Abstract
Abstract Oxygen isotope analysis of zircon, often combined with geochronology and Hf isotope analysis, has been pivotal in understanding the evolution of continental crust. In this contribution, we expand the use of underexplored accessory phases (titanite and apatite) by demonstrating that their oxygen isotope systems can be robust, and by developing geochemical indicators involving O isotopes and trace element concentrations to better constrain magma petrogenesis. These minerals have the advantage over zircon of being present in less evolved magmas and being more responsive to igneous processes and crustal metamorphism. We present new data on titanite, apatite and zircon from carefully-selected granitoids through geological time: the Phanerozoic high Ba-Sr granites (Caledonian province, Scotland), Archean sanukitoids (Karelia province, Finland) and a Neoproterozoic basalt-andesite-dacite-rhyolite suite (BADR; Guernsey, Channel Island). We demonstrate: (i) that δ 18O values of the studied accessory minerals are not affected by crystal fractionation, (ii) a strong correlation between δ 18O in all three accessory minerals, showing that apatite and titanite can faithfully record the magmatic δ 18O; (iii) that these accessory minerals can also record metamorphic and/or fluid circulation events during the syn- to post-magmatic history of granitoids.
Highlights
By way of U-Pb dates and Hf-O isotopes in particular, zircon (ZrSiO4) has provided evidence of crust formation processes even into the Hadean (4.4 Ga; e.g. Wilde et al, 2001; Bell et al, 2011) and plausible constraints on continental crust growth (Kemp et al, 2007)
Our results suggest that titanite and apatite oxygen isotope ratios can record useful information from various stages in the history of their host granitoids, from petrogenesis and source signature to later overprinting processes such as metamorphism and fluid circulation
Since it has been argued that the Caledonian high Ba-Sr granite suite is the result of progressive crystal fractionation from the appinitic compositions toward the most differentiated granites (Fowler et al, 2001, 2008), and apatite-titanite are at equilibrium, our data can be used to examine the influence of crystal fractionation on d 18Oapatite and d 18Otitanite
Summary
By way of U-Pb dates and Hf-O isotopes in particular, zircon (ZrSiO4) has provided evidence of crust formation processes even into the Hadean (4.4 Ga; e.g. Wilde et al, 2001; Bell et al, 2011) and plausible constraints on continental crust growth (Kemp et al, 2007). Notwithstanding these previous experimental studies, there is clearly still a lot to learn about oxygen isotope behaviour in natural systems For this contribution we have analysed O isotopes, major and trace elements in coexisting zircon, titanite and apatite from different sets of granitoids of different ages (Caledonian high Ba-Sr granites, Archean sanukitoids and Cadomian BADR). We can evaluate the isotopic information that accessory minerals record on petrogenesis and post-magmatic processes For this purpose, we first compare oxygen isotope behaviour in apatite and titanite on a well-constrained cogenetic high Ba-Sr suite (maficfelsic) of fresh, unmetamorphosed samples affected by crystal fractionation as indicated by trace element variations. Our results suggest that titanite and apatite oxygen isotope ratios can record useful information from various stages in the history of their host granitoids, from petrogenesis and source signature to later overprinting processes such as metamorphism and fluid circulation
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