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

Abstract

Please cite this article as: Coogan, L.A., O'Hara, M.J., MORB differentiation: In situ crystallization in replenishedtapped magma chambers, Geochimica et Cosmochimica Acta (2015), doi: http://dx.ABSTRACT The differentiation of mid-ocean ridge basalt (MORB) is investigated with a focus on intermediate-to fast-spreading ridges and two recently proposed differentiation mechanisms: (i) differentiation in replenished-tapped crystallizing (RTX) magma chambers, and (ii) chromatographic element separation during melt-rock reaction in the lower crust. There is compelling evidence in the petrology and geochemistry of MORB indicating that magma chambers at mid-ocean ridges behave as open systems, as required on thermal grounds in locations where a steady-state magma chamber exists. It hasrecently been suggested that the commonly observed over-enrichment of more-to-less incompatible elements during MORB differentiation can be explained by such an RTX model. However, the petrology of samples from the lower oceanic crust suggests an alternative mechanism could produce this over-enrichment. Clinopyroxene crystals in oceanic gabbros are commonly strongly zoned in incompatible elements with crystal rims apparently having grown from melts with very high incompatible element abundances.Elevated Zr/LREE in clinopyroxene rims, which has been interpreted as indicating growth from a melt in which these elements had been fractionated from one another by melt-rock reaction (chromatographic separation), is shown to be more simply explained by post-crystallization diffusive fractionation. However, the high incompatible element abundances in crystal rims demonstrates that the interstitial melt in crystal mush zones becomes highly differentiated. Disaggregation of such mush zones is indicated by the crystal cargo of MORB and must be accompanied by the return of interstitial melt to the eruptible reservoir -a form of in situ crystallization. Both a magma chamber undergoing closed system in situ crystallization, and a RTX magma chamber in which crystallization 3 occurs in situ, are shown to be capable of reproducing the differentiation trends observed in MORB. Simple stochastic models of the latter process suggest that significant variations of incompatible element abundances and ratios, at a constant MgO content, are likely to be generated from a single parental melt compositions. Additionally, parental melt compositions will generally be substantially more depleted than would be suggested if only fractional crystallization is considered. This has important implications for understanding the composition of the upper mantle. For example, the Sm/Nd of MORB are likely to be significantly lower than that of the Moho-crossing melt complicating analysis of the Nd-isotopic evolution of the upper mantle.

Geochemical Variability Along the Northern East Pacific Rise: Coincident Source Composition and Ridge Segmentation

“…Figure a shows a correlation between La/Sm ratios and Sr concentrations. Large variations in La/Sm at constant Sr content are expected if periodically replenished magma chambers as envisaged by O'Neill and Jenner () and Coogan and O'Hara () exist. Furthermore, as La or Ba contents increase, a replenished magma chamber should cause the Ba/La ratio to decrease, but data show that Ba/La ratio increases (see Figure b).…”

Elemental Systematics in MORB Glasses From the Mid‐Atlantic Ridge

“…The chemical information contained in incompatible element ratios is regarded to be dominated by mantle source depletion and enrichment processes with only minimal effects coming from partial melting or magma chamber processing that occur prior to eruption of MORB lavas. Some workers have questioned this long‐held notion arguing that magma chamber processes affect all but the most highly incompatible element ratios (e.g., Coogan & O'Hara, ; Lissenberg & MacLeod, ; O'Neill & Jenner, , ). Reactive porous flow in cumulate piles within MORB magma chambers potentially creates elemental fractionations among the least incompatible elements that differ from fractional crystallization due to buffering of the Mg# by olivine (Lissenberg & MacLeod, ).…”

“…A simple prediction for the deviation of the composition of an evolving intercumulus liquid within a MORB magma chamber from fractional crystallization is that the faster diffusive transport of Fe‐Mg versus La‐Yb would manifest itself in La becoming more incompatible than predicted at a given Mg#. Mixing of intercumulus liquid into the magma chamber (e.g., Coogan & O'Hara, ) would then produce mixed liquids that exhibit higher (La/Yb) N at a given Mg# than predicted by fractional crystallization. One might also expect that the longer magma resides in the magma chamber the more intercumulus liquid is mixed into it.…”

“…MORB glass compositions have been modified by fractional crystallization processes prior to eruption on the oceanic floor (e.g., Coogan & O'Hara, 2015;Grove & Bryan, 1983;Klein & Langmuir, 1987). Incompatible trace element ratios have been widely employed to examine variations in mantle source characteristics since the partitioning of these elements into both mantle and crustal mineral assemblages is minimal (Hofmann, 1988).…”

“…The abundances of chalcophile and siderophile elements in the Earth's mantle additionally reflect processes of accretion, volatile element depletion, core formation, and late accretion (e.g., Ballhaus et al, 2013;Mann et al, 2009;Righter et al, 2010Righter et al, , 2015Wade et al, 2012;Wood et al, 2014;Yang et al, 2015). MORBs sample ambient upper mantle, chemically complementary to the incompatible element-enriched continental crust, as partial melts subsequently modified by magma chamber processes (e.g., Coogan & O'Hara, 2015;Shorttle et al, 2016). Based on the abundances of incompatible elements MORBs can be categorized into enriched-(E-), normal-(N-), and depleted-(D-) MORBs (Gale et al, 2013;Perfit, 2001;Schilling, 1973).…”