Origin and Evolution of Mantle Lithosphere

Michael J. Walter1 & Dean C. Presnall2
1Institute for Study of the Earth's Interior, Okayama University, Misasa 682-0193, Japan
2Geophysical Laboratory, 5251 Broad Branch Rd NW, Washington D.C. 20015, United States

Mantle lithosphere beneath oceanic and continental crust is characterized by mineralogic and chemical variability resulting from extraction of partial melt. Normative mineral and major element composition trends from oceanic and off-craton subcontinental lithosphere intersect at an Mg# of about 89.3, indicating a common fertile lherzolite protolith. A model fertile upper mantle composition is calculated from these trends and has non-chondritic refractory lithophile major element ratios. Phase relations for melting of fertile peridotite are used to deduce melt extraction trends as a function of pressure and temperature. These trends are compared to compositional trends exhibited by mantle lithosphere to constrain the conditions of formation. Reconstructed abyssal peridotite compositional trends are best modelled by moderate degrees (5 to 25%) of combined batch and near-fractional melt extraction at an average pressure of about 1 GPa. Off-craton subcontinental mantle lithosphere, generally Proterozoic in age, is modelled by similar extents of melting but at a higher average pressure of about 2 GPa. Low SiO2 Archean cratonic lithosphere has compositional features indicating high-degree melt extraction (40 to 50%) at average pressures of 4 to 5 GPa. High-SiO2 cratonic lithosphere is modeled by melt extraction followed by addition of opx in a secondary process. Based on the assumption of nominally anhydrous melt extraction, estimates for average mantle potential temperature at the time of lithosphere formation indicate secular cooling of the mantle by about 350±50°C since the Archean. This amount of cooling is greater than predicted in models that assume whole-mantle convection, and could indicate transient periods of layered convection and mantle overturn during the first several billion years of Earth history.