Clay Minerals in Basalt-Hawaiite Rocks From Mururoa Atoll (French Polynesia). II. Petrography and Geochemistry

Abstract

The clay minerals formed in chilled margins and massive crystallized inner parts of three basalt-hawaiite bodies of Mururoa Atoll (French Polynesia) exhibit contrasting textures. Glass alteration textures are observed around fractures crosscutting the quenched margins: Fe-rich clays grow inward into the glass (retreating surface) while Mg-rich clays grow outward (open space). The textures of clay deposits filling the diktytaxitic voids (mesostasis) in the massive inner parts of the three volcanic bodies are different: unoriented clay matrix with embedded euhedral apatite and pyroxene microcrysts (submarine flow); pallisadic clays coating the void walls and the crystal surfaces of apatite and K-feldspar microcrysts (subaerial flow); and clay muffs covering all the apatite needles, with the central part of the void remaining empty (dike). The unoriented texture could result from the alteration of a glass precursor concomitant with the olivine phenocrysts (clay pseudomorphs). However, such an alteration implies important chemical transfers which are not observed. The pallisadic and muff textures form through heterogeneous nucleation on the solid surfaces and crystal growth from a saline solution. No glass precursor existed. As the center of the diktytaxitic voids in the dike is empty, the residual liquid was probably boiling. The amounts of light rare earth elements (LREE), Sr, and the most incompatible elements are greater in clays from diktytaxitic voids relative to the amounts formed in the altered glass of the chilled margins. Thus, diktytaxitic clays formed from a residual liquid which gave either an evolved glass or a saline solution after cooling (fractionation process). The δ18O variation vs. loss on ignition (LOI) indicates that sea water was involved either in rock alteration or magma contamination. This is confirmed by the 87Rb/86Sr ratio of bulk rocks and clay fractions from the quenched and massive inner parts of the three volcanic bodies which do not fit with the 11.5 Ma isochron indicating that the Rb-Sr system was not closed at any stage during the magmatic history.

Modeling of continental weathering under high-CO2atmospheres during Precambrian times

“…We can raise the question of how this clay fraction could contribute significantly to the weathering budget, even when it represents only at few percent of the whole rock. First, this clay fraction was probably formed by volcanic gas‐rock interactions just after solidifcation of the lava flow [ Meunier et al , 2008, and reference therein], and thus did not result from a weathering process (i.e., did not consume atmospheric CO 2 ). Second, even considering subsequent weathering, clay minerals are more stable than mafic minerals at the Earth's surface.…”

Rivers in transition

“…1a). The nucleation of mesostasis clays was extremely rapid and triggered by boiling of an over-saturated residual fluid 28 . 1b).…”

“…The clay particles cover all the unaltered surfaces of biotite or feldspar phenocrysts forming a palissade texture (Fig. The source of water in the basaltic magmas from Mururoa Atoll is either purely magmatic 30 or may have included contaminating sea water 28 . The mesostasis (saponite-chlorite mixed layer minerals; Supplementary Fig.…”

The Fe-Rich Clay Microsystems in Basalt-Komatiite Lavas: Importance of Fe-Smectites for Pre-Biotic Molecule Catalysis During the Hadean Eon

“…2d). It is to be noticed that these small-size mineral assemblages constituting the mesostasis are not formed through the alteration of a glassy precursor Meunier et al 2008). The proportion of mesostasis varies from 10 to 20 % (surface area % in thin sections) according to the type of volcanic body: dyke or submarine-subaerial lava flows respectively.…”