Pre-eruptive Conditions of the Huerto Andesite (Fish Canyon System, San Juan Volcanic Field, Colorado): Influence of Volatiles (C–O–H–S) on Phase Equilibria and Mineral Composition
JOURNAL OF PETROLOGY
VOLUME 49
NUMBER 5
PAGES 911^935
2008
doi:10.1093/petrology/egn011
Pre-eruptive Conditions of the Huerto Andesite
(Fish Canyon System, San JuanVolcanic Field,
Colorado): Influence of Volatiles (C^O^H^S)
on Phase Equilibria and Mineral Composition
1
MINERALOGISCH-GEOCHEMISCHES INSTITUT, ALBERTSTRASSE 23B, D-79104 FREIBURG, GERMANY
2
INSTITUT FU«R MINERALOGIE, UNIVERSITA«T HANNOVER, CALLINSTRASSE 3, D-30167 HANNOVER, GERMANY
RECEIVED NOVEMBER 8, 2007; ACCEPTED FEBRUARY 15, 2008
ADVANCE ACCESS PUBLICATION APRIL 10, 2008
KEY WORDS: experimental study; andesite; volatile; Fish CanyonTuff;
Huerto Andesite
Crystallization experiments at 400 MPa, oxidized condition
(logfO2 ¼ NNO þ1, where NNO is nickel^nickel oxide buffer)
and over a range of temperatures (850^9508C) and fluid composition (XH2Oin ¼ 03^1) have been carried out to constrain the storage conditions of the sulphur-rich magma of the Huerto Andesite
(an anhydrite, pyrrhotite, and S-rich apatite-bearing, post-Fish
Canyon Tuff mafic lava). The results are used to evaluate the role
of fluids released from the crystallization of magmas such as the
Huerto Andesite on the remobilization of the largely crystallized
dacitic Fish Canyon magma body. Experiments were performed
using the natural andesitic bulk composition with and without
added sulphur.The presence of sulphur slightly affects the phase equilibria by changing the phase proportions, stability fields of plagioclase, pyroxenes and ilmenite, and also affects the plagioclase
composition. Phase equilibria and mineral composition data indicate
that the magma may have contained 45 wt % water in the melt and
that the pre-eruptive temperature was 875 258C. Assuming that
the magma was in equilibrium with a fluid phase, the CO2 concentration of the melt is estimated to be in the range 2000^4000 ppm
(at 400 MPa). Before eruption, the andesite had an oxidation state
very close to, or slightly within, the co-stability field of anhydrite^
pyrrhotite at NNO þ11. At these conditions, the sulphur content in
the melt is 500 ppm. Assuming open-system degassing resulting
from continuing crystallization at depth, most of the CO2 dissolved
in the andesitic melt should be released after the crystallization
of 510 vol. % of the magma, corresponding to a cooling from
875 to 825^8508C.Thus, the fluids released owing to crystallization
processes should be mainly composed of water at temperatures
below 8258C.
The importance of volatile constituents (H2O, CO2 and S)
in magmatic processes is now well established. Volatiles
influence crystal^melt phase relations and the order
of crystallization of minerals from silicate melts, as well as
melt dynamics, and consequently processes such as mixing,
assimilation and differentiation. They may play an important role in subduction-related tectonic settings in the
generation of large silicic magma bodies. Although partial
melting of crustal materials and fractional crystallization
of more mafic parent magmas are the two mechanisms
commonly invoked to explain the generation of crystalpoor silicic magmas, reheating and partial remobilization
of a crystal mush has also been considered as a possible
mechanism for producing large silicic magma bodies
(Sisson & Bacon, 1999; Bachmann & Bergantz, 2003).
Upward percolation of a hot fluid phase through uppercrustal magma mushes may facilitate the segregation of
melt for the generation of voluminous rhyolite by their
reheating and partial remelting. Possible examples of thermally rejuvenated and remobilized crystal-rich magmas
include recent intermediate to silicic eruptions such as the
Montserrat andesite (e.g. Couch et al., 2001) and Pinatubo
dacite (Pallister et al., 1992), but also larger volcanic units
such as the Kos Plateau Tuff (Allen, 2001) and the
*Corresponding author.
E-mail:
**Present address: CODES-ARC - University of Tasmania, Hobart,
TAS 7001, Australia
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I N T RO D U C T I O N
FLEURICE PARAT1*, FRANCOIS HOLTZ2 AND SANDRIN FEIG2**
JOURNAL OF PETROLOGY
VOLUME 49
NUMBER 5
MAY 2008
5000 km3 Fish Canyon Tuff (Bachmann et al., 2002), one of
the most voluminous pyroclastic units worldwide.
The Fish Canyon Tuff of the San Juan volcanic field
in Colorado (USA) is a well-documented example of a
voluminous, unzoned, phenocryst-rich pyroclastic deposit
(Lipman et al., 1997) (Fig. 1). The water- and sulphur-rich
lavas of the Huerto Andesite (hornblende- and anhydrite^
pyrrhotite-bearing calc-alkaline lavas, Parat et al., 2005)
erupted after the emplacement of the Fish Canyon Tuff.
Bachmann & Bergantz (2003, 2004) suggested that the
release of volatiles from a hotter, more mafic magma
stored beneath the Fish Canyon Tuff magma would have
contributed to the rejuvenation of a partially solidified
batholith, causing the eruption of the Fish Canyon Tuff.
The Huerto Andesite lavas possibly could, therefore, represent the degassing mafic magma source. Assuming this
hypothesis to be correct, the main problem that needs to
be resolved is whether or not the pre-eruptive conditions
of the Huerto Andesite are compatible with the temperature and amount of volatiles required.
Phase equilibria experiments were conducted to constrain the conditions at which the phase assemblage of the
Huerto Andesite could be reproduced and to constrain
the composition of the fluid phase in equilibrium with the
melt. We investigated (1) how changing volatiles (H2O,
CO2 and S) affect the phase equilibria, crystalline phase
proportions and compositions, (2) the solubility of H2O,
CO2 and S in andesitic silicate melts, and (3) the partitioning of sulphur between silicate melt, minerals and fluid as
a function of temperature and the fugacities of the
volatile species.
Pre-eruptive conditions estimated for
the Huerto Andesite and Fish Canyon
magma system
The Huerto Andesite is a crystal-rich andesite (44 vol. %
phenocrysts) with a hyalopilitic texture and phenocrysts of
plagioclase (normally zoned from An60Or1 to An45Or3;
28 vol. %) and amphibole (11 vol. %), plus minor to sparse
augite [Mg-number ¼100 Mg/(Mg þ Fetotal) ¼ 76], apatite,
912
Fig. 1. Map of the Southern Rocky Mountains showing the location of the San Juan volcanic field (Colorado, USA) and the extent of the Fish
Canyon Tuff erupted from the La Garita caldera (LG) and the post-Fish Canyon lavas, the Huerto Andesite. After Lipman (2006).
PARAT et al.
HUERTO ANDESITE PRE-ERUPTIVE CONDITIONS
with and without sulphur, in the temperature range
850^9508C with different fluid composition (H2O^CO2).
The presence of both anhydrite and pyrrhotite in the
natural andesite constrains the oxygen fugacity at NNO
to NNO þ15, where NNO is nickel^nickel oxide buffer
(expressed as log fO2) (Carroll & Rutherford, 1988).
Table 1: Starting material (DA4)
Phase proportion (vol. %)
dry glas (...truncated)
