Zircon megacrysts from kimberlite: oxygen isotope variability among mantle melts

New isotopic constraints on age and magma genesis of an embryonic oceanic crust: The Chenaillet Ophiolite in the Western Alps

“…Similar phenomenon has been reported for the altered ocean-crust zircons that give δ 18 O zir values from 0.0 to 5.0‰ (Fu et al, 2012;Grimes et al, 2011). On the contrary, the least-altered zircon domains from the albitite give a mean δ 18 O zir value of 5.1 ± 0.4‰ (2SD) that is well consistent with the average δ 18 O zir value of 5.2 ± 0.5‰ determined for 107 normal magmatic zircons from plagiogranites (Grimes et al, 2011), and also indistinguishable within uncertainties from the average value (5.4 ± 0.4‰) for the troctolite sample ALP3-6.3 and the range of 5.3 ± 0.6‰ observed for zircons in high-temperature equilibrium with primitive mantle (Valley, 2003, Valley et al, 1998and Valley et al, 2005. Therefore, the least-altered albitite zircons do not show any detectable seawater signature, indicating that the albitite should have formed prior to significant interactions between the ocean-crust and seawater.…”

“…It is noted, despite being indistinguishable within analytical errors, the altered zircon domains have slightly lower δ 18 O zir values between 4.2‰ to 5.0‰ (averaging at 4.7 ± 0.6‰, 2SD) than the least-altered domains that give δ 18 O zir values between 4.8‰ to 5.4‰ (averaging at 5.1 ± 0.4‰, 2SD). Overall, the measured δ 18 O zir values for the troctolite zircons and the least-altered zircon domains from the albitite are consistent within analytical uncertainty with those of igneous zircons from the Mid-Atlantic and Southwest Indian Ridges gabbros, norites, and plagiogranites of modern oceanic crust (5.3 ± 0.8‰, Cavosie et al, 2009;5.2 ± 0.5‰, Grimes et al, 2011) and with igneous zircons in high-temperature equilibrium with primitive magma compositions of the mantle (5.3 ± 0.6%, Valley et al, 1998, Valley et al, 2005and Valley, 2003. These albitite zircons have exceptionally high 176 Yb/ 177 Hf ratios between 0.320 and 0.861 (Appendix Table 2 Table 2).…”

“…This is because (1) zircons, which are usually retentive of the magmatic oxygen isotopes, in equilibrium with mantle-derived magmas have very consistent δ 18 O zir value of 5.3 ± 0.6‰ (2ζ) (Valley, 2003, Valley et al, 1998and Valley et al, 2005. This mantle zircon value is insensitive to magmatic differentiation, because the increase in bulk rock δ 18 O value is compensated by an increase in zircon/liquid δ 18 O fractionation, (2) zircons are highly resistant to alteration except for extreme cases (e.g., hydrothermal alteration and metamictization), and commonly preserve primary magmatic δ 18 O values ( Grimes et al, 2011, Page et al, 2007, Valley, 2003and Valley et al, 2005.…”

Overlapping Sr–Nd–Hf–O isotopic compositions in Permian mafic enclaves and host granitoids in Alxa Block, NW China: Evidence for crust–mantle interaction and implications for the generation of silicic igneous provinces

“…As discussed above, however, the similar isotopic compositions of two-stage Alxa MMEs and host granitoids suggest that the underplated mafic rocks underwent remelting due to later basaltic magma intrusion. MMEs have also been discovered in other Alxa plutons that have Sr and Nd isotope compositions similar to the (Valley et al, 1998). Bayannuoergong batholith and Yamaitu pluton (our unpublished data).…”

“…Thus, the simplest and most plausible explanation is that the lithospheric mantle in the Alxa Block is locally heterogeneous, both in radioactive (Sr and Nd) and stable isotopes (O). The 270 Ma MMEs have low zircon δ 18 O values 5.59 ± 0.22‰, only slightly higher than the depleted mantle values of 5.3 ± 0.3‰ (Valley et al, 1998), indicating that their source region in the lithospheric mantle underwent only low degrees of fluid metasomatism. This suggestion is supported by the fluid-mobile trace element ratios, such as Ba/Th, which can be used to gauge the extent of fluid enrichment (Hawkesworth et al, 1997) …”