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numerical models of open-system heat and CO are tied to their protoliths, fluid budgets, tains high chemical potentials, driving de-
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mass transfer (e.g., Nabelek et al., 2014; Chu and reaction progresses (Fig. 1A). carbonation locally to completion (e.g., Chu
et al., 2019), providing accurate flux esti- Skarn rocks, composed of varying propor- et al., 2019, and references therein). For
mates. The drawback of these models, how- tions of garnet, pyroxene ± wollastonite, are example, a cubic meter of garnetite skarn
ever, is that they involve geologic specificity synonymous with decarbonation (Fig. 1B) signifies 1.01–1.05 metric tons of CO
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that belies a broad representation of meta- and are often associated with economic base released from calcite (Lee and Lackey, 2015).
morphism in continental arcs. To predict and precious metal deposits. Skarns epito- Skarns often form at shallow crustal
how metamorphic decarbonation has varied mize optimal conditions for releasing CO depths (3–5 km) and along the margins of
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through geologic time, a balance between where infiltration of water-rich fluid main- granitoid rocks that intruded into carbonates.
these common approaches needs to be found.
In this paper, we show that sedimentary,
igneous, and metamorphic rock evidence Retrograde CO 2
can be used to quantify the rates of meta- Carbonation CO 2 CO 2 CO 2
morphic decarbonation in continental arcs A Carbonate
through the Phanerozoic. Metamorphic
rocks in continental arcs can directly trace F
decarbonation rates, but the reactive trans- Fore Arc Back Arc
port processes involved in their formation is
not simple. We thus review common rocks Skarn, Calc-silicate
& Marble Decarbonation
that form through metamorphic decarbon-
ation in the shallow crust, the reactions and
conditions that generate them, and the CO
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amounts that they can release as a byproduct Assimilation Clinopyroxene
Decarbonation
of their formation. Additionally, through Asthenosphere
numerical modeling, we demonstrate that
the volume fraction of sedimentary rock that
undergoes decarbonation can be related to Slab Garnet
the relative volumes of sedimentary rock and a
ab
ab
ab
Slab
Slab
Slab
Slab
magma in continental arcs. This finding is
validated against the well-characterized rock C
record of the Cretaceous Sierra Nevada B Wollastonite
batholith (SNB). When compiled strati-
graphic sections of North America and arc
magma fluxes through the Phanerozoic are
imposed in our model, we predict how fluxes
of CO from metamorphic decarbonation
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changed through geologic time.
FIELD OBSERVATIONS OF
DECARBONATION AND RE-
CARBONATION IN THE ROCK
RECORD D E
There is abundant rock-hosted evidence Proportions: Rock CO 2
for CO liberation, transport, and immobili- CO 2 CO 2 CO 2 Contact CO 2 CO 2 Skarn Garnet
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zation in exhumed arc crust within circum- CO 2 CO 2 Metamorphic Calc-
Pacific batholiths, including the SNB. CO 2 CO 2 CO 2 Silicate
CO 2
Whereas the isolated screens and roof pen- CO 2 CO 2 CO 2 CO 2 Marble
dants of metamorphic rocks appear as slivers
in granitoid plutons, they are volumetrically Tremolite Calcite
underrepresented at Earth’s surface due to Diopside
erosion, overprinting by younger intrusions, Magma-hosted G
and/or downward transport to the sub-arc CO 2 (Assimilation) F
during pluton emplacement (e.g., Ducea et
al., 2015). These rocks show abundant evi- Figure 1. Arc decarbonation. (A) Schematic representation of plutons intruding carbonate-bearing
dence that carbonate-bearing rocks spanned crust at various depths in a magmatic arc (not to scale); (B) 30-cm-wide outcrop of garnet, clinopyrox-
from upper crustal contact aureoles to lower ene, and wollastonite (white) typical of Sierra Nevada batholith skarn; (C) 20-cm-wide slab of garnet-
wollastonite-diopside calc-silicate rock with folding of original sedimentary structures; (D) calc-silicate
crustal granulite facies domains (e.g., with garnet (red) showing traces of Al-rich domains in garnet-wollastonite calc-silicate (coin is 24 mm
Kerrick, 1977; Newberry and Einaudi, 1981). across); (E) laminated carbonate typical of rocks metamorphosed to form C and D (hammer is 28 cm
long); (F) cartoon depicting metamorphic decarbonation, common metamorphic rock types, their pro-
The capacities of these pendants to produce toliths, CO yields; (G) retrograde calcite deposited in 1-cm-wide cavity within garnet skarn.
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