Inferences on mid-ocean ridge thermal and magmatic structure from MORB compositions

Abstract

Systematic compositional variations among > 11,000 mid-ocean ridge basalts (MORB) from the global mid-ocean ridge spreading system demonstrate how the ocean crust regulates erupted lava compositions. MORB chemical differences reflect complex relationships among crustal thermal structure, magma temperatures, degree of magma differentiation, degree of magma homogenization, depth of seismically imaged crustal magma bodies and the ability of MOR volcanism to accurately map parent melt variations as functions of spreading rate and magma supply. High melt supply at the fastest spreading ridges produces relatively more differentiated magmas from shallow magma chambers and with a much greater variation in chemical signatures imparted by differentiation in the crust; however, such magmas also display narrow ranges in chemical attributes related to mantle source and melting style. In contrast, low melt supply promotes the eruption of less differentiated magma from deeper magma chambers of more uniform differentiation degree but more variable mantle parentage. Collectively, these observations indicate that in the oceanic crust the depth, style and rate of magma accumulation and the minimum pressure of differentiation reflect the thermal state of the upper crust, which is largely governed by the magnitude and continuity of regional melt supply. We propose a composite MOR magma chamber in which the number and size of melt segregations scale with spreading rate and magma supply, resulting in greater steady-state proportions of melt in the crust as supply increases. At high melt supply, shallow, large reservoirs are highly sensitive to temporal variations in cooling rate and produce magmas of greater and more variable degrees of fractionation. High melt supply promotes homogenization of parental magma compositions with increased degree of differentiation. At lower melt supplies, magma accumulates more episodically and resides in poorly connected, hotter, and more thermally insulated reservoirs deeper in the crust; eruptions at such settings produce lavas that are uniformly less differentiated but more likely to retain variations inherited from the underlying mantle.