Earth’s upper mantle, as sampled by mid-ocean ridge basalts (MORBs) at oceanic spreading centers, has developed chemical and isotopic heterogeneity over billions of years through focused melt extraction and re-enrichment by recycled crustal components. Chemical and isotopic heterogeneity of MORB is dwarfed by the large compositional spectrum of lavas at convergent margins, identifying subduction zones as the major site for crustal recycling into and modification of the mantle. The fate of subduction-modified mantle and if this heterogeneity transmits into MORB chemistry remains elusive. Here, we investigate the origin of upper mantle chemical heterogeneity underneath the Western Gakkel Ridge region in the Arctic Ocean through MORB geochemistry and tectonic plate reconstruction. We find that seafloor lavas from the Western Gakkel Ridge region mirror geochemical signatures of an Early Cretaceous, paleo-subduction zone, and conclude that the upper mantle can preserve a long-lived, stationary geochemical memory of past geodynamic processes.
The Arctic Ocean is roughly 14,000 km away from Melbourne (Monash University, Australia), where most of the action for our recent publication ‘An Early Cretaceous subduction-modified mantle underneath the ultraslow spreading Gakkel Ridge, Arctic Ocean’ in Science Advances took place.
In this publication we investigated rocks from the Gakkel Ridge in the Arctic Ocean, an area where tectonic plates move apart and hot magma rises from the upper mantle (~80 km depth) to the ocean floor where magma solidifies as basaltic rock (figure below). Typically, the magma inherits some of the unique signature of the parental mantle, which allows to draw conclusion on the source and the formation processes.
To understand the mantle source, the surroundings in which hot magma forms, we crushed up the rock sample to fine powder and processed it further to a rock glass and dissolved ~100 mg of the powder into a liquid to extract the geochemical fingerprint using mass spectrometer at the Monash Isotopia facility. The results we obtained from these rocks were unusual for samples from an area where tectonic plates move apart. It rather suggested a geochemical signature typically found in areas where plates collide and island arc volcanoes are formed, e.g. along the ring-of-fire in the Western Pacific.
At present there is no evidence for island arc volcanism or plates colliding in our study area. Therefore, we delved back in time using a tool called ‘GPlates’. This tool allowed us to look at the plate reconstructions hundred of millions years ago in the greater Arctic region.
By using these plate reconstructions we identified that arc volcanism existed 130 million years ago in our sample area and concluded that this event must have left a geochemical imprint in the mantle that persists in the geochemical signatures of basaltic magmas formed in this area today. The figure below shows a possible explanation of such a process.
In this contribution we combined geochemical observations with plate reconstructions and used geodynamic principles to explain an unusual geochemical signature found in basalts from the Arctic ocean ridge. This project was a collaborative work across many geoscientific disciplines and universities (Monash University, University of Melbourne, University of Sydney and Woods Hole Oceanography Institute).