Discussion of :
Boundary
layer fabric, dynamics and the origin of midplate
volcanoes,
by D.L. Anderson
16th March, 2011, Gillian R. Foulger
Don: Thank you for this contribution. Can you explain
where MORB comes from, in your LLAMA model?
18th March, 20011, Don L. Anderson
Ocean island basalts (OIB) derive from the boundary
layer (BL), between the plate and the so-called “convecting
mantle”. Although it is never precisely defined
the latter is what the conventional wisdom source of
midocean ridge basalts (MORB) is called. In the BL
model the source of MORB remains “the convecting
mantle”, also called DMM (depleted MORB mantle),
or DUM (depleted upper mantle). This is the mantle
that flows in as plates diverge at ridges; it is not
part of the upper BL of the mantle. LLAMA is neither
the “convecting mantle” nor is it DUM.
It is the sublayer beneath the plate (e.g., Elder,
1976). It is a laminated, laterally advecting mantle
assembage that includes the lithosphere, lid,
asthenosphere and melt-rich arrays. Its presence
resolves the paradoxes with conventional 1D geochemical
and thermal history models (e.g., Anderson,
2001, 2007, 2010; Hart et al., 1992; Korenaga,
2003).
BLs are broken up, diluted or dissipated in the vicinity
of ridges where deeper mantle flows in. At the onset
of rifting, or seafloor spreading, enriched material
from the BL provides the initial magmas. Ancient isotopic
signatures are more evident than at ridges where such
evidence is overwhelmed by the more voluminous MORB
magmas. This explains why MORB signatures are missing
in isotope mixing curves, and why they are not the
Common, or FOZO component of midplate magmas (e.g.,
Hart et al., 1992). Although the refractory lamellae
in LLAMA are virtually U-free and can retain ancient
helium isotope signatures (high 3He/4He)
because they are cold (low diffusivities), the enriched
lamellae may contain most of the “missing” (in
the standard model) heat-producing elements. Since
LLAMA is the shallowest layer in the mantle, away from
ridges, it provides the ubiquitous (albeit minor) components
(e.g., FOZO) in oceanic basalts. MORB only
appears in quantity at mature ridges and is purest
along fast spreading ridges. Recycling, delamination
and slab fluids contaminate the shallowest mantle but
not, to the same extent, the deeper NMORB source.
A basic question in mantle dynamics is whether BL
flow is mainly buoyancy-driven or shear-driven. The
fabric–i.e. the scales and statistical
properties–of BLs are different for the two cases
and thus should be resolvable by seismological techniques.
Shear-driven flows tend to be stratified and laminar
in the vertical direction and streaky in the horizontal
direction. Buoyancy-driven flows are dominated by a
few strong upwellings and the familiar Rayleigh-Benard
hexagonal patterns in plan view. In this case, one
does not expect long-range coherent anisotropy, a defining
characteristic of LLAMA, with its laminated lithology
and mesoscale anisotropy.
In buoyancy-dominated BL flow the primary flow structure
is determined by updrafts (plumes) while in shear-driven
BLs there are long streaks or “tracks” roughly
aligned with the mean motion direction (Moeng & Sullivan,
1994). Shear-flows tend to be layered. In buoyancy
driven BLs the structures generally span the entire
depth of the fluid while in the shear-driven case the
vertical extent of coherent structures is much less.
In both cases, the fluctuations in all properties and
motions are much larger in the BL than below it. Seismic
heterogeneity is therefore a defining characteristic
of a BL. Seismic anisotropy is a characteristic of
a shear-driven BL.
The azimuthal component of mantle anisotropy has its
fast axis aligned with the flow direction, which is
roughly parallel to surface fracture zones. This direction
is mainly EW in the eastern and central Pacific mantle.
These trends are interpreted as the long streaks evident
in shear-driven laboratory flows and are more robust
indicators of mantle flow directions that so-called
hotspot tracks. Seismic anisotropy does not confirm
that the famous Hawaiian-Emperor bend is a deep feature
or that it reflects an abupt change in plate motion.
BLs can be neutral, stable, shear-driven or buoyancy
driven. Seismology clearly indicates that LLAMA is
a ~200 km thick shear BL, and one therefore expects
shear-driven upwellings (and downwellings). Most models
of small-scale convection and midplate magmatism invoke
unstable or buoyancy-driven BLs. However, shear-driven
and passive upwellings dominate the dynamics of
LLAMA. These are mesoscale side-effects of the global
or synoptic buoyancy-driven plate-slab system. Just
as in the atmosphere and the oceans, upwellings do
not need to be driven locally by their own buoyancy.
BLs have their own convective patterns and fabrics,
and can also differ in composition from the main convecting
system since they collect the flotsam from the cooling
mantle, from the tops of slabs and from foundering
crust.
Basic questions in mantle geochemistry are, what is
ambient mantle, and what is anomalous? Non-MORB are
usually treated as anomalous and “therefore” not
from the upper mantle. However, “normal” midocean
ridge basalts (NMORB) may not be from “ambient” or “normal” mantle
except in regions where existing BLs have been entrained
and carried away by diverging plates.
Mantle potential temperatures are maximum near the
base of a convective BL (e.g., Elder, 1976)
and this effect is accentuated by internal heating,
secular cooling and recumbent cold slabs. Ridges appear
to tap colder mantle than exists deep in a BL (Anderson,
2010). Recall that the lower parts of a shear-driven
BL are almost stationary and sources there define a
relatively fixed reference system. Since mid-ocean
ridges are fuelled from “the convecting mantle” they
are not expected to be as stationary, or even as hot,
as volcanoes that are fuelled from deep in the surface
BL. LLAMA is a low-viscosity layer that decouples plate
motions from the interior. This low viscosity probably
makes plate tectonics possible. Narrow buoyant upwellings
from deep in the mantle, if they exist, are blown around
by winds in “the convecting mantle” and
will not be vertical or fixed, and will not create
parallel volcanic chains. These are three of the defining
characteristics of the original deep mantle plume hypothesis
but are more representative of shallow sources.
In summary, the source of normal midocean ridge basalt
may be mantle displaced by slabs or subplate mantle
that flows laterally toward the thinnest spots in the
plate, e.g., midocean ridges. This upward
and lateral flow provides the mass balance to diverging
and subducting plates, and completes the plate tectonic
cycle.
References
- Anderson, D. L. 2001. Topside Tectonics?, Science,
293, 2016-2018.
- Anderson, D. L. 2007. New Theory
of the Earth 2nd Edition. Cambridge: Cambridge
University Press, 384 pp; doi: 10.2277/0521849594.
- Anderson, D.L. 2010. Hawaii, boundary
layers and ambient mantle–geophysical constraints, J.
Petrology, published online December 2, 2010,
doi: 10.1093/petrology/egq068, available on-line
at Journal of Petrology, Advanced Access.
- Elder, John 1976. The Bowels of the
Earth, John Elder, Oxford University Press,
pp. 222.
- Hart, S. R. et al. (1992), Mantle plumes and entrainment—Isotopic
evidence, Science, 256, 517–520.
Korenaga, J., 2003. Energetics of mantle convection
and the fate of fossil heat, Geophys. Res. Lett.,
30, 1437, doi:10.1029/2003GL016982.
- Moeng, C.-H., and Sullivan, P.P. 1994.
A comparison of shear and buoyancy driven planetary-boundary-layer
flows. J. Atmospheric Sciences, 51, 999–1022.
31 July, 2013, Don L. Anderson
It has long been taught in geophysics and planetary physics courses, but not in mantle geochemistry courses, that the Earth started hot and was extensively differentiated during accretion. This knowledge goes back to the extremely influential papers of Francis Birch, including his classic 1952 paper, his 1965 Presidential Address and his energetics-of-core-formation papers. These papers form the foundations of modern geophysics but they are the antithesis of the Urey school of geochemistry (Urey, 1952) which produced many advocates of cold accretion, primordial Earth, crustal growth, continuous differentiation and undegassed mantle. Thus, 1952 was a pivotal year for both mantle geophysics and mantle geochemistry. The two sciences diverged from that point on.
Schilling (Nature 242, 1973, page 565) states "Contrary to earlier views (Birch 1965), the model implies that plumes are transporting to the Earth's surface more primordial mantle material(s) than present in the low-velocity layer lying beneath those mid-ocean ridge segments remote from plumes." This was the nucleus of the geochemical version of the plume hypothesis. Tatsumoto (1978) and O'Hara (1973) almost immediately showed the flaws in Schilling's argument, and Tozer (1973 ) demonstrated the fluid dynamic implausibility of the geochemical model. Birch’s papers had already demonstrated that classical physics ruled out the assumptions in what became the canonical model of geochemistry.
The Birch ideas were extended to mantle geochemistry by Tatsumoto, Armstrong and Kay who developed top-down models of geochemistry. The mass balance associated with hot accretion and early differentiation was developed in many early papers and summarized in Chapter 8 of Theory of the Earth and Chapter 13 of New Theory of the Earth. The mass balance shows clearly that the whole mantle had to be processed to form the crust plus the kimberlites and carbonatites.
