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< Supplementary Information >
< Supplementary Tables >
< Table 1 Major element and volatile composition of melt inclusions and glasses
from host lavas. We present the melt inclusion data corrected and uncorrected
for host olivine crystallization, and we report the fraction of olivine crystallized
(Xol). Major elements were determined by electron microprobe at the
Geophysical Laboratory, Carnegie Institution of Washington and the American
Museum of Natural History using a JEOL and a CAMECA microprobe
respectively. In both cases, analyses were performed using 15 kV accelerating
voltage, 10 nA beam intensity, a 10 µm defocused beam, and ZAF correction
procedures. The major element compositions of glass inclusions in olivine were
recalculated by reverse modeling of olivine fractional crystallization in steps of
0.1% until the inclusion and host olivine were in equilibrium. This calculation
Ol  melt
assumed a Fe+2/(Fe+3+Fe+2) ratio = 0.9 in the melt and K Fe/Mg
= 0.3. The
proportion of Fe+3 in the melt inclusions was estimated using of the
compositions of syngenetic Cr-spinels inclusions in the samples1. Trace
elements and volatiles were determined by ion probe2,3. The major element
compositions of the host lava glasses are from Perfit et al.4. K2O and P2O5
contents in inclusions and host glasses were reanalyzed by ion probe. When
only electron-probe data is reported for those two elements, the sample name
has an asterisk. The precision reported represents the average of the standard
deviation (2) of replicated analysis (3 to 4) of several melt inclusions and of
different chips from the host glasses during different sessions for a period of
one year. The reported pressure during eruption is based on depth of sample
collection. Pressures for volatile equilibration and vapor compositions were
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calculated using the model of Dixon et al.5. The errors are an upper bound
based on the uncertainty in depth of sample collection and a propagation of the
CO2 content error respectively. Log fS2 values were calculated using the model
of Wallace and Carmichael6. However, because the precise Fe+3 content in the
melt inclusions and host glasses is not known, there is an uncertainty in the
calculated fugacity of S. The estimated fugacity of oxygen is approximately 2
log units below the NNO (Nickel-Nickel Oxide) buffer, consistent with the
estimated fugacity of oxygen for MORB7. More oxidizing conditions would make
the inclusions more sulfide-undersaturated and vice-versa. In determining
volatile contents, we discarded data from melt inclusions and glasses with
evidence of devitrification. We also corrected the volatile content of melt
inclusions having shrinkage bubbles. This correction considers the volume of
the bubble to be spherical and the volume of the melt inclusion to be ellipsoidal.
The pressure (P) and the fraction of CO2 in the gas phase were calculated
using the model of Dixon et al.5 Assuming the bubbles were shrinkage bubbles
rather than syngenetic bubbles, we used P, T (assumed to be the melt-glass
transition at 700°C) and the gas and melt volumes, to calculate an upper bound
for the number of moles of CO2 and H2O in the bubbles and added them back to
the initially measured volatile content of the glass. Thus, the corrected volatile
concentrations in Table 1 are strictly upper limits.
Table 2 Representative trace element compositions of melt inclusions, and host
glasses (corrected and uncorrected for host olivine crystallization). The trace
elements of the melt inclusions and host glasses were determined by ion
probe2. The precision reported in the table represents the average of the
standard deviation (2) of replicated analysis (3 to 4). Average NMORB from
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the East Pacific Rise is from Donnelly, (personal communication). Values used
in the trace element models are also presented: Depleted Mantle
composition8,9, bulk partition coefficients (bulk D and P) compiled from the
literature, and source mineralogy and melting reaction from Longhi10. The major
element model compares temperature and average melt inclusion composition
with accumulated melts (1% increments) generated at ~10 kb from a previously
depleted mantle (10%)10. The slight difference in the FeO and MgO between the
model and the average melt inclusion composition is due to the different
 melt
KOl
KOl  melt = 0.33
Fe/Mg = 0.3 used in our correction compared with the Fe/Mg
used by Longhi melting model10. >
< Supplementary Figures >
< Figure 5 Major element compositions of Siqueiros picritic glasses and melt
inclusions. For comparison we added Mid-Ocean Ridge Basalts (MORB) from
the Northern EPR obtained from the MORB database
(http://petdb.ldeo.columbia.edu/petdb/) and other Siqueiros glasses that were
reported by Perfit et al.4. We also compared the melt inclusion data with two
melting models from Longhi10. Most of the major element compositions can be
reproduced using polybaric accumulative incremental mantle melting models
(Model 1) with small residual porosity (0 to 0.5 %). The calculations indicate that
15% polybaric (20 to 5 Kb) accumulative incremental melting with small residual
porosity (0 to 0.5 %) would be required to explain most of the major element
data. Model 2 represents a two-stage model of small extent of melting (1 to 3)
% melting at low pressure (~10 kbar) of a MORB mantle source after MORB
extraction (previously depleted by 10% melting).
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Figure 6 Primitive mantle normalized plot a, Representative Siqueiros melt
inclusions and host glasses showing the observed range in composition.
Average normal MORB from the EPR is plotted for comparison (see Table 2 in
Supplementary Information). Note depletions in the most incompatible trace
elements in the Siqueiros samples compared to those of normal MORB. b, The
modeled trace element composition. Two models can explain the trace element
data, either 15% batch melting (equivalent to 15% accumulated fractional
melting) of a normal MORB source or small extents of melting (0.5 to 5%) of the
residual mantle left after MORB extraction (after 10% melting). The modeled
trace element abundances bracket the range in compositions for the Siqueiros
samples (dark field). Rare earth elements with an asterisk were not measured
but calculated using the neighboring rare earth elements. Model parameters are
reported in Table 2 (Supplementary Information).
Figure 7 Volatile/refractory trace element ratios for elements with similar
incompatibility during MORB mantle melting for Siqueiros melt inclusions and
glasses: a, CO2/Nb; b, H2O/Ce; c, F/P and d, S/Dy ratios were plotted against
Na2O, which can be used as indicator of extent of melting. The volatileundersaturation of many Siquieros samples indicates that the samples were not
degassed during eruption. Therefore, the volatile contents, such as CO 2, will
depend mainly on their behavior during melting and their content in the mantle
source. The similar behaviors of CO2 to Nb, H2O to Ce, F to P and S to Dy in
the Siqueiros melt inclusions and glasses demonstrate the incompatible nature
of the volatile elements; especially the highly incompatible nature of CO 2, during
melting. The data for H2O are of somewhat lesser quality than the data for other
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volatiles, due to the very low concentrations in the melt inclusions and the
higher background of H2O in the ion probe. This is reflected in the poorer
reproducibility of H2O compared to other volatiles. Another process that could
have affected the H2O/Ce ratios in the melt inclusions is H diffusion through the
olivine lattice, which would tend to buffer the H2O contents of the inclusions to
that of the host magma. At magmatic temperatures, only a few days will be
necessary for H diffusion to have such an effect. Thus, the inclusions with the
lowest Ce probably started off with the lowest H2O, but H2O content increased
via diffusive exchange with the surrounding magma through the olivine.>
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