Petrologic and microstructural constraints on focused melt transport in dunites and the rheology of the shallow mantle

Braun, M. G.

doi:10.1575/1912/1856

Cited by 21 publications

(27 citation statements)

References 77 publications

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“…Compared to REE abundance in cpx from other ophiolites, the measured heavy rare earth elements (HREE) profiles in cpx from our 4 dunites are similar to the ranges reported by Kelemen et al [1995a] and Braun [2004] for the Oman ophiolite and by Suhr et al [2003] for the Bay of Islands ophiolite (Figure 15b). The light rare earth elements (LREE) span the range between the Oman data [ Kelemen et al , 1995a; Braun , 2004] and the Bay of Islands data [ Suhr et al , 2003]. The cpx REE profiles show a great deal of overlap between all three ophiolites.…”

Section: Discussionsupporting

confidence: 82%

“…CaO in olivine within the dunite is greater than CaO in the surrounding harzburgite. The increase of CaO in olivine within the dunite is a ubiquitous feature that has been observed in dunite bodies in other ophiolites (Trinity [ Quick , 1981a]; Iwanidake [ Kubo , 2002]; Bay of Islands [ Suhr et al , 2003]; Oman [ Braun , 2004]; and Josephine (this study)) and is likely a result of subsolidus reequilibration (see section 5.3 for a discussion).…”

Section: Resultsmentioning

confidence: 63%

“…The cpx REE profiles show a great deal of overlap between all three ophiolites. The Trinity dunite is similar to dunite from the Bay of Islands [ Suhr et al , 2003] and Oman [ Kelemen et al , 1995a; Braun , 2004], with the Bay of Islands having more HREE depleted samples. The Trinity lherzolite samples are more enriched than either the Bay of Islands or Oman.…”

Section: Discussionmentioning

confidence: 87%

“…As in peridotite dissolution experiments, records of melt‐peridotite interaction can be preserved or partly preserved in the dunite‐harzburgite or DHL‐PL sequence. Indeed, mineral compositional variations across dunite‐harzburgite or DHL‐PL sequences from ophiolites or peridotite massifs have been reported in a number of studies [e.g., Quick , 1981a; Obata and Nagahara , 1987; Kelemen et al , 1992; Takahashi , 1992; Takazawa et al , 2000; Kubo , 2002; Suhr et al , 2003; Braun , 2004; Lundstrom et al , 2005; Zhou et al , 2005]. In general, mineral compositional variations observed in the field are more complicated than those observed in peridotite dissolution experiments, reflecting more complicated geological processes involved in the nature.…”

Section: Introductionmentioning

confidence: 99%

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Significance of the concentration gradients associated with dunite bodies in the Josephine and Trinity ophiolites

Morgan

Liang

Kelemen

2008

Geochem Geophys Geosyst

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[1] Detailed transects were sampled across dunite bodies in the Josephine and Trinity ophiolites. The major peridotite lithologies sampled in the Josephine transect are a sequence of dunite and harzburgite and in the Trinity transect a sequence of dunite, harzburgite, lherzolite, and plagioclase lherzolite (DHL-PL). Major, minor, and selected trace element abundances in olivine, orthopyroxene, clinopyroxene, and spinel were measured. The composition profile from the Josephine transect has revealed a concentration gradient near the dunite-harzburgite contact. The composition profile from the Trinity transect has revealed several concentration gradients: two within the dunite, one in the harzburgite, and at least two in the plagioclase lherzolite. The composition profiles record complex histories of melt transport, melt-rock reaction, and subsequent subsolidus reequilibration. Analyses of closure distance suggest that compositional variation trends for a majority of major and minor elements in olivine, clinopyroxene, orthopyroxene, and spinel reported in this study were magmatic in origin. Subsolidus reequilibration may reduce the range or magnitude of variations for the 2+ cations such as Fe and Mg in olivine and spinel and significantly redistribute Ca and Li in coexisting minerals. Numerical simulations exploring the coupling of diffusion and advection in a porous matrix were used to explain compositional variations across the peridotite sequences. Melt flow from the host harzburgite into the dunite produces composition gradients near the dunite-harzburgite contacts similar to those from the Josephine transect. In contrast, melt flow from the dunite into the surrounding peridotite lithologies can produce concentration gradients similar to those observed in the Trinity transect. At least two chemically distinct episodes of melt flow within the same dunite channel system are proposed. Results from this study show that concentration gradients developed around the dunite-harzburgite and DHL-PL sequences can be used to infer part of the melt flow history of the dunite channel systems in the mantle. Results from this study and those from other ophiolite studies also demonstrate that concentration profiles in dunite and the surrounding peridotite lithologies are highly variable even among differing peridotite sequences within the same ophiolite, suggesting that the composition of instantaneous melt flowing through individual dunite channels is quite variable and the mantle source regions are heterogeneous.

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Section: Discussionsupporting

confidence: 82%

Section: Resultsmentioning

confidence: 63%

Section: Discussionmentioning

confidence: 87%

Section: Introductionmentioning

confidence: 99%

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Significance of the concentration gradients associated with dunite bodies in the Josephine and Trinity ophiolites

Morgan

Liang

Kelemen

2008

Geochem Geophys Geosyst

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“…Secondary phases limit olivine grain growth in the harzburgites, as has been observed in mylonites (Warren and Hirth, 2006) and in Oman Ophiolite harzburgites and dunites (Braun, 2004). At a smaller grain size, some deformation is by DisGBS, which accommodates deformation by the movement of dislocations within crystals and by grain boundary sliding (Hirth and Kohlstedt, 2003;Drury, 2005;Warren and Hirth, 2006 (Bai et al, 1991).…”

Section: Effect Of Secondary Phasesmentioning

confidence: 86%

Geochemical and rheological constraints on the dynamics of the oceanic upper mantle

Warren¹

2007

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Grain‐size dynamics beneath mid‐ocean ridges: Implications for permeability and melt extraction

Turner

Katz

Behn

2015

Geochem. Geophys. Geosyst.

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Grain size is an important control on mantle viscosity and permeability, but is difficult or impossible to measure in situ. We construct a two-dimensional, single phase model for the steady state mean grain size beneath a mid-ocean ridge. The mantle rheology is modeled as a composite of diffusion creep, dislocation creep, dislocation accommodated grain boundary sliding, and a plastic stress limiter. The mean grain size is calculated by the paleowattmeter relationship of Austin and Evans (2007). We investigate the sensitivity of our model to global variations in grain growth exponent, potential temperature, spreading-rate, and mantle hydration. We interpret the mean grain-size field in terms of its permeability to melt transport. The permeability structure due to mean grain size may be approximated as a high permeability region beneath a low permeability region. The transition between high and low permeability regions occurs across a boundary that is steeply inclined toward the ridge axis. We hypothesize that such a permeability structure generated from the variability of the mean grain size may focus melt toward the ridge axis, analogous to Sparks and Parmentier (1991)-type focusing. This focusing may, in turn, constrain the region where significant melt fractions are observed by seismic or magnetotelluric surveys. This interpretation of melt focusing via the grain-size permeability structure is consistent with MT observation of the asthenosphere beneath the East Pacific Rise.Key Points:The grain-size field beneath MORs can vary over orders of magnitude The grain-size field affects the rheology and permeability of the asthenosphere The grain-size field may focus melt toward the ridge axis

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Petrologic and microstructural constraints on focused melt transport in dunites and the rheology of the shallow mantle

Abstract: of ThesisObservations at mid-ocean ridges indicate that magmas are focused to the ridge

Cited by 21 publications

References 77 publications

Significance of the concentration gradients associated with dunite bodies in the Josephine and Trinity ophiolites

Significance of the concentration gradients associated with dunite bodies in the Josephine and Trinity ophiolites

Geochemical and rheological constraints on the dynamics of the oceanic upper mantle

Grain‐size dynamics beneath mid‐ocean ridges: Implications for permeability and melt extraction

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