Magmatic evolution of Hekla, Iceland

✍ Scribed by W. Scott Baldridge; Thomas R. McGetchin; Fred A. Frey; E. Jarosewich

• Publisher: Springer
• Year: 1973
• Tongue: English
• Weight: 837 KB
• Volume: 42
• Category: Article
• ISSN: 0010-7999
• DOI: 10.1007/bf00371589

✦ Synopsis

The extrusive rocks of ttekla are predominantly flows of basaltic andesite and andesite (ioelandite) but each eruptive cycle is initiated by production of tephra of andesitie, dacitie, and even rhyolitic composition. The evolution of basaltic andesites to dacites and rhyolites can be explained by crystallization and (presumably gravitative) separation of olivine, titaniferous magnetite, plsgioelase, and probably augite. No contamination by sialic crustal material is required.

Although basalts are never erupted from Hekla the origin of the basaltic andesites is probably best explained by separation of magnesian olivine, augite, and calcic plagioclase from an olivine tholeiite parent, producing an initial differentiation trend toward a high Fe/Mg ratio. The increase in Fe/Mg ratio is limited by the appearance of magnetite as a liquidus phase.

From the Fe/Mg ratios of the lavas and from compositions of the plagioclase phenocrysts the water pressure of the basaltic andesites is estimated to have been between 0.6 and 2.4 kb. Total pressure may have been significantly higher. A best estimate for the water content is approximately 21/2 to 6 weight percent. This high water content accounts for the explosive initiation of each eruptive cycle and is consistent with fractional crystallization in a shallow magma chamber.