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| Titel |
Interpreting plagioclase-melt (dis)equilibrium due to cooling dynamics: implications for thermometry, barometry and hygrometry |
| VerfasserIn |
Silvio Mollo, Keith Putirka, Gianluca Iezzi, Pierdomenico Del Gaudio, Piergiorgio Scarlato |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250047137
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| Zusammenfassung |
| Plagioclase is particularly useful for deciphering magma chamber dynamics due to the
sluggish diffusion of its major and minor constituents. However, the interpretation of
plagioclase crystallization conditions and plagioclase-liquid exchange reactions is often
ambiguous because cooling rate can change the primary plagioclase composition. Cooling
dikes and lava flows may affect the crystal growth and the chemistry of plagioclase during
post magmatic activity. Moreover, cooling kinetics may also have an impact on
plagioclases which crystallized before magma ascent from deeper magmatic chambers. In
consequence the study of plagioclase compositions that grow under dynamic crystallization
conditions are crucial to decipher the cooling history of magmas. In this study the
compositional variation of plagioclase and the partitioning of major elements between
plagioclase and melt have been experimentally measured as a function of cooling rate.
Crystals were grown from a basaltic melt at a pressure of 500 MPa under (i) variable
cooling rates of 0.5, 2.1, 3, 9.4, and 15 Ë C/min from 1250 Ë C down to 1000 Ë C,
(ii) quenching temperatures of 1025, 1050, 1075, 1090, and 1100 Ë C at the fixed
cooling rate of 0.5 Ë C/min, and (iii) isothermal temperatures of 1000, 1025, 1050,
1075, 1090, and 1100 Ë C. Our results show that euhedral, faceted plagioclases
form during isothermal and slower cooling experiments exhibiting idiomorphic
tabular shapes. In contrast, dendritic shapes are observed from faster cooled charges
due to a diffusion-controlled growth mechanism. As the cooling rate is increased,
concentrations of Al+Ca+Fe+Mg increase and Si+Na+K decrease in plagioclase
favouring higher An and lower Ab+Or contents. These exchange reactions occur
because the diffusion rate of components rejected by the crystal away from the
growing interface is much slower than the crystal growth rate; in other words, the
incompatible elements are rejected less efficiently during rapid crystal growth, thus they
are essentially trapped into the plagioclase crystal lattice (“diffusion-controlled”
growth mechanism). Significant variations of major element distribution coefficients
(pl-liqKd) are also observed by the comparison between isothermal and cooled charges;
notably,pl-liqKdAb-An, pl-liqKdCa-Na and pl-liqKdFe-Mg monotonically change with
increasing cooling rate. Therefore, crystal-melt exchange reaction has the potential to reveal
the departure from equilibrium for plagioclase-bearing cooling magmas. Finally,
thermometers, barometers, and hygrometers derived through the plagioclase-liquid
equilibria have been tested at these non-equilibrium experimental conditions. Since
such models are based on assumption of equilibrium, any form of disequilibrium
will yield errors. Results show that errors on estimates of temperature, pressure,
and melt-water content increase systematically with increasing cooling rate (i.e.
disequilibrium condition) depicting monotonic trends towards drastic overestimates. These
trends are perfectly correlated with those of pl-liqKdCa-Na, pl-liqKdAb-An, and
pl-liqKdFe-Mg, thus demonstrating the ability of models to test (dis)equilibrium conditions. |
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