Partial Melting of Depleted Peridotite in the Earth's Upper Mantle and Implications for Generation of Mid-Ocean Ridge Basalts

Peridotite in the earth's upper mantle undergoes polybaric, fractional melting as it rises adiabatically beneath mid-ocean spreading ridges. As liquid is continually extracted, peridotite becomes increasingly depleted in incompatible components. The amounts and compositions of partial melts of depleted peridotite are important parameters in models of MORB petrogenesis, but have not been well-constrained previously. I present partial melting experiments on a depleted peridotite composition at 10 kbar and 1250–1390°C. My experiments make use of small aggregates of glassy carbon particles into which partial melt is extracted at high temperature. I have been able to analyze low degree partial melts (<10%) and quantify the effects of incompatible element depletion on the melting behavior of peridotite. Special tests of the approach to equilibrium in this study confirm the validity of the aggregate melt extraction technique, which has sparked much debate in the literature (see Chapters 2 and 3 for details). Melts of depleted peridotite differ in important ways from melts of fertile peridotite, mostly due to lower alkali contents and chemical consequences thereof. At low melt fractions, melts of depleted peridotite have less SiO₂, more CaO, and higher CaO/Al₂O₃ than melts of fertile peridotite at the same melt fraction. According to these results and others in the literature, solidus temperature is a linear function of incompatible major element content. Melt fraction at cpx-out is proportional to normative cpx in source peridotite. Liquid compositions from this study are in good agreement with calculations using the quantitative models of Kinzler and Grove (1992a), Langmuir et al. (1992), and Ghiorso and Sack (1995). Calculations of polybaric, fractional melting of primitive mantle using the models of Langmuir et al. (1992) and Asimow (1997) indicate that about half of all liquid contributed to MORB is formed by partial melting of depleted peridotite. The data presented in this thesis provide information about amounts and compositions of partial melts formed from depleted peridotite, an important upper mantle constituent beneath mid-ocean ridges, and can be used to improve quantitative models of MORB primary magma formation and further our understanding of MORB petrogenesis.