Volatile characteristics of peralkaline rhyolites from Kenya: an ion microprobe, infrared spectroscopic and hydrogen isotope study

et al. 2010

Journal of Petrology

130

We present experimental phase equilibria carried out on a pantelleritic bulk-rock composition with a peralkalinity index [PI INTRODUCTIONPantelleria island is the type-locality of pantellerite, an iron-and sodium-rich peralkaline rhyolite. Pantellerites, by far the most abundant rock-type at Pantelleria, occur as pyroclastic flows, pumice fall deposits, or as obsidianaceous lavas (Mahood & Hildreth, 1986). After the Plinian eruption of the Green Tuff (45 ka BP), and the related caldera collapse, resurgent felsic magmatism was mainly centered inside the caldera, producing either low-energy explosive eruptions or lava flows.Recent experimental investigations (Scaillet & Macdonald, 2001 and melt inclusion studies (Kovalenko et al., 1988;Webster et al., 1993;Wilding et al Barclay et al., 1996;Gioncada & Landi, 2010) have shown that peralkaline melts may be rather H 2 O-rich, in contrast to previous inferences. These studies have also demonstrated the role of pressure, water, oxygen fugacity, and silica and sodium disilicate activity (aSiO 2 , aNds) on the phase equilibria of felsic peralkaline magmas from the east Kenyan rift zone (e.g. Scaillet & Macdonald, 2001Caricchi et al., 2006). In this study we present the hydrous phase equilibria of a pantellerite with a peralkalinity index PI [ ¼ molar (Na 2 O þ K 2 O)/Al 2 O 3 ] ¼ 1·4, established within the pressure range 25^150 MPa, for temperatures between 680 and 8008C, with oxygen fugacities below the Ni^NiO solid buffer (NNO), and melt water contents (H 2 O melt ) up to 6 wt %. We apply our experimentally derived P^TĤ 2 O melt grid to natural pantellerites to assess the pre-eruptive conditions (T, P,H 2 O melt )ofthemostpowerful event of the post-caldera period, the Fastuca eruption (Orsi et al.,1989). Knowledge of the pre-eruptive conditions sheds light on the nature of the magmatic plumbing system associated with recent felsic volcanism at Pantelleria, with several obvious implications for the eruptive style of pantellerite magmas. The determination of magma storage conditions of pantelleritic magmas can also provide constraints on the more general problem of the origin of silica-oversaturated peralkaline magmas, which has been extensively debated (see Bohrson & Reid, 1997;Civetta et al., 1998;Marshall et al., 1998;Marshall et al. 2009;Scaillet & Macdonald, 2001;Nekvasil et al., 2004;Macdonald et al., 2008). Much of the controversy has arisen because of the lack of modern phase equilibrium constraints for peralkaline magmas, in contrast to their metaluminous or peraluminous counterparts (e.g. Dall' Agnol et al., 1999;Klimm et al., 2003;Bogaerts et al., 2006). GEOLOGICAL AND PETROLOGICAL BACKGROUNDPantelleria (Italy) is an entirely volcanic island ( Fig. 1) located within the Sicily Channel continental rift system, in the northern part of the African plate (e.g. Rotolo et al., 2006). Pantelleria is well known in the petrological literature because its eruptive products represent a typical bimodal suite: a mafic end-member, which consists of mildly alkali basalts, a...

Section: Introductionmentioning

confidence: 92%

Section: Experimental Strategy and Starting Materialsmentioning

confidence: 99%

mentioning

confidence: 92%

See 1 more Smart Citation

Phase Equilibrium Constraints on Pre-eruptive Conditions of Recent Felsic Explosive Volcanism at Pantelleria Island, Italy

Carlo

Rotolo

et al. 2010

Journal of Petrology

130

“…Together with comendites, the less strongly peralkaline rhyolite variety (1 < AI < 1.4), pantellerites usually occur in extensional tectonic settings, from oceanic islands (Ascension Island) to continental rift zones (Ethiopia, Kenya) and back arc settings (Mayor Island, New Zealand), generally in close spatial and temporal association with transitional basalts. Recent analytical studies have emphasised that peralkaline rhyolites, once thought to be relatively anhydrous , may have in fact up to several wt.% dissolved water under pre-eruptive conditions (Harris, 1986, Kovalenko et al, 1988, Lowenstern and Mahood, 1991, Webster et al, 1993, Wilding et al, 1993and Barclay et al, 1996. Experimental work on comendites from the Olkaria volcanic complex, Kenya, has confirmed the water-rich nature of peralkaline rhyolites, as well as the usual corollary, the rather low temperatures of equilibration (Scaillet andScaillet andMacdonald, 2003).…”

Section: Introductionmentioning

confidence: 93%

Experimental constraints on pre-eruption conditions of pantelleritic magmas: Evidence from the Eburru complex, Kenya Rift

MacDonald

2006

Lithos

To cite this version:Bruno Scaillet, Ray Macdonald. Experimental constraints on pre-eruption conditions of pantelleritic magmas: Evidence from the Eburru complex, Kenya Rift. Lithos, Elsevier, 2006, 91, pp.1-4, 95- AbstractThe phase relationships and compositions of a pantellerite from the Eburru complex in the Kenya Rift Valley have been determined at 150 MPa and under reducing conditions, 2 log units below the Ni-NiO solid buffer. The effects of temperature and melt water content on phase relationships have been explored. Alkali feldspar and quartz crystallise alone at temperatures above 700 °C, irrespective of melt water content. Below 700 °C, sodic amphibole and clinopyroxene also crystallise; the amphibole being the liquidus phase under water-rich conditions. The coexistence of amphibole phenocrysts with alkali feldspar and quartz in a crystal-poor pantellerite implies temperatures below 700 °C and melt water contents higher than 4 wt.%, possibly up to 5-6 wt.%. Pantellerites have lower liquidus temperatures than associated comendites, which supports a parent-daughter relationship between the two magma types. The melts produced in the experiments extend the compositional trend displayed by the natural rock series, and reproduce some extreme compositions occasionally observed in alkaline volcanic series, with FeO contents above 12 wt.% and Na 2 O contents approaching 10 wt.%. Pantellerites are therefore the true nearminimum melt compositions of alkaline oversaturated magma series.

“…Although Bailey (1974) and Bailey and Macdonald (1987) argued on geological and petrological grounds that the Olkaria comenditic magmas were relatively anhydrous, data from melt inclusions in quartz phenocrysts (Wilding et al, 1993) and experimental evidence (Scaillet and Macdonald, 2001) showed that the pre-eruptive magmas contained up to nearly 6 wt.% H 2 O. The experiments also suggested that the comendite phenocryst assemblages crystallized at relatively low fO 2 (at or below FMQ), at very low temperatures (between 740 and 660 °C) and under near water-saturated conditions.…”

Section: Greater Olkaria Volcanic Complexmentioning

confidence: 99%

The central Kenya peralkaline province: Insights into the evolution of peralkaline salic magmas

Macdonald

2006

Lithos

The central Kenya peralkaline province comprises five young (< 1 Ma) volcanic complexes dominated by peralkaline trachytes and rhyolites. The geological and geochemical evolution of each complex is described and issues related to the development of peralkalinity in salic magmas are highlighted. The peralkaline trachytes may have formed by fractionation of basaltic magma via metaluminous trachyte and in turn generated pantellerite by the same mechanism. Comenditic rhyolites are thought to have formed by volatile-induced crustal anatexis and may themselves have been parental to pantelleritic melts by crystal fractionation. The rhyolites record very low temperatures of equilibration (≤ 700 °C) at low fO 2 (≤ FMQ). The development of compositional zonation within the magma reservoirs has been ubiquitous, involving up to tens of cubic km of magma at timescales of 10 3 -10 4 years. Magma mixing has also been commonplace, sometimes between adjacent centres. Isotopic evidence relating to rates and timescales of pre-eruptive residence times and crystal fractionation processes is summarized.