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| Titel |
Rapid growth of phosphorus-rich olivine in mantle xenolith from Middle Atlas
Mountains (Morocco, Africa) |
| VerfasserIn |
Ioannis Baziotis, Konstantinos Mavrogonatos, Stamatios Flemetakis, Angeliki Papoutsa, Stephan Klemme, Jasper Berndt, Paul Asimow |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250121704
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| Publikation (Nr.) |
 EGU/EGU2016-522.pdf |
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| Zusammenfassung |
| Phosphorus(P)-rich zones in olivine may reflect incorporation of P in excess of equilibrium
partitioning during rapid growth (e.g. Milman-Barris et al. 2008). We investigated a mantle
xenolith from Middle Atlas Mountains (Morocco) by optical microscopy and electron
microprobe. It contains spinel-bearing lherzolite and orthopyroxenite layers, cross-cut by
veins dominated by glass and secondary phases including P-rich olivines. The host lava,
presumed to be alkali basalt (El Messbahi et al. 2015), is present on the margins of the hand
sample but not included in our thin section.
The studied melt veins (MV) generally contain Ol+Gl+Cpx+Pl+Spl±Ap. Olivines in the
MV have (Fo72.1−83.4) with 0.02-0.3 wt.% P2O5; olivines with P2O5 >0.1 wt.% are
Fo75.3 −82.8. Some olivine grains are inclusion-free; others contain rounded glass inclusions
or subhedral spinel or ilmenite inclusions. Olivines is generally found in contact with
plagioclase and glass. Glass (5-15 vol%) has variable composition with P2O5 up to
1.52 wt.%, K2O 1.65-2.37 wt%, CaO 6.39-9.55 wt%, Na2O 0.78-6.70 wt% and
SiO2 45.2-49.6 wt%. Where glass is in contact with matrix olivine, Fe-rich outer
rims on olivine indicate mineral-melt reaction. In MgO variation diagrams, glass
compositions display a coherent single trend for all oxides, with the exception of
a discrete low-Na group. Clinopyroxene is present both as isolated subhedral to
euhedral crystals within the MV and as replacive rims on matrix minerals. Very
fine-grained dendritic clinopyroxene quench crystals up to 10 μm long are also
present. Plagioclase occurs as prismatic, flow-oriented crystals parallel or sub-parallel
to the layering. Spinel shows anhedral and euhedral shapes and occurs both as
inclusions in olivine and as discrete grains associated with plagioclase and glass.
Spinel in contact with glass shows a spongy outer rim and normal zonation towards
Fe-rich rim compositions. Apatite is found mostly as very small crystals embedded in
glass.
High-resolution X-ray mapping of P in olivine reveals narrow P-rich bands parallel to
crystal facets. P correlates negatively with Si4+, poorly with divalent cations (Mg+Fe+Ca),
and positively with Al3+, suggesting a substitution 2IV Si4+ = IV P5+ + IV R3+.
Furthermore, P is concentrated mainly at the rim of the olivine, in contact with surrounding
glass.
DPol∕melt has a wide range (0.02 to 1.6), with the lowest numbers thought to represent
equilibrium and higher numbers non-equilibrium partitioning via solute trapping during rapid
growth (e.g. Watson et al. 2015). The imperfect correlation between P and Al in our data
implies either diffusive relaxation of Al gradients or, judging by dynamic experiments (Grant
& Kohn, 2013), cooling rates ∼1-10˚ C/h that generate disequilibrium P solute trapping but
near-equilibrium Al incorporation. Early-crystallized olivine grew slowly enough to
incorporate P by equilibrium partitioning, suggesting that no P-rich boundary layer developed
despite slow diffusion of P in melts. Olivine rim crystallization, though was rapid
enough to over-enrich P, by more than can be associated with concentration of P
into a decreasing mass of residual melt (Shea et al. 2015). The apparent partition
coefficient between olivine rims and adjacent melt suggests DPol∕melt in the range
0.13-0.19.
References
El Messbahi et al. 2015. Tectonophysics 650, 34-52.
Grant, T. B. & Kohn, S. C. 2013. American Mineralogist 98, 1860-1869.
Milman-Barris et al. 2008. Contributions to Mineralogy and Petrology 155,
739-765.
Shea et al. 2015. Geology 43(10), 935-938.
Watson et al. 2015. American Mineralogist 100, 2053-2065. |
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