Basalt-Limestone and Andesite-Limestone Interaction in the Arc Crust – Implications for Volcanic Degassing of CO2

Abstract:

Volcanically emitted CO₂ is generally mantle-derived, but high degassing rates at some arcs (e.g. Merapi [1] and Colli Albani Volcanic District [2]) are thought to be affected by magma-carbonate interaction in the upper plate. However, the effects of depth, temperature, and composition on this process are poorly known.

We experimentally simulated magma (50%)-limestone (50%) wallrock interactions at 0.5-1.0 GPa, 1100-1200 °C using pure calcite and a hydrous (~3-5 wt.% H₂O) melt (basalt, andesite, or dacite). At 1.0 GPa, 1200 °C starting melts are superliquidus, whereas in the presence of calcite, Ca-rich cpx ± Ca-scapolite are produced.

With increasing T, basalt-calcite interaction causes the melt, on a volatile-free basis, to become silica-poor and Ca-rich with alumina decreasing as cpx becomes more CaTs-rich. The same trend is seen with all starting melt compositions as P decreases at a constant T (1200 °C), producing melts similar to ultracalcic (CaO/Al₂O₃>>1) melt inclusions found in arc settings. Shifting from basalt to andesite has little effect on SiO₂ and CaO of the reacted melt (e.g. 37 wt.% SiO₂, 42 wt.% CaO at 0.5 GPa, 1200 °C), whereas Al₂O₃ of andesite-derived reacted melt is lower, likely a result of lower alumina in the starting andesite. Wall-rock calcite consumption is observed to increase with increasing T, decreasing P, and increasing melt X_SiO2. At 0.5 GPa between 1100 and 1200 °C, our basalt experiments yield carbonate assimilation from 22 to 48 wt.%. This decreases to 20 wt.% at 1.0 GPa, 1200 °C, whereas an andesitic composition assimilates 59 to 52 wt.% from 0.5 to 1.0 GPa at 1200 °C. The higher assimilation in andesite-added runs at high-T is because of lower silicate liquidus as evidenced by lower modal proportion or absence of cpx ± scapolite.

Using a magma flux rate estimated for Mt. Vesuvius [3], we obtain a CO₂ outflux for a single such volcano experiencing arc magma-calcite reaction [4] of at least 2-4% of the present-day global arc flux of CO₂ [5]. Therefore, greater lengths of continental arcs with carbonate strata in some geologic time periods might have caused excess degassing of CO₂.

[1] Troll et al. (2012) GRL 11, 1-6; [2] Freda et al. (2008) Lithos 101, 397-415; [3] Scandone et al. (2008) JVGR 170, 167-180; [4] Iacono Marziano et al. (2009) Geol 37, 319-322; [5] Sano & Williams (1996) GRL 23, 2749-2752.