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The role of convection in the chemical composition of oceanic lavas

Two Amercian and French researchers, including a researcher from the Institut de Recherche en Astrophysique et Planétologie (Paul Sabatier University of Toulouse and CNRS), recently demonstrated, by means of numerical simulations, the role of convection at small scales in the chemical composition and variability of oceanic lavas of mid-ocean ridges. This study was published on July 20, 2014 in the journal Nature Geoscience.

It is generally accepted that oceanic lavas originate from sources with distinct composition located in the Earth's mantle.

If the movements of tectonic plates on a large scale are the main mechanism for mixing these sources, the geochemical signatures of the lavas from different mid-ocean ridges can vary greatly. Geochemical variability is often weak in regions where the plates are moving fast, which is consistent with mixing induced by plate motion. However, the slow-spreading centers, such as the Southwest Indian Ridge, are also associated with a homogenous geochemistry, which is inconsistent with a mixing from tectonic plates.

Our work is based on numerical simulations of mantle flow in order to study the efficiency of mixing in the vicinity of mid-ocean ridges, by varying the size and the speed of tectonic plates. Our simulations show that small-scale convection in the mantle contributes significantly to the mantle mixing at slow-spreading ridges. Faster spreading rates and smaller plates inhibit the development of small-scale convection. We conclude that if the lavas from fast mid-ocean ridges are well mixed by large-scale plate motion, slow mid-ocean ridges are also well mixed by small-scale convection.

These results have important significance for the interpretation of geochemical data at mid-ocean ridges and hot spots whose isotopic variability can be explained without invoking isolated geochemical reservoirs, but only by a variable efficiency of mixing processes in the Earth’s mantle.


Figure : Helium variability and predicted mixing times along mid-ocean ridges. a, Seafloor age and eight selected regions (labelled rectangles) considered in bdb, Standard deviation of R=3He/4He measured along spreading centres, normalized to the atmospheric ratio value, Ratm, versus ridge’s half-spreading rate. c,d, Predicted mixing times (c) and standard deviations (d) of chemical heterogeneities versus ridge’s half-spreading rate, assuming that mixing is entirely due to surface-plate motion (pink circles), or making the more reasonable assumption that mixing results from the combination of small-scale convection and surface-plate motion (blue stars). This blue trend in (d) is in much better agreement with the data (b) than the standard assumption (pink trend in d), thereby suggesting that small-scale convection combined with plate-driven flow can explain the observed Helium isotopic variability along ridges.

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  • Henri Samuel,
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