Fractionation of Peralkaline Silicic Magmas: the Greater Olkaria Volcanic Complex, Kenya Rift Valley

Marshall, A. S.; MacDonald, Raymond J.; Rogers, N. W.; Fitton, J. Godfrey; Tindle, A. G.; Nejbert, Krzysztof; Hinton, R. W.

doi:10.1093/petrology/egp001

Cited by 78 publications

(78 citation statements)

References 48 publications

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“…The majority of Boseti glasses fall below the obsidian field, suggesting that they have lost some Na relative to their pristine melt compositions. Data from Supplementary Table 1 (Marshall et al 2009) and there is also an excellent correlation between the parameters in the experimental glasses from Eburru (Scaillet and Macdonald 2006). Revised Na 2 O values for the Boseti glasses, were calculated from the FK/A ratio and are in good agreement (to within 0.5 wt.%) with those estimated from the Na 2 O-FeO* plot (Fig.…”

Section: Geochemistrysupporting

confidence: 54%

“…Eruption may require explosive activity; the most differentiated compositions may therefore occur as pyroclastic, especially ash fall, deposits. This has been shown at the Menengai trachyte volcano (Leat et al 1984) and the Olkaria rhyolitic complex (Marshall et al 2009) but we are unaware of any published, systematic geochemical study of the fall deposits in a pantellerite centre.…”

Section: Significance Of Glass Analysesmentioning

confidence: 88%

“…It is seldom clear in peralkaline rhyolites what stabilises one oxide over the other (Macdonald et al 2011). In the Mahood and Stimac (1990); Olkaria natural rocks, Marshall et al (2009); Olkaria experimental glasses, Scaillet and Macdonald (2003); Menengai, Macdonald et al (2011) Boseti rocks, an oxide phase occurs as phenocrysts only in B355, where it takes two forms: in one, titanomagnetite (X usp 70.4) rims ilmenite (X ilm 96.5) and in the second, it is a matrix phase (X usp 66.8) of much later crystallization than the ilmenite (X ilm 94.9-95.6) enclosed in an adjacent olivine. It might appear, therefore, that at Boseti the titanomagnetite is stabilised, relative to ilmenite, in lower temperature, more peralkaline melts.…”

Section: Feti-oxidesmentioning

confidence: 99%

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Evidence for extreme fractionation of peralkaline silicic magmas, the Boseti volcanic complex, Main Ethiopian Rift

et al. 2011

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Matrix glass and melt inclusions in phenocrysts from pantellerite lavas of the Boseti volcanic complex, Ethiopia, record extreme fractionation of peralkaline silicic magma, with Al 2 O 3 contents as low as 2.3 wt.%, FeO* contents up to 17 wt.% and SiO 2 contents~65 wt.%. The new data, and published data for natural and experimental glasses, suggest that the effective minimum composition for peralkaline silicic magmas has~5 wt.% Al 2 O 3 , 13 wt.% FeO* and 66±2 wt.% SiO 2 . The dominant fractionating assemblage is alkali feldspar + fayalite + hedenbergite + oxides±quartz. Feldspar -melt relationships indicate that the feldspar is close to the minimum on the albiteorthoclase solid solution loop through the entire crystallization history. There is petrographic, mineralogical and geochemical evidence that magma mixing may have been a common process in the Boseti rhyolites.

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Section: Geochemistrysupporting

confidence: 54%

Section: Significance Of Glass Analysesmentioning

confidence: 88%

Section: Feti-oxidesmentioning

confidence: 99%

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Evidence for extreme fractionation of peralkaline silicic magmas, the Boseti volcanic complex, Main Ethiopian Rift

et al. 2011

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“…This is a complex of peralkaline rhyolite lava flows erupted from at least 13 centers over the last 20 kyear (Marshall et al, 2009). The youngest of these is the Ololbutnot flow which has a C 14 date of 180 ± 50 year BP (1720-1820) derived from carbonized wood associated with a pumice flow.…”

Section: Olkaria (∼1800)mentioning

confidence: 99%

Historical Volcanism and the State of Stress in the East African Rift System

et al. 2016

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Crustal extension at the East African Rift System (EARS) should, as a tectonic ideal, involve a stress field in which the direction of minimum horizontal stress is perpendicular to the rift. A volcano in such a setting should produce dykes and fissures parallel to the rift. How closely do the volcanoes of the EARS follow this? We answer this question by studying the 21 volcanoes that have erupted historically (since about 1800) and find that 7 match the (approximate) geometrical ideal. At the other 14 volcanoes the orientation of the eruptive fissures/dykes and/or the axes of the host rift segments are oblique to the ideal values. To explain the eruptions at these volcanoes we invoke local (non-plate tectonic) variations of the stress field caused by: crustal heterogeneities and anisotropies (dominated by NW structures in the Protoerozoic basement), transfer zone tectonics at the ends of offset rift segments, gravitational loading by the volcanic edifice (typically those with 1-2 km relief) and magmatic pressure in central reservoirs. We find that the more oblique volcanoes tend to have large edifices, large eruptive volumes, and evolved and mixed magmas capable of explosive behavior. Nine of the volcanoes have calderas of varying ellipticity, 6 of which are large, reservoir-collapse types mainly elongated across rift (e.g., Kone) and 3 are smaller, elongated parallel to the rift and contain active lava lakes (e.g., Erta Ale), suggesting different mechanisms of formation and stress fields. Nyamuragira is the only EARS volcano with enough sufficiently well-documented eruptions to infer its long-term dynamic behavior. Eruptions within 7 km of the volcano are of relatively short duration (<100 days), but eruptions with more distal fissures tend to have lesser obliquity and longer durations, indicating a changing stress field away from the volcano. There were major changes in long-term magma extrusion rates in 1977 (and perhaps in 2002) due to major along-rift dyking events that effectively changed the Nyamuragira stress field and the intrusion/extrusion ratios of eruptions.

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“…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.…”

mentioning

confidence: 99%

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

Carlo

Rotolo

Scaillet

et al. 2010

Journal of Petrology

130

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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...

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Fractionation of Peralkaline Silicic Magmas: the Greater Olkaria Volcanic Complex, Kenya Rift Valley

Cited by 78 publications

References 48 publications

Evidence for extreme fractionation of peralkaline silicic magmas, the Boseti volcanic complex, Main Ethiopian Rift

Evidence for extreme fractionation of peralkaline silicic magmas, the Boseti volcanic complex, Main Ethiopian Rift

Historical Volcanism and the State of Stress in the East African Rift System

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

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