TY - JOUR

T1 - Peridotitic lithosphere metasomatised by volatile-bearing melts, and its association with intraplate alkaline HIMU-like magmatism

AU - Scott, James

AU - Brenna, Marco

AU - Crase, Jordan

AU - Waight, Tod Earle

AU - van der Meer, Quinten

AU - Cooper, Alan

AU - Palin, J. Michael

AU - Le Roux, Petrus

AU - Münker, Carsten

PY - 2016

Y1 - 2016

N2 - 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, exceptfor 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 metasomaticstyles 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 samemantle 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, whichprobably 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 meltingprocess. Although not observed in the studied xenoliths, amphibole is associated with mantle metasomatism by carbonatitic or CO2-bearing melts elsewhere in the world and has been found as xenocrysts with HIMU-like isotope compositions in some Zealandia basalts. The melt modelling results also imply that amphibole could buffer the trace element budgets of a low-degree melt regardless of the source peridotite composition; therefore, provided that hydrous metasomatized lithospheric mantle can be perturbed to melt, the contribution of amphibole would explain the similarities of alkaline ocean island basalt-like magmas in continental and oceanic settings. HIMU-like reservoirs formed by percolation of young volatile-rich (CO2þH2O) melts are widespread within the Earth’s lithospheric mantle and are a potential source for intraplate alkaline basaltic magmatism.

AB - 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, exceptfor 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 metasomaticstyles 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 samemantle 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, whichprobably 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 meltingprocess. Although not observed in the studied xenoliths, amphibole is associated with mantle metasomatism by carbonatitic or CO2-bearing melts elsewhere in the world and has been found as xenocrysts with HIMU-like isotope compositions in some Zealandia basalts. The melt modelling results also imply that amphibole could buffer the trace element budgets of a low-degree melt regardless of the source peridotite composition; therefore, provided that hydrous metasomatized lithospheric mantle can be perturbed to melt, the contribution of amphibole would explain the similarities of alkaline ocean island basalt-like magmas in continental and oceanic settings. HIMU-like reservoirs formed by percolation of young volatile-rich (CO2þH2O) melts are widespread within the Earth’s lithospheric mantle and are a potential source for intraplate alkaline basaltic magmatism.

U2 - 10.1093/petrology/egw069

DO - 10.1093/petrology/egw069

M3 - Journal article

VL - 57

SP - 2053

EP - 2078

JO - Journal of Petrology

JF - Journal of Petrology

SN - 0022-3530

IS - 10

ER -