Geology topics

R.S.J. Sparks

Publications and source records attributed to R.S.J. Sparks.

3 recordsLinked to original sources

USGS2018

The 1875 eruption of Askja volcano, Iceland: Combined fractional crystallization and selective contamination in the generation of rhyolitic magma

Major and trace element and Sr, Nd and O isotopic data are presented for ferrobasalts, icelandites, rhyolites, mixed pumices and silicic xenoliths of the 1875 eruption of Askja. Trace element modelling and Sr and Nd data largely confirm previous major element calculations that fractional crystallization was dominant in the generation of the basalt-ferrobasalt-icelandite-rhyolite suite. Relative enrichment in Rb (and Th and U?), depletion in Cs, and low values of δ 18 O/ 16 O, in the rhyolites are not explained by this mechanism alone. The silicic magmas were selectively contaminated by diffusion from partially molten granitic wall rocks, now found as xenoliths in the eruptive products, the process being particularly marked by lower δ 18 O and Cs/Rb ratios in the rhyolites than in the associated basalts. This is the first record of a combined fractional crystallization-selective contamination process in an Icelandic silicic complex.

Askja volcano↗

USGS1999

Chapter 2: Geological and tectonic setting of Santorini

Santorini lies in an area of complex extensional and subduction-related tectonics in a continental environment (Jackson 1994). The region has had a long geological history with large changes occurring during the Alpine orogeny and in the Late Tertiary as a consequence of post-collisional extension and major re-adjustments of the plate boundaries. The Aegean region is thought to be moving towards the southwest where the Aegean microplate overrides the Eastern Mediterranean sea floor. The collision has created the Hellenic trench to the south of Crete where the eastern Mediterranean sea floor is subducting beneath the Aegean Sea at 5-6 cm a-1. Santorini is one of several Quaternary volcanoes that define the present day active volcanic arc related to this subduction zone (Fig. 2.1). We here provide a synopsis of the broader geological and tectonic context of Santorini within the Aegean area. The geology, isotope geochemistry and geochronology of the crystalline basement of the southern Aegean is of particular relevance as it is likely that the Santorini magmas have interacted extensively with the continental crust. Geochemical evidence presented in Chapters 6 and 7 confirms this prediction. We also give a synopsis of the Quaternary Aegean Island Arc, of which Santorini is the most active centre.

Santorini↗

USGS1999

Appendix 1: Analytical methods and errors

Thirty eight K-Ar and eight 40Ar/39Ar high-precision age determinations were made at the US Geological Survey, Menlo Park, on a total of 22 rocks from the entire volcanic field. Duplicate or triplicate determinations were carried out on 14 samples in order to improve analytical precision. All ages were measured on whole-rock samples selected after thin-section examination. Most of the samples meet the usual criteria for whole-rock dating (Mankinen © Dal-rymple 1972), but some contain minor amounts of glass and a few samples are very glassy. The samples selected for dating were crushed to 0.5-lmm (-18 to +35 mesh). For K-Ar dating aliquots weighing c. 25 g were used for the Ar measurements. A 10 g aliquot was ground to -200 mesh and splits of the powder were used for K20 measurements, which were made in duplicate on each of two separate splits of sample powder by flame photometry after lithium metaborate fusion and dissolution (Ingamells 1970). Ar analyses were by isotope-dilution mass spectrometry using a high-purity (>99.9%) 38Ar tracer and techniques and equipment described previously (Dalrymple & Lanphere 1969). All samples for Ar extraction were baked overnight at 280°C. Mass analyses were done on a 22.68 cm radius, multiple-collector mass spectrometer with a nominal 90° sector magnet, using automated data collection (Stacey etal 1981). Errors given for the calculated K-Ar ages of individual measurements are estimates of the standard deviation of analytical precision. The errors were calculated using formulae derived by Cox & Dalrymple (1967) and Dalrymple & Lanphere (1969).

Geological Society, London, Memoirs↗