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Any carbonate that was not converted to water-rich metapelitic units, CO can diffuse metasomatism of originally pure carbonate
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skarn coarsens into marble. Marbles are out of the marble and thus export nontrivial rocks, calc-silicates release similar amounts
more abundant than skarns (Fig. 2B) and can amounts of CO (e.g., Vidale and Hewitt, of CO because interbedded layers of carbon-
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appear to be relatively unaffected by meta- 1973; Ague, 2000). ate, silica, and clay minerals predispose
morphic decarbonation. Yet, small amounts Calc-silicate rocks, with white, green, or mixed carbonate-siliciclastic rocks to fully
of reaction progress are enabled by trace red laminations inherited from sedimentary decarbonate (Fig. 1F). Water-rich fluids are
quantities (<5 modal %) of quartz present, laminations, are also composed of micro- still necessary to fully decarbonate these
producing considerable amounts of CO crystalline wollastonite, pyroxene, and gar- rocks, but their laminated character can
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(~32–46 kg CO per cubic meter of rock; net (Fig. 1C–1E). Whereas skarns see copi- cause a positive feedback that enables near-
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Ferry, 1989). Further, if marble bodies abut ous CO release by fluid infiltration and complete decarbonation. The enhancement
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of permeability during decarbonation pro-
motes CO transport, enabling further decar-
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bonation (e.g., Zhang et al., 2000).
Even if thermodynamic conditions enable
decarbonation to proceed, not all CO
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produced makes it out of the crust. CO is
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most often immobilized when low crustal
permeability inhibits fluid flow or magma
production rates decrease. Secondary car-
bonate deposition represents CO immobili-
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zation in arcs, often occurring away from
hotter, deeper areas of the arc crust, and
down temperature gradients. Examples
include calcite veins that cross-cut skarn
rocks and precipitate in vugs and brittle frac-
tures (Fig. 1G). Other silicates that form
when CO is mineralized, including retro-
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grade serpentine and tremolite, occur appre-
ciably (up to 2 wt%), even when crustal per-
meability is high enough to promote contin-
ued CO removal (Nabelek et al., 2014).
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Veins and deposits at shallower levels in the
crust are further evidence that CO can be re-
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precipitated. Even granitoids in arc crust are
noted to contain regular but small amounts
of calcite (0.2–1.0 wt%; White et al., 2005).
Overall, these observations suggest that
seemingly trace amounts of retrograde CO
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mineralization can manifest in large masses
of CO left behind after prograde metamor-
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Pluton phic decarbonation.
The journey of an individual molecule of
CO can be complicated by a series of pro-
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grade and retrograde reactions at different
times and locations in the arc crust. Yet, at a
fundamental level, the amount of metamor-
phically derived CO that exits from conti-
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nental arcs directly relates to the composi-
tion of rocks that comprise the arc and the
amount of magma that is emplaced over a
given time. These underlying principles,
while still considering the intricacies of
metamorphic decarbonation and its geo-
logic record, motivate our model design.

Figure 2. (A) Predicted volume fraction of aureole as a function of volume ANALOGUE MODEL FOR
fraction of intrusion. Error is 1σ. (B) Skarn-marble area distributions for a METAMORPHIC DECARBONATION
corridor of the Sierra Nevada batholith (SNB). (C) Surface area addition The basis of the analogue model is to
rates for intrusions and metamorphic pendants in the SNB through the
40 m.y. of elevated magmatic activity. determine the volume of sedimentary rock

6 GSA Today | May 2020