Travertine mounds form at the mouth of springs where CO 2 degassing drives carbonate precipitation from water fl owing from depth. Building of such mounds commonly involves the successive "stratigraphic" deposition of carbonate layers that precipitate from waters rising from depth along vertical to horizontal open fi ssures that are episodically sealed by radiating crystals. Much more intriguing structures can also be observed, such as widespread horizontal white veins of carbonate with vertical aragonite fi bers, parallel or oblique to the "stratigraphic" travertines, which extend laterally over distances of several tens of meters and could represent up to 50% of the total volume of the travertine mound. Using highly precise U-Th dating, the growth direction of these horizontal veins is shown to be from top to bottom, and this fact clearly indicates that they developed within the mound over a period of ~1000 yr for the vein analyzed. A vein growth mechanism is proposed that is able to uplift the rock above the vein thanks to the force of crystallization. The possible development of such structures in other places and the consequences of reverse growth direction when interpreting travertine data are discussed.

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The subduction of seamounts and ridge features at convergent plate boundaries plays an important role in the deformation of the overriding plate and influences geochemical cycling and associated biological processes. Active serpentinization of forearc mantle and serpentinite mud volcanism on the Mariana forearc (between the trench and active volcanic arc) provides windows on subduction processes.  Here, we present (1) the first observation of an extensive exposure of an undeformed Cretaceous seamount currently being subducted at the Mariana Trench inner slope; (2) vertical deformation of the forearc region related to subduction of Pacific Plate seamounts and thickened crust; (3) recovered Ocean Drilling Program and International Ocean Discovery Program cores of serpentinite mudflows that confirm exhumation of various Pacific Plate lithologies, including subducted reef limestone; (4) petrologic, geochemical and paleontological data from the cores that show that Pacific Plate seamount exhumation covers greater spatial and temporal extents; (5) the inference that microbial communities associated with serpentinite mud volcanism may also be exhumed from the subducted plate seafloor and/or seamounts; and (6) the implications for effects of these processes with regard to evolution of life. This article is part of a discussion meeting issue ‘Serpentine in the Earth system’.

International audienceActive and fossil endogenic travertine mounts scattered along the Little Grand Wash fault are studied asrecords of Quaternary CO2-enriched fluid leakage. This study focusses on a particular area where a fossilmount formed in a near-surface setting by successive circulation/sealing episodes from Late Pleistoceneto Mid-Holocene and where a modern surface travertine is still being formed by a CO2-enriched fluidsource. The fossil mount is composed of horizontal and vertical veins whereby the vertical veins recordednumerous cycles of circulation/sealing/dissolution events and were used as conduits for the CO2-enriched fluid circulation from the depth to the surface or along sub-horizontal fractures where successiveprecipitation events are recorded. The modern travertine is being built at the surface by successiveeruption of Crystal Geyser, an anthropic geyser active since the 1930's.d13C and d18O signatures and U/Th datings, ranging from 11.5 ky till present-day allows calibrating indetail the CO2 enriched fluid leakage along a single fault segment and in a post glacial context, as lastglaciations in the study area took place 15 ky ago. The dataset shows a high decrease of the oxygen stableisotope values till about 6 ky, then the variations reflect a constant range until present-day. This tends torestrain the period of local increase of the meteoric water input in the aquifer that is sourcing the CO2-enriched water.The fossil travertine represents a 7 ky-long record of CO2 leakage above a natural reservoir, from LatePleistocene to Mid-Holocene. The flux of CO2 leakage through time and the total escaping volume havebeen computed and appears to be low in comparison with an anthropogenic leak provoked, for instance,by a non-sealed well

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