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| Titel |
Morphology of melt-rich channels formed during reaction infiltration
experiments on partially molten mantle rocks |
| VerfasserIn |
Matěj Peč, Benjamin Holtzman, Mark Zimmerman, David Kohlstedt |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250129620
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| Publikation (Nr.) |
 EGU/EGU2016-9758.pdf |
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| Zusammenfassung |
| Geochemical, geophysical and geological observations suggest that melt extraction from
the partially molten mantle occurs by some sort of channelized flow. Melt-solid
reactions can lead to melt channelization due to a positive feedback between melt
flow and reaction. If a melt-solid reaction increases local permeability, subsequent
flow is increased as well and promotes further reaction. This process can lead to
the development of high-permeability channels which emerge from background
flow.
In nature, anastomozing tabular dunite bodies within peridotitic massifs are thought to
represent fossilized channels that formed by reactive flow. The conditions under which such
channels can emerge are treated by the reaction infiltration instability (RII) theory (e.g.
Szymczak and Ladd 2014).
In this contribution, we report the results of a series of Darcy type experiments designed
to study the development of channels due to RII in mantle lithologies (Pec et al. 2015). We
sandwiched a partially molten rock between a melt source and a porous sink and
annealed it at high-pressures (P = 300 MPa) and high-temperatures (T = 1200˚
or 1250˚ C) under a controlled pressure gradient (∇P = 0-100 MPa/mm) for up
to 5 hours. The partially molten rock is formed by 50:50 mixtures of San Carlos
olivine (Ol, Fo ∼ 88) and clinopyroxene (Cpx) with either 4, 10 or 20 vol% of alkali
basalt added. The source and sink are disks of alkali basalt and porous alumina,
respectively.
During the experiments, silica undersaturated melt from the melt source dissolves Cpx
and precipitates an iron rich Ol (Fo ∼ 82) thereby forming a Cpx-free reaction layer at the
melt source – partially molten rock interface. The melt fraction in the reaction layer increases
significantly (40% melt) compared to the protolith, confirming that the reaction increases the
permeability of the partially molten rock. In experiments annealed under a low pressure
gradient (and hence slow melt flow velocity) the reaction layer is planar and no channels
develop. However, if the melt migration velocity exceeds ∼5 μm/s the reaction
layer locally protrudes into the partially molten rock forming finger-like melt-rich
channels. The morphology and spacing of the channels depends on the initial melt
fraction. With 20 vol% melt, multiple and voluminous channels with an elliptical core
formed of pure melt develop. At lower melt contents, fewer and thinner channels
develop.
Our experiments demonstrate that melt-rock reactions can lead to melt channelization in
mantle lithologies. The morphology of the channels seems to depend on the initial
permeability perturbations present in the starting material. The observed lithological
transformations are in broad agreement with natural observations. However, the resulting
channels lack the tabular anastomozing shapes which are likely caused by shear deformation
in nature. Therefore, both reaction-driven as well as stress-driven melt segregation have to
interact in nature to form the observed dunite channels.
Szymczak, P., and A. J. C. Ladd (2014), Reactive-infiltration instabilities in rocks. Part 2.
Dissolution of a porous matrix, J. Fluid Mech., 738, 591–630.
Pec, M., B. K. Holtzman, M. Zimmerman, and D. L. Kohlstedt (2015), Reaction
infiltration instabilities in experiments on partially molten mantle rocks, Geology, 43(7),
575–578, doi:10.1130/G36611.1. |
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