Intraplate Volcanism Research Articles

The distribution of intraplate volcanism in the Pacific Ocean basin: a spectral approach

Summary. Although the major linear island chains in the Pacific Ocean basin have been extensively studied, relatively little attention has been given to the origin of the numerous seamounts and oceanic islands that are concentrated in the western Pacific. Spectral analysis of shipboard gravity and bathymetry data provides a means for statistically estimating the distribution of this volcanism in space and time. Estimates of the power spectrum of the bathymetry FB and the average gravitational admittance z (cross-spectrum correlation of gravity and bathymetry) were obtained as functions of age of the oceanic lithosphere by dividing the Pacific plate into eight age regions A-H. The power spectrum of the bathymetry is indicative of the overall bathymetric roughness (primarily volcanic) in each region. This roughness shows three gradations: regions A-D ( 140Myr), with many large volcanic features, are very rough. The average gravitational admittance is indicative of the degree of isostatic compensation of bathymetric features in each region and can be used to estimate the proportionh of total bathymetric power that was emplaced on relatively old lithosphere and the proportion that formed on young lithosphere at or near a mid-ocean ridge crest. The highest proportion of bathymetry that formed on old lithosphere occurs in region H (fI- 0.90) and the lowest proportion occurs in regions G and E (fI - 0.40-0.60). The power spectrum of the bathymetry and the gravitational admittance allow an estimate of the total bathymetric power PI = ~IPB accounted for by intraplate volcanism ('intraplate power') and the power PR = (1 - fI)PB which is of on-ridge origin ('ridge power'). At long wavelengths in regions E-G, the amount of on-ridge volcanism exceeds the amount of intraplate volcanism (PR > PI), indicating the presence of several large plateaus and rises which formed at ridge crests. This suggests the

Convection generated stress field and intra-plate volcanism

Convection generated stresses in the lithosphere are inferable from the high degree harmonics of the geopotential. Hot spots on the earth's surface, marked by anomalous volcanism, could be surface expressions or flaws of stress concentration in tension caused by the hotter upwelling mantle convection currents. Widespread Cenozoic intra-plate volcanism in Africa, Asia, Australia and the Western United States is almost exclusively restricted to areas of the convection generated tensional stress field. This non-random distribution indicates that during Cenozoic the convection generated stresses in the lithosphere have exerted a control on the locations of these volcanic areas. In this case intraplate volcanism would be associated with tensional tectonics. The absence of volcanism in the convection generated compressional stress regimes implies that the environment of compression in the lithosphere is unfavorable for basalt magmas to migrate from the asthenosphere to the surface of the earth. Based on the results of this study, it seems reasonable to suggest that the convection generated subcrustal tensional stress field as inferred from the satellite and surface gravity data may be the genesis of the Cenozoic intra-plate volcanism.

Convection-generated stress field and intra-plate volcanism : Liu, Han-Shou, 1980. Tectonophysics, 65(3/4): 225–244

Convection-generated stress field and intra-plate volcanism : Liu, Han-Shou, 1980. Tectonophysics, 65(3/4): 225–244

Quaternary alkalic and sub-alkalic volcanism in South Auckland, New Zealand

A significant feature of Quaternary volcanism in New Zealand is the presence of several compact fields of monogenetic basaltic volcanism, in many respects comparable to back-arc volcanism of Japan and intra-plate volcanism in S.E. Australia. Volcanism in South Auckland between 1.56 m.y. (B.P.) and 0.51 m.y. has produced some 20 km3 of lava and scoria from at least 70 centres. Older lavas are alkalic (basanite and alkali-olivine basalt), and are associated with maars and small scoria cones and many contain mafic and ultramafic xenoliths and megacrysts. Younger subalkalic lavas (mainly transitional tholeiite) form large scoria cones with extensive lava aprons.

Intra-plate volcanism

A significant fraction of the volcanism on the Earth’s surface cannot be associated with plate margin processes. The mantle plume hypothesis is one explanation for this volcanism. Convective plumes beneath rigid plates could be the result of turbulent thermal convection within the upper mantle. Although the hypothesis of nearly fixed mantle plumes is reasonably successful in explaining the direction and velocity of migration of intra-plate volcanism, there are a number of difficulties. It is particularly difficult to explain continued volcanism over extended linear distances. An alternative hypothesis for intra-plate volcanism is that magmas flow to the surface through lithospheric fractures. In this case intra-plate volcanism would be associated with tensional tectonics. Intra-plate volcanism and seismicity in Africa and the western United States are discussed in terms of these hypotheses.

Stress accumulation in the lithosphere

It is shown that stress can accumulate in the lithosphere for 10 8–10 9 years. It is hypothesized that intraplate ruptures caused by tensional stresses are responsible for intraplate volcanism, seismicity, and tectonics. There are a number of mechanisms leading to the accumulation of stress in the lithosphere. Because the earth is an oblate spheroid, changes in latitude of the surface plates cause membrane stresses. Changes in temperature cause thermal stresses. Addition or removal of overburden generates stress. The driving mechanism for plate tectonics results in stress. And stresses are generated by changes in crustal thickness. Oceanic island chains such as the Hawaiian—Emperor are attributed to thermal stresses in the cooling, contracting oceanic lithosphere. Localized centers of continentalmargin volcanism in the central and south Atlantic are attributed to a combination of thermal stresses and stresses due to changes in crustal thickness. Continental volcanism such as in the East African Rift system is attributed to membrane stresses. It is concluded that tensional fracturing of the lithosphere is a more likely explanation for intraplate volcanism than deep mantle plumes.

East pacific ridge (2°S–19°S) versus nazca intraplate volcanism: Rare-earth evidence

Basalts from seamounts within the Nazca Plate representing intraplate volcanism, and the East Pacific Ridge between 19°S and 2°N have similar light rare earth depleted abundance patterns. Both intraplate and ridge basalts appear to have been derived from the low-velocity layer apparently depleted in large lithophile elements (DLVL). Nepheline-normative basalts and ferrobasalts occasionally occurring on the East Pacific Rise are shown to have also been derived from the same DLVL source. Furthermore, the rare-earth pattern similarity of nepheline-normative and tholeiitic basalts from the East Pacific Rise is best explained by distinct, pressure induced, conditions of partial melting of the DLVL source; whereas total rare-earth pattern enrichment and relative europium depletion of the ferrobasalts are consistent with shallow depth fractional crystallization during ascent.

30. Evaporation and sputtering

We report the Sr, Nd and Pb isotopic ratios of clinopyroxene separates from spinel peridotite xenoliths hosted by late Cenozoic alkali basaltic rocks from Baengnyeong Island, Jogokni, and Jeju Island, all near the southeastern margin of the Sino-Korean Craton in South Korea. Measured Sr, Nd and Pb isotopic compositions in the clinopyroxene separates show, in general, simple trends between depleted MORB-like mantle (DMM) and relatively enriched components, which can be explained by old event(s) of infiltration by metasomatic melts/fluids that enriched a precursor time-integrated depleted mantle in incompatible trace elements and overprinted its isotopic composition. Comparison of the xenoliths with the host basalts suggests that infiltration of carbonatitic melts can explain the characteristics of the Baengnyeong xenoliths while reaction with residual slab-derived melt/fluid subjected to amphibole and phlogopite fractionation explains the Jogokni xenoliths best, ruling out direct influence of the magmatism that produced the host basalts. On the other hand, the Jeju clinopyroxene separates display a mixing hyperbola between DMM and the host basalts in Sr–Pb and Nd–Pb isotope correlation diagrams, suggesting some influence by liquids similar to the host basalts. The Korean xenoliths tend to be similar in the overall range of their mineralogic modes, elemental compositions, and Sr–Nd–Pb isotopic ratios to those of the fertile peridotite xenoliths derived from the post-Archean lithospheric mantle of eastern China, and yet without any compositional evidence for a refractory Archean mantle root. Overall comparison of the measured isotopic compositions for these Korean xenoliths with available data on lavas from eastern China, indicates that the dominant enriched mantle source components for the Cenozoic intraplate volcanism in the East Asia (EM1 for northeast China, and EM2 for Southeast Asia) are absent in the lithospheric mantle beneath the region. The Cenozoic intraplate volcanism, therefore, must have originated in the asthenosphere.