The main features of the greater
Ontong Java Plateau (OJP) may be summarized as follows.
It is presently obducting along
the Solomon Islands trench (Figure 2) (e.g
It includes the Ontong Java Plateau
and neighboring basin basalts, including Nauru,
E. Mariana, possibly Pigafetta and other contemporaneous
flood basalts (Coffin & Eldholm, 2004).
It is the most voluminous large
igneous province (LIP) on Earth, at ~ 60 million
cubic km (Gladczenko
et al., 1997
It is structurally homogeneous
– there is no evidence of continental lithosphere
It is geochemically homogeneous,
involving large degrees (> 30%) of batch partial
melting followed by extensive fractional crystallization
It is widely believed to have
been formed by a large mantle plume head (e.g.
Figure 1: Plate tectonic reconstruction
for Ontong Java Plateau (OJP), Nauru (NAU), East Mariana
(MAR), Pigafetta (PIG) basin flood basalts and Manihiki
Plateau (MAN) (118.7 Ma – Chron M0: after Eldholm
& Coffin, 2000).
Figure 2: Predicted bathymetry
with drill sites that penetrated flood basalts
superimposed (Smith & Sandwell, 1997). Click
on figure for enlargement.
Figure 3: Satellite-derived
free-air gravity field with same drill sites as
Figure 2 superimposed (Sandwell & Smith, 1997).
Click on figure for enlargement.
of mantle plume theory vs. observations for the greater
prior to magmatism
Widespread subaerial volcanism is predicted
to be caused by the arrival of hot, buoyant ascending
plume material at the base of the oceanic lithosphere
- Uplift of the seafloor from 1000 m to 2000 m
is predicted due to the arrival of a 1000-2000-km-diameter
plume head (Griffiths
- Uplift of the seafloor from 3000 m to 4000 m
is predicted based on OJP-specific dynamic plume
- Uplift of the seafloor from 1000 m to 3000 m
is predicted based on OJP-specific isostatic plume
Observation: Actual uplift
depends on the model chosen, but it is obvious from
the data that uplift of the oceanic crust falls far
short of predictions, regardless of the wide range for
predicted uplift models (Figure 4).
Figure 4: Model-predicted paleodepth
ranges for the top of OJP crust (open symbols) compared
to actual paleodepth ranges (filled symbols) of Aptian
sediments overlying the crust for DSDP and ODP sites.
The error bars for all data give the range based on
minimum and maximum estimates (predicted) or calculations
(actual). See Ingle
& Coffin (2004) for details.
- According to simple isostasy, emplacement of
the 35-km-thick crust of the OJP following uplift
of the preexisting ~ 7-km-thick oceanic crust, should
have resulted in a minimum culminating elevation
of ~ 700 – 4200 m on the crest of the OJP
- Significant subaerial volcanism should have been
- Most oceanic plateaus, submarine ridges, and
seamount chains have experienced subaerial volcanism
during construction (Detrick et al., 1977;
Observation: All sampled OJP
basalts, both drilled & obducted, were erupted well
below sea level (Figure 5).
Figure 5: Mass fluxes and peak eruption
levels of currently active hot spots and the OJP. Note
that of all the hot spots, only Iceland coincides with
a mid-ocean ridge. For the OJP, the error bars represent
the mass flux range assuming an emplacement duration
of 1 - 5 Myr.
- Plateaus within oceanic lithosphere should subside
as a result of thermal conduction (Detrick &
Crough, 1978) or continuous viscous spreading
of the anomalous mantle material (Richards et
al., 1989) (Figure 6).
- Many oceanic plateaus and submarine ridges have
subsided in this manner (Detrick et al.,
- Predicted total subsidence for ~120 Ma oceanic
crust is ~3000 – ~3800 m
Observation: Based on paleoenvironments
interpreted from sediment overlying basalt (Andrews
et al., 1975; Sliter & Leckie, 1993)
and basalt eruption depths calculated from water and
carbon dioxide concentrations in basaltic glasses (Roberge
et al., 2003) it appears that the OJP crust subsided
only 1000 – 2800 m (Ingle
& Coffin, 2004) (Figure 6).
Figure 6: Total subsidence vs. age
for Ontong Java Plateau basalts (error bars on symbols
denote range) compared to lithospheric subsidence curves
from: BL and PS77 (Parsons & Sclater, 1977) and
SS92 (Stein & Stein, 1992).
to a hot spot source
- the OJP was originally proposed to have been created
by the Louisville hot spot (e.g., Tarduno
Observation: No link to any
hot spot track or presently active hot spot can be argued
for on the basis of geochemistry (Mahoney
1993), plate reconstructions (Neal
1997), paleomagnetic data (Riisager
2003), or combined hot spot motion and true polar
wander (Antretter et al., in press)
The mantle plume model predicts:
- An “enriched” mantle source, preferably
isotopically comparable to one of the mantle endmembers
(e.g., EMI, EMII, HIMU).
- Large degrees of melting – melting at a
plume-ridge intersection or a “wet”
- High concentrations of elements or element isotope
ratios that reflect a possible core input (Neal
- Noble gas data that reflect a primitive mantle
source, typically associated with the lower mantle.
Observation: Geochemical data
are ambiguous in their support for a mantle plume origin.
Isotopic compositions are intermediate between primitive
mantle and “mid-ocean ridge” or “depleted”
upper mantle compositions (e.g., Tejada
2002). Platinum group element concentrations are
high relative to predicted values (Ely
& Neal, 2003). OJP basalts suggest low-water
contents in their mantle source (Michael,
1999) ruling out a “wet” plume.
head residue or mantle root
- Centered beneath the OJP is a 1200 km-diameter,
~ 300-km-deep mantle root, that extends well into
the asthenosphere (Richardson
2000). It corresponds to an S-wave
velocity anomaly of ~ 5% and is less dense than
the surrounding mantle (Klosko
- This root has been interpreted as being a “frozen
plume head” (Klosko
- Depleted, restitic material may be less dense
if the extracted magmas were extremely Fe-rich but
the residue should then have the high seismic wave
speeds observed for roots beneath both Archean cratons
(Zhang & Tanimoto, 1993) and modern-day
hotspots such as Pitcairn (Rost & Williams,
Observation: The root appears
to be rheologically strong and not related to a thermal
anomaly. Its volume is far too large to be explained
by mantle material remaining after extracting the volume
needed to form the OJP basalts.
summary many geophysical, geodynamic, and geochemical
results from the greater OJP are at odds with
various mantle plume models for its origin. As
discussed below, we suggest that an extraterrestrial
impact model is much more consistent with existing
data and results.
impact of a large meteorite explain better the observations
for the greater OJP as has been suggested for the Siberian
- ~20 km diameter bolide of chondritic composition.
- Impact velocity ~20 km/s.
- Vertical impact assumed and instantaneous
vaporization of the ~ 4 km deep water column.
- Preexisting lithosphere thickness of ~50
- Penetration depth of ~ 60 km (O’Keefe
& Ahrens, 1993) and initial crater
diameter of ~ 200 km (Grieve & Pesonen,
1992) (Figure 7).
- Massive decompression melting in the upper
mantle, to a minimum depth of 300 km, assuming
100% partial melting. This results from the
removal of the lithospheric overburden.
- Solid asthenospheric mantle, moving laterally
inward and upward from below to replace the
extracted mantle may have experienced a catastrophic
decrease in pressure during its emplacement
beneath the OJP, resulting in the low shear-wave
velocities observed by Richardson
Conceptual model showing possible sequence of
events if a ~20 km-diameter bolide caused the
creation of the OJP, from the model of Jones
et al. (2002; see also Impacts
page). t=1: Moment of impact, water column
is vaporized, 20-Myr-old oceanic lithosphere (pink
layer) at impact site is obliterated, uppermost
asthenosphere is penetrated, and surrounding lithosphere
fractures; t=2: Moment of maximum penetration,
the crater is completely formed and the melting
region becomes focused; t=3: Infill of void from
bottom and sides, melt migrates out along fractures
in lithosphere, refractory surrounding mantle
fills space vacated by outflowing magmas; t=4:
~120 Ma, the OJP at the end of emplacement (pink
layer represents ~35 km thick crust); t=5: ~90
Ma, tectonism causing new pressure release melting.
Scale is maintained throughout the diagram.
to validate an impact model
- Quantitative modeling of the impact event, including
evolution of the crater morphology, melt volumes
and compositions induced by the impact, emplacement
of the melt and OJP uplift and subsidence are critical
to validate the model.
- Spherules and highly-siderophile element anomalies
might be present in contemporaneous, local sediments.
At the time of writing, such evidence has not been
- Discovery of distal sediments of Aptian age including
spherules, tsunamis deposits and disturbed abyssal
sediments, would help to validate this model. Tektites
from the impact site would be unlikely to survive
as shocked mafic minerals are significantly less
stable than shocked felsic minerals, e.g.,
phenomena related to a ~120 Ma impact
- Onset of the Cretaceous normal magnetic polarity
et al., 1991).
- The Selli oceanic anoxic event (Larson &
- Significant marine faunal extinctions (Sepkoski,
- A worldwide radiogenic Sr isotopic excursion
in marine sediments that suggests a rapid change
from terrigenous-dominated values to mantle-dominated
values (Jones & Jenkyns, 2001).
- Obvious terrestrial evidence for an impact of
this magnitude at ~120 Ma is lacking, e.g.,
no major land faunal extinctions are known to have
ocurred. Possible explanations might be the large
distance between the impact site and any land mass
(Figure 1). The environment of emplacement might
also have played a role. This was deep marine, compared
with shallow sediments at the Chicxulub site.
- It may seem logical to anticipate normal, Pacific
mid-ocean-ridge-like basalts in the impact scenario
(for more extensive discussion of this, see the
Ontong Java page) but the
isotopic ratios in basalts of the OJP are more typical
of ocean islands. However, there are now compelling
arguments that suggest that the upper mantle is
not as homogeneous as once believed to be (e.g.,
2003; see also Mantle
reservoirs page and He-Os
page). Considering the volume of the OJP requires
melting to at least 300 km depth in the mantle and
large extents of melting, many local heterogeneities
could have been tapped and mixed in with the depleted
matrix to explain the geochemistry of OJP basalts.
The absence of recognized impact craters
on Earth’s deep ocean floor is remarkable considering
that estimates based on the present-day terrestrial
cratering rate predict that up to three bolides ~10
km in diameter should have struck the deep ocean basins
in post-Jurassic times (Proveda et al., 1989).
The OJP and the neighboring ocean basin flood basalts
may be the evidence of one such impact.
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