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Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variations in the formation of oceanic lithosphere (Articolo in rivista)

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Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variations in the formation of oceanic lithosphere (Articolo in rivista) (literal)

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BONATTI E.; LIGI M.; BRUNELLI D.; CIPRIANI A.; FABRETTI P.; FERRANTE V.; GASPERINI L.; OTTOLINI L. (2003)
Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variations in the formation of oceanic lithosphere
in Nature (Lond.); Nature Publishing Group, New York (Stati Uniti d'America)
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BONATTI E.; LIGI M.; BRUNELLI D.; CIPRIANI A.; FABRETTI P.; FERRANTE V.; GASPERINI L.; OTTOLINI L. (literal)

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At the Vema anomaly, the Mid-Atlantic Ridge is slow spreading, and our study, published on Nature, adds to our understanding of how the oceanic crust grows in these circumstances. The exposure of the Vema tranverse ridge has allowed to dredge up and analyse rock that would normally be deeply buried. It has been possible to measure an apparent time-delay between changes in crustal thickness, as estimated from gravity data, and in residual peridotite chemistry (investigated by EMPA and SIMS) over the 20-million-year timescale. The signals indicating crustal thickness and the extent of peridotite depletion show oscillations that have a wavelength of about 60 km (corresponding to a 3-4-million-year frequency), superposed on an overall 20-million-year trend of increasing peridotite depletion and crustal thickness. The variations seen in the two signals are correlated, but there is a phase lag of 22 km (or some 1.3 million years of spreading time) between the signals. Impact Factor: 30.979 (literal)

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The oceanic lithosphere covers two/thirds of our Planet: understanding how it forms and evolves is a major challenge in the Earth sciences. It is generally agreed that the oceanic lithosphere forms along mid ocean ridges (MOR), where mantle material upwells and undergoes decompression and partial melting. The melt rises rapidly and freezes producing the crust, while the peridotitic residue forms the lithospheric mantle. MOR topography, structure and composition indicate that near zero age processes of lithosphere formation vary along ridge axis1-3. Less is known, however, of how these processes vary through time, a question critical for our understanding of how ocean basins evolve. Variations through time of MORs thermal regime and/or composition should be recorded in lithosphere lying at increasing distances from ridge axis along sea floor spreading flow lines. However, older lithosphere is normally covered by sediment and not easily accessible to high-resolution observation and sampling. An uplifted sliver of oceanic lithosphere (Fig. 1), exposing in the central Atlantic a ~20 Myr long record of creation of lithosphere at a Mid Atlantic Ridge (MAR) segment, gave us the opportunity to study temporal variability in the formation of lithosphere, to estimate the upwelling velocity of the mantle below the Ridge and to determine whether passive or dynamic models of creation of oceanic lithosphere prevail at slow-spreading ridges. (literal)

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BONATTI E. 1),2),3); LIGI M.1); BRUNELLI D. 1),3); CIPRIANI A. 2); FABRETTI P. 1); FERRANTE V. 1),3); GASPERINI L. 1); OTTOLINI L. 4). 1) ISMAR-CNR, Sezione di Geologia Marina, Bologna. 2) LDEO, Columbia University, Palisades, USA. 3) Univ. La Sapienza, Roma. 4) IGG-CNR, Sezione di Pavia, Pavia. (literal)

Titolo

Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variations in the formation of oceanic lithosphere (literal)

Abstract

A 20-Myr record of creation of oceanic lithosphere at a segment of the central Mid-Atlantic-Ridge is exposed along an uplifted sliver of lithosphere. The degree of melting of the mantle that is upwelling below the ridge, estimated from the chemistry of the exposed mantle rocks, as well as crustal thickness inferred from gravity measurements, show oscillations of about 34 Myr superimposed on a longer-term steady increase with time. The time lag between oscillations of mantle melting and crustal thickness indicates that the solid mantle is upwelling at an average rate of about 25 mm per year, but this appears to vary through time. Slow-spreading lithosphere seems to form through dynamic pulses of mantle upwelling and melting, leading not only to along-axis segmentation but also to across-axis structural variability. Also, the central Mid-Atlantic Ridge appears to have become steadily hotter over the past 20 Myr, possibly owing to northsouth mantle flow. (literal)

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