3. Constraints on the density contrast and volume of the anomalous bodies
3.1. Bouguer Anomaly
Inversion modelling of the new gravity dataset provides the relationship between
density contrast and the relative volume of the anomalous bodies, and shows the
minimum possible density contrast. We show these relationships for inversions done
following a Free setting (as explained above) and for a regional constant of -320
mGal. This produces a local anomaly that is mostly negative: -44 to +12 mGal. We
repeat the inversion assuming that the anomalous bodies extend from 3 km a.s.l. down
to: -75 km, -55 km -35 km and -25 km (Figure 3).
3.2. Densities
We use the petrological data [Lucassen et al., 2001] and geophysical constraints
[Prezzi et al., 2009; Zandt et al., 1994] that calculate an overall density of 2700 kg m3
for the mid-upper crust of the Central Andes. We add to this value a density of 2730
kg m-3 for a Gneiss xenolith erupted from Uturuncu, taken to represent a piece of old,
regional metamorphosed basement [Sparks et al., 2008], that we have calculated
using the algorithm of Hacker and Abers [2004].
We extract a range of composition of the dacites erupted from Uturuncu from Sparks
et al. [2008] and use the algorithm of Ochs and Lange [1999] with a general adiabat,
assuming a water content of 3.5% and a depth range above the APMB, to calculate a
density of the melt between 2287±46 kg m-3 and 2329±44 kg m-3. In the main text we
use a value of 2300 kg m-3 to be representative of all dacitic melts from Uturuncu.
Using the algorithm of Hacker and Abers [2004] we calculate a density of 2650 kg m3
for the crystallised dacite.
Similarly, we use modal analysis data of norite xenoliths from Sparks et al. [2008]
and calculate a density of 3020 kg m-3. These norite adcumulates segregated from
parental andesite [Sparks et al., 2008], and mass balance on linear SiO2 and major
element plots (Figure 12 in Sparks et al. [2008]) suggest that parental andesite
differentiated to produce 30-50% norites and 50-80% dacitic melt.
3.3. Applying both constraints
Using the constrains from the Bouguer Anomaly and the Density, in Figure 3 we
show the relationship between the volume of the anomalous bodies and their density
contrast. The density contrast can be related to a fraction of melt based on the
densities calculated as shown above: dacitic melt 2300 kg m-3, crystallised dacite
2650 kg m-3 and the overall density of the crust 2700 kg m-3. If we included the norite
adcumulates in our calculation, an additional 0.5 melt fraction for the two-phase
mixture would be required to produce density contrasts comparable to those in Figure
3.
As an example in Figure 3, if the anomaly is assumed to occur at shallow levels, e.g.
the top 25 km, the minimum volumes for a crystallised granitoid (density 2650 kg m3
) and a 15 vol.% partially molten granitoid (density = 2598 kg m-3) are 30% and
17%, with density contrasts of -70 and -121 kg m-3 respectively.
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