 |
| Titel |
An empirical hygrometer for trachybasaltic melts: applications to the kinetics of magma ascent at Mt. Etna. |
| VerfasserIn |
Pietro Armienti, Cristina Perinelli, Keith D. Putirka |
| Konferenz |
EGU General Assembly 2011
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250054645
|
|
|
|
|
|
| Zusammenfassung |
| Water is possibly the most important chemical component in magmas, affecting liquidus
temperatures, crystal fractionation trends, melt rheology and in turn the dynamics of ascent
and eruption of magmas. Hence, it is of broad interest to determine the initial abundances of
water in magmas and its variations during magma ascent and differentiation. Solubility
models satisfactorily account for the partitioning of water between exsolved vapor
the water dissolved in silicate melts provided that magma water contents prior to
exsolution are known. However crystal-hosted melt inclusions rarely capture the most
shallow, late-stage episodes of magma ascent, just before water exsolution, when
undercoolings are moderate and so provide little or no record of water content variations
during the to be used for understanding the triggering mechanisms for volcanic
erutpions.
Melt inclusions from different volcanic products erupted by Mt. Etna indicate that the
primitive melts rising from the mantle are particularly rich in water (1 - 3.8 wt%) making this
volcano a natural laboratory to investigate on relations between water variations and the
relevant aspects of magma ascent. In this work we calculate P(H2O)-T-t paths of magma
ascent in the Mt. Etna volcanic feeding system, combining thermochemical and kinetic data
on Mt. Etna clinopyroxene. Due to the its early appearance on the liquidus of Etnean
magmas, clinopyroxene can be used as a tracer of polybaric crystallization processes that take
place mostly well below depths of water exsolution. Since clinopyroxene abundances and
compositions are controlled by magma water solubility we have developed a new hygrometer
based on the composition of clinopyroxene phenocrysts. It links clinopyroxene compositions
found in controlled experimental runs to H2O concentration, temperature, pressure and
melt composition. Our empirical method is calibrated with a resolution < 0.5 wt%
H2O, and relies on a set of experiments on hydrous liquids representative of Etnean
volcanics.
The formulation of the model is:
K* Â Wt% H2Omelt = a * DiHd b* EnFs + c * CaTs + d * Jd + e *CaTi + f * P + g /T
(1)
where the end-member of clinopyroxene (DiHd, EnFs, CaTs, Jd, CaTi) are calculated
following the Putirka et al.’s (2003) geothermometer. The parameters K, a,b,c,d,e,f,g were
determined by regression are.:
K= 3.925495108; a= 19.44971977; b= -0.618948565; c= 8.387657214
d= 49.33424833; e= -86.02044893; f= -0.366040062; g= -0.3706621
P is in GPa and T is in Ë C.
This technique thus holds promise as a method for determining the water content of
virtually any volcanic rock, whose composition is similar to those considered for the
calibration of the hygrometer. |
|
|
|
|
|
|