Search

The role of lithospheric mantle metasomatized by CO2-bearing melts in the genesis of HIMU-like alkaline intraplate basalts is investigated using a suite of peridotite xenoliths from New Zealand. The xenoliths have Sr-Nd-Pb-Hf isotope compositions (Sr-87/(86) Sr=0.7029, epsilon Nd = +5 to +6, Pb-206/(204) Pb = 20.4 and epsilon Hf = +5 to +8) indistinguishable from the host low-silica basalts and, except for Pb-207/Pb-204, overlapping with the HIMU mantle reservoir. Laser line scans across grain boundaries in the xenoliths show, however, that the host magma contribution is restricted to minor degrees of melt infiltration along grain boundaries during ascent, with the distinctive peridotite isotopic compositions having been imparted earlier by mantle metasomatism. Two mantle metasomatic styles are distinguished from pyroxene trace element concentrations (in particular, rare earth elements, Ti, Zr and Hf) and are interpreted to be the result of reaction of peridotite with CO2-bearing magmas. The occurrence of two subtly chemically different but isotopically indistinguishable styles of metasomatism in rocks with the same equilibrium temperatures within the same mantle column may be due to separate volatile-rich melts formed by different degrees of melting of a deeper carbonated peridotitic +/- pyroxenitic source, or due to metasomatism having been imparted to different degrees on a variably depleted protolith. In either case, the formation of the HIMU-like enriched lithospheric mantle was achieved by percolation of volatile-rich melts, which probably rose from the asthenosphere. Melt modelling of representative depleted and subsequently enriched samples shows that low-degree melting of a CO2-bearing melt-metasomatized peridotite could yield a melt with a trace element composition very similar to that of the Zealandia HIMU-like alkaline basalts, but only if small volumes (similar to 5%) of amphibole participated in the melting process. Although not observed in the studied

The role of lithospheric mantle metasomatized by CO2-bearing melts in the genesis of HIMU-like alkaline intraplate basalts is investigated using a suite of peridotite xenoliths from New Zealand. The xenoliths have Sr–Nd–Pb–Hf isotope compositions (87Sr/86Sr =0.7029, eNd = +5 to +6, 206Pb/204Pb = 20.4 and eHf 0 +5 to +8) indistinguishable from the host low-silica basalts and, except for 207Pb/204Pb, overlapping with the HIMU mantle reservoir. Laser line scans across grain boundaries in the xenoliths show, however, that the host magma contribution is restricted to minor degrees of melt infiltration along grain boundaries during ascent, with the distinctive peridotite isotopic compositions having been imparted earlier by mantle metasomatism. Two mantle metasomatic styles are distinguished from pyroxene trace element concentrations (in particular, rare earth elements, Ti, Zr and Hf) and are interpreted to be the result of reaction of peridotite with CO2- bearing magmas. The occurrence of two subtly chemically different but isotopically indistinguishable styles of metasomatism in rocks with the same equilibrium temperatures within the same mantle column may be due to separate volatile-rich melts formed by different degrees of melting of a deeper carbonated peridotitic6pyroxenitic source, or due to metasomatism having been imparted to different degrees on a variably depleted protolith. In either case, the formation of the HIMU-like enriched lithospheric mantle was achieved by percolation of volatile-rich melts, which probably rose from the asthenosphere. Melt modelling of representative depleted and subsequently enriched samples shows that low-degree melting of a CO2-bearing melt-metasomatized peridotite could yield a melt with a trace element composition very similar to that of the Zealandia HIMU-like alkaline basalts, but only if small volumes (5%) of amphibole participated in the melting process. Although not observed in the studied xenoliths, amphi

Dispersed volcanism in intraplate settings produces volcanic fields that may erupt over millions of years to produce hundreds to thousands of scoria and spatter cones, tuff cones and maars, as well as lava flows. Many aspects of this globally ubiquitous process are poorly known, ranging from the tectonic drivers to the mechanisms controlling magma accumulation and ascent. To investigate magma generation processes leading to a variety of individual eruption types at volcanic fields and to understand the spatio-temporal evolution of these whole-systems, knowledge of the geochemical and petrological properties of erupted products must be linked to the geologic and tectonic framework. This study was based on detailed stratigraphic sampling of small- (<0.01 km3) and large-volume (>1 km3) eruptive sequences in the Jeju Island Volcanic Field, Korea at both individual exposed eruption centres and from deep drill cores. This island is the subaerial representation of a volcanic field developed above continental crust over the last 1.8 Ma. Pyroclastic and lava samples were analysed for whole-rock major-, trace-elements and Sr-Nd-Pb isotopes, and for mineral compositions and Sr-Nd-Pb isotopes. The Jeju magmatic system started with small-volume alkali basaltic eruptions sourced at mantle depths equivalent to c. 2.5 GPa in partially hydrous peridotite. These magmas passed through the crust and erupted rapidly, with minor modification. Intrusions and eruptions accommodated regional tectonic strain, and excess melts became stalled to fractionate toward trachyte compositions in both the lower and upper crust. Trachyte erupted sporadically, with the first episode at c. 750 ka. After this, the system started to erupt with volumetric rates two orders of magnitude higher. This accelerated magma production involved alkali basalt melts derived from greater depths/pressures (3.5 GPa) than earlier, along with subalkali basalts derived from c. 2.5 GPa. Despite prevailing extensional tectonic