Geochemistry, Geophysics, Geosystems
Supporting Information for
What processes control the chemical compositions of arc front
stratovolcanoes?
Stephen J Turner1* and Charles H Langmuir1
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Department of Earth and Planetary Sciences, Harvard University,
20 Oxford St., Cambridge, MA 02138
Contents of this file
Text S1
Additional Supporting Information (Files uploaded separately)
Captions for Table S1
Captions for Dataset S1
Introduction
The supporting information for this manuscript includes a details of the models
referenced in the main text, as well as separate files with a table of partition
coefficients used in mantle melting calculations, and a database of rear-arc
samples.
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Text S1. Additional Model Details
1) Non-modal mantle melting model description
The calculation of partition coefficients for mantle melting is based on a set of four
independent parameterizations, which are automated using MATLAB scripts that are available
for download as Software S1:
1) Subsolidus mantle modes are extrapolated for each model pressure. At pressures
<2GPa, we use the parameterization found in Tenner et al. (2009). At pressures greater
than 2 GPa, model clinopyroxene (cpx), modal orthopyroxene (opx) and modal garnet
(gt) are extrapolated linearly based on 17% cpx and 30% opx at 1.8 GPa, 1.5% gt at 2
GPa, and 30% cpx, 12% opx, and 2.5% gt at 3 GPa, consistent with the results of Walter
(1998), Baker and Stolper (1994), and Baker et al. (1995). Subsolidus olivine is held
constant at 53%, following Tenner et al. (2009).
2) The extent of mantle melting is calculated using the equations of Katz et al. (2003)
taking into account an enthalpy of fusions of 180 cal/g, as described in the main text.
3) Resulting mantle modes after melting are calculated by logarithmically extrapolating
reaction coefficients based on the pressure of melting from the experiments of Walter
et al. (1995) between 1.7 and 3.5 GPa. If garnet is present, reaction coefficients for cpx,
opx, and ol are reduced to accommodate a reaction coefficient for garnet of -0.115.
This value is somewhat lower than the garnet reaction coefficient determined by Walter
(1998), but is likely more consistent with melting of a hydrated mantle wedge, as shown
by Hall (unpublished thesis, 1999). If cpx is exhausted, melting proceeds using reaction
coefficients of -.03 for olivine, and -.97 for opx, following Walter (1998).
4) Partition coefficients for each mode are linearly extrapolated based on pressure from a
variety of literature sources (found in a separate Online Supporting Information
document), and bulk partition coefficients are then calculated based on the results of
step 3.
2) Slab melting model description
Calculation of slab melt compositions was based on the experimental work of Hermann
and Rubatto (2009) and Kessel et al. (2005). The process described below is automated using
MATLAB scripts that are also available in the Online Supporting Information. The partition
coefficients for elements between slab sources and slab melts, if not mentioned explicitly
below, are determined by direct linear extrapolation from partition coefficients from
experiments C-2446 (750 °C), C-1848 (800 °C), C-1578 (900 °C), and C-1868 (1050 °C) from
Hermann and Rubatto (2009) for sediment melting, and the 4 GPa experiments from Kessel et
al. (2005) for AOC melting.
For the LREE, in order to ensure smooth, consistent behavior across a range of
temperatures, the D value for Nd was extrapolated exponentially from D=3 at 800 °C to D=1.5 at
1000 °C, which produces the formula [DNd=8E9*T-3.244]. Because LREE may become saturated
beyond a certain concentration in sediment melts, maximum concentrations of Nd in melts
were also extrapolated from 13 ppm at 800 C to 45 ppm at 1000 °C. Remaining LREE and Sm
concentrations in slab melts were calculated using starting sediment and AOC compositions, as
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described in the main text, and average (D(s/l)LREE)/(D(s/l)Nd) values calculated from Hermann
and Rubatto (2009), which were held constant. These values were .73, .71, 1.6 for La, Ce, and
Sm, respectively. The D value for Sr was held at 0.15 for all calculations. Slab melt H2O contents
are extrapolated from 6% H2O at 980 °C to 25% H2O at 800 °C.
The extent of slab melting depends on the pressure, temperature, and water content of
the melt, and so it is essentially unconstrained. For the model calculations in Figure 10, the
extent of melting of the AOC was set at F=.3, and for sediment at F=.4. For Figure 14, the extent
of melting for the AOC was set at F=.35, and sediment at F=.5. For the calculations in Figures 1617, F AOC was allowed to vary from .2 to .4, while F Sediment was allowed to vary from .4 to .6.
These ranges were chosen in order to be consistent with the range of extents of melting found
in the experiments that were used to constrain partition coefficients.
3) Sediment composition used to generate Figures 10 and 14 in the main text
The model outputs in Figures 10 and 14 are meant to demonstrate the compositional
variability of arc lavas independent of variation in sediment compositions, and so a single
sediment composition is used for the model calculations in these figures. The sediment
composition used is the mean of sediment compositions from each of the segments in the
dataset, which for Central American segments are the hemipelagic sediment composition from
Patino et al. (2000) and for all other segments are the estimated subducting sediments from
Plank et al. (2013). The hemipelagic sediment from Central America was used for relevant
segments because including the underlying carbonate section results in Sr enrichments that are
much greater than observed, and because a sediment flux dominated by hemipelagic sediment
for most Central American volcanic centers is indicated by the mixing lines in Patino et al.
(2000).
4) Equations used to relate subduction parameters with thermal conditions
The WTS model tests the hypothesis that some simple relationship exists between
crustal thickness and pressure and temperature of melting beneath the arc. As described in the
main text, as successful model was found in which changes in crustal thickness from 15 to 50 km
correspond linearly to changes in P and T from 2 to 3.5 GPa and temperature decreases from
1450 to 1300 °C, resulting in the equations [P=CT*-0.43+13.5; T=CT*4.3+1514]. Note however,
that by changing the assumed H2O contents of the mantle, these equations would also need to
be updated to provide a good fit to the data. An explicit equation can also be derived from the
STS model for the relationship between Φ and % slab addition, on the basis of Figure 14, which
is [%SlabAddition=log(Φ/100)*(-0.143)+0.336].
References:
Baker, M., Hirschmann, M., Ghiorso, M., Stolper, E., 1995. Compositions of near-solidus
peridotite melts from experiments and thermodynamic calculations. Nature 375, 308-311.
Baker, M.B., Stolper, E.M., 1994. Determining the composition of high-pressure mantle melts
using diamond aggregates. Geochimica et Cosmochimica Acta 58, 2811-2827.
Hermann, J., Rubatto, D., 2009. Accessory phase control on the trace element signature of
sediment melts in subduction zones. Chemical Geology 265, 512-526.
Katz, R.F., Spiegelman, M., Langmuir, C.H., 2003. A new parameterization of hydrous mantle
melting. Geochemistry, Geophysics, Geosystems 4.
Kessel, R., Schmidt, M.W., Ulmer, P., Pettke, T., 2005. Trace element signature of subductionzone fluids, melts and supercritical liquids at 120–180 km depth. Nature 437, 724-727.
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Patino, L.C., Carr, M.J., Feigenson, M.D., 2000. Local and regional variations in Central American
arc lavas controlled by variations in subducted sediment input. Contributions to Mineralogy
and Petrology 138, 265-283.
Plank, T., 2013. The chemical composition of subducting sediments. The Crust, Treatise on
Geochemistry 4.
Tenner, T.J., Hirschmann, M.M., Withers, A.C., Hervig, R.L., 2009. Hydrogen partitioning
between nominally anhydrous upper mantle minerals and melt between 3 and 5 GPa and
applications to hydrous peridotite partial melting. Chemical Geology 262, 42-56.
Walter, M.J., 1998. Melting of garnet peridotite and the origin of komatiite and depleted
lithosphere. Journal of Petrology 39, 29-60.
Table S1.
This table provides partition coefficients used for mantle melting and their literature sources,
and is available as a separate file.
Data Set S1.
This data set contains the rear-arc data referenced in the main text, which is presented in
GEOROC file format, available in a separate file.
Software S1.
This zip file contains model scripts and xls files for the WTS and STS models described in the
main text. See separate readme file included in the zip for more information.
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