
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Geochem. Persp. Let. (2017) 4, 13-18 | doi: 10.7185/geochemlet.1729 | Published 5 August 2017

No direct contribution of recycled crust in Icelandic basalts

Abstract:
Using Melt-PX to model the decompression melting of a heterogeneous mantle, I investigated the role of major-element composition of the lithologies present in the source on magmatic productivity, and trace element and isotopic melt compositions, independently of the bulk mantle composition. My calculations demonstrate that the volume of magma produced is not significantly affected by the nature of the lithological heterogeneity, but depends on the bulk mantle composition. However, an isochemical bulk mantle can produce contrasting trace element and isotopic melt compositions depending on the major-element compositions of the lithologies present in the source. Results show that the observed crust thickness of the Icelandic rift zones is consistent with about 10 % of recycled crust in the source, but also demonstrate there is no need to involve the contribution of melts derived from a recycled basalt component to explain the compositional variability of the Icelandic basalts in rift zones, and rather advance the contribution of olivine-bearing hybrid lithologies formed by solid-state reactions between the recycled crust and the peridotite.


S. Lambart
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Geochem. Persp. Let. (2017) 4, 7-12 | doi: 10.7185/geochemlet.1728 | Published 12 July 2017

Tracking continental-scale modification of the Earth’s mantle using zircon megacrysts

Abstract:
Metasomatism, the chemical alteration of rocks by a variety of melts and fluids, has formed a key concept in studies of the Earth’s mantle for decades. Metasomatic effects are often inferred to be far-reaching and yet the evidence for their occurrence is usually based upon individual hand specimens or suites of rocks that display considerable heterogeneity. In rare cases, however, we are offered insights into larger-scale chemical modifications that occur in the mantle. Here we utilise the Lu–Hf systematics of zircon megacrysts erupted in kimberlite magmas to discern two temporally and compositionally discrete metasomatic events in the mantle beneath southern Africa, each having an influence extending over an area exceeding one million km2. These data provide unambiguous evidence for metasomatic processes operating at continental scales and seemingly unperturbed by the age and composition of the local lithospheric mantle. The most recent of these events may be associated with the major Jurassic-Karoo magmatism in southern Africa.


J. Woodhead, J. Hergt, A. Giuliani, D. Phillips, R. Maas
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Geochem. Persp. Let. (2017) 4, 1-6 | doi: 10.7185/geochemlet.1727 | Published 10 July 2017