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

Lee, M.R.

Lindgren, P.

Simpson, S.S.L.

English

Enlighten: Publications

 
 
 
 
 
 
 
 
Simpson, S.S.L., Lee, M.R., and Lindgren, P. (2014) Impact-generated 
hydrothermal circulation and metasomatism of the rochechouart 
astrobleme: mineralogy and major and trace element distribution. In: 45th 
Lunar and Planetary Science Conference, 17-21 March 2014, The 
Woodlands, TX USA. 
 
Copyright © 2014 The Authors 
 
 
 
 
 
http://eprints.gla.ac.uk/92042/ 
 
 
 
 
Deposited on: 27 February 2014 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Enlighten – Research publications by members of the University of Glasgow 
http://eprints.gla.ac.uk 
IMPACT-GENERATED HYDROTHERMAL CIRCULATION AND METASOMATISM OF THE 
ROCHECHOUART ASTROBLEME: MINERALOGY AND MAJOR AND TRACE ELEMENT DISTRI-
BUTION. Sarah L Simpson, Martin R. Lee and Paula Lindgren, University of Glasgow, University Ave, Glasgow, 
Lanarkshire G12 8QQ, UK; S.Simpson.1@research.gla.ac.uk, Martin.Lee@Glasgow.ac.uk, 
Lindgren.Paula@Glasgow.ac.uk 
 Introduction:    The energy released during a hyper-
velocity impact on Earth can generate high temperat-
ures 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 re-
sources such as hydrocarbons, precious metals and ore 
minerals.
    The Rochechouart impact structure:   The 
Rochechouart impact event has been dated to 201 ± 2 
Ma [2]. Its target rocks are primarily of granitic com-
position and are exposed in the Limousin region of 
south-central France. The structure has been eroded to 
the basement and little to none of the original morpho-
logical features or allochthonous units have been pre-
served. The latest research has identified the projectile 
to have been an iron meteorite [3]. The fact that this 
structure has been so eroded gives researchers a unique 
opportunity to observe the effects of hypervelocity im-
pacts on the target below the transient crater floor. It 
also can provide an insight into the deeper levels of the 
subsequently produced hydrothermal systems.
    Previous work:    Previous work has focused on de-
tailed classification of the Rochechouart target and 
autochthonous and allochthonous impactites [4, 5], 
identification of the projectile component [3], and dat-
ing the structure employing Ar-isotope techniques [2]. 
They have also noted geochemical evidence of metaso-
matism, which is pronounced throughout all lithologies 
as enrichment in K2O and depletion in CaO and Na2O 
[3, 4, 5]. This indicates a pervasive hydrothermal sys-
tem, whose effects throughout the structure have yet to 
be studied in detail, particularly in those parts below 
the transient floor.
    Study purpose and methods:    The purpose of this 
study is to classify the mineralogical and geochemical 
effects of the hydrothermal system on what remains of 
the overlying impactites as well as the basement below 
the crater floor. By classifying the resulting mineral as-
semblages and comparing them to that of the protolith, 
an attempt will be made to constrain the temperature 
and compositional conditions of the system responsible 
for the alteration, mobilization and reprecipitation of 
minerals. 
   Samples were collected directly from the Rochechou-
art impact structure via permission from the Réserve 
Naturelle de l'Astroblème de Rochechouart-Chassenon 
[6]. Sample selection was based on the presence of sec-
ondary mineralization in hand sample. They were pre-
pared for SEM/EDS and Raman imaging and analysis 
using the facilities available through the University of 
Glasgow.
    Distribution of sulphides, carbonates, oxides, 
phyllosilicates and trace elements:   Sulphides, 
sulphates and carbonates The majority of the sulph-
ides are constrained to mineral veins below the crater 
floor, and are found in a variety of basement rocks. In 
allochthonous samples, authigenic barite has been ob-
served within vesicles of melt-rich rocks. Breccia dikes 
have been found with a “cement”, or secondary intra-
clastic mineralization, composed of coarse crystalline, 
euhedral, authigenic carbonates and sulphides. Due to 
their nature and location within the structure, these 
dikes have been classified as impact-generated, and 
will be used for forthcoming sulphur and oxygen stable 
isotope analysis. Oxides Euhedral Fe and Ti oxides are 
the most abundant, and appear to have formed from the 
cooling of vapour phases within vesicles and in situ 
with the secondary phase of K-feldspar. Phyllosilicates 
Fractured granitic target rocks below the transient floor 
show evidence of sericitization, which is highly charac-
teristic of potassic alteration. Melt-rich, vesicular al-
lochthonous lithologies show signs of heavy argilliza-
tion and some vesicles have been completely occluded 
by clay mineralization. Trace elements The concentra-
tion of REEs has been observed within the primary K-
feldspar phase of the melt rocks, and accumulates 
around the periphery of vesicles. These aggregates are 
not believed to be a direct product of hydrothermal 
processes, but it is clear from the distribution pattern 
that they have concentrated within the intracrystalline 
voids left by depletion of potassium in the primary 
groundmass phase of the melt rocks. 
   Alteration texture of impact melt rocks:    The 
most pronounced type of alteration observed in melt-
rich impactites is consistent with low temperature, po-
tassium metasomatism in the form of a secondary 
phase of authigenic K-feldspar. The groundmass exhib-
its a “skeletal” texture with large intracrystalline voids, 
which appears to have been the source of alteration for 
1039.pdf45th Lunar and Planetary Science Conference (2014)
a further generation of authigenic K-feldspar that fills 
veins and vesicles (Figure 1). REE’s have accumulated 
in this void space around the periphery of vesicles. A 
similar texture has been observed within the Reis crater 
melt rocks [7]. The unusual texture and phases ob-
served in this lithology appear to be the result of a dis-
solution-recrystallization regime, but further study is 
required.  
Figure 1: Melt-rich impactite, type Babaudus, showing unusual 
“skeletal” texture of altered K-feldspar groundmass (Kfp-1), a sec-
ondary phase of euhedral K-feldspar filling vesicles (Kfp-2) and the 
accumulation of REE’s around vesicle peripheries (white). 
Figure 2: Vesicle in melt-rich impactite, type Babaudus, also dis-
playing “skeletal” texture of K-feldspar groundmass (Kfp-1) and 
secondary phase of euhedral K-feldspar (Kfp-2), with euhedral bar-
ite (white).
  Implications for system conditions:     Based on the 
distribution and types of assemblages found within the 
structure, an attempt will be made to classify the condi-
tions of the hydrothermal system. Many trace elements, 
particularly REE’s, are very useful in constraining en-
vironments of alteration and petrogenesis, as they tend 
to be highly immobile and immiscible under most con-
ditions [8]. Contamination from the projectile may also 
have influenced the geochemistry of alteration as-
semblages, which will also be a component of this 
study. 
     Implications for the search for extra-terrestrial 
life:    Hypervelocity impacts have long been of in-
terest in the search for extraterrestrial life, as the condi-
tions they produce are associated with the release of 
heat and water, whether held in the crystalline structure 
of target rocks or frozen close in proximity to the sur-
face. While the amount of energy released is dependent 
on size and velocity of the projectile, even smaller 
complex craters (>2-4km in diameter on Earth, >5-
10km on Mars [1]) are capable of initiating the warm, 
wet environments favourable to the formation and sup-
port of life [1, 9]. Hydrothermal systems in particular 
are believed to be “cradles of life” here on Earth, and 
possibly on other terrestrial planets, as some of the 
most seemingly minimalist organisms are now known 
to thrive in the hot, chemically extreme environments 
produced therein [1, 10].
    Acknowledgements:   Special thanks to the 
Réserve   Naturelle de l'Astroblème de   Rochechouart  - 
Chassenon in France for their permission to collect 
samples, and also to Philippe Lambert for his guidance 
and advice throughout this project.
    References:  [1] Osinski, G. R. and Tornabene, L. 
L. (2013) Icarus 224, 347–363 [2] Schmeider, M. et al. 
(2010) Meteoritics & Planetary Science 45, Nr 8, 
1225–1242 [3] Tagle, R. and Schmitt, R. T., (2009) 
Geochimica et Cosmochimica Acta 73, 4891–4906  [4] 
Lambert, P., (1977) Earth and Planetary Science Let-
ters, 3, 258-268 [5] Lambert, P., (2010) The Geologic-
al Society of America Special Paper 465 [6] 
Réserve   Naturelle de l'Astroblème de   Rochechouart  - 
Chassenon, CCPM Mairie-Place-du-Chateau, 87600, 
Rochechouart, France  [7] Osinski, G. R., (2004) 
Earth and Planetary Science Letters 226, 529 – 543 
[8] Parsapoor, A., (2009) Journal of Asian Earth Sci-
ences 34, 123–134 [9] Schwenzer, S.P. et al., (2012) 
Earth and Planetary Science Letters 335–336, 9–17 
[10] Stetter, K. O., (2009) FEBS Letters, Volume 452, 
Issues 1–2, 22–25
1039.pdf45th Lunar and Planetary Science Conference (2014)


Impact-generated hydrothermal circulation and metasomatism of the rochechouart astrobleme: mineralogy and major and trace element distribution

http://eprints.gla.ac.uk/92042/1/92042.pdf

Impact-generated hydrothermal circulation and metasomatism of the rochechouart astrobleme: mineralogy and major and trace element distribution

Abstract

Similar works

Full text

Available Versions

Enlighten: Publications