The oceanic mantle may comprise more than 60% of the mass of the Earth. The modern oceanic mantle is known to be chemically and isotopically heterogeneous, primarily documented through studies of mid-ocean ridge basalts (MORB). The processes that resulted in the creation of the heterogeneities were most likely dominated by a combination of partial melting (melt removal), metasomatism (melt/fluid addition) and incorporation of recycled crust (via subduction). The relative impacts of each of these processes, the timing of the processes, and the length scales of the resulting heterogeneities remain topics of considerable debate. Clarifying the causes, timing and spatial extent of chemical and isotopic heterogeneities in the oceanic mantle is, therefore, critical for refining our knowledge of the present bulk composition of this major terrestrial reservoir, for achieving a more complete understanding of how the chemical and isotopic composition of Earth's mantle evolved through time, and for assessing the efficiency by which the oceanic mantle is convectively mixed today.
To tackle these issues, sections of the oceanic mantle accessible in four ophiolite complexes (tectonically emplaced slivers of oceanic lithosphere) will be studied. The targeted ophiolites range in age from 1.95 billion years old to 6 million years old. The study will primarily focus on examining the absolute and relative abundances of siderophile (iron-loving) elements in the mantle rocks exposed in these locations. The study will seek to answer fundamental questions about the present and past oceanic mantle including: 1) What is the geometry of long-term, exceptionally melt-depleted portions of the oceanic mantle? 2) Do common oceanic mantle lithologies, such as pyroxenites and dunites, have chemical compositions and sufficient volume to be implicated in the generation of certain volcanic rocks with unusual compositions, for example some Hawaiian lavas? 3) Have key geochemical parameters in the oceanic mantle changed through time? If so, can they be correlated with the rates of processes that may have acted on the mantle through time. In addition to answering these questions about the chemical structure of the oceanic mantle, comparison of data from ophiolites spanning nearly two billion years will begin the process of documenting secular changes in the chemical structure of this mantle.
