Intracrystalline melt migration in deformed olivine revealed by trace element compositions and polyphase solid inclusions
<p>Melt transport mechanisms have an important impact on the chemical composition of the percolated host rock and the migrating melts. Melt migration is usually assumed to occur at grain boundaries. However, microstructural studies revealed the occurrence of polyphase inclusions along dislocat...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-08-01
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| Series: | European Journal of Mineralogy |
| Online Access: | https://ejm.copernicus.org/articles/33/463/2021/ejm-33-463-2021.pdf |
| Summary: | <p>Melt transport mechanisms have an important impact on the chemical
composition of the percolated host rock and the migrating melts. Melt
migration is usually assumed to occur at grain boundaries. However,
microstructural studies revealed the occurrence of polyphase inclusions
along dislocations, subgrain boundaries and microcracks in single mineral
grains. The inclusions are interpreted as crystallized melt pockets
suggesting that melts can migrate within deformed crystals. Intracrystalline
melt migration and diffusive re-equilibration can lead to significant mineral
trace element enrichments when associated with dissolution–precipitation
reactions. In this contribution, we study a body of replacive troctolites
associated with the Erro-Tobbio ophiolitic mantle peridotites (Ligurian
Alps, Italy). The replacive formation of the olivine-rich troctolite
involved extensive impregnation of a dunitic matrix, i.e. partial
dissolution of olivine and concomitant crystallization of interstitial
phases. The olivine matrix is characterized by two distinct olivine
textures: (i) coarse deformed olivine, representing relicts of the pre-existing
mantle dunite matrix (olivine<span class="inline-formula"><sub>1</sub></span>), and (ii) fine-grained undeformed olivine, a product
of the melt–rock interaction process (olivine<span class="inline-formula"><sub>2</sub></span>). Previous studies documented a
decoupling between olivine texture and trace element composition, namely
enriched trace element compositions in olivine<span class="inline-formula"><sub>1</sub></span> rather than in
olivine<span class="inline-formula"><sub>2</sub></span>, as would be expected from the dissolution–precipitation process.
Notably, the trace element enrichments in deformed olivines are correlated
with the occurrence of elongated 10 <span class="inline-formula">µm</span> size polyphase inclusions
(clinopyroxene, Ti-pargasite, chromite) preferentially oriented along
olivine crystallographic axes. These inclusions show irregular contacts and
have no crystallographic preferred orientation with the host olivine, and
the phases composing the inclusions show similar chemical compositions to
the vermicular phases formed at the grain boundaries during late-stage
reactive crystallization of the troctolite. This suggests that the
investigated inclusions did not form as exsolutions of the host olivine but
rather by input of metasomatic fluids percolating through the deformed
olivine grains during closure of the magmatic system. We infer that strongly
fractionated volatile-rich melts were incorporated in oriented
microfractures within olivine<span class="inline-formula"><sub>1</sub></span> and led to the crystallization of the polyphase
inclusions. The presence of intracrystalline melt greatly enhanced diffusive
re-equilibration between the evolved melt and the percolated olivine<span class="inline-formula"><sub>1</sub></span>, in turn
acquiring the enriched character expected in neoformed olivine crystals.
Intracrystalline melt percolation can have strong geochemical implications
and can lead to efficient re-equilibration of percolated minerals and rocks.</p> |
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| ISSN: | 0935-1221 1617-4011 |