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that undergoes decarbonation in a continen- pendants and skarns within a portion of the both mixed carbonate-siliciclastic rocks and
tal arc. The model simulates two-dimen- SNB to amounts of produced CO along the platform carbonates, contribute 59% of all
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sional fluid flow, heat transfer, and fluid- entire arc. This estimate is compared to our generated CO . Paleozoic sections such as
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rock oxygen isotope exchange after the model prediction to assess its utility in esti- the Morrison block in the eastern SNB,
emplacement of an intrusion as a proxy for mating CO fluxes. which are composed predominantly of silici-
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metamorphic decarbonation (further model clastic rocks and 23% carbonate, contribute
setup and assumptions are described in GROUND-TRUTHING THE 41%. Notably, this net flux agrees within 2σ
Ramos et al., 2018, and in the Supplementary ANALOGUE MODEL WITH THE error of the net decarbonation estimate from
Information ). The d O values of carbonates GEOLOGIC RECORD FROM THE (1) SNB skarn outcrops (1 Mt/yr) and when
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1
decrease during progressive decarbonation CRETACEOUS SNB (2) North American sedimentary rock infor-
(e.g., Bowman, 1998). Therefore, in each The area distribution of skarns (Fig. 2B) mation is used (40 Mt/yr) instead of SNB-
simulation, we track the changes in host-rock varies considerably along the SNB corridor specific stratigraphic sections (Fig. 3C at ca.
d O values during hydrothermal fluid flow we examined, with some exposures contain- 100 Ma time marker). Although location-
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to highlight areas around a magma body that ing <10 m of skarn and others containing >1 specific geology will always yield more
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meet likely conditions for decarbonation. km . The skarn area is generally dwarfed by accurate flux estimates, these findings sup-
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Once hydrothermal activity has ceased, the the marble area and only comprises 4% of port the utility of North American sedimen-
d O values of the host rock define a volume the total mapped area. If we assume a main- tary rocks as a globally representative
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of rock that undergoes decarbonation, which tained skarn-marble ratio within pendants archive of sedimentary rock types.
we term the aureole volume. along the entire SNB, an average carbonate
Our numerical model considers effects of fraction in sedimentary rocks in the arc of PHANEROZOIC METAMORPHIC
crustal permeability and magma volume on 20%, and skarn occurrence over 7 km of DECARBONATION RATES
aureole volumes. The model domain remains depth in the SNB, we compute a total skarn The variational growth rate of sedimen-
constant across each simulation (i.e., V host rock volume in the SNB of 19,000 km . This vol- tary rock and granitoid volumes underpins
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+ V intrusion = constant; Fig. DR3, see footnote ume, if decarbonated over a 40 m.y. time the changes in decarbonation rates in conti-
1). A series of model runs predicts aureole interval, produces an average CO flux of ~1 nental arcs through time (negative slope of
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volumes as a function of intrusion volume Mt/yr. This value, which excludes CO from lines in Fig. 3A). Once corrected for erosion
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and crustal permeability where the largest calc-silicates and marbles and fluxes from (assuming an erosional half-life of 400 m.y.
volumes of decarbonated host rock (V aureole ) magma degassing and assimilation, is five- sensu Cao et al., 2017), sedimentary rocks
are at intermediate relative intrusion vol- fold less than measurements of modern from North America, granitoids (from Cao
umes (Fig. 2A). Effectively, as magma vol- global continental arcs that intersect plat- et al., 2017), and their volumetric distribu-
umes exceed the volume of host rock in an form carbonates (5 Mt/yr; Aiuppa et al., tions grow unsteadily. Cambrian through
arc, the aureole volume diminishes, concom- 2019) and nearly two orders of magnitude Devonian time (542–400 Ma) is marked by
itant to a diminished aureole decarbonation less than previous estimates for net CO similar rates of growth of different rock
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flux. This result counters common thought, fluxes from all Cretaceous continental arcs types, highlighting the voluminous deposi-
where continental arc flare-ups (i.e., times of (Lee et al., 2013; Lee and Lackey, 2015). This tion of carbonate throughout the Phanerozoic.
maximum intrusion volume) are thought to disparity can largely be attributed to the The volume of mixed carbonate-siliciclastic
be times of maximum CO output from arcs sparse distribution of metamorphic pendants rocks surpasses that of pure carbonate by lat-
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(Lee and Lackey, 2015). in the SNB and the difficulty of computing est Pennsylvanian (ca. 300 Ma) when the
The mineralogy of the host rocks in which assimilation fluxes from the geologic record. growth rate of siliciclastic rocks increases
the magma intrudes controls the magnitude Thus, this skarn CO flux is considered a well beyond all other rock types. Sediment
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of the decarbonation flux it produces. We minimum estimate for metamorphic CO deposition rates plateau in the Triassic (245–
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thus amass magma addition rates and sedi- fluxes from the Cretaceous SNB. 206 Ma) after the assembly of Pangea (Cao et
mentary rock information—including rock When sedimentary rock volumes and pro- al., 2017) and subsequently increase upon its
types, depositional ages, and stratigraphic portions from SNB-specific sites (Fig. DR2, breakup in the Jurassic (ca. 180 Ma). Car-
thicknesses—for the SNB and the entirety of see footnote 1) are compared with granitoid bonate and mixed carbonate-siliciclastic
North America. Details about how we com- volumes emplaced in North America from rocks grow in volume in the Cretaceous but
pare sedimentary and magma volumes 125 to 85 Ma (Cao et al., 2017), we predict are dwarfed by increases in the siliciclastic
through time are given in the Supplementary the net metamorphic CO flux from North deposition rate. These trajectories of growth
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Information, but in short, our model predicts American arcs to be 32.3 ± 28.4 Mt/yr during remain constant through the Cenozoic.
a metamorphic CO flux—which includes the Cretaceous, with 13% of the flux deriv- Change in area addition rates of granitoid is
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CO produced via metamorphism in the ing from assimilated wall rock and 87% out of phase with the deposition of sedimen-
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aureole and by assimilation of host rock in coming from decarbonation in the aureole tary rocks (Fig. 3A). Globally, granitoid vol-
the intrusion—based off this volume com- (see Supplementary Information for further umes grow at a roughly consistent rate until
parison. Independent of the model, we com- details on the flux calculation). Western the breakup of Pangea, whereupon their
pute a CO flux for the Cretaceous SNB by SNB rocks, typified by the Triassic–Jurassic cumulative volumes grow rapidly. Continental
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scaling the area distribution of metamorphic Kings Sequence (Fig. DR2), which contains arc activity in North America is quiescent
1 GSA Data Repository item 2020150, including model descriptions and data sources for rock type information, is available online at https://www.geosociety.org/
datarepository/2020.
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