DO POST-MAGMATIC CLAYS EXIST?

1. DO POST-MAGMATIC CLAYS EXIST? Alain MEUNIER, Antoine MAS, Daniel BEAUFORT, Patricia PATRIER & Patrick DUDOIGNON clays HydrASA - University of Poitiers - INSU CNRS Lafayette Mars meteorite EETA 79001 Catling (2007) Nature, 448, 31-32. 20 µm

2. NONTRONITE ON MARS ’SURFACE Phyllosilicates on Mars – implication on early climate. Poulet et al. (2005) Nature, 438, 623-627; Nontronite as indicator of alteration by liquid water Chevrier et al. (2007) Nature, 448, 60-63.

3. Is the first term of the following equation realistic or too simplistic : clays = alteration = presence of liquid water = life? In other words: do nontronite in basaltic rocks systematically result from an alteration process? TO ANSWER THAT QUESTION NEEDS TO STUDY TERRESTRIAL BASALTIC ANALOGUES. Microtextures can be used as signatures of the origin of clays. The goal of the present study is to compare the clay microtextures which undoubtedly result from an alteration process with those forming the mesostasis in the massive inner parts(diktytaxitic voids).

4. WHAT IS TYPICALLY A GLASS ALTERATION TEXTURE? Drief A. & Schiffman P. (2004). Clays Clay Mineral, 52, 622-634. Crystallization of clay minerals at the fluid-glass interface (large particles – geometrical selection). INTERFACE-TYPE ALTERATION Crystallization of clay minerals in the altered glass (small particles – randomly distributed) INNER-TYPE ALTERATION

5. THE MURUROA ATOLL (FRENCH POLYNESIA): a potential terrestrial analogue The volcanic rock basalt-hawaiite series from Mururoa atoll give an opportunity to compare the origin of Fe-rich clay minerals (nontronite-like) in different geological environments: 1 - alteration zones of basaltic glass (chilled margins of volcanic bodies) 2 - mesostasis (diktytaxitic voids) of the massive inner parts • 4 volcanic bodies: • aerial flow, • subaerial flow, • submarine flow, • dyke.

6. METHOD FOR CLAY MINERALOGY air dried glycol °2 Cu K °2 Cu K °2 Cu K °2 Cu K heated 300°C °2 Cu K °2 Cu K °2 Cu K IR spectroscopy X-ray diffraction Electron microprobe analyses

7. ALTERATION MICROTEXTURE – 1 – the zoning SUBMARINE FLOW altered olivine zeolites clays palagonite 20µm unaltered glass Alteration occurs along fractures in the massive glass of the chilled margins SUBAERIAL FLOW zeolites palag. unalt. glass 20 µm clays

8. ALTERATION MICROTEXTURE – 2 – the retreating glass surface GLASSY MARGIN OF THE SUBMARINE FLOW altered glass Ti oxides Fe-rich clays Fe-Mg clays altered glass zeolites 10 µm Ti oxides Sap80/Chl20 + No55/Chl45 + Chl 10 µm

9. ALTERATION MICROTEXTURE – 3 –organization of the clay particles pla pla zeolites clays alt. glass alt. glass glass plagioclase 10 µm Mg-rich clays Fe-rich clays plagioclase 10 µm plagioclase GLASSY MARGIN SUBAERIAL FLOW Saponite + nontronite

10. THE FORMATION OF CLAYS BY GLASS ALTERATION a possible model fracture volume dissolved volume first interface final interface Initial fluid/glass interface Mg-clays grow outwards in the free space Fe-clays grow inwards follwing the retreating surface Ti-oxides accumulate on the dissolution surface of the glass

11. MESOSTASIS AND DIKTYTAXITIC VOIDS 20µm px px px mesostasis pl op ap 10 µm Example: the subaerial lava flow (Mururoa) mesostasis

12. MESOSTASIS - AERIAL LAVA FLOW (HAWAIITE) 1 – clay microtexture Surprisingly, clays grow on unaltered biotite surfaces. To what process this zoning is related? alt. ol pla pla mes bio cpx mes cpx K-fels biotite 20µm Ce + Mg-clays alt. ol pla Sap55/Chl45 + Sap22/Chl78 + Ce K-fels 10 µm

13. MESOSTASIS - AERIAL LAVA FLOW (HAWAIITE) 2 – the geometrical selection (Grigor’ev D. P., 1965) celadonite + Mg-clays K-felds 2nd step: center-oriented crystal growth large particles 1st step: non-oriented crystal growth small particles 10 µm The geometrical selection pattern is typical of crystal growth in free space not of glass inner alteration-type. Why? Possibly, no glass precursor, that is to say, no alteration.

14. MESOSTASIS - SUBAERIAL LAVA FLOW (HAWAIITE) clay microtexture No alteration features on the clay-apatite or clay-K-feldspar contacts. 1- Apatite and K-feldpar co-precipitation in free space; 2- clay nucleation and growth on all the crystal surfaces. Sap + Sap35/Chl65 + Cel K-fels clays ap Palissadic texture Possibly, no glass precursor 10 µm

15. MESOSTASIS - SUBMARINE LAVA FLOW (BASALT) microtexture of clay deposits px px pla clays clays 10 µm Sap70/Chl30 + No30/Chl70 + Cel No alteration features on the clay-pyroxene or clay-plagioclase contacts. co-precipitation in free space of pyroxene and clays (Decarreau et al., 2004). The presence of a glass precursor is not discarded but chemically unprobable.

16. MESOSTASIS – DYKE (HAWAIITE) 1 – microgeoda and diktytaxitic voids clays ap 10 µm cpx No(2Gl)35/No(1Gl)65 + Ce67/No33 + Sap diktytaxitic voids microgeoda felds-K cpx pl ap 1 - The diktytaxitic voids are partially filled by the clay-rich mesostasis. Their central part remains empty. 2 – The thickness of the clay coating varies with the size of the diktytaxitic void. clays 10 µm

17. MESOSTASIS – DYKE (HAWAIITE) 2 – microtexture of clay deposits in microgeoda no glass precursor but a boiling residual solution. 10 µm clays apatite 10 µm

18. CONCLUSION 1 - no glass precursor 2 - no alteration CLAYS PRECIPITATED DURING THE POST-MAGMATIC STAGE DIRECTLY FROM THE RESIDUAL FLUIDS TRAPPED IN THE DIKTYTAXITIC VOIDS. CONSEQUENTLY,LIQUID WATER DID NOT FLOW INSIDE BUT ESCAPE OUTSIDE THE LAVAS AS VAPOR.

19. SO WHAT? … clays Because the presence of nontronite does not mean that water has circulated inside the basaltic rocks , it may be better to chose another criterium for the future Mars landing sites. It should be safer to select areas where other clays are detected (kaolinite for instance). To that point, it is even more important to study terrestrial analogues. Crédit : NASA/JPL/JHUAPL/University of Arizona/Brown University.