Magmatic processes under mid‐ocean ridges: A detailed mineralogic study of lavas from East Pacific Rise 9°30′N, 10°30′N, and 11°20′N

Abstract

[1] Detailed petrologic study has been made for lavas from the northern East Pacific Rise (EPR) 9°30 0 N has nonrobust magma supply and no shallow melt lens. Lavas from all three localities are sparsely phyric and glassy, containing plagioclase ± olivine ± pyroxene. Typically, the lavas contain several to many (up to seven) distinct chemical groups of plagioclase that are not always distinct texturally. The lavas may also contain up to three chemically distinct groups of olivine and two groups of pyroxene. The lavas contain both individual crystals and groups comprising reticulate and dendritic clots that we interpret to represent bits of crystal networks forming in mushy zones of an axial magma chamber. 21 of the 23 samples studied in detail have diverse crystal compositions and require mixing, which most likely occurs when mostly liquid magma passes through mushy zones. We find no significant systematic differences between robust and nonrobust segments in terms of their crystal content, proportion of texturally distinct xenocrysts, crystal size, aspect ratio, roundness, modal abundance, magma residence time, number of diverse mineral-chemical groups, and characteristics of mixing like the total range of composition of disequilibrium minerals present, and the magnitude of the chemical gaps between disequilibrium and equilibrium compositions. This unexpected and remarkable similarity suggests that the presence or absence of a seismically imaged shallow melt lens has essentially no effect on the mineralogy of erupted lavas. We conclude that the fundamentally important magmatic process under mid-ocean ridges, from slow to fast, is the formation of crystal networks and their subsequent compaction; the seismically detected shallow melt lenses likely contain highly evolved magma, formed by expelled interstitial melt during crystal network compaction, and play very little role in crustal accretion. Our conclusion implies that the thick (>3 km) low velocity zone near the base of the crust produces the thin (<30 m) shallow melt lenses, not the other way around.

Magmatic processes in developing oceanic crust revealed in a cumulate xenolith collected at the East Pacific Rise, 9°50′N

“…Data presented here are consistent with the Pan and Batiza [2003] model and strongly support complex crystal‐melt interactions between various melts and accumulating crystals in the lower crust and upper mantle. Such interactions have also been proposed based on mineralogic evidence in gabbroic sections formed at fast spreading centers (e.g., Hess Deep [ Natland and Dick , 1996]) and from some ophiolites, e.g., Oman [ Kelemen et al , 1997].…”

“…Primitive NMORB melts have molar Ca/Na ratios that are too low to crystallize anorthite under equilibrium or near‐equilibrium and this is illustrated by the absence of highly calcic plagioclase phenocrysts in NMORB lavas. Consequently, the common presence of anorthite‐bytownite xenocrysts in erupted NMORB lavas [ Sinton et al , 1993; Nielsen et al , 1995; Pan and Batiza , 2003] and anorthite‐bytownite cumulates in ophiolites is enigmatic. Similarly, the cumulate described here contains a significant amount of anorthite, particularly as a cumulate phase during the Stage 1 precipitation of plagioclase.…”

“…Observations from NMORB lavas that have crystallized plagioclase and phase equilibrium considerations indicate that NMORB lavas that are nearly anhy- Table S5. [Sinton et al, 1993;Nielsen et al, 1995;Pan and Batiza, 2003] and anorthite-bytownite cumulates in ophiolites is enigmatic. Similarly, the cumulate described here contains a significant amount of anorthite, particularly as a cumulate phase during the Stage 1 precipitation of plagioclase.…”

MORB differentiation: In situ crystallization in replenished-tapped magma chambers

“…This can be determined texturally (e.g., Dungan and Rhodes, 1978) and/or compositionally; most olivine and plagioclase crystals in MORB are more primitive than those in equilibrium with the host basalt ( Fig. 1d; Bryan and Moore, 1977;Rhodes et al, 1979;Pan and Batiza, 2003;Coogan, 2014). In many cases it can be shown that mixing occurred within weeks to months before eruption (e.g., Pan and Batiza, 2002;Costa et al, 2009;Moore et al, 2014).…”

Consequences of a crystal mush-dominated magma plumbing system: a mid-ocean ridge perspective

“…Mush destabilization by replenishment can account for either of these. It can also account for the observation that MORB commonly contains diverse populations of phenocrysts and antecrysts, each recording a different petrogenetic history [74][75][76][78][79][80], as well as xenoliths of crystal mush [56,57]: when mush destabilizes and melts are transported upwards through it, they may pick up crystals and crystal clots from different parts of the system. Furthermore, even when crystals originate from a single location, the dynamics of mixing of melt and mush can be such that each crystal may take a different composition-time path, leading to the generation of multiple populations [64].…”