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| Titel |
Melt fractionation during pāhoehoe flow lobe emplacement, Heiðin há lava, SW Iceland |
| VerfasserIn |
Paavo Nikkola, Thorvaldur Thordarson  |
| 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 |
250131148
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| Publikation (Nr.) |
 EGU/EGU2016-11520.pdf |
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| Zusammenfassung |
| Melt segregations are vesicular formations of evolved melts generated by in situ
closed system fractionation of a host lava. Although they are common in p¯a hoehoe
flows, pillow basalts, lava lakes and shallow intrusions, their development is not
fully understood. In addition, as the melt segregations are often confined to the
scale of a single outcrop, they can be seen as an easily approachable analogue to
the crystal-melt fractionation processes generating evolved magmas in the Earth’s
crust.
An eight meter high p¯a hoehoe flow lobe in Heiðin há lava, SW Iceland, was sampled in
order to understand the development of the elaborate segregation structures within. The
sampled outcrop is a cross-section of a typical Icelandic p¯a hoehoe lava, belonging to
a large post-glacial lava shield on Reykjanes Peninsula. The lava core is striped
by melt segregations in the form of vertical vesicle cylinders 1–7 cm in diameter,
which feed horizontal vesicle sheets higher up in the upper lava core and lower
crust.
Whole-rock major and trace element results for the 20 samples from the Heiðin há lava reveal
a homogenous olivine tholeiitic host lava intersected by segregations of varying composition.
The vesicle cylinders in the flow core are only mildly differentiated, but the segregated melt
evolves upwards to horizontal vesicle sheets, from which some have experienced an
additional enrichment possibly by a gas filter-pressing of the residual liquid in the horizontal
sheet. The most evolved segregations are extremely Fe-rich with 19.5 % FeOtot in
comparison to the average of 12.4 % FeOtot in the host lava. Consequently, MgO drops from
the host lava’s 9.5 % to 4.4 % in the segregation sheets. In addition, segregations are enriched
by a factor of ∼2–2.5 in TiO2, K2O, P2O5 and incompatible elements Zr, Nb, Y
and V. As a consequence of the closed system behavior, geochemical trends are
evident between the host lava, vesicle cylinders, and vesicle sheets of different
types. |
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