Layered Lithospheric Mantle Beneath the Ontong Java Plateau: Implications from Xenoliths in Alnöite, Malaita, Solomon Islands

Abstract

A varied suite of mantle xenoliths from Malaita, Solomon Islands, was investigated to constrain the evolution of the mantle beneath the Ontong Java Plateau. Comprehensive petrological and thermobarometric studies make it possible to identify the dominant processes that produced the compositional diversity and to reconstruct the lithospheric stratigraphy in the context of a paleogeotherm. P–T estimates show that both peridotites and pyroxenites can be assigned to a shallower or deeper origin, separated by a garnet-poor zone of 10 km between 90 and 100 km. This zone is dominated by refractory spinel harzburgites (Fo91–92), indicating the occurrence of an intra-lithospheric depleted zone. Shallower mantle (∼Moho to 95 km) is composed of variably metasomatized peridotite with subordinate pyroxenite derived from metacumulates. Deeper mantle (∼95–120 km) is represented by pyroxenite and variably depleted peridotites that are unevenly distributed; the least-depleted garnet lherzolite (Fo90–91) lies just below the garnet-poor depleted zone (∼100–110 km), whereas the presence of pyroxenite is restricted to the deepest region (∼110–120 km), together with relatively Fe-enriched garnet lherzolite (Fo87–88). This depth-related variation (including the depleted zone) can be explained by assuming that the degree of melting for a basalt–peridotite hybrid source was systematically different at each level of arrival depth within a single adiabatically ascending mantle plume: (1) the depleted zone at the top of the mantle plume, where garnet was totally consumed in the residual solid; (2) an intermediate part of the plume dominated by the least-depleted garnet lherzolite just above the depth of the peridotite solidus; (3) the deepest pyroxenite-rich zone, whose petrochemical variation is best explained by the interaction between peridotite and normative quartz-rich basaltic melt, below the solidus of peridotite and liquidus of basalt. We explain the obvious lack of pyroxenites at shallower depths as the effective extraction of hybrid melt from completely molten basalt through the partially molten ambient peridotite, which caused the voluminous eruption of the Ontong Java Plateau basalts. From these interpretations, we conclude that the lithosphere forms a genetically unrelated two-layered structure, comprising shallower oceanic lithosphere and deeper impinged plume material, which involved a recycled basaltic component, now present as a pyroxenitic heterogeneity. This interpretation for the present lithospheric structure may explain the seismically anomalous root beneath the Ontong Java Plateau.

Lithospheric mantle duplex beneath the central Mojave Desert revealed by xenoliths from Dish Hill, California

“…T Ca-Opx rim temperatures are within error similar to T BKN rim at the hotter end of the series, while a distinct negative correlation between T BKN rim and the T Ca-Opx rim -T BKN rim difference is visible at lower temperatures. Similar correlations between T Ca-Opx and T BKN have been observed in low-temperature xenoliths world wide (e.g., Kaapvaal craton [Brey and Köhler, 1990], Basin and Range and Colorado Plateau , and OntongJava Plateau [Ishikawa et al, 2004]), and have been ascribed to the influence of Na on Ca incorporation in orthopyroxene. The correlation between T Cr/Al-Opx rim and T BKN rim is rather poor, and T Cr/Al-Opx rim tend to underestimate T BKN rim at medium and high temperatures, but overestimate them at low temperatures.…”

Formation and evolution of Shatsky Rise oceanic plateau: Insights from IODP Expedition 324 and recent geophysical cruises

“…Figure 16 also indicates that the T P of Shatsky Rise is ~100°C lower than that estimated for the OJP and ~50°C higher than MORB. Similar to Shatsky Rise, crust and lithosphere beneath the OJP is thick (e.g., Ishikawa et al, 2004). The higher T P estimation for the OJP magma resulted from the higher melt fraction estimated for the OJP magma (~30 %).…”

Revision of Scheumann’s classification of melilitic lamprophyres and related melilitic rocks in light of new analytical data

“…Rare ultramafic melilite-bearing olivine nephelinite to olivine melilitite volcanic rocks and their plutonic equivalents (mostly of the melteigite-ijolite-urtite series) and associated lamprophyric alnöite represent low-volume mantle-derived melts (Ishikawa et al 2004;Melluso et al 2011). Rocks of melilitic volcanic series occur in intraplate rift settings (Siberia -Egorov 1970;Zhabin and Surina 1970;the Balcones Province, Texas, and Tasmania -Brey 1978;Africa -Dawson et al 1985;Bailey et al 2005;Keller et al 2006;Greenland -Bernstein et al 2000;Madagascar -Melluso et al 2011;Euro-Mediterranean region -Lustrino and Wilson 2007;Piromallo et al 2008 and citations therein), at continental margins (the western Canary and Madeira islands are situated on oceanic lithosphere, whereas the eastern Canary Islands on transitional, oceanic/continental lithosphere -Hoernle and Schmincke 1993) and in ocean island settings (Hawaii -Wilkinson and Stolz 1983;Maaloe et al 1992;French Polynesia -Chauevel et al 1992).…”

“…Potassic to ultrapotassic melilitic rocks of exceptional subductionrelated orogenic settings are represented by the Roman Magmatic Province (Di Battistini et al 2001 and citations therein) and a diatreme filling of polzenite (modlibovite) composition in California (Morgan et al 1985). Alnöite dykes are rare in rift-related settings, e.g., Alnö Island, Sweden (Eckermann 1961), Monteregian Hills, Canada (Hamois and Mineau 1991), MaimechaKotui Province, Siberian Platform (Egorov 1970;Zhabin and Surina 1970), Kola Peninsula (Beard et al 1998;Rass 2008), Gardar Province, Greenland (Pearce et al 1997), Sierra Subandinas, Argentina (Barbieri et al 1997) but also in an island-arc setting on the Solomon Islands, Ontong Java Plateau (Ishikawa et al 2004).…”