The origin of gases that caused the Permian–Triassic extinction - Volcanism and Global Environmental Change - page 155

mantle peridotite. Using the same criteria Sobolev et al .( 2009b , 2011 ) suggested
that the Ayan River picrites were generated from a pyroxenite-dominated source
(80% pyroxenite) while the meimechites were generated from an almost pure
peridotite. It was further proposed that altered recycled crust in the Siberian plume
was a major source of volatiles, and that release of these volatiles caused both
the end-Permian mass extinction and the carbon isotope excursion (Sobolev et al .,
2011 ). If this hypothesis is true, the new data suggest that the pyroxenite source
had been exhausted in Cl and thus most probably in other volatiles at the early
stage of the Siberian Traps eruption.
Crustal contaminated lavas possess variable Cl/K ratios from very low values,
as for the basalts of the Onkuchaksky suite of the Maymacha Kotuy province

(Black et al ., 2012 ) to very high or even extreme as for the Maslovskoe intrusion
or sills (Black et al ., 2012 ). Where available, data show strong correlation of
Cl excess and crustal contamination. Like Sobolev et al . ( 2009a ) and Black et al .
( 2012 ), we propose a crustal source for the Cl excess in these melts.

-

10.2.8 Calculation of total magmatic CO 2 , HCl and SO 2 release
Following Sobolev et al .( 2011 ), we assume that the mantle source for the Siberian

flood basalts consisted of two components: (1) pyroxenite derived from melting
of the recycled oceanic crust that constituted 15 wt% of the thermochemical
mantle plume; and (2) peridotite that made up the remainder of the mantle plume.
We obtain concentrations of Cl, S and H 2 O of the pyroxenitic source from melt
inclusions in olivine from uncontaminated Gudchikhinsky picrites (sample
4270/13). Inclusions in olivine from these magmas have been shown to represent
primary melts and thus their Cl, S and H 2 O concentrations can be used to estimate
the contents of these volatiles in the mantle source. This yields the following
values for the composition of recycled oceanic crust: Cl

135 ppm
after normalization of the values to K. In the deep mantle, these amounts of
Cl and S could reside in chloride and sul

¼ 137 ppm, S ¼

de. In contrast with Cl, S and H 2 O,
the amount of CO 2 in relatively shallow melts does not represent the primary
concentration, because of almost complete degassing at high pressures. Thus,
for an assessment of CO 2 in the recycled oceanic crust we use global estimations
of 3000 ppm CO 2 for the bulk 7-km-thick oceanic crust and its maximum outgas-
sing rate through arc volcanism of 70% (Dasgupta and Hirschmann, 2010 ).
This gives a conservative minimum estimate of 900 ppm CO 2 in the deeply
recycled oceanic crust. The maximum estimate would be about 1800 ppm using
the same initial bulk concentrations of CO 2 and minimal outgassing of 40% during
subduction (Dasgupta and Hirschmann, 2010 ). In the deep mantle this amount of
CO 2 could reside in carbonates or diamond.