Chemical differentiation of the silicate earth: insights from the distribution of the element Eu

    The formation of the Earth’s continental crust is one of the main outcomes of planetary differentiation. The composition of the crust provides insights into how the crust formed. It’s been generally agreed that the Earth continental crust was extracted from the mantle billions of years ago. During these processes, incompatible elements were removed from the mantle and preferentially partitioned into the continental crust, leaving the residual mantle depleted. Thus the knowledge of elemental distribution and fractionation may improve our understanding of the Earth differentiation and evolution.

Fig. 1 Cross section of the Earth. The layered structure of the Earth results from a series of physical and chemical differentiation.

    REE (rare earth element) is a group of elements that behave quite similarly to each other. Significant REE fractionation only occurs under certain circumstances. The REE abundance pattern is thus very useful to indicate geological processes. Contrary to all other REEs, which are trivalent elements (except Ce⁴⁺), Eu is unique in that it can also exist in the reduced (2+) valence state under reducing conditions. This feature, therefore, makes Eu easily fractionated from other REEs during Earth chemical differentiation, as divalent Eu is generally more compatible than other trivalent REEs, and may be stuck in the depleted mantle.

    The REE abundance pattern of the upper continental crust has, on average, a negative Eu anomaly (Eu is depleted relative to its neighboring rare earth elements). Some models for the composition of the bulk continental crust also indicate a negative Eu anomaly. However, to precisely constrain the Eu abundance relative to other REEs has often proved formidable by crustal sampling alone. Assume the bulk Silicate Earth (SE) has no Eu anomaly, that is, Eu anomaly is absent in the combination of the continental crust and depleted mantle. The depleted upper mantle, as a chemically complementary reservoir to the continental crust, provides a potential approach to unraveling this issue. Huge amount of basaltic magma erupts along the mid-ocean ridges–spreading centers of plate tectonics. These basalts are known as mid-ocean ridge basalts (MORBs), which came directly from the depleted mantle. MORBs thus open the window for studying the depleted mantle. My study will focus on MORB glasses from the Atlantic Ocean, Pacific Ocean and Indian Ocean, and examine whether the depleted mantle holds excessive Eu.

Fig. 2 Sampling locations of my research cover the mid-ocean ridges of the Pacific Ocean, Atlantic Ocean and Indian Ocean.