Research Interests

Zeolite facies mineralisation in the Hvalfjördur area, Iceland

Research Collaboratore:
Dr. Rune Selbekk, Natural Museum Oslo, Norway

The Hvalfjördur area, 30 km north of Iceland´s capital Reykjavik, belongs to the sequence of late Tertiary to early Quaternary flood basalts with minor interlayer of hyaloclastites and rhyolites. Rutten 1958). The basalts are affected by a low temperature zeolite facies metamorphism, caused by the burial of the lava succession and higher heat flow influenced by the Laxárvogur and the Hvalfjördur located central volcano (Weisenberger & Selbekk 2009). Low-grade zeolite facies metamorphism of basaltic lavas in the Hvalfjördur field area results in two distinct mineral paragenesis that can be correlated to events in the burial and hydrothermal history of the lava pile. Stage 1a marks near surface alteration in which celadonite and silica were precipitated along primary pores. During burial, hydrolysis of olivine and glass led to the formation of mixed layers chlorite/smectite clays. The chlorite content of stage 1b precipitation increases with increasing burial depth, respectively temperature. Stage 2 occurred after burial and is marked by the zeolite mineralisation, caused by higher heat flow, from the Laxárvogur central volcano. Altogether eleven different zeolites were found in the Hvalfjördur area: analcime, chabazite, epistilbite, heulandite, laumontite, levyne, mesolite, stilbite, stellerite, thomsonite and yugawaralite Selbekk & Weisenberger 2005, Weisenberger & Selbekk 2009). Based on the work done by Walker (1960), zeolites were grouped into zeolite zone. In total three separate depth and temperature-controlled “zeolite zones” are described in the Hvalfjördur area: the upper chabazite/thomsonite zone, the middle mesolite zone and the lowest laumontite zone. The mineralisation temperature for zeolites increases from the upper chabazite/thomsonite zone to the lower laumontite zone. Empirical correlation between the depth distribution of zeolite zone and the temperatures of formation of zeolites in the geothermal system, a geothermal gradient of 133°C/km can be estimated, usual for central volcanoes. This indicate the occurrence of a Laxárvogur central volcano, which can be supported by geochemistry of  volcanic rocks and tectonic features in the Hvalfjördur area.

Rutten M.G.  (1958) Geological reconnaissance of the Esja-Hvalfell-Armannsfell area, southwestern Iceland. Verhandelingen van het Koninklijk Nederlands Geologisch-Mijnbouwkundig Genootschap, Geologische Seris XVII, 219-298

Walker G.P.L. (1960) Zeolite zones and dike distribution in relation to the structure of the basalts of eastern Iceland. Journal of Geology 68, 515-528

Selbekk R.S. & Weisenberger T.  (2005) Stellerite from the Hvalfjördur area, Iceland. Jökull 55, 49-52

Weisenberger T. & Selbekk R.S. (2009) Multi-stage zeolite facies mineralization in the Hvalfjördur area, Iceland. International Journal of Earth Sciences, 98, 985-999 (pdf)

Fig. 1: Geology of Iceland

Fig. 2: Simplified geological map of the Hvalfjördur area

Fig. 3: Zeolite stability

Fig. 4: Spatial and temporal development of pore-filling mineral assemblages in the Hvalfjördur area. The vertical axis depicts depth below land surface at the time of each event depicted in the figure. Time elapsed after eruption increases to the right. No scales are implied on the axis

Iceland spar from Helgustadir: geology, mineralogy and the influence to science development

Research Collaboratores:
Dipl. Geol. Simon Spürgin, Hauri, Mineralstoffwerk, Bötzingen, Germany,
Dr. Runes S. Selbekk, Natural Museum Oslo, Norway

The rock formations exposed in Iceland consists mostly of basalt lava sequences erupted from fissures or within central-volcanoes. Low-temperatures alteration minerals like zeolites and calcite are abundant in the tertiary basalts, especially in zones of alteration associated with central volcanoes. A site at Helgustadir (65°0´, 14°01´), Reydarfjördur  supplied large quantities of transparent cleavage rhombs of calcite (commonly called Iceland spar). These crystals played a significant role in the early development of several fields in the physical sciences.

Weisenberger T., Spürgin S. & Selbekk R.S. (2008) Die Fundstelle Helgustadir (Island): Geologie, Mineralogie und die bedeutende Geschichte des Isländischen Doppelspats für die Wissenschaft. Aufschluss 1, 53-63

Fig. 1: Helgustadir road sign

Fig. 2: Map of low temeprature alteration in East Iceland

Fig. 3: Iceland spar

Fluid control on low-temperature mineral formation in volcanic rocks of Kahrizak, Iran

Research Collaboratores: 
Kousehlar M., Tutti F., & Mirnejad H.
, University of Teheren, Iran

The Kahrizak volcanic field, south Tehran, in Iran, is composed dominantly of basalt and basaltic andesite that have experienced variable degrees of alteration due to the low-grade metamorphism (Stage I) and hydrothermal activity (Stage II). Stage I alteration, which occurred in response to the burial of volcanic rocks and their interaction with heated groundwater, is characterized by the formation of low-temperature zeolite facies minerals in vesicles consisting mainly of fine-grained mafic phyllosilicate (smectite, chlorite/smectite mixed layer) and zeolites (thomsonite, chabazite, gonnardite, natrolite, analcime, heulandite, and mordenite). Stage II mineralization occurred due to the activity of hydrothermal fluids that formed large crystals of heulandite, stilbite, mesolite/scolecite, natrolite, and analcime along with quartz and calcite in cavities and fractures. The elements necessary for the formation of these alteration minerals in Kahrizak were derived from the hydrolysis of olivine and volcanic glass as well as the alteration of plagioclase. Various mineral assemblages formed during stages I and II reflect changes in temperature, pressure, and fluid composition. The change from mafic phyllosilicates to zeolites species is caused by the decrease of Mg and Fe fluid activities. Zeolite assemblages of stage I, known to be formed at lower temperatures, show the general sequential order from older to younger: chabazite, thomsonite, gonnardite, and natrolite. This sequence is consistent with a hypothetical fluid evolution path with increasing Na+ relative to Ca2+ activity. The change to stage II, which consists of zeolites species that formed at higher temperatures, can be attributed to a temperature increase and fluid influx caused by hydrothermal activity related to a later magmatic event in the region.

Kousehlar M., Weisenberger T.B., Tutti F., & Mirnejad H. (2012) Fluid control on low-temperature mineral formation in volcanic rocks of Kahrizak, Iran. Geofluids 12 (4), 295-311. doi: 10.1111/gfl.12001 (pdf)