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Tracking the Earliest Stages of Aqueous Alteration in the Mildly Altered CM Chondrite EET 96029
The earliest stages of CM carbonaceous
chondrite aqueous alteration are very poorly
understood as mildly altered CMs are extremely rare.
The Paris meteorite (CM2.7) [1-3] and QUE 97990
(CM2.6) [4,5] are among the least aqueously altered
CMs described to date. However, neither of them contain
the pristine attribute of chondrule mesostasis glass.
Glass is highly reactive and so among the very first
phases to undergo aqueous alteration [6]. Therefore,
the CM carbonaceous chondrite EET 96029 is very
unusual as it has been shown to have retained
mesostasis glass in at least one chondrule [7]. According
to the new CM classification scheme of [8], which
is based on H content, EET 96029 has an index of 2.0
(data in [9]), meaning that it is less altered than all but
one of the fifty CMs analysed by [8]. A caveat is that a
low H content could be due to mild heating as well as a
low degree of aqueous processing [9]. However, the
bulk O oxygen isotope composition of EET 96029 (as
determined by [10]) is consistent with a low degree of
alteration as it is slightly closer to that of the CO3 falls
(possible representatives of the anhydrous progenitors
of the CMs) than even the least altered lithology of
Paris (Fig. 1).<p></p>
To better understand the earliest stages of CM
aqueous alteration and its impact on mesostasis glass,
we have undertaken a detailed study of chondrule
mesostasis textures and compositions in the mildly
aqueously altered CM chondrite EET 96029.<p></p>
Glassy chondrule mesostasis in EET 96029: a CM3 component of a minimally altered CM2 carbonaceous chondrite
No abstract available
Impact-generated hydrothermal circulation and metasomatism of the rochechouart astrobleme: mineralogy and major and trace element distribution
The energy released during a hypervelocity
impact on Earth can generate high temperatures
in the target rock. There are currently 170 known
impact structures worldwide, of which over one-third
contain fossil hydrothermal systems [1]. Results from
the analysis of these hydrothermal systems have many
implications for the study of the origin of life on Earth
and potential thereof on Mars. Hypervelocity impacts
are also of particular economic interest as they may
produce, expose or concentrate high commodity resources
such as hydrocarbons, precious metals and ore
minerals
Impact fracturing and aqueous alteration of the CM carbonaceous chondrites
Aqueous alteration of the CM carbonaceous
chondrites has produced a suite of secondary
minerals, and differences between meteorites in
their abundance defines a progressive alteration sequence
[e.g. 1, 2]. The means by which this water
gained access to the original anhydrous constituents of
the meteorites is the subject of considerable debate.
Studies of rock texture, mineralogy and bulk chemical
composition have concluded that solutions were generated
by the melting of water ice in situ, and remained
essentially static as a consequence very low intergranular
permeabilities [e.g. 3, 4]. By contrast, results of
oxygen isotope work and modelling have suggested
that the fluids moved considerable distances within the
parent body [5, 6]. Given the intergranular permeability
of the CMs, an extensive fracture network would be
required to support such flow.
Clues to how the two very different models for
aqueous alteration of the CMs can be reconciled have
been recently provided by Rubin [7]. He recognised a
good correlation between the magnitude of impact-induced
compaction of CM meteorites and their degree
of aqueous processing, with the more highly deformed
meteorites being more altered. Here we have asked
whether compaction was accompanied by the development
of fracture networks that could have provided the
conduits for aqueous solutions that mediated all or
some of the alteration
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