provinces imply relatively deep asthenospheric melting.
For example if, as seems likely, the Ontong Java Plateau
was the result of ~25 % peridotite partial melting, then
the spherical mantle volume required to form this amount
of melt would be more than 700 km across (i.e., deeper
than the 660 km discontinuity) (Kerr and Mahoney, 2007).
The generation of such large volumes of magma as are
observed in oceanic plateaus means that the source region
must either be hotter, more fertile, or have a higher volatile
content than ambient upper mantle, or undergo a large
amount of decompression, or some combination of the
above. However, the most common explanation for these
high melt production rates is the mantle source temperatures significantly higher than the ambient upper mantle
(see review in Campbell, 2007). Most geoscientists regard
the source of this hot material to be mantle plumes that
have risen from a thermo-geochemical boundary layer
such as the core mantle boundary (see Hot Spots and
Mantle Plumes). The evidence for elevated mantle source
temperatures for oceanic plateaus is extensive:
Firstly, as reviewed by Kerr and Mahoney (2007), the
eruption of high-MgO (>15 wt%) magmas (picrites
and komatiites) in some oceanic plateaus, particularly
in the Caribbean Plateau and in Precambrian accreted
plateaus, provides evidence of mantle source temperatures well in excess of the pressure-corrected temperature
of
the
normal
upper
asthenosphere
(1,300–1,400
C). Herzberg and Gazel (2009) calculated that high-MgO magmas from the Caribbean Plateau were derived from mantle sources with
temperatures (corrected for pressure) up to 1,620
C.
Secondly, Herzberg (2004) has used mantle melting phase
relations to calculate that the Ontong Java primary
magmas contained 17–19 wt% MgO and were derived
from a mantle source with a temperature range of
1,500–1,560
C. These figures are confirmed by trace
element-based estimates of the mean degree of melting
in the source region of the Ontong Java Plateau that
range from 25 % to 30 %, and these are consistent with
derivation from a mantle source with a temperature
greater than >1,465
C (Fitton and Godard, 2004).
Thirdly, numerical modeling of the petrological characteristics of large igneous provinces (e.g., Farnetani and
Richards, 1994) indicates that mantle source temperatures well in excess of ambient upper mantle are
required in order to generate the large melt volumes of
these oceanic plateaus from peridotite.
Furthermore, in addition to these petrological constraints, several Cretaceous oceanic plateaus display significant evidence for subaerial or shallow marine
eruption, resulting from seafloor uplift. This uplift is due
to the thermal buoyancy of the plume and is consistent
with sources hotter than the ambient upper mantle. In the
Caribbean Plateau, these include shallow water sedimentary sequences with carbonized tree trunks, overlying
accreted Caribbean Plateau basalts in Colombia (Kerr
and Mahoney, 2007). In addition to this, volcaniclastic
deposits near the top of the accreted Caribbean Plateau
sequence on Aruba, in the southern Caribbean, comprise
tuffs and ignimbrites that are consistent with eruption in
a shallow marine or subaerial environment (Kerr and
Mahoney, 2007).
Many of the drill holes into the Cretaceous Kerguelen
Plateau show evidence of subaerial eruption, including
oxidized flow tops, aa and pahoehoe flows, pyroclastic
flows, and fluvial conglomerates. Furthermore, the presence of terrestrial and shallow marine sediments
containing terrestrial plant remains, overlying the igneous
basement at ODP Leg 183, Site 1138, provides substantial
evidence for a prolonged period of subaerial exposure of
the Kerguelen Plateau (Frey et al., 2003).
Drilling on the Ontong Java Plateau at Site 1184 during
ODP Leg 192 discovered extensive basaltic
phreatomagmatic deposits that, in conjunction with oxidized horizons and wood fragments, indicate a substantial
phase of subaerial eruption on the eastern lobe of the plateau (Fitton and Godard, 2004), although evidence of significant uplift is lacking for other parts of the plateau (e.g.,
Tejada et al., 2002).
This evidence of subaerial exposure of oceanic plateaus
is entirely consistent with upwelling from the deep mantle
of hot buoyant material and associated dynamic uplift of
the lithosphere above the ascending plume. This, in combination with the evidence from petrology and geochemistry, provides significant support for mantle plume models
for the formation of oceanic plateaus (see Kerr, 2003).
Geochemical features of oceanic plateaus
Geochemically, most oceanic plateau rocks can be
subdivided into three main groups:
1. Basalts and dolerites with a narrow compositional
range and flat to slightly light rare earth elementenriched chondrite-normalized patterns. Most igneous
rocks from Cretaceous oceanic plateaus have MgO
contents ranging from ~6 to 11 wt%, with most lavas
possessing a relatively restricted MgO range of
6.5–8.5 wt%. Concentrations of incompatible elements are similar to those in mid-ocean-ridge basalts,
whereas radiogenic isotope ratios are broadly oceanisland-like (see reviews in Kerr and Mahoney, 2007;
Kerr, 2014). Virtually all sampled Ontong Java Plateau
lavas are of this type, as are many of the lavas of the
Caribbean and Kerguelen plateaus.
2. Basalts with high La/Nb ratios and high initial
87 Sr/
86
Sr (>0.705), along with low
143
Nd/
144
Nd
(<0.5127). Many early Kerguelen Plateau lavas possess this signature that is consistent with contamination
by continental crust (see review in Kerr, 2014).
3. Picrites and komatiites with high-MgO contents
(>12 wt%), which have a wider range of isotope ratios
and incompatible trace element concentrations and
560
OCEANIC PLATEAUS
For example if, as seems likely, the Ontong Java Plateau
was the result of ~25 % peridotite partial melting, then
the spherical mantle volume required to form this amount
of melt would be more than 700 km across (i.e., deeper
than the 660 km discontinuity) (Kerr and Mahoney, 2007).
The generation of such large volumes of magma as are
observed in oceanic plateaus means that the source region
must either be hotter, more fertile, or have a higher volatile
content than ambient upper mantle, or undergo a large
amount of decompression, or some combination of the
above. However, the most common explanation for these
high melt production rates is the mantle source temperatures significantly higher than the ambient upper mantle
(see review in Campbell, 2007). Most geoscientists regard
the source of this hot material to be mantle plumes that
have risen from a thermo-geochemical boundary layer
such as the core mantle boundary (see Hot Spots and
Mantle Plumes). The evidence for elevated mantle source
temperatures for oceanic plateaus is extensive:
Firstly, as reviewed by Kerr and Mahoney (2007), the
eruption of high-MgO (>15 wt%) magmas (picrites
and komatiites) in some oceanic plateaus, particularly
in the Caribbean Plateau and in Precambrian accreted
plateaus, provides evidence of mantle source temperatures well in excess of the pressure-corrected temperature
of
the
normal
upper
asthenosphere
(1,300–1,400
C). Herzberg and Gazel (2009) calculated that high-MgO magmas from the Caribbean Plateau were derived from mantle sources with
temperatures (corrected for pressure) up to 1,620
C.
Secondly, Herzberg (2004) has used mantle melting phase
relations to calculate that the Ontong Java primary
magmas contained 17–19 wt% MgO and were derived
from a mantle source with a temperature range of
1,500–1,560
C. These figures are confirmed by trace
element-based estimates of the mean degree of melting
in the source region of the Ontong Java Plateau that
range from 25 % to 30 %, and these are consistent with
derivation from a mantle source with a temperature
greater than >1,465
C (Fitton and Godard, 2004).
Thirdly, numerical modeling of the petrological characteristics of large igneous provinces (e.g., Farnetani and
Richards, 1994) indicates that mantle source temperatures well in excess of ambient upper mantle are
required in order to generate the large melt volumes of
these oceanic plateaus from peridotite.
Furthermore, in addition to these petrological constraints, several Cretaceous oceanic plateaus display significant evidence for subaerial or shallow marine
eruption, resulting from seafloor uplift. This uplift is due
to the thermal buoyancy of the plume and is consistent
with sources hotter than the ambient upper mantle. In the
Caribbean Plateau, these include shallow water sedimentary sequences with carbonized tree trunks, overlying
accreted Caribbean Plateau basalts in Colombia (Kerr
and Mahoney, 2007). In addition to this, volcaniclastic
deposits near the top of the accreted Caribbean Plateau
sequence on Aruba, in the southern Caribbean, comprise
tuffs and ignimbrites that are consistent with eruption in
a shallow marine or subaerial environment (Kerr and
Mahoney, 2007).
Many of the drill holes into the Cretaceous Kerguelen
Plateau show evidence of subaerial eruption, including
oxidized flow tops, aa and pahoehoe flows, pyroclastic
flows, and fluvial conglomerates. Furthermore, the presence of terrestrial and shallow marine sediments
containing terrestrial plant remains, overlying the igneous
basement at ODP Leg 183, Site 1138, provides substantial
evidence for a prolonged period of subaerial exposure of
the Kerguelen Plateau (Frey et al., 2003).
Drilling on the Ontong Java Plateau at Site 1184 during
ODP Leg 192 discovered extensive basaltic
phreatomagmatic deposits that, in conjunction with oxidized horizons and wood fragments, indicate a substantial
phase of subaerial eruption on the eastern lobe of the plateau (Fitton and Godard, 2004), although evidence of significant uplift is lacking for other parts of the plateau (e.g.,
Tejada et al., 2002).
This evidence of subaerial exposure of oceanic plateaus
is entirely consistent with upwelling from the deep mantle
of hot buoyant material and associated dynamic uplift of
the lithosphere above the ascending plume. This, in combination with the evidence from petrology and geochemistry, provides significant support for mantle plume models
for the formation of oceanic plateaus (see Kerr, 2003).
Geochemical features of oceanic plateaus
Geochemically, most oceanic plateau rocks can be
subdivided into three main groups:
1. Basalts and dolerites with a narrow compositional
range and flat to slightly light rare earth elementenriched chondrite-normalized patterns. Most igneous
rocks from Cretaceous oceanic plateaus have MgO
contents ranging from ~6 to 11 wt%, with most lavas
possessing a relatively restricted MgO range of
6.5–8.5 wt%. Concentrations of incompatible elements are similar to those in mid-ocean-ridge basalts,
whereas radiogenic isotope ratios are broadly oceanisland-like (see reviews in Kerr and Mahoney, 2007;
Kerr, 2014). Virtually all sampled Ontong Java Plateau
lavas are of this type, as are many of the lavas of the
Caribbean and Kerguelen plateaus.
2. Basalts with high La/Nb ratios and high initial
87 Sr/
86
Sr (>0.705), along with low
143
Nd/
144
Nd
(<0.5127). Many early Kerguelen Plateau lavas possess this signature that is consistent with contamination
by continental crust (see review in Kerr, 2014).
3. Picrites and komatiites with high-MgO contents
(>12 wt%), which have a wider range of isotope ratios
and incompatible trace element concentrations and
560
OCEANIC PLATEAUS