Mantle magmatic convective storms – a mechanism for intraplate volcanism?
E.M. Parmentier, Department of Geological Sciences, Brown University
Oceanic intraplate volcanism is one of the most visible consequences of mantle
dynamics. Linear, long-lived, and age-progressive volcanic chains have been explained
as the manifestation of fixed hotspots possibly generated by buoyantly plumes of rising
material originating deep in the mantle. However, the fixed hotspot model fails to
explain observed short-lived chains, violations of predicted along-chain age vs. distance
behavior, or extended periods of volcanism at individual volcanoes. A significant fraction
of intraplate volcanism does not occurs in chains aligned with plate motion.
Furthermore, volcanism is often observed to be coincident with, and perhaps controlled
by, lithosphere structure.
Theoretical models are used to explore the hypothesis that melting may occur in
convective upwellings that result from the buoyancy associated with the melting itself,
what might be termed ‘buoyant decompression melting’. Since the melting temperature
of the mantle increases with pressure or depth more rapidly than an adiabat, any small
upward displacement of a parcel of upper mantle that is sufficiently near its melting
temperature will cause melting. This melting creates buoyancy because the presence of
melt reduces the overall density and because the residual mantle, after melt is extracted,
is less dense than undepleted mantle. Buoyancy forces due to melting cause additional
upwelling, which induces further melting creating a process that may become selfsustaining. One can thus envision the development of convective “storms” in regions of
the mantle warm enough to be at their melting temperature and thus susceptible to
spontaneous buoyant decompression melting.
Since the oceanic upper mantle may have previously melted beneath a spreading
center and subsequently cooled, spontaneous buoyant decompression melting should not
be possible everywhere in the upper mantle – only in regions where some larger scale
upwelling offsets conductive cooling or where triggered by some initial upwelling. This
triggering may provide a physical explanation for volcanism that is controlled by
lithosphere structure. Lithospheric structure may trigger buoyant melting in several
ways. For example, uppermost mantle that flows beneath a fracture zone from older,
thicker lithosphere to younger, thinner lithosphere will rise and could undergo some
small initial amount of decompression melting. This also might occur as mantle flows
beneath lithosphere of variable thickness under an older hotspot track.