HR: 10:20h
AN: T51H-01 INVITED     [PDF]
TI: Melting Processes Beneath Mid-Ocean Ridge Spreading Centers
AU: * Grove, T L
EM: tlgrove@mit.edu
AF: Mass. Inst. of Tech., Dept. EAPS, 54-1220, Cambridge, MA 02139 United States
AB: The variations in major element composition of mid-ocean ridge basalts (MORB) provide significant insight into the temperature structure in the upper mantle beneath ocean ridges. In their landmark paper Klein and Langmuir (1987, JGR 92, 8089) showed that there were global correlations in MORB chemical composition that reflected extent and the depth interval of melt generation (notably Na and Fe, respectively). These parameters were also shown to correlate with ridge bathymetry and variations in oceanic crustal thickness. These observations led to a renaissance in understanding MORB generation. When all the evidence is considered, near fractional adiabatic decompression melting is required in order to produce the major compositional characteristics of the global MORB array. In near fractional melting increments of melt are extracted continuously from a melting column and aggregated. The result is an average of the melts produced over a range of pressures from a mantle that is progressively depleted as it melts. Experimental studies have calibrated the range of melting conditions. In particular, the work of Kinzler (1997, JGR 102, 853) provides a robust set of numerical models for predicting physical conditions. Melting is controlled by variations in mantle potential temperature, which range from 1460 $\deg$C to 1325 $\deg$C. Melting begins when the adiabat crosses the solidus and ends at very shallow mantle depths ($\sim$ 9 km or 0.3 GPa). At the high potential temperature end of the spectrum melting starts at a pressure of 3 GPa, resulting in 11 % melting and production of 9 km of oceanic crust. At the low potential temperature end melting begins at 1.5 GPa, resulting in 7 % melting and production of 3 km of oceanic crust. In contrast to the MORB environment, Hawaii is our best example of a hotspot melt generation environment. Rare picritic magmas reveal a melting environment that is complicated by interaction of the mantle melt with a thick, cooler lithosphere (Wagner and Grove, 1998, CMP, 131,1). The mantle potential temperature beneath Hawaii is 1535 $\deg$C, but assimilation of lithospheric mantle modifies the picrite primary magma composition and lowers magma temperature to 1400 $\deg$C. Therefore, mantle melts record a 200 $\deg$C potential temperature variation in the Earth's mantle today.
DE: 1025 Composition of the mantle
DE: 3035 Midocean ridge processes
DE: 3630 Experimental mineralogy and petrology
DE: 8439 Physics and chemistry of magma bodies
SC: Tectonophysics [T]
MN: 2003 Fall Meeting