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Global MORB Chemistry and Ridge Axial Depth - A New Interpretation

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The paper by Klein and Langmuir [JGR, 1987] is a milestone on MORB genesis. They showed that MORB chemistry correlates with ridge axial depth on a global scale: CaO/Al2O3 and Fe8 (FeO corrected for fractionation to MgO = 8.0 wt%) increase whereas Na8 decreases as the ridge shallows. They interpreted such correlations as resulting from varying pressures and degrees of melting caused by mantle potential temperature (TP) variation of up to 250°C from beneath cold deep ridges to hot shallow ridges. This interpretation is reasonable because a hotter rising mantle begins to melt deeper (high Fe8), has a taller melting column, and melts more (high CaO/Al2O3, low Na8) than a cooler mantle. The validity of this interpretation depends heavily on Fe8. HIDDEN in this interpretation is the FACT that at MgO = 8 wt%, the inverse Fe8-depth correlation equals a positive Mg#-depth correlation. That is, Mg# decreases from 0.66 at deep ridges (e.g., Cayman Trough, or CT, > 5 km below sea level) to 0.56 at shallow ridges (e.g., Reykjanes Ridge, RR, close to sea level). This means that by using Fe8 (total range: 7 – 11) one examines the progressively more evolved melt from deep ridges to shallow ridges, which does not tell pressures of melting, thus provide no TP information.

By correcting for fractionation to Mg# = 0.72, one examines largely the mantle signals of MORB melts. In this case, the range of Fe72 is reduced (7.5 – 8.5), and the Fe72-depth correlation essentially disappears. IF one used Fe72 to estimate TP, then 50°C variation may be reasonable beneath global ridges. That is, degrees of mantle melting may not vary significantly with varying ridge depth. However, significant Na72-depth (+) and Ca72/Al72-depth (-) trends remain. Assuming spreading rate effect is small and melting region shape effect is averaged out, then Na72 and Ca72/Al72 largely reflect fertile mantle composition. Deeper ridges are underlined by more fertile mantle with higher Al2O3 and Na2O that make denser garnet and jadeiite-rich cpx, thus greater bulk density in the mantle than shallower ridges. In order to explain the > 5 km ridge depth variation, we can use CT as a reference point to calculate isostatic compensation depth: DC (km) = 339.82X(-0.79355), where X is mantle density reduction. This says that the 5 km elevation of RR (vs. CT) results from its sub-ridge mantle density reduction of 0.5 (equivalent to 150°C hotter) with DC = 600 km, or 1% ( 300°C hotter) with DC = 334 km. Obviously, density reduction due to variation in composition is more realistic than temperature beneath global ocean ridges.

This talk is part of the Bullard Laboratories Wednesday Colloquia series.

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