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   Ontong Java Plateau

Maria Luisa G. Tejada1 & John J. Mahoney2

1National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City, 1101 Philippines

2School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawai, 96822, USA

mtejada@nigs.upd.edu.ph & jmahoney@hawaii.edu

The Ontong Java Plateau (OJP), with an area of 2 x 106 km2 and a maximum crustal thickness of more than 30 km (e.g. Gladczenko et al., 1997; Richardson et al., 2000; Miura et al., 2004; J.G. Fitton & M.F. Coffin, pers. comm. 2003), is the largest of several massive oceanic plateaus that appeared at equatorial-to-mid-southern latitudes in the Pacific Basin between the latest Jurassic and Late Cretaceous. Of these, the OJP is by far the best sampled by drilling (Figure 1). Four ODP Leg 192 sites on the main or high plateau penetrated 65-217 m of basaltic basement, and three earlier DSDP and ODP sites penetrated 9-149 m. In comparison, only 1-2 basement sites each, with penetrations of 0.9-46 m, have been drilled on the Manihiki Plateau, Magellan Rise, Hess Rise, and Shatsky Rise. In addition to the OJP drill sites, plateau basement sections as thick as 3.5 km have been sampled in the islands of Malaita, Santa Isabel, and San Cristobal in the Solomon Islands along the southern margin of the OJP.

Figure 1: Basement drill sites on the OJP. Squares represent ODP drill sites 1183, 1186, 1185 and 1187 and pink dots represent previous DSDP and ODP drill sites. Click on figure for higher-resolution enlargement.

The origin of the OJP and the other Pacific plateaus is debated. Among the mechanisms proposed are:

  1. cataclysmic melting in the inflated heads of newly risen mantle plumes (e.g., Richards et al., 1989) or even a single “super” plume (Larson, 1991),
  2. formation above near-ridge plume tails over much longer periods of time (e.g., Mahoney & Spencer, 1991; Ito & Clift, 1998),
  3. plate separation above extensive, near-solidus, but non-plume regions of shallow asthenosphere (e.g. Anderson et al., 1992; Smith & Lewis, 1999; Hamilton, 2002), and
  4. asteroid or comet impact (Rogers, 1982).
The variety of models that has been applied in part reflects a lack of detailed knowledge of Pacific plateau composition and age. This in turn is a result of the very sparse sampling of crustal basement.

The OJP lavas are low-K tholeiitic basalts with only a small range of major element, trace element and Nd-Pb-Sr-Hf isotopic composition (Figure 2), a rather surprising result in view of the immensity of the plateau (e.g., Mahoney et al., 1993; Tejada et al., 1996, 2002, 2004; Babbs, 1997; Neal et al., 1997; Fitton & Godard, 2004). All of these basalts are distinct from both OIB (ocean island basalt) and Pacific N-MORB (normal mid-ocean-ridge basalt). They have low, N-MORB-like concentrations of many incompatible elements but OIB-like isotopic signatures comparable to those of the Hawaiian shield volcanoes of Kilauea and Mauna Loa; moreover, unlike either N-MORB or OIB, their incompatible-element patterns (normalized to estimated primitive mantle) and rare-earth patterns are relatively flat (Figure 3). Dating by 40Ar-39Ar suggests most of the OJP was emplaced in a short period around 120 Ma (Mahoney et al., 1993; Tejada et al., 1996, 2002; Parkinson et al., 2001; Chambers et al., 2004).

Figure 2: Sr-Nd-Pb-Hf isotope data for the OJP showing the two isotopic groups of lavas (orange outline) and the new data from Leg 192: Site 1183 (yellow circles); Site 1186 (diamond); Site 1185 (red circles) and Site 1187 (blue squares).

Figure 3: Primitive-mantle-normalized-incompatible-element pattern for OJP lavas. Normalising values are taken from Sun & McDonough (1989). The patterns for Kwaimbaita-type basalt are reproduced, as orange outlines, on all the other diagrams for comparison. Note the similarity in the shape of the patterns for Kwaimbaita- and Kroenke-type basalt and the slight relative enrichment in the more incompatible elements shown by the Singgalo-type basalts (Site 807, unit A). Modified from Fitton & Goddard (2004).

Despite the limited compositional variability, two isotopically distinct, stratigraphically separate groups of basalt, termed the Kwaimbaita and Singgalo types (Tejada et al., 2002), have been identified at widely separated locations. A third, rather primitive magma type termed the Kroenke-type was found at ODP Sites 1185 and 1187; isotopically, Kroenke-type basalts are indistinguishable from the Kwaimbaita type and are likely to be parental to the Kwaimbaita. Both Singgalo and Kwaimbaita/Kroenke magma types appear to be the products of high fractions of partial melting. The most recent estimates of the average amount of partial melting for the Kwaimbaita and Kroenke types are between 25 and 31% (Fitton & Godard, 2004; Herzberg, 2004), whereas the Singgalo type appears to represent slightly lesser amounts (Mahoney et al., 1993; Neal et al., 1997; Tejada, 1998). On the basis of present sampling, it appears that Kwaimbaita-type magmas dominate the upper portion of the plateau’s crust. Furthermore, Kwaimbaita-type lavas fill the large Nauru and East Mariana basins adjacent to the OJP on the north and east (e.g., Mahoney, 1987; Castillo et al., 1991, 1994). The small isotopic range defined by lavas from the OJP and surrounding basins indicates an enormous, isotopically homogeneous (relative to the scale of melting) mantle source (Tejada et al., 1996, 2002; Neal et al., 1997).

What might be the explanation for the particular combination of isotopic and incompatible-element characteristics in the mantle source of the OJP?

The flat primitive-mantle-normalized patterns defined by alteration-resistant incompatible elements in the Kwaimbaita- and Kroenke-type basalts (see Fitton & Godard, 2004) point to a mantle source not too different from estimated primitive mantle in most of its inter-element ratios. However, the observed isotopic values (e.g., eNd(t) ~ +6) are clearly far-removed from those estimated for primitive mantle (eNd = 0). For some oceanic plateaus, the general explanation given is in terms of variable mixing of OIB mantle end-member types, with or without involvement of MORB-type mantle (e.g., Kerr et al., 2002). However, no evidence for involvement of MORB-type mantle has yet been found in the OJP. Although mixing involving, for example, an EM-1-like end-member and anciently recycled oceanic lithosphere can explain the observed OJP isotopic ratios, there are no discernible mixing arrays in the data, which would be expected for this model, and no support is provided by trace or major element modeling (Tejada et al., 2002).

Could the Kwaimbaita/Kroenke source represent originally primitive mantle (i.e., originally of bulk silicate Earth composition) that underwent minor fractionation long ago and evolved isotopically in an essentially closed-system manner until the early Aptian? A simple, two-stage evolution model that assumes a mantle fractionation event in the 3-4 Ga range can reproduce the Sr, Pb, Hf, and Nd isotopic characteristics of the Kwaimbaita- and Kroenke-type basalts (Figure 4). In this model, the first stage of isotopic evolution occurs in a reservoir of primitive-mantle chemical and isotopic composition until about 3 Ga, when a fractionation event, perhaps the removal of a small (~ 1%) partial melt fraction occurs, modifying the reservoir’s chemical composition slightly. Subsequent closed-system isotopic evolution transpires in this slightly modified mantle reservoir until the eruption of the OJP lavas at 120 Ma (for details of this model, see Tejada et al., 2004). Trace element modeling is consistent with this model (Fitton & Godard, 2004).

Figure 4: Two-stage modeling results for the OJP. The first stage of isotopic evolution occurs until about 3 Ga when 1% partial melting modified its composition slightly. The second stage is a closed-system evolution of this modified mantle until it melted at 120 Ma ago to form the OJP.

Arguably the most likely part of the planet in which a large volume of ancient, chemically near-primitive material might survive would be the lower mantle. Recent seismological results suggest the bottom ~1000 km of today’s mantle may be chemically distinct, separated from the rest of the mantle by a thermochemical interface with large undulations, and that most of the time little mixing occurs across the interface (e.g., Kellogg et al., 1999). Hypotheses involving an ultimately lower-mantle origin for the OJP, of course, inevitably are coupled to plume-head or mantle-overturn models. The isotopic homogeneity of the Kwaimbaita- and Kroenke-type lavas is consistent with recent models of plume formation in the lower mantle that suggest large plume heads should be well mixed, should entrain little non-plume mantle during ascent, and thus should be significantly more homogeneous than plume tails (e.g., Van Keken, 1997; Farnetani et al., 2002). The OIB-like isotopic signature and the apparently rapid formation of the bulk of the OJP around 120 Ma by high-degree partial melting also are consistent with predictions of plume-head models (e.g., Richards et al., 1989; Campbell & Griffiths, 1990; Campbell, 1998).

The problem with the plume hypothesis for the OJP is that……

However, several significant discrepancies exist between observation and model, including the following (Tejada et al., 2002):

  1. the recurrence of compositionally Kwaimbaita-like tholeiitic basalts at ~90 Ma, after an apparent eruptive hiatus of ~30 m.y.;
  2. the lack of a post-plateau seamount chain corresponding to the plume-tail stage of hotspot development theorized to follow the plume-head stage;
  3. the lack of any presently active hotspot that can be linked unambiguously to the plateau;
  4. that post-eruptive subsidence of the plateau appears to have been much less than expected for oceanic lithosphere; and
  5. the substantial water depths at which most OJP lavas appear to have been erupted (below the calcite compensation depth in some cases) are contrary to predictions of shallow or subaerial emplacement. The calcite compensation depth is the depth in the ocean where the rate of calcium carbonate dissolution exceeds the rate of deposition. This means that if the oceanic floor or seamount top is above the calcite compensation depth, limestone layers may be deposited on it, but if they are below, limestone does not accumulate. In the present Pacific ocean, this level is at 4,000-5,000 meters.

Many of these discrepancies may provisionally be accommodated by assuming a number of case-specific modifications to the standard plume-head model, but the sheer number of ad hoc modifications needed indicates to us that the model may not be appropriate at all for the OJP.

Can plate tectonic processes and perisphere explain the origin of the OJP?

Non-plume enthusiasts have suggested mechanisms for the origin of large igneous provinces that include focusing of melt flow, unusually fertile mantle (see also PT Processes page), large-scale melt ponding, “EDGE” and rift-related processes (see also EDGE page), and a partially molten shallow mantle (e.g., Anderson, 1998). The plate-separation hypothesis (e.g., Anderson et al., 1992; Smith & Lewis, 1999) posits an extensive layer of shallow, volatile-rich, near-solidus, OIB-like (but not plume derived) asthenosphere (“perisphere”) to have resided beneath a region of the Pacific lithosphere that was rifted suddenly by a ridge jump around 120 Ma, causing cataclysmic melting. However, the pre-120 Ma seafloor within several hundred kilometers of the OJP is not much older than the plateau itself, having been formed only ~ 2 – 35 m.y. earlier (e.g., Taylor, 1978; Nakanishi et al., 1992). Thus, during this period, a spreading system was not too distant from the (future) location of the OJP. It is difficult to understand why such perisphere, assuming it existed, was not drained earlier by the nearby ridge. Also, the hypothesis predicts that normal MORB-type mantle lying just beneath the perisphere should have been tapped progressively more as OJP volcanism proceeded; yet, as noted above, no evidence of MORB-type mantle has been found thus far in OJP basalts. Indeed, the Singgalo-type basalts that compose the topmost part of the lava pile in several widespread locations are even less MORB-like than the Kwaimbaita and Kroenke types. Furthermore, OJP lavas are notably poor, not rich, in volatiles (Roberge et al., 2004).

Why not meteorite impact instead of plume impact?

Well before either the plume-head or plate-separation hypotheses were proposed, Rogers (1982) suggested that the Pacific plateaus represent massive outpourings of basalt formed by the cataclysmic excavation of asthenosphere by large but rare meteorite impacts. We find this hypothesis attractive in that it can explain the absence of a post-plateau seamount chain and any obvious present-day hotspot that can be linked with the OJP. Also, the apparent lack of large areas at shallow water depths during the construction of the OJP is not necessarily a problem, because inherently hotter than normal mantle is not required for widespread magmatism. Nor are huge amounts of volatile-rich (or eclogitic) mantle necessary. Moreover, the limited range of elemental and isotopic variation in both the Singgalo- and Kwaimbaita-type basalts might be attributable in part to melt homogenization in extensive magma pools created by the impact. Another potential advantage of the impact hypothesis is that it may help explain the enigmatic 300-km-thick, seismically slow mantle “root” beneath the OJP (Ingle & Coffin, 2004).

However, just as with the plate separation hypothesis, this hypothesis does not explain why OJP lavas lack any resemblance to Pacific MORB of the same age. Large-volume, high-degree partial melting of the upper few hundred kilometers of sub-seafloor mantle resulting from a large impact would ordinarily be expected to produce basalt with essentially N-MORB-type isotopic and incompatible element characteristics. Although sampled in relatively few places, pre-OJP Pacific MORB are isotopically and chemically indistinguishable from modern Pacific MORB (Janney & Castillo, 1997; Mahoney et al., 1998). In contrast, the enormous volumes of OJP basalt have OIB-like isotopic signatures and rather flat incompatible-element patterns, and there is as yet no evidence for any involvement of MORB-type mantle in the plateau (see above). Note also that neither chondritic, achondritic, or iron meteorites have a suitable combination of Sr-Pb-Nd isotopic characteristics (e.g., Kerridge & Matthews, 1988, and references therein) to explain the isotopic signature of the OJP basalts via contamination of MORB-type mantle with meteoritic material, even assuming a significant amount of such material remained at the impact site.

To reconcile the geochemical evidence with the impact hypothesis, the impact site can be assumed to have fortuitously been located above a geochemically anomalous region of asthenosphere containing a large amount of OIB-type mantle (cf. Ingle & Coffin, 2004). However, suitable isotopic compositions are notably rare among modern hotspots and non-hotspot volcanic areas in the South Pacific, where plate reconstructions put the original location of the OJP. It can be speculated further that a large impact triggered a deep mantle plume beneath the impact site (e.g., Alt et al., 1988; Glikson, 1999), although it is not clear whether significant initial uplift of the plateau's surface would result or not.

An additional challenge for the impact hypothesis is the reportedly systematic patterns in the gravity field and bathymetry of the OJP that Winterer & Nakanishi (1995), Neal et al. (1997), and Kroenke et al. (2004) have suggested represent a seafloor spreading fabric. This interpretation remains to be evaluated rigorously but, if correct, is very difficult to reconcile with the expected widespread destruction and disruption of preexisting oceanic lithosphere by a large impact, or with the short-lived outpouring of magma following an impact, which would be too rapid to allow formation of significant amounts of new seafloor.

To our knowledge, key features diagnostic of other large impact events, such as microspherules and siderophile element anomalies, have not been found in terrestrial or marine sediments of OJP age. Moreover, in contrast to the impact-linked Cretaceous-Tertiary boundary, no mass extinction occurred at the time of OJP formation. Further, it is not clear that a sufficiently large object has been available in Earth’s vicinity in the last few hundred million years. The Cretaceous-Tertiary boundary impact created the largest known Phanerozoic impact crater, the ~ 200 km-wide crater at Chicxulub, Mexico, which is thought to have been formed by an object ~ 10 km in diameter (Grieve & Therriault, 2000, and references therein). Any object that could trigger the formation of the huge OJP must have been considerably larger. However, no near-Earth objects larger than 40 km in diameter are observed today, and only two are larger than 20 km (Binzel et al., 2002). Venus, with a surface age estimated at ~ 600 Ma (e.g., Nimmo & McKenzie, 1998), lacks any impact craters larger than Chicxulub (Schaber et al., 1992), whereas all lunar craters larger than ~ 100 km in diameter appear to be older than ~ 800 Ma (e.g., Eberhardt et al., 1973; Grier et al., 2001). Nevertheless, despite these potentially serious problems with the impact hypothesis, we regard it as deserving of further study in view of the difficulties encountered with any simple form of the plume-head or other proposed models for the OJP.

Back to square one…

Presently, it appears that no single model for the origin of the OJP can explain all of the geochemical, geophysical, and geodynamical observations. This apparent failure makes the OJP one of the most challenging and potentially fruitful sites for further scientific investigation in the near future.

References

last updated 31st December, 2006
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