Evidence for Moderate Mantle Temperature Anomalies Associated with
Department of Geology and Geophysics
Woods Hole, MA
of the characteristics of
deep-rooted mantle plumes and the excess volcanism of hotspots that
represents their surface expression is the presence of anomalously hot
asthenosphere underlying the lithosphere. Such asthenosphere is
predicted to cause faster subsidence of hotspot crust than normal
oceanic crust. However, studies of the sedimentary cover from a range
of seamounts, plateaus and ridges of various ages from all major ocean
basins show no or only moderate anomalous uplift that can be linked to
hot underlying mantle during hotspot magmatism. Assuming all the uplift
is caused by excess mantle heat, the temperature anomalous rarely
exceeds 100˚C and could be somewhat lower. In many cases subsidence is
slower than for normal oceanic lithosphere, suggesting either colder
mantle temperatures, or more likely the emplacement of buoyant
lithospheric root under the magmatic province at the time of its
Evidence that the Earth’s mantle is
in a state of circulation is well
documented, yet the relative influence of deep-seated mantle plumes,
some possibly originating at the core-mantle boundary is still hotly
contested (e.g., Anderson, 2003; Foulger and Natland, 2003). Areas of
elevated topography linked to greater than normal magmatism in the
ocean basins have been historically been linked to greater melting
above a steady state plume, or to the impacting of a newly initiating
plume head on the base of the lithosphere (e.g., Campbell and
Griffiths, 1990). New models however advance excess melting due to
lithospheric processes above essentially normal upper mantle (Anderson,
2003). Resolution of this debate is essential to understanding the
major controls on terrestrial magmatism and the geochemical evolution
of the Earth.
Although different plume models propose different
magnitudes of thermal anomaly, ranging up to +350˚C (Farnetani and
Richards, 1994), a common characteristic of deep-seated mantle plumes
is their excess heat relative to the surrounding ambient asthenosphere.
Greater heat results in the plume mantle being less dense than normal,
allowing this material to rise to the base of the lithospheric plates.
This same excess heat and buoyancy is also inferred to cause
long-wavelength uplift and shallowing of the seafloor, which can be
seen in modern bathymetry around active hotspots or superswell areas
(Crough, 1978; Detrick and Crough, 1978; McNutt and Judge, 1990).
Alternatively, ancient plume-driven uplift can be preserved in the
sedimentary rocks deposited in hotspot regions (e.g., White and Lovell,
1997). As a result of plate motion crust that is affected by plume
activity is removed from over the mantle thermal anomaly. Continued
upper mantle convection then removes the hotter, less dense material
and allows the crust to subside to normal depths over periods of 10’s
of millions of years (Olson, 1990; Griffiths and Campbell, 1991).
Fig. 1. Predicted subsidence patterns for oceanic lithosphere of 110 Ma
age, based on the temperature of the underlying mantle asthenosphere
over which it was emplaced. Model assumes that the anomalously hot
asthenosphere is allowed to dissipate with time and that the crust was
fully constructed at the start of the subsidence. Mantle thermal
anomalies are seen to have a major effect on the vertical motions of
effects of plume activity have been
interpreted from sedimentary records from continental margins, most
notably in the NE Atlantic and Kerguelen Plateau (Brodie and White,
1994; White and Lovell, 1997; Clift et al., 1995; Coffin and Gahagan,
1995), anomalous subsidence linked to plume activity is most readily
identified within oceanic crust because the subsidence behavior of
oceanic lithosphere is relatively simple and well characterized
(Parsons and Sclater, 1977; Stein and Stein, 1992). Although the
absolute depth of any given piece of oceanic crust will vary depending
on the thickness and density of the crust, the rate at which subsidence
occurs after the end of crustal accretion can be used to identify the
effects of mantle buoyancy, because this is controlled by cooling and
thickening of the mantle lithosphere (Parsons and Sclater, 1977).
Isolating the effect of hot asthenosphere on plate
subsidence is simplest if the hotspot volcanic crust was emplaced close
to the crest of a mid ocean ridge, as this allows a simple comparison
between the subsidence reconstructed from the sedimentary record and
that of normal oceanic crust. Clift (1997) used this approach to show
anomalously shallow depths within the oldest oceanic crust adjacent to
the volcanic rifted margins of the NE Atlantic, shortly after
continental break-up in the Eocene. If all the anomalous depth is
assigned to temperature rather than compositional buoyancy or dynamic
flow then the degree of misfit with the oceanic model can be used to
estimate maximum temperature anomalies.
The subsidence of lithosphere affected by plume
activity has been modeled in a variety of ways, treating the plume as
either an isostatic anomaly or as a region of dynamic flow (e.g., Ribe
and Christensen, 1994; Sleep, 1987, 1990). Dynamic models generate
uplift through the viscous normal stresses that would be imposed on the
base of the lithosphere by an upwelling column of material and mimic
modern bathymetry accurately with no requirement for a temperature
anomaly. However, results consistent with the evolving bathymetry along
the Hawaiian seamount chain have also been generated by simple
isostatic, conductive cooling models (Sleep, 1987), such as that of Ito
and Clift (1998), which uses a one-dimensional thermal diffusion
approach. The model allows the hot cushion of plume material to diffuse
away after the initial emplacement, resulting in a gradually reducing
thermally induced buoyancy with time. Figure 1 shows a series of models
generated from this type of model that shows the difference in
subsidence behavior predicted for oceanic crust emplaced above normal
mantle, above a plume of moderate strength (T = 150˚C, 100 km
thick plume head) and above a plume at the high end of current
estimates (T = 350˚C, 200 km thick). In each case the end point
of the subsidence history is known, because it is the present day depth
of the basement, but the initial elevation is predicted to differ
significantly depending on the original mantle temperature. As a result
the predicted subsidence curve for crust generated above a plume mantle
might be expected to be steeper than that above normal oceanic mantle.
Fig. 2. Map showing the
distribution of ODP and DSDP drill sites
considered in this study, spanning all major ocean basins and large
periods of geologic time.
subsidence history is slightly more
complex if the emplacement of hot mantle material and associated
hotspot volcanism occurs away from an oceanic spreading center.
In this case the water depth shallows compared to equivalent age crust
away from the hotspot, as a result of crustal thickening and the
buoyancy of the mantle plume material. The rate of subsidence of the
crust after removal from the area of the hotspot and its swell is
faster than crust of similar age that was not influenced by the
hotspot. Detrick and Crough (1978) noted such behavior in the Hawaiian
seamount chain, a phenomenon that they assigned to a thermal erosion of
the Pacific lithosphere after it passes over the Hawaiian plume.
Although the Hawaiian Islands are much shallower than the surrounding
abyssal seafloor they subside at a faster rate after they pass away
from the immediate influence of the thermally erosive plume.
Alternatively Phipps Morgan et al. (1995) explained the bathymetry of
the Hawaiian Islands as being the result of melt extraction in the
asthenosphere producing a buoyant mantle pillow centered under the
hotspot. This buoyant mantle was partially entrained with the
lithospheric plate as this moved away from the hotspot. In the case of
old hotspot provinces it is not always clear whether a hotspot edifice
was emplaced on axis or not, and this may be decided by comparing
hotspot and lithospheric ages. In this study I infer the age of the
surrounding oceanic lithosphere on the basis of the nearest identified
magnetic anomaly, while the age of hotspot volcanism is taken from the
radiometric age of the basement lavas, or from the biostratigraphic age
of the overlying sediment if the former is not available.
In order to assess the magnitude of mantle
temperature anomalies associated with hotspot magmatism I have
reconstructed the subsidence history of a series of oceanic plateaus,
ridges or seamounts using their sedimentary covers. These features span
all the major ocean basins and a wide period of geologic time (Fig. 2).
Their analysis is made possible through the sampling of the Deep Sea
Drilling Project (DSDP) and the Ocean Drilling Program (ODP), which
recovered the sediments overlying the volcanic sequences. The sediments
and fossils found in these cores allow the depositional environment,
and especially the water depth of deposition to be constrained. These
data were compiled from the cruise reports and associated postcruise
papers, particularly those on benthic foraminifer fauna, which are
considered sensitive water depth indicators. Water depth estimation is
the principle uncertainty in the reconstruction of the vertical motions
of the igneous basement and is generally better where water depths are
shallow (<200 m), as this is the range where sedimentary structures
and benthic foraminifers shows the greatest resolution. Nonetheless,
some resolution is possible at greater depths. For the purpose of
this analysis I assumed that the present depth of the features is the
maximum during their history. The stratigraphy at each drill site was
“backstripped” in order to isolate the tectonic component of the
subsidence history. This process involves the correction of sediment
loading and restoration of the seafloor to the estimated depth in the
past for each dated interval, assuming local isostacy (Sclater and
Christie, 1980). In doing so the vertical motion of the igneous
basement at each drill site can be reconstructed since the end of
volcanism. No correction was made for sealevel variations because of
debate about the timing and magnitude of this effect. However, the
uncertainties introduced by this omission are small (~100 m) compared
to the much greater values of total subsidence and the general
uncertainties in the water depths.
Figures 3, 4 and 5 shows the results of the
analysis and compares each reconstructed history with that of normal
oceanic crust (Stein and Stein, 1992). In each case the normal oceanic
subsidence curve was shifted to match the modern depth to basement in
order to compare rates, and not the total depth of subsidence. It is
rate of subsidence that is affected by the mantle thermal state.
Absolute depth is influenced by crustal thickness and thus the degree
of melting, which is partially controlled by mantle temperature, but
also by the chemical state of the mantle, and the nature of upwelling
under the hotspot (Mutter et al.). In the case of features emplaced
off-axis a second oceanic subsidence curve is shown indicating the
predicted rate of subsidence if the lithosphere had been totally reset
to zero age at the time of eruption of the basement. Some of the drill
sites show clear evidence for shallower than predicted depths early in
their history, such as would be predicted for emplacement above hotter
than normal mantle. Sites in the NE Atlantic, the Hatton Bank, SE
Greenland and the Vøring Plateau and even SE Newfoundland show
early anomalously shallow depths, i.e. steeper than expected subsidence
curves, similar to that seen in modern Hawaii. Modeling by Clift (1997)
indicated that the moderate nature of the depth anomaly was consistent
with temperature anomalies of around +100˚C. Outside this region
definitive evidence for shallower depths following eruption is scarce,
as many of the sites fall within the predicted trend of crust overlying
normal upper mantle temperatures. Where water depth uncertainties are
large the possibility of hot mantle exists but is not required.
There appears to be as many if not more sites where the reconstructed
depth to basement after emplacement is shallower than predicted for
even normal mantle temperatures. This raises the possibility that the
asthenosphere under the sites has actually heated, not cooled since
emplacement, though this is not considered likely.
Figure 3. Reconstructed
basement subsidence curves from the North Atlantic showing the depths
of the basement in SE Newfoundland and the Vøring Plateau of
Norway, which are shallower than predicted by a normal oceanic
subsidence curve (Stein and Stein, 1992) after the initial
eruption. This is consistent with rapid crustal emplacement above
a hot mantle plume.
A number of models
have been proposed to account for
the lack of subsidence in old hotspot regions. The relative lack
of subsidence and initial submarine eruption of lavas in the Ontong
Java and Manihiki Plateaus lead Ito and Clift (1998) to propose growth
of these features in two or more stages. In practice they argued that
the plateaus are inflated through time by underplating stretching from
120 to 90 Ma. In this case uplift caused by crustal growth counteracted
the subsidence driven by lithospheric cooling. This model is
practical in that setting because these features remained approximately
stationary over the proposed plume source during that time.
However, this model is not appropriate when applied to many other
igneous provinces, which move rapidly relative to their source
hotspots, e.g., NinetyEast Ridge. The MIT Guyot and the Mid Pacific
Mountains are good examples of features that appear to represent excess
melting, move rapidly within the hotspot reference frame after
emplacement and which are not associated with a hotspot track. Their
subsidence histories are slower than predicted for normal oceanic crust
of their age, the opposite of that predicted for emplacement over hot
asthenosphere. Long-term crustal accretion is not a reasonable
explanation for the slow subsidence seen in this type of hotspot.
Instead mantle processes must be inhibiting subsidence. Most likely the
presence of a buoyant, depleted mantle residue from melting is acting
as a cushion under these features. Mantle from which melt has been
extracted is known to have a lower density than normal depleted upper
mantle. If this material was not advected away from under the seamounts
then this could slow subsidence (Robinson, 1988). Such a
compositional-derived density model for hotspots was invoked by Phipps
Morgan et al. for the Hawaiian swell and may be applicable to many
Pacific and Indian Ocean hotspot features. What is unusual is that
although Phipps Morgan et al. (1995) consider the depleted residue to
be more viscous that normal asthenosphere, it is predicted to disperse
over long periods of geologic time, resulting in a reduction in the
height and width of the Hawaiian Swell. That prediction is at odds with
the long-term lack of subsidence seen in many of the reconstructions
presented here, which would require much of the buoyant residue to
remain under the hotspot edifice until the present day, i.e. that it
had a high viscosity.
Figure 4. Reconstructed
basement subsidence curves from the Mid Pacific Mountains and MIT
Guyot. Basement depths are approximately the same as those predicted by
a normal oceanic subsidence curve
(Stein and Stein, 1992). There is no need to invoke a hotter than
normal asthenosphere to account for the subsidence of these features.
shows the predicted curve for crust of the same age as the igneous
basement at each site because this is different from the oceanic
patterns of hotspot features in the
modern oceans only occasionally fit simple models for rapid crustal
generation over hot mantle, followed by faster than normal subsidence
as the ridge or seamount was removed from over the thermal anomaly by
plate motion. The normal or slower than normal subsidence seen in many
hotspots may signify the presence of a buoyant mantle root. Such
subsidence patterns do not preclude hotter than normal mantle at the
time of emplacement, but they do not require this either. Many hotspots
are compatible with emplacement over mantle of normal temperature,
followed by conductive cooling of the type associated with regular
Figure 5. Reconstructed
basement subsidence curves from the Shatksy Rise and the NinetyEast
Ridge showing that the depths
of the basement is deeper than predicted by a normal oceanic subsidence
(Stein and Stein, 1992). This pattern can be explained by either
gradual growth, emplacement above colder than normal mantle, or
buoyancy linked to emplacement of depleted residual mantle root.
The North Atlantic
margins where subsidence
anomalies do exist are consistent with mantle temperature anomalies of
~100˚C, but preclude very hot plume models. This conclusion is
supported by inversion modeling of rare earth element data from hotspot
volcanic rocks that predict temperature anomalies not exceeding 150˚C
(e.g., Watson, 1993; Kent and McKenzie, 1994; Kerr, 1994). If any of
the anomalous shallow depths seen after emplacement are caused by
buoyancy resulting from mantle composition or dynamic mantle upwelling
then the temperature anomalies inferred from the subsidence anomalies
would be lower. The subsidence behavior of the North Atlantic margins
and the Hawaiian seamount chain appears to differ from many other
hotspots. This may be linked to a different origin that is linked to
hot mantle that is not required in many other areas. Fast subsidence in
these regions begs the question of why these provinces are not affected
by a buoyant mantle residual root. Perhaps vigorous mantle circulation
of the type proposed by Mutter et al. to increase melting was
responsible for the removal of any residue that did form. There is
little evidence to support the presence of a buoyant root in the NE
Atlantic related to Tertiary magmatism. Temporary uplift of the NW
European shelf away from the immediate vicinity of the volcanic margins
during their formation is consistent with the moderate thermal anomaly
derived from the oceanic crust subsidence histories (Clift et al.
1998), modern heatflow (Stein and Stein, 2003) and the modeling of
volcanic rare earth element data (Kerr, 1994).
In summary, the sedimentary records of hotspot
provinces worldwide frequently do not require hotter than normal mantle
to be present at the time of magmatic crustal growth. In the few
examples where hotter mantle is required the degree of the temperature
anomaly is usually no more than ~100˚C and could be much less if
dynamic flow and compositional buoyancy is an major factor in driving
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This research used data provided by the Ocean Drilling Program
ODP is sponsored by the U.S. National Science Foundation (NSF) and
participating countries under management of Joint Oceanographic
Institutions (JOI), Inc.