The global mid-ocean ridge, with a total length over 65,000 km, is the longest volcanic chain on Earth. Hot magmas are extracted from the upwelling asthenosphere beneath the ridge axis to produce ocean crust, which covers two-thirds of Earth's surface. According to the plate motion rate at each side of ocean ridges, seafloor spreading can be classified in two different modes, i.e., symmetrical or asymmetrical, which show much difference in ocean crustal thickness and seafloor morphology. The fast- and intermediate-spreading ridges (with a full spreading rate of >80 mm/yr and 55-80 mm/yr, respectively) are dominated by symmetric spreading. Nevertheless, both symmetric and asymmetric spreading occur along the slow- to ultrslow-spreading ridges (with a full spreading rate of 55-20 mm/yr and <20 mm/yr, respectively).
Change in spreading mode reflects the variation in magma flux at ocean ridges, which have commonly related to mantle temperature or spreading rate. An implicit assumption in these interpretations is that the asthenosphere with homogeneous compositions passively uprises beneath ocean ridges and gives rise to magma. Nevertheless, numerous studies have demonstrated that the asthenosphere is not compositionally homogeneous at all, in which refractory mantle domains with experience of ancient melt depletion are preserved. A plausibility is that the volumetric content of the ancient refractory mantle domains within the asthenosphere could control the magma flux and thus play an active role in modulate the spreading mode.
The slow-spreading Mid-Atlantic Ridge (MAR) at 23°N is an ideal place to test such a hypothesis. The 23°N MAR has experienced a shift from asymmetric to symmetric spreading in the past 3.3 million years (Ma), during which neither mantle temperature nor spreading rate shows remarkable variations. More importantly, abyssal peridotites, residues of the asthenosphere after partial melting, are quasi-continuously outcropped along the strike of spreading, which could provide well constraints on the temporal change in compositions of the asthenosphere.
An international team led by Prof. Chuan-Zhou Liu of the Laoshan Laboratory has conducted a multi-disciplinary study using geophysical data, geochemical analyses of mantle peridotites and basalts, and numerical modeling. They found that the asthenosphere contained higher volumes of ancient refractory mantle domains during the asymmetric spreading phase (3.3-0.4 Ma) than the asthenosphere during the symmetrical spreading phase (<0.4 Ma). Such a simultaneity in the change of mantle composition and spreading mode provides a robust support for the hypothesis that the asthenosphere's composition can modulate magma flux and thus the spreading mode.
Moreover, the discovery in this study corroborates that the asthenosphere plays an active role in constructing the ocean lithosphere rather than being a passive actor as commonly assumed. The reason lies in the mantle heterogeneity of the asthenosphere, which has been widely demonstrated but underestimated by the community. "Earth's upper mantle is like a mixed stew shaped by plate tectonics," said Prof. Chuan-Zhou Liu. "Over billions of years in Earth's history, old mantle from different tectonic settings can be recycled into the asthenospehere. They are not completely homogeneized by mantle convection at all."
This study exemplifies that effects of the mantle heterogeneity on magma genesis and seafloor spreading can be enlarged at slow- to ultraslow-spreading ridges. More attentions should be paid to mantle heterogeneity of the asthenosphere in the future studies. We thus call upon a paradiam shift from a homogeneous asthenosphere to a heterogeneous asthenosphere.
