Publication Date:
2017-04-04
Description:
The compositional variation of plagioclase and the partitioning of major elements between plagioclase and
melt have been experimentally measured as a function of the 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 °C/min from 1250 °C
down to 1000 °C, (ii) quenching temperatures of 1025, 1050, 1075, 1090, and 1100 °C at the fixed cooling rate
of 0.5 °C/min, and (iii) isothermal temperatures of 1000, 1025, 1050, 1075, 1090, and 1100 °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. As the
cooling rate is increased, concentrations of Al+Ca+Fe+Mg increase and Si+Na+K decrease in plagioclase
favoring higher An and lower Ab+Or contents. Significant variations of pl–liqKd are also observed by the
comparison between isothermal and cooled charges; notably, pl–liqKdAb–An, pl–liqKdCa–Na and pl–liqKdFe–Mg
progressively change with increasing cooling rate. Therefore, crystal–melt exchange reactions have 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 their ability
to test (dis)equilibrium conditions.
Description:
Published
Description:
221–235
Description:
2.3. TTC - Laboratori di chimica e fisica delle rocce
Description:
JCR Journal
Description:
reserved
Keywords:
Cooling rate
;
Partition coefficients
;
Thermometers
;
04. Solid Earth::04.04. Geology::04.04.05. Mineralogy and petrology
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
