Enriched Pt-Re-Os Isotope Systematics in Plume Lavas Explained by Metasomatic Sulfides

Abstract

To explain the elevated osmium isotope (186Os-187Os) signatures in oceanic basalts, the possibility of material flux from the metallic core into the crust has been invoked. This hypothesis conflicts with theoretical constraints on Earth's thermal and dynamic history. To test the veracity and uniqueness of elevated 186Os-187Os in tracing core-mantle exchange, we present highly siderophile element analyses of pyroxenites, eclogites plus their sulfides, and new 186Os/188Os measurements on pyroxenites and platinum-rich alloys. Modeling shows that involvement in the mantle source of either bulk pyroxenite or, more likely, metasomatic sulfides derived from either pyroxenite or peridotite melts can explain the 186Os-187Os signatures of oceanic basalts. This removes the requirement for core-mantle exchange and provides an effective mechanism for generating Os isotope diversity in basalt source regions.

Mantle Sulfides and their Role in Re–Os and Pb Isotope Geochronology

“…Base metal sulfides have been observed in pyroxenites in several studies over the last 40 years from continental localities as diverse as Australia, the French Massif Central, SW USA and Taiwan (e.g., Irving 1974;Peterson and Francis 1977;Andersen et al 1987;Dromgoole and Pasteris 1987;Luguet et al 2008;Wang et al 2009). Several groups of workers have attributed the formation of Al-augite pyroxenite-hosted BMS, and BMS hosted within clinopyroxene megacrysts and clinopyroxenites, to sulfur-saturation resulting from fractional crystallization of mafic magmas at mantle depths (e.g., Andersen et al 1987;Dromgoole and Pasteris 1987;Guo et al 1999).…”

“…For example, Luguet et al (2008) ). For BMS grains of < 100 µm in diameter, this corresponds to Os amounts of 10s-100s pg.…”

“…This review represents the first time that BMS in mantle rocks and diamonds are discussed together for the purposes of age determination. The interested reader should also consult other articles that have discussed these topics individually (e.g., Shirey and Walker 1998;Burton et al 1999Burton et al , 2012Pearson and Shirey 1999;Luguet et al 2001Luguet et al , 2003Luguet et al , 2004Luguet et al , 2007Luguet et al , 2008; Richardson et al 2001Richardson et al , 2004Richardson et al , 2009Shirey et al 2001Shirey et al , 2002Shirey et al , 2004aShirey et al ,b, 2013Alard et al 2002Alard et al , 2005Aulbach et al 2004aAulbach et al ,b, 2009aAulbach et al ,b,c, 2011Harvey et al 2006Harvey et al , 2010Harvey et al , 2011 …”

“…This makes the retention of ancient concentrations are now routinely performed using LA ICPMS (e.g., Alard et al 2000Alard et al , 2002Alard et al 2005Lorand and Alard 2001;Luguet et al 2001;. For Os isotope ratios, in situ LA MC ICPMS typically employs a Nd:YAG 266-nm or 213-nm laser microprobe coupled to the mass spectrometer (e.g., Nowell et al 2008). The new generation of Excimer lasers are also eminently suitable for this purpose and are routinely employed for PGE analysis in metals, BMS grains and PGM (e.g., van Acken et al 2012).…”

“…Although the values obtained by plasma machines may drift over the course of a day, this can easily be corrected for by analyzing a sulfide standard at regular intervals during the analytical session. Typically, instrument induced drift is limited to 1-2 % over a 24-hour period Nowell et al (2008), who suggested that the much lower 187 Re/ 188 Os value of < 0.5 is more appropriate to ensure an accurate correction of the isobaric overlap. Pragmatically, this 187 Re/ 188 Os upper limit rules out the analysis of eclogitic sulfides that have been in equilibrium with basaltic and more evolved liquids-leaving peridotitic sulfides to be tractable.…”