THE FERTILE BLOB MODEL OF MANTLE GEOCHEMISTRY AND GEODYNAMICS; THE PARALLELISM & FIXITY ISSUES

Don L. Anderson
Seismological Laboratory
California Institute of Technology, MC 252-21
Pasadena, CA 91125, USA
Tel: 626.395.6901 ( no messages )
Fax: 626.564.0715
E-mail: dla@gps.caltech.edu
Cambria 805 927 1635  phone, FAX, and voice mail
Altadena 626 797 7426 phone, FAX, and voice mail

ABSTRACT

Much of mantle geochemistry is based on the assumption of chemical 
and mineralogical homogeneity of the shallow mantle, with so-called 
Normal Midocean Ridge Basalt (N-MORB) being  representative of the 
homogeneity and depletion of the entire upper mantle source (" the 
convecting upper mantle ). The entire upper mantle is perceived to 
be a homogeneous depleted olivine-rich lithology approximating 
pyrolite (pyroxene-olivine-rich rock) in composition which,incidently, 
is buoyant compared to most of the mantle of roughly constant potential 
temperature.  Melting anomalies are attributed to narrow  patches of 
high absolute temperature. Acess to the lower mantle  and deep hot 
thermal boundary layers are requirements of these models.  Venerable 
concepts such as reservoirs, plumes, & temperature-crustal thickness 
correlations are products of these perceived constraints. It is 
increasingly clear, however,  that the upper mantle is heterogeneous 
in all parameters at all scales. The parameters include seismic 
velocities, mineralogy, major and trace element chemistry, isotopes, 
melting point, and temperature. An isothermal homogeneous upper mantle, 
however, has been the underlying assumption in much of mantle 
geochemistry for the past 35 years (e.g. Zindler et al, 1984; 
Meibom and Anderson, 2003).  Derived parameters such as degree and 
depth of melting and the age and history of mantle reservoirs are 
based on these assumptions. 

The locations of volcanicic chains, including midocean ridges and 
island arcs , involves some combination of lithospheric stress and 
architecture, fissures, mantle fertility and homologous temperature. 
Mature midocean ridges are well understood but incipient and reactivated 
ones  are often attributed to different physics (hotspot tracks). Global 
plate reorganizations are an attribute of plate tectonics and ridges are 
therefore ephemeral and new one must form and old ones can be reactivated. 
In a chemically stratified mantle with a low-viscosity asthenosphere the 
mantle return flow is slower than plate motions and is roughly parallel 
to the overlying plate. Surface melting anomalies will drift slowly with 
respect to one another. 

The recycling and thermal equilibration of subducted young plates and 
seamount chains, and tensile stress in the plate, are the causes of 
melting anomalies.  In the shallow return flow model asthenosphere 
hosted fertile blobs generate melting anomalies at the surface that 
are parallel , on a given plate, and slowly moving with respect to other 
hotspots.

Because of buoyancy considerations, the most refractory products of 
mantle differentiation harzburgite, lherzolite, and pyrolite collect 
at the top of the mantle .  The volume fractions and the dimensions of 
the fertile components basalt, eclogite, piclogite of the 
mantle are unknown but seismic scattering may solve the problem. There 
is no reason to suppose that the upper mantle is equally fertile 
everywhere or that the fertile patches in hand specimens and outcrops 
are representative of the scale of heterogeneity in the mantle.  The 
heat required for volcanism in the plume hypothesis is from the core. 
Alternatively, mantle fertility, melting point, ponding, focusing, and 
edge effects, i.e. plate tectonic and near-surface phenomena, may control 
the volumes and rates of magmatism. I attribute the chemical heterogeneity 
of the upper mantle to subduction of young plates and seamount chains. 
As slabs, these warm up past the melting point of eclogite and become 
buoyant low-velocity diapirs that undergo further adiabatic decompression 
melting as they encounter thin or spreading regions of the lithosphere. 
The heat required for the melting of cold subducted material is extracted 
from the essentially infinite heat reservoir of the mantle, not the core. 
Melts from recycled oceanic crust, and seamounts and possibly 
even plateaus pond beneath the lithosphere, particularly 
beneath basins and suture zones, with locally thin, weak or young 
lithosphere. The characteristic scale lengths 150 km to 600 km 
of variations in bathymetry and magma chemistry, and the variable 
productivity of volcanic chains, probably reflect compositional 
heterogeneity of the asthenosphere, not the scales of mantle 
convection or the spacing of hot plumes.