Electrical conductivity of hydrous basaltic melts: implications for partial melting in the upper mantle

We use both seismology and geobarometry to investigate the movement of melt through the volcanic crust of Iceland. We have captured melt in the act of moving within or through a series of sills ranging from the upper mantle to the shallow crust by the clusters of small earthquakes it produces as it forces its way upward. The melt is injected not just beneath the central volcanoes, but also at discrete locations along the rift zones and above the centre of the underlying mantle plume. We suggest that the high strain rates required to produce seismicity at depths of 10–25 km in a normally ductile part of the Icelandic crust are linked to the exsolution of carbon dioxide from the basaltic melts. The seismicity and geobarometry provide complementary information on the way that the melt moves through the crust, stalling and fractionating, and often freezing in one or more melt lenses on its way upwards: the seismicity shows what is happening instantaneously today, while the geobarometry gives constraints averaged over longer time scales on the depths of residence in the crust of melts prior to their eruption. This article is part of the Theo Murphy meeting issue ‘Magma reservoir architecture and dynamics'.

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Petrology of ultramafic xenoliths from Rayfield River, south-central British Columbia

Canadian Journal of Earth Sciences ◽

10.1139/e87-161 ◽

1987 ◽

Vol 24 (8) ◽

pp. 1679-1687 ◽

Cited By ~ 12

Author(s):

Dante Canil ◽

Mark Brearley ◽

Christopher M. Scarfe

Keyword(s):

British Columbia ◽

Phase Equilibrium ◽

Partial Melting ◽

Upper Mantle ◽

Mantle Xenoliths ◽

Spinel Lherzolite ◽

Equilibrium Constraints ◽

Host Magma ◽

South Central ◽

Ultramafic Xenoliths

One hundred mantle xenoliths were collected from a hawaiite flow of Miocene–Pliocene age near Rayfield River, south-central British Columbia. The massive host hawaiite contains subrounded xenoliths that range in size from 1 to 10 cm and show protogranular textures. Both Cr-diopside-bearing and Al-augite-bearing xenoliths are represented. The Cr-diopside-bearing xenolith suite consists of spinel lherzolite (64%), dunite (12%), websterite (12%), harzburgite (9%), and olivine websterite (3%). Banding and veining on a centimetre scale are present in four xenoliths. Partial melting at the grain boundaries of clinopyroxene is common and may be due to natural partial melting in the upper mantle, heating by the host magma during transport, or decompression during ascent.Microprobe analyses of the constituent minerals show that most of the xenoliths are well equilibrated. Olivine is Fo89 to Fo92, orthopyroxene is En90, and Cr diopside is Wo47En48Fs5. More Fe-rich pyroxene compositions are present in some of the websterite xenoliths. The Mg/(Mg + Fe2+) and Cr/(Cr + Al + Fe3+) ratios in spinel are uniform in individual xenoliths, but they vary from xenolith to xenolith. Equilibration temperatures for the xenoliths are 860–980 °C using the Wells geothermometer. The depth of equilibration estimated for the xenoliths using geophysical and phase equilibrium constraints is 30–40 km.

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