Effect of melt composition on crustal carbonate assimilation: Implications for the transition from calcite consumption to skarnification and associated CO₂ degassing

Abstract

Skarns are residue of relatively low-temperature magma-induced decarbonation in the crust largely associated with silicic plutons. Mafic magmatic intrusions are also capable of releasing excess CO 2 due to carbonate assimilation. However, the effect of mafic to silicic melt evolution on the decarbonation processes, in addition to temperature controls on carbonate-intrusive magmatic systems, particularly at continental arcs, remains unclear. In this study, experiments performed in a piston cylinder apparatus at midcrustal depth (0.5 GPa) at supersolidus temperatures (900-12008C) document calcite interaction with andesite and dacite melts at equilibrium under closed-system conditions at calcite saturation in a 1:1 meltcalcite ratio by weight. With increasing silica content in the starting melt, at similar melt fractions and identical pressure, assimilation decreases drastically (65% andesite-calcite to 18% dacite-calcite). In conjunction, the CaO/SiO 2 ratio in melts resulting from calcite assimilation in andesitic starting material is >1, but 0.3 in those formed from dacite-calcite interaction. With increasing silica-content in the starting melt skarn mineralogy, particularly wollastonite, increases in modal abundance while diopsidic clinopyroxene decreases slightly. More CO 2 is released with andesite-calcite reaction (2.9 3 10 11 g/y) than with more skarn-like dacite-calcite interaction (8.1 3 10 10 g/y, at one volcano assuming respective calcite-freesuperliquidus conditions and a magma flux of 10 12 g/y). Our experimental results thus suggest that calcite assimilation in more mafic magmas may have first degassed a significant amount of crustal carbon before the melt evolves to more silicic compositions, producing skarn. Crustal decarbonation in long-lived magmatic systems may hence deliver significant albeit diminishing amounts of carbon to the atmosphere and contribute to long-term climate change. Key Points:Limestone assimilation increases with infiltration of hydrous andesitic melt over basaltic but decreases with dacite Skarn minerals like wollastonite increase in abundance in more silicic magma-calcite interaction Assimilation can release <0.4 Gt CO 2 during intruding magma evolution from basalt to granite

Crustal CO2 contribution to subduction zone degassing recorded through calc-silicate xenoliths in arc lavas

“…This temperature is consistent with the clinopyroxene saturation temperatures calculated from the magmatic skarn xenolith glasses (755–917 °C, Equation 34 57 ). This thermometer reproduces experimental low temperature 900 °C carbonate assimilation data 48 within the published error of 45 °C. Varying temperatures (e.g.…”

“…Our modelling results, and the general paucity of relict calcite in the xenoliths, demonstrate that magma-carbonate interaction at Merapi is very efficient at remobilising crustal CO 2 into the magmatic system and ultimately the atmosphere. Crustal carbonate assimilation has been shown to be an important contributor to CO 2 output at Merapi 11,12,48 and a widespread occurrence in arc volcanoes, which may even dwarf contributions from source contamination 1,2 . We have used a mass balance model 49 (see methods) to place constraints on the amount of crustal CO 2 produced at Merapi.…”

Carbonatitic versus hydrothermal origin for fluorapatite REE-Th deposits: Experimental study of REE transport and crustal “antiskarn” metasomatism

“…In terms of phase relations, this reaction zone might not be different from skarns. Whereas some skarns form when silicate magma or derived fluids intrude a carbonate rock (see for example, Carter and Dasgupta, 2016), our experiments show the reaction of carbonatite magma with silicate rock: an "antiskarn". The mineral assemblage of our "antiskarns" (diopside, calcite, vesuvianite, wollastonite, also cuspidine, monticellite and magnesioferrite in run D2332) highly resembles the assemblage of hydrothermal skarns, raising the possibility that some skarns or mineralogically similar rocks may in fact be paleoconduits of carbonatite magma as it traversed the crust.…”

“…However, the genetic term "endoskarn" implies the presence of a silicate intrusion and an aqueous fluid -neither of which are present in antiskarns. Alternatively, endoskarns may form by direct assimilation of carbonates by the silicate magmas at high temperatures (Di Rocco and others, 2012;Carter and Dasgupta, 2016). Like fenitic fluids, the skarn-forming metasomatic aqueous fluid exsolves due to decompression and crystallization of a pluton at shallow depths (Ͻ 5 kbar, Meinert, 1993), whereas the metasomatism of antiskarns results from direct reaction of a carbonatite with silicate wall rocks at mid-crustal depths (Ͼ 5 kbar).…”

Remnants and Rates of Metamorphic Decarbonation in Continental Arcs

“…Without information regarding the rocks in which the magma intrudes, only magmatic CO 2 fluxes from continental arcs can be approximated. Experiments replicating sub-arc and lower crustal conditions show that carbonate rock can be almost wholly decarbonated (Carter and Dasgupta, 2016), which has been corroborated by observations of extremely low 13 C/ 12 C ratios of calc-silicate xenoliths from the Merapi volcano (Whitley et al, 2019). The degree to which continental arc magmas completely decarbonate their host rocks is unknown, but given the relatively open-system nature of continental arcs, these findings likely reflect upper limits for decarbonation rates.…”