Major and trace element variations during the initial cooling of an Icelandic lava

Watkins, N. D.; Gunn, Bernard M.; Coy-Yll, R.

doi:10.2475/ajs.268.1.24

Cited by 25 publications

(6 citation statements)

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“…The author agrees with Watkins, Gunn & Coy-yll (1970) in their view that changes in Fe-Ti-oxides due to differences in initial-cooling oxygen fugacities play a very important role in the development of basic extrusives during solidification. Unfortunately the behaviour of the Fe-Ti-oxides is frequently ignored in petrological work, and the way in which some petrologists have chosen to get around this problem, namely the freezing of the Fe 2 0 3 /FeO ratio at some arbitrary value when comparing different rocks, is not in the opinion of the author an acceptable solution.…”

Section: Discussionsupporting

confidence: 85%

“…Amongst previous studies of variations within single extrusive bodies, post-eruptional processes have been inferred mainly in studies on lava lakes (Peck, Wright & Moore, 1966;Richter & Moore, 1966;Moore & Evans, 1967;Evans & Moore, 1968), while variations within lava flows have been ascribed to pre-eruptional differentiation -the most differentiated magma being first extruded, and later fractions becoming more basic -by , MacDonald (1967). Watkins, Gunn & Coy-yll (1970) ascribe the variations within an Icelandic lava as being partly due to the effects of initial-cooling oxygen fugacity gradients, but find it necessary also to invoke the possibility that the magma may have changed composition during eruption.…”

Section: Discussionmentioning

confidence: 99%

“…The oxidation development described can be accompanied by the oxidation of titanomagnetite to titanomaghemite, this oxidation, however, being restricted to Oxidation Indices I-III. Watkins, Gunn & Coy-yll (1970) used the Oxidation Indices of Watkins & Haggerty (1967) but extended the classification to include the development of ilmenite in discrete grains. They state that the oxidation development of discrete ilmenite grains corresponds to the development of ilmenite lamellae in titanomagnetite, starting at Oxidation Index IV, discrete ilmenite grains thus being unaltered through Oxidation Indices I-III.…”

Section: Opaque Mineralsmentioning

confidence: 99%

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Mineralogical and geochemical variations across three basaltic lava flows from the Faeroe Islands

Jensen

1980

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Three basaltic lava flows from the Faeroe Islands, one from the lower, one from the middle and one from the upper lava series, have been sampled with a maximum vertical separation between samples of 2 m, and with two parallel sample profiles for each flow. The investigations based on this close sampling have shown that significant mineralogical and chemical variations are to be found within single extrusive bodies. On the basis of 97 chemical analyses, microscopic investigations comprising modal analyses and determination of the oxidation stages of the Fe-Ti-oxides, and electron microprobe analyses of plagioclase, pyroxenes and Fe-Ti-oxides, the mineralogical and geochemical variations across the flows are described. Only averages of the chemical analyses for each flow are presented, but the results of all the 97 analyses can be seen in the figures, which give the variations across the flows for the main oxides, except P2O5 which hardly varies. In a TiO,-K20-P2O5 diagram the samples from the flows from the lower and upper series fall in the oceanic field, whereas the samples from the flow from the middle series cross the border between the oceanic and non-oceanic fields. In a F1-F2 diagram the samples from the lower and upper series fall in the within-plate basalts field, whereas the samples from the middle series fall in the field common to calc-alkali basalts and lowpotassium tholeiites. In a F,-F3 diagram the samples from the middle series fall mainly in the field of low-potassium tholeiites; the contact samples fall in the middle of this field, while most of the other samples trend towards the field of ocean-floor basalts, and two samples from the middle of the flow actually reach the field of ocean-floor basalts. Plotted in a total alkalies versus silica diagram the samples all fall in the tholeiitic field well below the dividing line of Macdonald & Katsura ( 1964 ). The variations in the flows from the lower and middle series are interpreted as being due mainly to pre-eruptional processes whereas the variations in the flow from the upper series are believed to be due entirely to post-eruptional processes.

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Section: Discussionsupporting

confidence: 85%

Section: Discussionmentioning

confidence: 99%

Section: Opaque Mineralsmentioning

confidence: 99%

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Mineralogical and geochemical variations across three basaltic lava flows from the Faeroe Islands

Jensen

1980

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“…The aim of this study is to provide information about the extent and location of mixing and the associated length scales of variability by study of the geochemical variation within a single lava flow. Detailed sampling of single flows on submerged parts of the mid‐ocean ridge system is difficult and expensive [ Perfit and Chadwick , 1998; Rubin et al , 2001], so most studies of variability within flows have been limited to ocean islands such as Hawaii [ Rhodes , 1983] and Iceland [ Watkins et al , 1970; Lindstrom and Haskin , 1981; Sigmarsson et al , 1991]. The only detailed study of compositional variability in a single recent flow from Iceland to date is that of the 1783–1784 Lakagigar eruption in southern Iceland [ Sigmarsson et al , 1991].…”

Section: Introductionmentioning

confidence: 99%

Geochemical variability in a single flow from northern Iceland

Maclennan¹,

McKenzie²,

Hilton³

et al. 2003

J. Geophys. Res.

102

116

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[1] Compositional variability is present for almost every element analyzed in 70 whole rock samples and 40 olivine-hosted melt inclusions from the Borgarhraun lava flow in NE Iceland. The variation of incompatible element concentrations can be produced by incomplete mixing of fractional melts, while the compatible element variations in the whole rock samples can be explained by addition/removal of crystals found in the flow. The melt inclusion incompatible element compositions are more variable than the whole rock compositions, and the magnitude of variability of the whole rock samples can be matched if each hand specimen is made of magma that was formed by mixing of 20-30 batches of melt with the composition of the inclusions. Clinopyroxene barometry and the major element composition of the whole rock samples suggest that mixing took place at pressures of $0.9 GPa in sub-Moho magma chambers. The spatial distribution of the concentrations of incompatible elements is not random in Borgarhraun and samples with a separation of <4 km have concentrations that are more similar than expected from a random distribution of geochemistry. This separation distance corresponds to a lava volume of 0.014-0.14 km 3 . This volume may be controlled by magma mixing and episodic eruption of melts from a sub-Moho chamber. The concentration variations of compatible elements with position in the flow are little different to those expected for random variation. It is likely that the crystal removal and addition processes that control major element compositions generate variability on short length scales.

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“…After the removal of all obvious signs of surface contamination, the four minicores were crushed into small pieces and combined to form a single composite sample to represent the unit. We consider the use of a composite sample to be justified because previous workers have demonstrated that individual Icelandic tholeiitic lava flows are relatively homogeneous in trace element composition [Watkins et al, 1970;Hart et al 1971;Wood et al, 1976] displaying little primary vertical variation in the abundances of the relatively immobile elements, and because the samples were taken very close to each other. The relative homogeneity of individual flows is confirmed by our own studies (see section on intraflow trace element variation).…”

Section: Introductionmentioning

confidence: 99%

The trace element composition of the lavas and dikes from a 3‐km vertical section through the lava pile of eastern Iceland

Gibson¹,

Kirkpatrick

Emmerman³

et al. 1982

J. Geophys. Res.

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The Tertiary lava succession of eastern Iceland dips at shallow angles towards and beneath the axial Quaternary volcanic zone. A 3‐km vertical section through part of this Tertiary pile, studied during the Iceland Research Drilling Project, contains flows ranging in composition from basalt to icelandite. This succession can be divided into three distinct stratigraphic groups: upper and lower groups, 1.7 and 0.6 km thick with Zr/Y ratios of about 4.5, and a 0.7‐km‐thick middle group with a Zr/Y ratio of about 6.5. The middle group also has a significantly higher Ce/Yb ratio and higher elemental Sr abundances. The data on within‐group variation are equally compatible with high‐level fractionation of a plagioclase‐olivine‐clinopyroxene assemblage with accessory apatite and opaque minerals or with open system fractionation models. Dikes occur throughout the 3‐km section and are comparable in trace element composition to the lavas of the upper group. The dikes are part of a N‐S trending swarm originating from the Breiddalur volcanic center some 15 km to the south. These dikes were injected laterally, northward into the Reydarfjordur lava pile at the time of the eruption of the upper group lavas. The lower two major lava groups may be related to similar, but unidentified, discrete volcanic centers perhaps buried downdip. The new data on this deeper section corroborate earlier studies which indicated that on a gross scale, the eastern Iceland lava pile becomes less mafic with depth. This vertical change occurs because each major volcanic center erupted more silicic lavas close to the center and more magnesium‐rich compositions toward the periphery. The formation of the lava pile occurred by tilting toward the spreading center and overlapping repetition of such major units which resulted in the more mafic edges of the lava groups forming the topographically higher parts of the crustal section.

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Major and trace element variations during the initial cooling of an Icelandic lava

Cited by 25 publications

References 0 publications

Mineralogical and geochemical variations across three basaltic lava flows from the Faeroe Islands

Mineralogical and geochemical variations across three basaltic lava flows from the Faeroe Islands

Geochemical variability in a single flow from northern Iceland

The trace element composition of the lavas and dikes from a 3‐km vertical section through the lava pile of eastern Iceland

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