ICATE   21876
INSTITUTO DE CIENCIAS ASTRONOMICAS, DE LA TIERRA Y DEL ESPACIO
Unidad Ejecutora - UE
artículos
Título:
GLASSES IN METEORITES AND THE PRIMARY LIQUID CONDENSATION MODEL
Autor/es:
VARELA, M.E; KURAT, G
Revista:
Mitteilugen Der Osterreischischen Mineralogischen Gessellschaft
Editorial:
Österreichischen Mineralogischen Gesellschaft
Referencias:
Lugar: Innsbruck; Año: 2009 vol. 155 p. 279 - 320
ISSN:
1680-8959
Resumen:
Abstract
Meteoritic glasses are quenched samples of the silicate liquid that was involved in the formation
of chondritic constituents and achondritic rocks. Conventional genetic models see them as
residual liquids from which the associated minerals crystallized as demonstrated by terrestrial
igneous rocks or as locally produced impact melts. These models are all closely related to our
experiences with terrestrial geology and petrology and, consequently,make planetary processes,
such asmixing andmelting of solid precursors and planetary differentiation primarily responsible
for the formation of the large variety of meteoritic rocks. However, different types of glasses
(e.g., glass inclusions in minerals, mesostasis glasses, glass pockets, glass veins) in a variety of
meteorites (chondrites and achondrites) have particular chemical features that cannot be
reconciled with these models:
1)Glasses do not showthe chemical signature of crystallization of theminerals they are associated
with a geochemical impossibility;
2) All types of glasses in all types of meteorites reported here show very similar trace element
abundance pattern with the refractory lithophile element abundances at ~ 1020 x CI chondrite
abundances.
3) All refractory element abundance patterns in primitive glasses have unfractionated solar
relative abundances (they are flat) and medium refractory and volatile elements are depleted
relative to the refractory elements.
4) Medium volatile and volatile elements, when present, display chaotic abundance patterns.
The ubiquitous pattern for refractory elements signals vapor fractionation rather than geochemical
(igneous) fractionation or stochasticmixing of precursorminerals (as in shockmelts).
It indicates that the same process must have been involved in the origin of all glasses in chondritic
constituents aswell as achondritic rocks and, consequently, in the formation of allmeteorite
types investigated. The chaotic abundances of volatile elements signal that chaotic processes
were involved during condensation.
Herewe present a newmodel - the Primary Liquid Condensation (PLC)model as an alternative
to the currently acceptedmodels for the formation ofmeteoritic rocks. The PLCmodel is capable
of accommodating all observational and chemical data accumulated so far on meteorites with
the exception of enstatite and SNC meteorites, which record physico-chemical conditions that
were different fromthose of themajority ofmeteoritic rocks (the processes, however, could have
been the same).
The new model identifies a new role for silicate liquids in cosmochemistry as being an essential
phase for the formation of early crystalline condensates from the hot solar nebula. Liquids are
identified to have been the first major phase to condense from the solar nebula. In order to be
capable to produce early liquid condensates, the nebula must have been either enriched in
condensable elements over solar abundances (> 500 times) or was at total pressuresmuch higher
than the canonically predicted ones (> 500 times, > 0.5 bar ). Our data require that this liquid
we named it universal liquid (UL) - had a refractory composition (Ca-Mg-Al-silicate or
CMAS) and facilitated condensation of the major minerals for chondritic constituents as well as
for achondritic rocks. The process possibly was a variant of the vapor-liquid-solid (VLS) condensation
process, which is utilized in industrial crystal growth. Thereby, the liquid condenses
first, then nucleates a crystal of the species that is oversaturated in the vapor in the case of the
solar nebula usually olivine. As this olivine grows from the liquid, it depletes the liquid in Mg
and Si. The liquid tries to maintain equilibrium with the solar nebula. Thereby, Mg and Si are
replenished by condensation from the gas phase and all incompatible elements are kept at an
equilibrium concentration by condensation-evaporation equilibrium. Thus, the contents of incompatible
refractory elements are kept at an approximately constant level throughout crystallization
of the major minerals olivine and pyroxene. This way not only the abundances of incompatible
refractory elements are kept at a constant level but also their relative abundances remain
unfractionated solar.
The UL also represents the long sought for refractory component of chondritic constituents and
appears also to be the source of achondritic igneous rocks. Variations in the amount of liquid
available, the liquid condensation and crystal nucleation rates, as well as different crystal-liquid
mixing proportion will allow the formation of objects of highly variable composition. The final
composition (chemical and isotopic) of any chondritic object or achondrite, as well as that of
the associated glasses, will be determined by different degrees of post-formational metasomatic
elemental exchange processes taking place between solids and the cooling nebular gas. These
processes add medium volatile and volatile elements to the products of high temperature
condensation. As these processes usually dont run to completion, an infinite number of chaotic
compositional variations are produced and this is exactly what we observe in meteorites.
to the currently acceptedmodels for the formation ofmeteoritic rocks. The PLCmodel is capable
of accommodating all observational and chemical data accumulated so far on meteorites with
the exception of enstatite and SNC meteorites, which record physico-chemical conditions that
were different fromthose of themajority ofmeteoritic rocks (the processes, however, could have
been the same).
The new model identifies a new role for silicate liquids in cosmochemistry as being an essential
phase for the formation of early crystalline condensates from the hot solar nebula. Liquids are
identified to have been the first major phase to condense from the solar nebula. In order to be
capable to produce early liquid condensates, the nebula must have been either enriched in
condensable elements over solar abundances (> 500 times) or was at total pressuresmuch higher
than the canonically predicted ones (> 500 times, > 0.5 bar ). Our data require that this liquid
we named it universal liquid (UL) - had a refractory composition (Ca-Mg-Al-silicate or
CMAS) and facilitated condensation of the major minerals for chondritic constituents as well as
for achondritic rocks. The process possibly was a variant of the vapor-liquid-solid (VLS) condensation
process, which is utilized in industrial crystal growth. Thereby, the liquid condenses
first, then nucleates a crystal of the species that is oversaturated in the vapor in the case of the
solar nebula usually olivine. As this olivine grows from the liquid, it depletes the liquid in Mg
and Si. The liquid tries to maintain equilibrium with the solar nebula. Thereby, Mg and Si are
replenished by condensation from the gas phase and all incompatible elements are kept at an
equilibrium concentration by condensation-evaporation equilibrium. Thus, the contents of incompatible
refractory elements are kept at an approximately constant level throughout crystallization
of the major minerals olivine and pyroxene. This way not only the abundances of incompatible
refractory elements are kept at a constant level but also their relative abundances remain
unfractionated solar.
The UL also represents the long sought for refractory component of chondritic constituents and
appears also to be the source of achondritic igneous rocks. Variations in the amount of liquid
available, the liquid condensation and crystal nucleation rates, as well as different crystal-liquid
mixing proportion will allow the formation of objects of highly variable composition. The final
composition (chemical and isotopic) of any chondritic object or achondrite, as well as that of
the associated glasses, will be determined by different degrees of post-formational metasomatic
elemental exchange processes taking place between solids and the cooling nebular gas. These
processes add medium volatile and volatile elements to the products of high temperature
condensation. As these processes usually dont run to completion, an infinite number of chaotic
compositional variations are produced and this is exactly what we observe in meteorites.
to the currently acceptedmodels for the formation ofmeteoritic rocks. The PLCmodel is capable
of accommodating all observational and chemical data accumulated so far on meteorites with
the exception of enstatite and SNC meteorites, which record physico-chemical conditions that
were different fromthose of themajority ofmeteoritic rocks (the processes, however, could have
been the same).
The new model identifies a new role for silicate liquids in cosmochemistry as being an essential
phase for the formation of early crystalline condensates from the hot solar nebula. Liquids are
identified to have been the first major phase to condense from the solar nebula. In order to be
capable to produce early liquid condensates, the nebula must have been either enriched in
condensable elements over solar abundances (> 500 times) or was at total pressuresmuch higher
than the canonically predicted ones (> 500 times, > 0.5 bar ). Our data require that this liquid
we named it universal liquid (UL) - had a refractory composition (Ca-Mg-Al-silicate or
CMAS) and facilitated condensation of the major minerals for chondritic constituents as well as
for achondritic rocks. The process possibly was a variant of the vapor-liquid-solid (VLS) condensation
process, which is utilized in industrial crystal growth. Thereby, the liquid condenses
first, then nucleates a crystal of the species that is oversaturated in the vapor in the case of the
solar nebula usually olivine. As this olivine grows from the liquid, it depletes the liquid in Mg
and Si. The liquid tries to maintain equilibrium with the solar nebula. Thereby, Mg and Si are
replenished by condensation from the gas phase and all incompatible elements are kept at an
equilibrium concentration by condensation-evaporation equilibrium. Thus, the contents of incompatible
refractory elements are kept at an approximately constant level throughout crystallization
of the major minerals olivine and pyroxene. This way not only the abundances of incompatible
refractory elements are kept at a constant level but also their relative abundances remain
unfractionated solar.
The UL also represents the long sought for refractory component of chondritic constituents and
appears also to be the source of achondritic igneous rocks. Variations in the amount of liquid
available, the liquid condensation and crystal nucleation rates, as well as different crystal-liquid
mixing proportion will allow the formation of objects of highly variable composition. The final
composition (chemical and isotopic) of any chondritic object or achondrite, as well as that of
the associated glasses, will be determined by different degrees of post-formational metasomatic
elemental exchange processes taking place between solids and the cooling nebular gas. These
processes add medium volatile and volatile elements to the products of high temperature
condensation. As these processes usually dont run to completion, an infinite number of chaotic
compositional variations are produced and this is exactly what we observe in meteorites.