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The control of cooling rate on titanomagnetite composition: implications for a geospeedometry model applicable to alkaline rocks from Mt. Etna volcano
Language
English
Obiettivo Specifico
2.3. TTC - Laboratori di chimica e fisica delle rocce
Status
Published
JCR Journal
JCR Journal
Title of the book
Issue/vol(year)
/165 (2013)
ISSN
0010-7999
Electronic ISSN
1432-0967
Publisher
Springer Verlag Germany
Pages (printed)
457-475
Issued date
2013
Keywords
Abstract
In this study, we have investigated the control
of cooling rate on the composition of titanomagnetite
formed from a trachybasaltic melt. Results show that disequilibrium
growth conditions exert a primary control on
the abundance, texture, and composition of the crystals. As
the degree of cooling is increased, titanomagnetites show
immature textures and are progressively enriched in
Al ? Mg and depleted in Ti. Thus, early-formed titanomagnetite
nuclei do not re-equilibrate with the melt over
faster cooling rates; instead, their compositions are far from
equilibrium. On the basis of the different intra-crystal
redistribution rates for Ti, Al, and Mg, we have calibrated a
geospeedometer that represents the first quantitative
description of the effect of cooling rate on titanomagnetite
composition. This model was tested using the compositions
of titanomagnetites in lava and dike samples from Mt. Etna
volcano whose crystallization conditions resemble those of
our experiments. Cooling rates calculated for lava samples
are comparable with those measured in several volcanic
complexes. At Mt. Etna, compositional variations of titanomagnetite
grains from the innermost to the outermost
part of a dike testify to progressively higher degrees of
cooling, in agreement with numerical simulations of thermal
gradients in and around magmatic intrusions.
of cooling rate on the composition of titanomagnetite
formed from a trachybasaltic melt. Results show that disequilibrium
growth conditions exert a primary control on
the abundance, texture, and composition of the crystals. As
the degree of cooling is increased, titanomagnetites show
immature textures and are progressively enriched in
Al ? Mg and depleted in Ti. Thus, early-formed titanomagnetite
nuclei do not re-equilibrate with the melt over
faster cooling rates; instead, their compositions are far from
equilibrium. On the basis of the different intra-crystal
redistribution rates for Ti, Al, and Mg, we have calibrated a
geospeedometer that represents the first quantitative
description of the effect of cooling rate on titanomagnetite
composition. This model was tested using the compositions
of titanomagnetites in lava and dike samples from Mt. Etna
volcano whose crystallization conditions resemble those of
our experiments. Cooling rates calculated for lava samples
are comparable with those measured in several volcanic
complexes. At Mt. Etna, compositional variations of titanomagnetite
grains from the innermost to the outermost
part of a dike testify to progressively higher degrees of
cooling, in agreement with numerical simulations of thermal
gradients in and around magmatic intrusions.
Type
article
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