Abstract
Abstract There is abundant evidence for significant H2O in evolved melts from the platinum-rich UG2 chromitite and the Merensky Reef of the Bushveld Complex (South Africa), but there is no consensus about the source of H2O. We report triple-oxygen and hydrogen isotope ratios of interstitial, late-magmatic phlogopite from three localities of the UG2 layer. The phlogopite yielded δD values of −43 to −23, which is >30 higher than previously known from Bushveld rocks and far above the mantle values of ~−75. The phlogopite triple-oxygen isotope ratios are the first to be reported for Bushveld rocks, with values of Δ′17O0.5305 (17O excess relative to the reference line 0.5305) from –0.069 to –0.044 (δ18O 5.2–6.2). The oxygen data support existing models of as much as 30%–40% contamination of mantle-derived magmas in the lower to middle crust. However, the high δD values require a second step of contamination, which we attribute to brines from the marine sediments in the Transvaal Basin at the emplacement level.
Highlights
Understanding the petrogenesis of mafic layered intrusions and their mineralization remains a huge challenge, and among many open questions is the role of H2O in these essentially anhydrous igneous bodies (Charlier et al, 2015)
We report the first co-registered H and triple-O (16O, 17O, and 18O) isotopic ratios in late-magmatic phlogopite from the Upper Group 2 (UG2) horizon at three localities spaced throughout the complete Rustenburg Layered Suite (RLS) (Fig. 1)
Phlogopite forms fresh, reddish-brown, interstitial grains (0.1–1 modal%, ∼50–500 μm in size) in the UG2 chromitite and its silicate wall rocks
Summary
Understanding the petrogenesis of mafic layered intrusions and their mineralization remains a huge challenge, and among many open questions is the role of H2O in these essentially anhydrous igneous bodies (Charlier et al, 2015). The latter might be suggested by long-established evidence for crustal contamination of RLS magmas from high Sr- and Os-isotope ratios (Hart and Kinloch, 1989; McCandless and Ruiz, 1991; Kruger, 1994) and from δ18O values higher than the mantle composition (Schiffries and Rye, 1989; Harris et al, 2005).
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