The Ontong Java Plateau – the Largest and the Most Puzzling Jun Korenaga Department of Geology and Geophysics, Yale University, P.O. Box 208109, New Haven, CT 06520-8109 jun.korenaga@yale.edu

For discussion of this page, click here.

This short essay is based on my recently published paper:

Korenaga, J., Why did not the Ontong Java Plateau form subaerially?, Earth Planet. Sci. Lett., 234, 385-399, 2005.

Please read this paper for more details. Here, I emphasize two issues in the broader context of currently ongoing debates on the existence of mantle plumes.

Figure 1. Location of the Ontong Java Plateau with previous drilling sites. 4000-m bathymetric contour is shown as a line to indicate the spatial extent of this plateau.

First, it is essential to consider ALL available data. Of course, not all data are of the same quality and reliability, so one always has to be careful to not weight equally strong and weak constraints. Nevertheless, it is usually beneficial to look at something from different perspectives (e.g., geochemical vs. geophysical). The generation of hotspots and large igneous provinces involves chemical differentiation due to mantle melting, which has both geochemical and geophysical aspects. If one's working hypothesis is entirely geochemical or entirely geophysical, one may miss something critical.

Second, it is important to realize that we are all limited by our imaginations when searching for the truth with incomplete data. We may not be able to reach conclusions through a process of elimination. In the case of the Ontong Java Plateau, for example, at the time I wrote my recent paper (Korenaga, 2005), there were two major hypotheses: the plume hypothesis and the bolide impact hypothesis (Ed: See also Plume vs. Alternative Hypotheses for the Origin of the Ontong Java Plateau and Impact Origin for the Greater Ontong Java Plateau?). The plume hypothesis, at least in its classical form, cannot explain the submarine eruption of the Ontong Java Plateau nor its anomalous subsidence history (Ed: See also Sedimentary Evidence for Moderate Mantle Temperature Anomalies Associated with Hotspot Volcanism). But this does not mean that the bolide impact hypothesis is more likely. As it turns out, the bolide impact hypothesis is even less likely than the plume hypothesis, given the currently available geophysical and geochemical observations.

Plume vs. non-plume debates tend to be formulated as competition between two contrasting hypotheses, but there may be many other possibilities. Consider linear differential equations vs. nonlinear differential equations. The latter encompass everything that is not linear, so in a sense, they are not well defined. This may frustrate discussion because non-plume explanations such as chemically heterogeneous mantle may appear to be ad hoc; one can choose source composition to be just right to explain many observations, if not everything. However, chemical heterogeneity is a plausible idea with growing observational support. It is thus worth putting effort in to understand better the melting of heterogeneous media and resulting mantle dynamics. The interpretation of observations depends heavily on our theoretical understanding.

With these two points in mind, I reiterate the main conclusions of my recent paper on the Ontong Java Plateau (Korenaga, 2005). Key observations to be considered are the following:

1. The bulk of the plateau formed at ~120 Ma (less than a few million years).
2. The plateau was formed on relatively young seafloor (15-30 Ma).
3. Average crustal thickness is greater than 30 km (the total crustal volume for OJP is estimated to be greater than 40-50 x 10⁶ km³).
4. The bulk of the plateau was formed in a submarine environment, probably more than 1 km below sea level.
5. The plateau has subsided by only ~1-2 km since its formation.
6. There was minor magmatism at ~90 Ma.
7. The plateau is underlain by unusually low seismic velocities.
8. Most of the erupted lava is isotopically very uniform over a vast area (~1000 km).

The first three observations are fairly robust, and also not very surprising just by themselves. Other oceanic plateaus in the Pacific are similar in these respects. However, the rest of the observations listed are extremely difficult to explain if the first three are correct. The (thermal) plume hypothesis cannot explain observations 4-7, and probably not observation 8, if one considers that a convecting plume head is expected to have entrained ambient heterogeneous mantle. The bolide impact hypothesis also cannot explain observations 4-7, and probably not observation 8 either. More seriously, the impact hypothesis does not explain observation 3, which is fatal (Ed: See also Impact volcanism as a possible origin for the Ontong Java Plateau (OJP)). The alternative hypothesis I proposed in my paper, which calls for the entrainment of dense recycled oceanic crust by rapid seafloor spreading, can potentially explain observations 1-6. It is still not clear to me how to explain observations 7 and 8. As far as I know, none of the (now) three existing models can explain observation 7 in a convincing manner. The first thing to do may be to study in more detail the mantle velocity structure and its uncertainty with new seismic data.

The origin of the Ontong Java Plateau is thus still not completely resolved. Even though this plateau is, to date, the best sampled oceanic plateau, we can still learn much more by collecting new data, both geochemical and geophysical. Equally importantly, we need to improve our understanding of the mantle dynamics associated with the melting of heterogeneous sources (Ed: See also The layered mantle revisited and The eclogite engine: Chemical geodynamics as a Galileo thermometer). The Ontong Java Plateau may have been formed by some kind of plume, but if so, it must have been very different from what we envisage a mantle plume to be. New data acquisition, better theoretical understanding, and feedback between observation and theory are the best ways of making further progress.

Discussion

25th April, 2006, Stephanie Ingle
Several of the conclusions listed in this webpage have been known for quite a while. It might be helpful to bring to the readers' attention some relevant references:

1. ..bulk of the plateau formed at ~120 Ma...
   References: Mahoney et al., Geophys. Monogr. 77, 1993; Tejada et al., J. Petrol., 1996, 2002.
2. ..the plateau formed on ... young seafloor...
   References: Andrews et al., DSDP Init. Reports 30, 1975; Sliter & Leckie, ODP Init. Reports Leg 130, 1993; Mahoney et al., Proc. ODP Init. Reports 192, 2001; Ingle & Coffin, EPSL, 2004.
3. Average crustal thickness...
   References: Gladczenko et al., JGR, 1997; Richardson et al., Phys. Earth Planet. Interior, 2000.
4. .. plateau formed in a submarine environment...
   References: ODP Leg 192 Shipboard Scientific Party, 2001; Tejada et al., J. Petrol. 2002.
5. Plateau has subsided only 1-2 km since formation.
   References: Andrews et al., DSDP 30, 1975; Sliter & Leckie, ODP Init. Reports Leg 130, 1993; Shipboard Scientific Party, ODP Leg 192, 2001; Ingle & Coffin, 2004; Roberge et al., Geology, 2004.
6. Minor phase of magmatism at ~90 Ma.
   References: Tejada et al., J. Petrol. 1998; 2002
7. ... unusually low seismic velocities.
   Reference: Richardson et al., Phys. Earth Planet. Interior, 2000.
8. .. isotopically very uniform...
   References: Mahoney, Geophys. Monogr. 43, 1987; Castillo et al., EPSL 1991; Castillo et al., Proc. ODP, Sci. Res. 129, 1992; Castillo et al., EPSL, 1994; Tejada et al., J. Petrol., 1998, 2002. [and others]

25th April, 2006, Jun Korenaga
Thanks much, Stephanie, for listing up relevant references, most of which are, of course, cited in my EPSL paper. Importantly, however, the list of key observations is not my own "conclusion" list – it's just a list of observations. What follows after the list is the conclusion of my paper.