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| Titel |
Constraining metasomatism in the oceanic lithosphere |
| VerfasserIn |
Oliver Plümper, Andreas Beinlich, Håkon Austrheim |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250042965
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| Zusammenfassung |
| Serpentinization is the most prominent fluid-mediated alteration process in the
oceanic lithosphere, but the physical and chemical conditions of this process are
difficult to constrain. It is crucial to establish a framework of mineralogical markers
that constrain (a) whether the reaction proceeded without substantial addition of
elements from the fluid (isochemical), (b) the reaction is isovolumetric generating no
internal stresses and (c) if the overall system was closed with respect to certain
elements. We have examined ophiolitic metaperidotites from Norway, combining
microtextural and microchemical observations to gain further insight into the complex
fluid-mediated phase transformations occurring during the alteration of the oceanic
lithosphere. Serpentinization can be isovolumetric, resulting in pseudomorphic
mineral replacement reactions (e.g. Viti et al., 2005), or produce an observable
volume increase (e.g. Shervais et al., 2005). In the case of olivine, the ideal reaction is
commonly written as forsteritic olivine reacting to lizardite and brucite, i.e. 2 Mg2SiO4 +
3 H2O - Mg3[Si2O5](OH)4 + Mg(OH)2, implying a total volume increase of
approximately 20%. However, if Mg was lost from the system, the reaction can
also be written as 2 Mg2SiO4 + 2 H+ + H2O - Mg3[Si2O5](OH)4 + Mg2+. This
suggests that the solid volume is preserved and no internal stresses are generated.
Therefore, the presence of brucite could be used to constrain volumetric changes during
serpentinization. However, the small size and sparse distribution of brucite makes it
difficult to find in serpentinized metaperidotites. Here we show that micro-Raman
spectroscopy is a reliable tool to identify even nanometer-sized brucite in serpentine. In
addition, we also used the electron backscatter diffraction (EBSD) technique to
identify volume increase illustrated by the progressive change of olivine orientation
at the tip of a crack induced by serpentinization. Furthermore, it is important to
constrain the degree of system openness and the transport of elements through
the fluid phase. Observations from fractures in metapyroxenite layers from the
Røragen-Feragen ultramafic complex provide closer insight into the late stage alteration of
the oceanic lithosphere. Detailed electron microscopy reveals that these fractures
are filled with polyhedral serpentine, indicating late stage open system conditions
(Andreani et al., 2007). However, microtextures and reactive transport modeling
suggest that Ca from clinopyroxene dissolution in the metapyroxenite layers was
instantaneously precipitated as andradite within the fracture, without major Ca
transport. Hence, although the overall system can be regarded as open for water, Ca
exhibits closed system behavior on the decimeter scale within the metapyroxenite
layers. Our observations show that mineralogical and microtextural markers, such as
characteristic phases, their spatial relationship and stress generation associated with
replacement, provide an insight into the metasomatic conditions of oceanic lithosphere
alteration.
References:
Andreani et al. (2007), Geochem. Geophys. Geosyst., 8 (2).
Shervais et al. (2005), Int. Geol. Rev., 47, 1-23.
Viti et al. (2005) Min. Mag., 69 (2), 491-507. |
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