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

Ridley, W. I.; Perfit, Michael R.; Smith, Matthew C.; Fornari, Daniel J.

doi:10.1029/2006gc001316

Cited by 48 publications

(32 citation statements)

References 73 publications

(116 reference statements)

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“…Observations in the field and thin sections of clots of normal gabbros being smeared within anorthosite laminae suggest that a freshly settled layer of gabbro could have been dismembered and remobilized in an anorthosite‐rich suspension sweeping the floor. This interpretation may explain how anorthosite cumulates can be present as xenoliths in lavas extruded at EPR, fitting the conclusion of Ridley et al [2006] that they represent fragments from a melt‐bearing plagioclase cumulate.…”

Section: Discussionsupporting

confidence: 69%

Structure and dynamics of ridge axial melt lenses in the Oman ophiolite

Nicolas¹,

Boudier²

2011

J. Geophys. Res.

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[1] The Oman ophiolite is regarded as derived from a fast spreading oceanic ridge. As in its present-day marine analogs, expectedly, the gabbro unit crystallized in a large magma chamber underneath a small melt lens. In ophiolites, this melt lens is reduced to a horizon mapped in the field, where the floor and the roof of the melt lens joined. The magmatic activity of the former melt lens has been studied in gabbros located below the melt lens horizon. Beneath a stable melt lens, gabbros subside and drift through the steep walls of the magma chamber as steeply foliated gabbros. In contrast, in unstable melt lenses more specifically considered here, flat-lying gabbros are exposed beneath the lens. These gabbros were settling on the floor while the melt lens was retreating. Their strong magmatic foliations and lineations point to a dynamic deposition on the floor, and lineation trends record the structure of convection rolls within the lens. Time constraints suggest a ∼100 year periodicity of melt recharge by short and powerful melt pulses followed by a shorter time of convection activity after recharge and a longer period of static magma settling, as recently proposed for the East Pacific Rise. At each new melt pulse, the lens nearly doubles its volume by surface expansion, within a short time lapse, before slowly and progressively retreating to feed crustal accretion. Large horizontal surface inflation of the lens better explains the seawater hydration of the accreting crust than melt lens vertical motion through the lid.Citation: Nicolas, A., and F. Boudier (2011), Structure and dynamics of ridge axial melt lenses in the Oman ophiolite,

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Section: Discussionsupporting

confidence: 69%

Structure and dynamics of ridge axial melt lenses in the Oman ophiolite

Nicolas¹,

Boudier²

2011

J. Geophys. Res.

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“…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). Additionally, glomerophyric clusters of crystals, and small plutonic xenoliths containing interstitial melt, are found in some MORB suites (e.g., Hekinian et al, 1985;Ridley et al, 2006) suggesting disruption of mush zones accompanies eruption in some instances; this provides a mechanimsm for interstitial melt to be returned to an eruptible melt reservoir.…”

Section: Observational Constraints On Morb Differentiationmentioning

confidence: 99%

“…3) shows that interstitial melt within crystal mush zones at mid-ocean ridges can become highly fractionated. Evidence for disruption of such crystal mush zones prior to, and during, 20 eruptions come from the crystal cargo in MORB (e.g., Hekinian et al, 1985;Ridley et al, 2006;Costa et al, 2009;Moore et al, 2014) indicating that evolved interstitial melt must become mixed back into the eruptible magma reservoir. These observations lead us to explore whether in situ crystallization (Langmuir, 1989;Nielsen and DeLong, 1992;O'Hara and Fry, 1996b) can explain the observed differentiation trends in MORB.…”

Section: Crystallizationmentioning

confidence: 99%

“…compaction, compositional convection) but the crystal cargo of MORB suggests that at least some fraction of the interstitial melt is returned because of mush zone disaggregation associated with replenishment (e.g. Hekinian et al, 1985;Ridley et al, 2006;Moore et al, 2014). In this model, eruptions triggered by replenishment will produce basalts with compositions that are mixtures of the resident melt in the AML, the interstitial melt in the mush zone surrounding the AML and the replenishing melt; the latter may be modified by crystallization prior to or during mixing (e.g., Huppert and Sparks, 1980).…”

Section: A View Of Morb Differentiation and Broader Implicationsmentioning

confidence: 99%

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

Coogan

O’Hara

2015

Geochimica et Cosmochimica Acta

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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.

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“…For example, primitive olivine crystals carried by evolved liquids have been interpreted as entrained macrocrysts in samples from Iceland (Thomson and Maclennan 2013), Hawai'i (Vinet and Higgins 2010), Réunion (Albarède and Tamagnan 1988), and mid-ocean ridges (Donaldson and Brown 1977). Basalt-hosted high-anorthite plagioclase macrocrysts have also been understood as disaggregated mush remnants in a range of geological settings (Hansen and Grönvold 2000;Ridley et al 2006). Moreover, isotope and trace element disequilibria between entrained macrocrysts and their carrier liquids indicate that crystals and melts are often derived from different mantle melt distributions (Halldórsson et al 2008;Winpenny and Maclennan 2011;Lange et al 2013b).…”

Section: Introductionmentioning

confidence: 99%

Continuous mush disaggregation during the long-lasting Laki fissure eruption, Iceland

Neave

Buisman

Maclennan

2017

American Mineralogist

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Plagioclase textures were investigated in the products of the voluminous 1783-1784 CE Laki eruption from the Eastern Volcanic Zone (EVZ) of Iceland to establish whether mush disaggregation occurred solely at the onset of the eight-month eruption or throughout its whole duration. Phase proportions and plagioclase size distributions were determined using standard optical and manual techniques as well as automated approaches based on Quantitative Evaluation of Minerals by SCANing electron microscopy (QEMSCAN). Based on optical microscopy and the explicit combination of textural and compositional information in QEMSCAN images, plagioclase crystals were divided into two populations: small (<0.5 mm long), high-aspect ratio (length/width >4) microcrysts with low-anorthite (0.5 mm long), low-aspect ratio (length/width = 2-3) macrocrysts with high-anorthite (An 84 -An 92 ) cores. Small microcrysts grew from their carrier liquid during the final phase of pre-eruptive crystallization while large macrocrysts, which are out of geochemical equilibrium with their carrier liquids, were entrained from crystal mushes. Changes in phase proportions and plagioclase size distributions between eruptive episodes demonstrate that macrocryst entrainment efficiency varied substantially during the eruption; material erupted in later episodes contain proportionally more mush-derived material. Using stereologically corrected plagioclase size distributions, we estimate that the pre-eruptive residence times of microcrysts in the Laki carrier liquid were probably of the order of 2-20 days. Because microcryst crystallization was concurrent with macrocryst rim growth, these day-to-week residence times also indicate that macrocryst entrainment occurred on much shorter timescales than the eruption's eight-month duration. In line with constraints from independent geochronometers, macrocryst entrainment and mush disaggregation thus appears to have continued throughout the eruption. Magmas were assembled on an episode by episode basis, and the volume of eruptible magma in the plumbing system at any given time was probably closer to 1-2 km 3 than the final erupted volume of 15.1 km 3 .

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

Cited by 48 publications

References 73 publications

Structure and dynamics of ridge axial melt lenses in the Oman ophiolite

Structure and dynamics of ridge axial melt lenses in the Oman ophiolite

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

Continuous mush disaggregation during the long-lasting Laki fissure eruption, Iceland

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