| Summary: | Alkali basalts are silica undersaturated rocks, essentially containing clinopyroxene, plagioclase and olivine phenocrysts in a fine grained groundmass. In Iceland, alkali basalts are mainly found in Snæfellsnes belonging to the Snæfellsnes Volcanic System and in Vestmannaeyjar belonging to the East Volcanic System. Samples were collected from the alkali basalt in Búðahraun, Snæfellsnes, for chemical analysis in order to determine the processes responsible for the complex zonation patterns of clinopyroxene crystals. All hand specimens have a considerable amount of vesicles and phenocrysts of olivine, plagioclase and clinopyroxene, with clinopyroxene being the largest and olivine the most abundant. 4 thin sections were made from selected hand specimens and 21 points in 9 cpx were analysed by electron microprobe. Chemical compositions were compared to those from the Ph.D thesis of Harðarson (1993) which show a similar distribution. The anhedral and subhedral shapes of various phenocrysts suggest melting and recrystallization. In order to determine the crystallization environment, the thermobarometry models provided by Putrika (2008) were applied. These show that crystallization of clinopyroxene cores and rims happened in a high pressure environment at great depths, probably around 10-20 km. Based on the standard errors for the equations the cores and rims of the analysed phenocrysts, probably formed in a similar P environment, with an exception of one pyroxene rim, which might have crystallized upon when magma ascent from the magma reservoir. The various zoning patterns observed are therefore best explained by magma mixing in large depth. Alkalí basalt er kísilsnautt berg sem inniheldur klínópýroxen, plagíóklas og ólivín díla í fínkornóttum grunnmassa. Á Íslandi er alkalí basalt helst að finna á Snæfellsnesi, sem tilheyrir Snæfellsnes eldstöðvakerfinu og í Vestmannaeyjum sem tilheyrir Austur eldstöðvakerfinu. Bergsýnum var safnað úr Búðahrauni á Snæfellsnesi til efnagreiningar. Niðurstöðurnar voru notaðar til að ...
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