Bonatti Enrico

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Last Name
Bonatti
First Name
Enrico
ORCID
0000-0001-6162-9572

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  • Article
    Nonvolcanic tectonic islands in ancient and modern oceans
    (John Wiley & Sons, 2013-10-24) Palmiotto, Camilla ; Corda, Laura ; Ligi, Marco ; Cipriani, Anna ; Dick, Henry J. B. ; Douville, Eric ; Gasperini, Luca ; Montagna, Paolo ; Thil, Francois ; Borsetti, Anna Maria ; Balestra, Barbara ; Bonatti, Enrico
    Most oceanic islands are due to excess volcanism caused by thermal and/or compositional mantle melting anomalies. We call attention here to another class of oceanic islands, due not to volcanism but to vertical motions of blocks of oceanic lithosphere related to transform tectonics. Sunken tectonic islands capped by carbonate platforms have been previously identified along the Vema and Romanche transforms in the equatorial Atlantic. We reprocessed seismic reflection lines, did new facies analyses and 87Sr/86Sr dating of carbonate samples from the carbonate platforms. A 50 km long narrow paleoisland flanking the Vema transform, underwent subsidence, erosion, and truncation at sea level; it was then capped by a 500 m thick carbonate platform dated by 87Sr/86Sr at ∼11–10 Ma. Three former islands on the crest of the Romanche transverse ridge are now at ∼900 m bsl; they show horizontal truncated surfaces of oceanic crust capped by ∼300 m thick carbonate platforms, with 10–6 Ma Sr isotopic ages. These sunken islands formed due to vertical tectonics related to transtension/transpression along long-offset slow-slip transforms. Another tectonic sunken island is Atlantis Bank, an uplifted gabbroic block along the Atlantis II transform (SW Indian Ridge) ∼700 m bsl. A modern tectonic island is St. Peter and St. Paul Rocks, a rising slab of upper mantle located at the St. Paul transform (equatorial Atlantic). “Cold” tectonic islands contrast with “hot” volcanic islands related to mantle thermal and/or compositional anomalies along accretionary boundaries and within oceanic plates, or to supra-subduction mantle melting that gives rise to islands arcs.
  • Article
    Postmelting hydrogen enrichment in the oceanic lithosphere
    (American Association for the Advancement of Science, 2021-06-09) Le Roux, Véronique ; Urann, Benjamin M. ; Brunelli, Daniele ; Bonatti, Enrico ; Cipriani, Anna ; Demouchy, Sylvie ; Monteleone, Brian D.
    The large range of H2O contents recorded in minerals from exhumed mantle rocks has been challenging to interpret, as it often records a combination of melting, metasomatism, and diffusional processes in spatially isolated samples. Here, we determine the temporal variations of H2O contents in pyroxenes from a 24-Ma time series of abyssal peridotites exposed along the Vema fracture zone (Atlantic Ocean). The H2O contents of pyroxenes correlate with both crustal ages and pyroxene chemistry and increase toward younger and more refractory peridotites. These variations are inconsistent with residual values after melting and opposite to trends often observed in mantle xenoliths. Postmelting hydrogen enrichment occurred by ionic diffusion during cryptic metasomatism of peridotite residues by low-degree, volatile-rich melts and was particularly effective in the most depleted peridotites. The presence of hydrous melts under ridges leads to widespread hydrogen incorporation in the oceanic lithosphere, likely lowering mantle viscosity compared to dry models.