Publication Date:
2017-04-04
Description:
Deep-sea exploration is rapidly improving our understanding of volatiles geochemistry in mid-ocean-ridge igneous products.
It is also placing greater constraints on degassing processes of the Earth’s mantle, with the result that degassing models
based on vapour–melt equilibrium are no longer able to explain the increasing number of data. In fact, such models force to
postulate an upper mantle strongly heterogeneous at any scale, and cannot account for the widespread carbon supersaturation
of the recovered igneous products. Here we review the global He–Ar–CO2 dataset of fluid inclusions in mid-ocean-ridge glasses
using the framework of advanced modelling of multicomponent bubble growth in magmas. We display that non-equilibrium
fractionations among He, Ar and CO2, driven by their different diffusivities in silicate melts, are common in most of the
natural conditions of magma decompression and their signature strongly depends on pressure of degassing. Due to the comparable
Ar and CO2 diffusivity, magma degassing at low pressure fractionates both the He/Ar and He/CO2 ratio by a similar
extent, while the slower CO2 diffusion at high pressure causes early kinetic effects on Ar/CO2 ratio and dramatically changes
the degassing path. On this ground, the very different geochemical signatures among suites of data coming from different ridge
segments mainly depend on the depth of the magma chamber where the melt was stored. Besides, the variations inside a single
suite highlight variable ascent speed and cooling rate of the emplaced lava. The large variations in both the He/CO2 and
Ar/CO2 ratios at almost constant He/Ar, displayed in glasses coming from the Mid-Atlantic Ridge 24–30 N segment and
the Rodriguez Triple Junction, are therefore interpreted as a high-pressure signature. In contrast, the simultaneous increase
in both He/CO2 and He/Ar of the East Pacific Rise, Pito Seamount and South-East Indian Ridge data sets suggests the dominance
of low-pressure fractionation, implying that the shallow magma chambers are at a lower depth than those of the Mid-
Atlantic Ridge 24–30 N and Rodriguez Triple Junction. Our conclusions support the presence of a relationship between
spreading rate and depth of high-temperature zones below ridges, and are consistent with the depth of magma chambers
as suggested from seismic studies. Non-equilibrium degassing explains the volatile systematics of mid-ocean-ridge basalts
by starting from a single mantle-derived magma, dispensing with the supposed need for heterogeneities in abundance ratios
of volatiles in the mantle below oceanic ridges.
Description:
Published
Description:
1747–1763
Description:
1.2. TTC - Sorveglianza geochimica delle aree vulcaniche attive
Description:
JCR Journal
Description:
reserved
Keywords:
nonequilibrium degassing
;
MORB mantle
;
geochemical modeling
;
fluid inclusions
;
04. Solid Earth::04.08. Volcanology::04.08.99. General or miscellaneous
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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