The Meaning of Global Ocean Ridge Basalt Major Element Compositions

Abstract

Mid-ocean ridge basalts (MORB) are arguably the most abundant and also the simplest igneous rocks on the Earth. A correct understanding of their petrogenesis thus sets the cornerstone of igneous petrogenesis in general and also forms the foundation for studying mantle dynamics. Because major element compositions determine the mineralogy, phase equilibria and physical properties of rocks and magmas, understanding global MORB major element systematics is of prime importance. The correlated large MORB major element compositional variations are well understood as the result of cooling-dominated crustal-level processes (e.g. fractional crystallization, magma mixing, melt–rock assimilation or reaction, and other aspects of complex open-magma chamber processes), but it remains under debate what messages MORB major elements may carry about mantle sources and processes. To reveal mantle messages, it is logical to correct MORB melts for the effects of crustal-level processes to Mg# ≥ 0·72 to be in equilibrium with mantle olivine of ≥Fo90. Such corrected MORB major element (e.g. Si72, Ti72, Al72, Fe72, Mg72, Ca72 and Na72) compositional variations thus reflect fertile mantle compositional variation, composition-controlled mantle physical property variation (e.g. density and solidus), variation in the extent and pressure of melting, and uncertainties associated with the correction. The correction-related uncertainties can be removed through justified heavy averaging. Because ridge axial depth variation (∼0 to ∼6000 m below sea level) and plate spreading rate variation ( 150 mm a–1) are the two largest known physical variables along the global ocean ridge system, possible correlations of MORB major element compositions at Mg# ≥ 0·72 with these two physical variables are expected to reveal intrinsic controls on global MORB petrogenesis and ocean ridge dynamics. Indeed, global MORB major element data averaged with respect to both ridge axial depth intervals and ridge spreading rate intervals show significant first-order correlations. These correlations lead to the conclusion that the ridge axial depth variation and MORB chemistry variation are two different effects of a common cause, induced by fertile mantle compositional variation. The latter determines (1) variation in both composition and mode of mantle mineralogy, (2) variation of mantle density, (3) variation of ridge axial depth, (4) source-inherited MORB compositional variation, (5) density-controlled variation in the maximum extent of mantle upwelling, (6) apparent variation in the extent of melting, and (7) the correlated variation of MORB chemistry with ridge axial depth. These correlations also confirm the recognition that the extent of mantle melting increases with, and is caused by, increasing plate spreading rate. Mantle temperature variation could play a part, but its overstated role in the literature results from a basic error (1) in treating ridge axial depth variation as solely caused by mantle temperature variation by ignoring the intrinsic control of mantle composition, (2) in treating mantle plume-influenced ridges (e.g. Iceland) as normal ridges of plate spreading origin, and (3) in treating seismic low velocity at great depths (>300 km) beneath these mantle plume-influenced ridges as evidence for hot ridge mantle. There is no evidence for large mantle temperature variation beneath ridges away from mantle plumes. The suggested conclusions of this study may continue to be debated, but they are most objective, and are most consistent with petrological, geochemical, geological and geophysical principles and observations.