Alonso-Azcárate, J.

Benito-Moreno, M.I.

Lee, M.R.

Moyano-Cambero, C.E.

Trigo-Rodríguez, J.M.

English

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ANCIENT MARTIAN FLOODS IN A PLAUSIBLE VARIABLE CLIMATIC ENVIRONMENT AS 
REVEALED FROM THE SEQUENTIAL GROWTH OF ALLAN HILLS 84001 CARBONATE 
GLOBULES.   J.M. Trigo-Rodríguez1,2, C.E. Moyano-Cambero1,2, M.I. Benito-Moreno3, J. Alonso-Azcárate4, and 
M. R. Lee5. 1Institute of Space Sciences (ICE, CSIC), Campus UAB, c/Can Magrans s/n. 08193 Cerdanyola del Val-
lés, Barcelona, Catalonia, Spain, e-mail:      trigo@ice.csic.es 2Institut d’Estudis Espacials de Catalunya (IEEC), C/ 
Gran Capità, 2-4, Ed. Nexus, desp. 201, 08034 Barcelona, Catalonia, Spain,   3Departamento de Estratigrafía-IGEO, 
Facultad de Ciencias Geológicas, Universidad Com-plutense de Madrid-CSIC, José Antonio Nováis, 12, 28040 Ma-
drid, Spain, 4Universidad de Castilla-La Mancha (UCLM) Campus Fábrica de Armas, 45071 Toledo, Spain, 5School 
of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow G12 
8QQ, UK. 
 
Introduction: Until samples are returned from 
Mars, the achondrite meteorites are the only Martian 
rocks available for study in our laboratories. They pro-
vide valuable information about current and ancient 
environmental conditions on Mars because they have 
different crystallization ages and relevant information 
about their formation regions on the red planet [1]. 
Here we focus in the Allan Hills 84001 meteorite 
(hereafter ALH 84001) that can be used for constrain-
ing conditions on early Mars because it formed more 
than 4 Gyr ago. Due to its age and long exposure to the 
Martian environment, ALH 84001 has unique features 
capable of recording early processes: a highly fractured 
texture, gases trapped during the ejection event or dur-
ing formation of the rock, and the presence of spherical 
Fe-Mg-Ca carbonates. Here we describe the impor-
tance of carbonates within the ALH 84001,82 thin sec-
tion (Fig. 1) [2]. 
 
 
Figure 1. High-resolution mosaic of the ALH 
84011,82 section in transmitted light and crossed nich-
ols. A grid to identify the regions of interest (ROIs) is 
superimposed. 
 
Instrumental procedure: As the focus of this 
study is the carbonates in the thin section (Fig. 2), we 
have used several SEM and microprobe techniques to 
determine the chemical composition of the carbonate 
globules. In particular, quantitative chemical analyses 
and BSE images of the carbonate globule walls were 
obtained using a JEOL JXA-8900 electron microprobe 
equipped with five wavelength-dispersive spectrome-
ters at the UCM. 
 
 
 
Figure 2. Cathodoluminescence image obtained of 
the D5 ROI in Fig. 1 (note the curved crack for further 
identification) using an accelerating voltage of 20-24 
keV. As Fe is a typical quencher, the right red areas 
correspond to Fe-poor and Mn-rich carbonate layers 
that nicely delineate the carbonate globule walls. 
 
Results: The carbonate globule chemical data, pre-
viously presented in [2], are clear evidence for a se-
quential growth of the carbonate. Such progressive 
growth could be explained by the precipitation of the 
ALH 84001 carbonates after (or during) at least 
two/three different events while the rock was forming 
part of the ancient Mars’ crust about 4 Gyr ago. In-
deed, the two main fracture systems of the analysed 
thin section and the presence of fracturing and corro-
sion predating precipitation of some layers [2] point 
towards two or even three different formation episodes 
with clearly distinct bulk chemistry. For the sake of 
1448.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)
 brevity, such overall results could be exemplified in 
one of the studied carbonate globules (see Fig. 3). 
 
 
Figure 3. BSE and schematic images showing lay-
ers #3 to #7 in one carbonate globule in D5, plus the 
Fe, Ca, Mn, and Mg microprobe line profiles from one 
side to the other.   
Discussion: We notice a sharp change in composi-
tion that suggests a change in solution chemistry and 
saturation state (e.g., the concentration in CO2 or pH) 
between formation of the inner (#1 to #4) and outer (#5 
to #7) layers. The microprobe data thus reveal two 
trends that are consistent with the idea of at least two 
episodes of precipitation to form the carbonate glob-
ules. The distribution coefficients for Fe and Mn into 
carbonates are higher than 1, meaning that any Fe2+ or 
Mn2+ in the fluid will preferentially partition into the 
carbonate [3]. Assuming that the system was closed so 
that there was no continuous influx of fluid, Fe and Mn 
would be depleted relative to Mg, and this process may 
be responsible for creating the geochemical patterns 
observed in the layers. The studied carbonates display 
two compositional patterns (at layers #1 to #4 and 
again at #5 to #7), which implies that a second carbon-
ate formed as an overgrowth on the first generation. 
Indeed, the contrasting trends imply that the fluids re-
sponsible for carbonate precipitation differed in com-
position, possibly pointing to distinct aqueous altera-
tion events, which could be related with the already 
mentioned necessity of at least two episodes of fractur-
ing. Our previous globule growth interpretation is also 
in accordance with experimental data [4]. 
 
Conclusions: The petrographic features and com-
positional properties of these carbonates indicate that a 
Mg- and Fe-rich solution saturated the rock, leading to 
their precipitation in at least two different episodes. 
This is supported by the presence of distinct chemical 
trends indicating that these carbonates grew in two or 
more stages. In conclusion, 1) we have found clear 
evidence of carbonate globule formation when a fluid 
soaked the host rock of this meteorite, with chemical 
variations probably associated with atmospheric 
changes and volcanic outgassing. 2) The presence of 
these carbonates suggests that secondary minerals pro-
duced by aqueous alteration should be more common 
in the oldest Martian terrains, as suggested by recent 
remote-sensing studies and their study can provide 
valuable clues on the long-standing action of water on 
the Martian environment. 
 
Acknowledgements: We acknowledge support 
from the Spanish Ministry of Science and Innovation 
(project AYA 2015-67175-P). 
 
References: [1] Moyano-Cambero C.E. et al. 
(2013), In The Early Evolution of the Atmospheres of 
Terrestrial Planets, J.M. Trigo-Rodríguez, F. Raulin, 
C. Muller and C. Nixon (eds.), Springer, New York, 
165–172, 2013. [2] Moyano-Cambero C.E. et al., 
(2017) MAPS 52, 1030-1047. [3] Rimstidt J. D., et al. 
(1998) GCA 62:1851-1863. [4] Golden D. C., et al. 
(2001) American Mineralogist 86:370-375. 
1448.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)


Ancient Martian Floods in a Plausible Variable Climatic Environment as Revealed from the Sequential Growth of Allan Hills 84001 Carbonate Globules

Enlighten

ANCIENT MARTIAN FLOODS IN A PLAUSIBLE VARIABLE CLIMATIC ENVIRONMENT AS REVEALED FROM THE SEQUENTIAL GROWTH OF ALLAN HILLS 84001 CARBONATE GLOBULES.   J.M. Trigo-Rodríguez1,2, C.E. Moyano-Cambero1,2, M.I. Benito-Moreno3, J. Alonso-Azcárate4, and M. R. Lee5. 1Institute of Space Sciences (ICE, CSIC), Campus UAB, c/Can Magrans s/n. 08193 Cerdanyola del Val-lés, Barcelona, Catalonia, Spain, e-mail:      trigo@ice.csic.es 2Institut d’Estudis Espacials de Catalunya (IEEC), C/ Gran Capità, 2-4, Ed. Nexus, desp. 201, 08034 Barcelona, Catalonia, Spain,   3Departamento de Estratigrafía-IGEO, Facultad de Ciencias Geológicas, Universidad Com-plutense de Madrid-CSIC, José Antonio Nováis, 12, 28040 Ma-drid, Spain, 4Universidad de Castilla-La Mancha (UCLM) Campus Fábrica de Armas, 45071 Toledo, Spain, 5School of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow G12 8QQ, UK.  Introduction: Until samples are returned from Mars, the achondrite meteorites are the only Martian rocks available for study in our laboratories. They pro-vide valuable information about current and ancient environmental conditions on Mars because they have different crystallization ages and relevant information about their formation regions on the red planet [1]. Here we focus in the Allan Hills 84001 meteorite (hereafter ALH 84001) that can be used for constrain-ing conditions on early Mars because it formed more than 4 Gyr ago. Due to its age and long exposure to the Martian environment, ALH 84001 has unique features capable of recording early processes: a highly fractured texture, gases trapped during the ejection event or dur-ing formation of the rock, and the presence of spherical Fe-Mg-Ca carbonates. Here we describe the impor-tance of carbonates within the ALH 84001,82 thin sec-tion (Fig. 1) [2].   Figure 1. High-resolution mosaic of the ALH 84011,82 section in transmitted light and crossed nich-ols. A grid to identify the regions of interest (ROIs) is superimposed.  Instrumental procedure: As the focus of this study is the carbonates in the thin section (Fig. 2), we have used several SEM and microprobe techniques to determine the chemical composition of the carbonate globules. In particular, quantitative chemical analyses and BSE images of the carbonate globule walls were obtained using a JEOL JXA-8900 electron microprobe equipped with five wavelength-dispersive spectrome-ters at the UCM.    Figure 2. Cathodoluminescence image obtained of the D5 ROI in Fig. 1 (note the curved crack for further identification) using an accelerating voltage of 20-24 keV. As Fe is a typical quencher, the right red areas correspond to Fe-poor and Mn-rich carbonate layers that nicely delineate the carbonate globule walls.  Results: The carbonate globule chemical data, pre-viously presented in [2], are clear evidence for a se-quential growth of the carbonate. Such progressive growth could be explained by the precipitation of the ALH 84001 carbonates after (or during) at least two/three different events while the rock was forming part of the ancient Mars’ crust about 4 Gyr ago. In-deed, the two main fracture systems of the analysed thin section and the presence of fracturing and corro-sion predating precipitation of some layers [2] point towards two or even three different formation episodes with clearly distinct bulk chemistry. For the sake of 1448.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) brevity, such overall results could be exemplified in one of the studied carbonate globules (see Fig. 3).   Figure 3. BSE and schematic images showing lay-ers #3 to #7 in one carbonate globule in D5, plus the Fe, Ca, Mn, and Mg microprobe line profiles from one side to the other.   Discussion: We notice a sharp change in composi-tion that suggests a change in solution chemistry and saturation state (e.g., the concentration in CO2 or pH) between formation of the inner (#1 to #4) and outer (#5 to #7) layers. The microprobe data thus reveal two trends that are consistent with the idea of at least two episodes of precipitation to form the carbonate glob-ules. The distribution coefficients for Fe and Mn into carbonates are higher than 1, meaning that any Fe2+ or Mn2+ in the fluid will preferentially partition into the carbonate [3]. Assuming that the system was closed so that there was no continuous influx of fluid, Fe and Mn would be depleted relative to Mg, and this process may be responsible for creating the geochemical patterns observed in the layers. The studied carbonates display two compositional patterns (at layers #1 to #4 and again at #5 to #7), which implies that a second carbon-ate formed as an overgrowth on the first generation. Indeed, the contrasting trends imply that the fluids re-sponsible for carbonate precipitation differed in com-position, possibly pointing to distinct aqueous altera-tion events, which could be related with the already mentioned necessity of at least two episodes of fractur-ing. Our previous globule growth interpretation is also in accordance with experimental data [4].  Conclusions: The petrographic features and com-positional properties of these carbonates indicate that a Mg- and Fe-rich solution saturated the rock, leading to their precipitation in at least two different episodes. This is supported by the presence of distinct chemical trends indicating that these carbonates grew in two or more stages. In conclusion, 1) we have found clear evidence of carbonate globule formation when a fluid soaked the host rock of this meteorite, with chemical variations probably associated with atmospheric changes and volcanic outgassing. 2) The presence of these carbonates suggests that secondary minerals pro-duced by aqueous alteration should be more common in the oldest Martian terrains, as suggested by recent remote-sensing studies and their study can provide valuable clues on the long-standing action of water on the Martian environment.  Acknowledgements: We acknowledge support from the Spanish Ministry of Science and Innovation (project AYA 2015-67175-P).  References: [1] Moyano-Cambero C.E. et al. (2013), In The Early Evolution of the Atmospheres of Terrestrial Planets, J.M. Trigo-Rodríguez, F. Raulin, C. Muller and C. Nixon (eds.), Springer, New York, 165–172, 2013. [2] Moyano-Cambero C.E. et al., (2017) MAPS 52, 1030-1047. [3] Rimstidt J. D., et al. (1998) GCA 62:1851-1863. [4] Golden D. C., et al. (2001) American Mineralogist 86:370-375. 1448.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)

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Ancient Martian Floods in a Plausible Variable Climatic Environment as Revealed from the Sequential Growth of Allan Hills 84001 Carbonate Globules

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