Shield Lavas Research Articles

The geochemistry of basalts from the Wallis Islands, northern Melanesian Borderland: Evidence for a lithospheric origin for Samoan-type basaltic magmas?

The Wallis Islands are located on the western extension of the Samoan seamount chain and are composed largely of basaltic flows and pyroclastics with ages that are less than 0.5 Ma. The youngest eruptives are tholeiitic in composition and are relatively depleted in large ion lithophile and high field strength trace and minor elements. In contrast, the older basaltic flows and pyroclastics, which make up the bulk of exposed volcanic rocks, have transitional alkalic chemistry and are relatively enriched in incompatible trace and minor elements. The younger tholeiites have lower 87Sr/ 86Sr (0.7045–0.7048) and tend to have higher 143Nd/ 144Nd (0.512844–0.512910) than the older alkalic rocks ( 87Sr/ 86Sr: 0.7049–0.7052; 143Nd/ 144Nd: 0.512800–0.512895). Major- and trace-element modelling based on relationships inferred from the isotopic and trace-element compositions of stratigraphically related samples supports the hypothesis that some of the evolved basalts could have been generated by crystal fractionation from less evolved parental basalts. The Quaternary volcanism on the Wallis Islands is too young to be directly related to passage of the Pacific Plate over the Samoan hot spot, but isotopically Wallis basalts are very similar to shield lavas of the Samoan Islands. This suggests that Wallis basaltic magmas were generated in the lithosphere by a later thermal disturbance that was probably related to deformation along the transform boundary between the Pacific and Indo-Australian plates. A model is proposed whereby the basalts derived from a complex, heterogeneous, lithospheric mantle by progressive melting which initially extracted relatively alkalic melts from mantle domains dominated by an enriched component. With time, as melting proceeded, the enriched component was diluted by a more depleted, and more refractory component. The relationship of the mantle plume, that generated the Samoan seamount chain, to the volcanism on the chain may be largely thermal with very little geochemical input from the plume into the erupted magmas. The northern Melanesian Borderland may be a region where unusual lithospheric mantle has existed for some time and the shield lavas of the Samoan chain, along with the Wallis basalts, may have been extracted largely from this lithosphere.

Geochemical and experimental study of the genesis of magmas of Reunion Island, Indian Ocean

Major element, trace element, and Sr and Nd isotopic compositions were determined on a suite of lavas and intrusives from Piton des Neiges and Piton de la Fournaise volcanoes of Reunion Island. The temperatures of appearance of mineral phases in six basalt samples were determined by melting and recrystallization experiments at pressures from 1‐atm to 2000 MPa. The compositions of minerals, both in the starting basalts and in the experimental run products, were analyzed by electron microprobe and compared to determine the temperature and depth of their formation in the basalts. The Sr and Nd isotopes and ratios of highly incompatible trace elements of basalt samples from both volcanoes are virtually identical, indicating that the mantle source for the magmas is chemically homogeneous, both in time and space. The lavas of the active volcano (Piton de la Fournaise) and the late stage Differentiated Series of Piton des Neiges are slightly more alkali‐rich than the primitive shield lavas of Piton des Neiges. The alkali enrichment could be the result of lower degrees of partial melting of the mantle for the young eruptions, but Zr and the rare earths do not fit this model. An alternative explanation is that the alkali enrichment resulted from contamination of a Piton des Neiges‐type magma with the low‐melting component of the lava pile. The melting experiments and least squares modeling indicate that augite was an important fractionating phase and that the fractionation of olivine plus augite occurred at depths of at least 5 km. The temperature of the magma chamber at this depth was between 1200° and 1300°C. The parent magma composition for Reunion had between 15 and 17% MgO, and all Piton des Neiges magmas can be derived from it by fractional crystallization at 5 to 15 km depth. Piton de la Fournaise magmas can be derived from this parent magma if they are also enriched with alkali elements by an independent process.

Geochemistry of the Hawi lavas, Kohala Volcano, Hawaii

Hawi lavas form the late stage alkalic cap on Kohala Volcano and range in composition from hawaiite to trachyte. New, detailed field mapping of Kohala and reinterpretation of previously published age data suggest that there was no significant eruption hiatus between the Hawi and underlying Pololu shield lavas as was previously suggested. Mineral and whole-rock chemical data are consistent with a crystal fractionation origin for the hawaiite to trachyte compositional variation observed within the Hawi lavas. Plagioclase, clinopyroxene, Ti-magnetite, olivine and apatite fractionation are needed to explain this variation. The clinopyroxene fractionation may have occurred at moderate pressure because it is virtually absent in these lavas and is not a near liquidus phase at pressures of less than 8 Kb. Plagioclase would be buoyant in the Hawi hawaiite magmas so a mechanism like dynamic flow crystallization is needed for its fractionation and to account for the virtual absence of phenocrysts in the lavas. Hawi lavas are distinct in Sr and Nd isotopic ratios and/or incompatible element ratios from the Pololu lavas. Thus they were derived from compositionally distinct sources. Compared to other suites of Hawaiian alkalic cap lavas, Hawi lavas have anomalously high concentrations of phosphorus and rare earth elements. These differences could be due to greater apatite content in the source for the Hawi lavas.

The origin of Samoa: new evidence from Sr, Nd, and Pb isotopes

We report Sr, Nd and Pb isotope ratios and parent and daughter element concentrations in 34 volcanic rocks from Samoa. The highly undersaturated post-erosional volcanics, which have erupted in Recent to Historic time along a 250-km-long fissure, have isotopic compositions that define fields distinct from those of the tholeiitic to alkalic lavas of the older Samoan shield volcanoes. Most shield lavas have 206Pb/ 204Pb of 18.9–19.4, 87Sr/ 86Sr of 0.7045–0.7055 and 87Sr/ 86Sr (to 0.7075). In general, isotopic compositions of the shield lavas are similar to those of the Marquesas and Society Islands. Post-erosional samples have lower 206Pb/ 204Pb and 143Nd/ 144Nd and higher 87Sr/ 86Sr than most shield lavas. The most striking feature of the post-erosional data is a negative correlation between 207Pb/ 204Pb and 206Pb/ 204Pb. This suggests that post-erosional lavas are derived from mixtures of the shield source and a high- 207Pb/ 204Pb, 87Sr/ 86Sr, low- 206Pb/ 204Pb and 143Nd/ 144Nd post-erosional source which may contain recycled ancient sediment. This enriched mantle domain may also underlie the Ontong-Java and Manihiki Plateaus to the north and west. Although both the Samoan shield and post-erosional lavas show chemical characteristics often associated with mantle plumes, only the shield volcanism can plausibly be related to a plume. The post-erosional eruptions appear to be the result of flexure and rifting due to plate bending at the northern termination of the Tonga Trench.

Submarine slope micromorphology and volcanic substructure of the Island of Hawaii inferred from visual observations made from U.S. Navy deep-submergence vehicle (DSV) “Sea Cliff”

Based on visual observations made during forty-four submersible dives around the Island of Hawaii we are able to identify distinct morphological provinces which comprise the submarine slopes of the island. The micromorphology of each submarine slope is primarily a function of the distance from active volcanism, and age and structure of the adjacent subaerial volcanic shield. The ages of the submarine slopes around Hawaii progress from young submarine extrusive terrane off the east coast on Puna Ridge, clockwise to older carbonate-mantled submarine slopes and terraces off the northwest and northern coastlines. The transition between each submarine slope morphology is gradational and depends on the mechanical and chemical weathering of the pillow lavas and the structural setting of the adjacent shield. An exception to the clockwise age-progression of the submarine slopes was discovered west of Kealakekua Bay, midway along the west coast of Hawaii. Very fresh submarine lava flows and possible submarine vents overlie older submarine extrusive terrane and normal, volcanic rubble and sediment-covered submarine slope terrane. Extrapolation of lateral and vertical contact relationships, and submarine slope micromorphologies into the center of Hawaii, coupled with previously published geological and geochemical data allows us to construct a schematic model of the volcanic substructure of Hawaii. This model calls for the three-dimensional interfingering of volcaniclastic wedges with subaqueously extruded pillow lavas below and subaerial shield lavas above. Visual observations and our substructure model indicate that the major portion of the volcanic pedestal which underlies Hawaii is composed of pillow lava sequences.

Magmatic evolution of the basaltic shield volcanoes of Reunion Island

Chemical data are presented for the basic lavas of the two volcanic shields, Piton des Neiges and Piton de la Fournaise, which comprise Reunion Island. In addition, data for cumulate xenoliths have been used to predict mineral/melt distribution coefficient values for the Reunion magmas. The younger volcanic shield, Piton de la Fournaise, comprises two lava sequences, the >0.5−0.2-m.y. B.P. Primary Shield lavas, and the <0.2-m.y. B.P. Caldera Series lavas. Fractional crystallization models for these lavas indicate that olivine is the major fractionating phase during the evolution from the parental basalt composition to the average basaltic liquid. Only during the evolution of the older, Primary Shield lavas has the common fractionation of an ol + cpx + plag + mt assemblage resulted in the eruption of hawaiitic, ankaramitic and feldspar-phyric lavas. The restriction of the Caldera Series liquids predominantly to olivine fractionation and the extensive cotectic fractionation during the evolution of the Primary Shield sequences is interpreted in terms of the maturity of the volcanic center. The younger stages of evolution involve high magma input into a well-developed feeder and reservoir system, thus maintaining the liquids above a cotectic surface. Whereas, during the evolution of the Primary Shield lavas, lower magma input rates into a less well-developed feeder system increased the probability of the fractionating liquid attaining a cotectic surface. Fractional crystallization accounts for all the chemical variation observed for the Piton de la Fournaise basaltic magmas. The analytical data are closely comparable to the rare earth element (REE) and trace element fractionation curves predicted by least-squares calculations, this supports the use of such models in quantitative evaluation of fractional crystallization. A preliminary survey of Sr isotope values indicates that the oldest (>2 m.y. B.P.) lava sequences of Piton des Neiges may be derived from a source which was isotopically distinct from that of the <2 m.y. B.P. lavas of both volcanic shields. These latter sequences are remarkably consistent in both isotopic and trace element abundance implying a homogeneous source material and an invariable partial melting process. Partial melting calculations indicate that the basaltic lavas have been derived by 5–10% melting of a garnet-poor peridotite (cpx/gt ∼ 9). Systematic differences in the light- and heavy-REE patterns between similar basaltic provinces are interpreted to be a result of variation in the nature of the phases buffering the entry of light- and/or heavy-REE into the melt during partial fusion.

Proceedings

Ordinary General Meeting 21 October 1970 at 5.00 p.m. The Twenty-fifth William Smith Lecture: A return to William Smith, was delivered by Professor W. S. Fyfe, F.R.S., Royal Society Professor of Geochemistry in the University of Manchester, in the Wellcome Lecture Theatre at the apartments of The Royal Society. Ordinary General Meeting 11–12 November 1970 A two-day Discussion Meeting: Volcanism and the structure of the earth, arranged jointly by the Volcanic Studies Group of the Geological Society, and The Royal Society, organized by Professor J. Sutton, F.R.S., Dr P. S. Sabine &amp; Dr R. R. Skelhorn, was held in the apartments of The Royal Society. The following papers were read (* denotes the speaker): 1. Aspects of magmatic evolution on Reunion Island By W. J. Wadsworth* and B. G. J. Upton The ‘primitive’ shield lavas of the two Reunion volcanoes are olivine-tholeiites whose compositions are dictated by olivine addition or subtraction, down to a limiting value of approximately 5% MgO. They differ from those of Hawaiian shields (a) in being more alkalic and (b) in containing dunite-wehrlite-gabbro nodule suites. The elder of the two has a ‘Kohala-like’ structure with a veneer of more highly differentiated volcanic rocks. These, ranging from feldsparphyric basalts to trachytes, are dominantly hypersthene-normative with more extreme compositions carrying modal quartz. Fractionation of basic feldspar (An 85 ) and diopsidic pyroxene, as well as olivine, is believed to have controlled the low pressure evolution of the differentiated lavas. The differentiated magmas were (a) less siliceous and (b) slightly