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Remnants and Rates of Metamorphic

Decarbonation in Continental Arcs

Evan J. Ramos*, Dept. of Geological Sciences, University of Texas, Austin, Texas 78712, USA; Jade Star Lackey, Geology Dept.,
Pomona College, Claremont, California 91711, USA; Jaime D. Barnes, Dept. of Geological Sciences, University of Texas, Austin, Texas
78712, USA; Anne A. Fulton**, Geology Dept., Pomona College, Claremont, California 91711, USA

ABSTRACT et al., 2004; Lee et al., 2013; McKenzie et al., and lower crustal conditions show that car-
Metamorphic decarbonation in magmatic 2016). CO fluxes from continental arcs are bonate rock can be almost wholly decarbon-
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arcs remains a challenge to impose in models the cumulative expression of magmatism, ated (Carter and Dasgupta, 2016), which has
of the geologic carbon cycle. Crustal reser- contact metamorphism and assimilation of been corroborated by observations of
voirs and metamorphic fluxes of carbon vary sedimentary rocks by magmas, and fluid extremely low C/ C ratios of calc-silicate
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with depth in the crust, rock types and their flow through the crustal column. Because of xenoliths from the Merapi volcano (Whitley
stratigraphic succession, and through geo- its connection to magmatic and hydrother- et al., 2019). The degree to which continen-
logic time. When byproducts of metamor- mal systems (e.g., Baumgartner and Ferry, tal arc magmas completely decarbonate
phic decarbonation (e.g., skarns) are exposed 1991), metamorphic CO production in con- their host rocks is unknown, but given the
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at Earth’s surface, they reveal a record of tinental arcs remains a challenge to quantify relatively open-system nature of continental
reactive transport of carbon dioxide (CO ). In and has thus been on the periphery of most arcs, these findings likely reflect upper lim-
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this paper, we discuss the different modes of studies. The movement of CO during meta- its for decarbonation rates.
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metamorphic decarbonation at multiple spa- morphism is further complicated by meta- The geochemical and isotopic composition
tial and temporal scales and exemplify them morphic reaction progress, fluid availability, of volcanic emissions from active continental
through roof pendants of the Sierra Nevada geothermal gradients, and chemical poten- arcs reveal CO generated by metamorphism.
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batholith. We emphasize the utility of ana- tials. Nonetheless, the strides made in stud- A global compilation of CO /S measure-
T
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logue models for metamorphic decarbon- ies of continental arcs position us to advance ments shows that arcs where magma intrudes
ation to generate a range of decarbonation our understanding of metamorphic decar- platform carbonates often produce large CO
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fluxes throughout the Cretaceous. Our bonation through geologic time, its role in fluxes (Aiuppa et al., 2019). Moreover, the
model predicts that metamorphic CO fluxes the carbon cycle, and its influence on isotopic composition of volcanic emissions
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from continental arcs during the Cretaceous past climates. from these continental arcs further suggests
were at least 2 times greater than the present Maps of fossilized magmatic systems and input of sedimentary carbon (Mason et al.,
cumulative CO flux from volcanoes, agree- experiments replicating sub-arc and lower 2017). Despite these advancements, measure-
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ing with previous estimates and further sug- crustal environments have been employed ment uncertainty in these data hampers a
gesting that metamorphic decarbonation was to estimate CO fluxes from continental quantitative assessment of the metamorphic
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a principal driver of the Cretaceous hothouse arcs. In general, these studies establish proportion of continental arc CO outputs. By
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climate. We lastly argue that our modeling upper and lower estimates for CO fluxes focusing on active systems, this approach
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framework can be used to quantify decar- from continental arcs, but questions remain cannot convey how continental arc magma-
bonation fluxes throughout the Phanerozoic regarding the proportion of CO produced tism and concomitant CO fluxes have
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and thereby refine Earth systems models for via metamorphism. For example, estimates changed through geologic time.
paleoclimate reconstruction. of area addition rates of magma through Numerical models have been useful in
geologic time proxy for magma fluxes (Cao understanding metamorphism in continental
INTRODUCTION et al., 2017; Ratschbacher et al., 2019), arcs. Studies have typically scaled observa-
How much “bark” was in the arc? The which are critical parameters that set the tions, such as changes in the length of conti-
question of CO contribution from magmatic tempo and duration of metamorphic decar- nental arcs through time, to fluxes of meta-
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arcs, especially continental arcs that poise bonation (e.g., Cathles et al., 1997). Without morphic CO (e.g., Mills et al., 2019; Wong et
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platform carbonates in the paths of ascend- information regarding the rocks in which al., 2019). Although these methods provide
ing magmas (Lee et al., 2013), is important the magma intrudes, only magmatic CO meaningful boundary estimates, they do not
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given the power of tectonically outgassed fluxes from continental arcs can be approx- fully consider the thermodynamics of re-
CO to modulate Earth’s climate (e.g., Royer imated. Experiments replicating sub-arc active transport. Other studies have used
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GSA Today, v. 30, https://doi.org/10.1130/GSATG432A.1. Copyright 2020, The Geological Society of America. CC-BY-NC.
*Corresponding author: ejramos@utexas.edu.
**Now at Dept. of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado 80401, USA.

4 GSA Today | May 2020