Le mécanisme de la précipitation du Co3 Ca et la Détermination de L'épaisseur du Dépôt annuel de la vase dans le lac de Neuchâtel

Sans résumé     

Geochemical fingerprint of hyaloclasts in glassy fragmental rocks of Macquarie Island (Southern Ocean): implications for volcanogenic sedimentary processes at a waning mid-ocean ridge

Two main types of glassy fragmental rocks formed along the Proto-Macquarie Spreading Ridge: (i) hyaloclastite breccia; and (ii) pillow-fragment breccia. Examples now exposed on Macquarie Island, Southern Ocean, were largely sourced from proximal pillow lavas. In each of seven samples examined, hyaloclasts (basaltic glass grains) have a narrow major- and trace-element geochemical range, consistent with derivation of each sample from a single volcanic eruption event. Moreover, every sample analysed within the one stratigraphic section (at three sites) displays distinctive major- and trace-element geochemistry compared with the other two sections. This suggests that hyaloclasts at each site represent discrete magma batches. A single source for these glassy fragmental rocks contrasts with the dominant fault-scarp-derived polymict sedimentary rocks on Macquarie Island. We suggest that the hyaloclasts analysed in this study were deposited in small basins between the slopes of growing pillow cones along the mid-ocean ridge. The geochemical analyses presented here encompass (weakly) fractionated (e.g. Bauer Bay) to near-primitive (e.g. Pyramid Peak) compositions. All samples presented here lie within the range of the enriched- to normal-MORB suites previously reported for the island that include the least fractionated MORB melts known globally. The interpretation of geochemically distinct magma batches over the small area of the island suggests very limited magma mixing consistent with an immature or waning magmatic system. We relate these geochemical characteristics to: (i) volcanism near a very long offset transform; and (ii) genesis of magmas during the waning stages of slow seafloor spreading within a very narrow (<50 km-wide) spreading corridor.     

La formation du sédiment calcaire du lac de Neuchâtel

Aquatic Sciences - The present paper studies the calcareous sediments of lake Neuchâtel. The thickness of the annual deposit of mud has been estimated by various methods (pp. 206 to 224)....     

Vitriclastic lithofacies from Macquarie Island (Southern Ocean): compositional influence on abyssal eruption explosivity in a dying Miocene spreading ridge

Macquarie Island is composed of a complete section of oceanic crust that formed in a slow-spreading mid-ocean ridge 2.0 to 3.5 km below sea level. Vitriclastic facies preserved on the island have both pyroclastic and hyaloclastic characteristics. Monomict hyaloclastic breccia facies are widespread across the island and are predominantly composed of near-primitive (~7.9 wt% MgO) subalkaline/transitional (~0.7 wt% K₂O) sideromelane shards and crystalline basalt clasts with low vesicularity (LV, < 15% vesicles). Breccias are thick bedded and structureless with matrix-supported angular pillow fragments, bomb-sized fluidal mini-pillows, and globular glass lapilli. Clasts are lithologically similar to interbedded pillow basalts and laterally grade into fine-grained sandstone facies. These sandstones are normal-graded, well-laminated, thin bedded, and interstratified with red pelagic mudstone. Lithofacies associations indicate that the hyaloclastic breccias were formed proximal to a source vent via quench-fragmentation, and subsequently reworked by ocean-bottom currents into distal epiclastic sandstone facies. During eruption, co-genetic pillow lava and hypabyssal intrusions mingled with the breccia, forming fluidal peperite. Rare polymict pyroclastic facies only occur in the highest stratigraphic levels and are mostly composed of highly vesicular (HV, 15–50% vesicles) sideromelane shards and crystalline basalt clasts with alkaline (~1.0 wt% K₂O) fractionated (~6.8% MgO) compositions. Minor lithic grains are composed of subalkaline (~0.7 wt% K₂O) to very highly alkaline (~1.7 wt% K₂O) LV sideromelane shards, and amphibole-bearing diabase. The pyroclastic facies contains medium to thick beds of lapilli-tuff that exhibit both reverse and normal grading, diffuse lamination, and planar-grain fabric. These beds are locally overlain by thin fine-grained tuff beds entirely composed of cuspate to very thin elongate bubble-wall shards. These characteristics indicate that explosive deep-marine eruptions produced high-density coarse-grained gravity flows that were covered by slower suspension settle-out of delicate bubble-wall shards. Stratigraphic relationships suggest that explosive eruptions started during the waning stages of more alkaline volcanism along the proto-Macquarie spreading center.