Ancient sediments in the magma sources of volcanoes in the Lesser Antilles
Volcanoes on Earth are made of solidified magma that is usually produced within the Earth’s mantle at depths of a few tens of kilometers beneath the surface. However, a closer look at the composition of volcanic rocks reveals additional components that originate from the continents rather than the mantle. How this continental material is added to the magma sources is often unclear. Our new study, published in Geology, focuses on volcanic rocks from the Lesser Antilles islands in the Caribbean where some magmas have strongly ‘continental’ compositions. We trace the origin of these magmas using the newly established molybdenum isotope system.
The Lesser Antilles islands formed within a subduction zone where the tectonic plate underlying the Atlantic ocean is subducting into the underlying mantle. On top of the Atlantic plate are layers of ancient sediments that accumulated there over the past ca. 120 million years. These sediments, mostly originating from the continents and transported into the ocean via rivers, are also subducting into the mantle together with the Atlantic plate.
One line of evidence attributes the ‘continental’ component in the Lesser Antilles magmas to the subducted sediments. Once these reach a depth of about 100 km within the mantle they become hot enough to melt. Another model suggests that the magma produced in the mantle is melting the crust beneath the volcanoes while on the way to the surface. Melting of the crust can impart a similar continental signature on the magmas.
Our new study uses the molybdenum isotope system to distinguish between these scenarios. The sedimentary rocks on top of the Atlantic plate contain a type of sediment called ‘black shales’ that formed when the ancient Atlantic ocean became depleted of oxygen during rare events about 80- 90 million years ago. The black shales are highly enriched in the element molybdenum and they have unusual molybdenum isotope ratios. Analyses of Lesser Antilles magmas show that they have the same unusual molybdenum isotope composition as the black shales. Black shale type sediments are not present within the crust beneath the Lesser Antilles volcanoes. These observations allow us to attribute the continental component in the Lesser Antilles magmas to the addition of sediment melts to the mantle beneath the volcanic islands rather than melting of the crust.
Full paper citation is: Freymuth, H., Elliott, T., van Soest , M., & Skora , S. (2016). Tracing subducted black shales in the Lesser Antilles arc using molybdenum isotope ratios. Geology. doi: 10.1130/G38344.1