Pin-pricking the elephant: evidence on the origin of the Ontong Java Plateau from Pb-Sr-Hf-Nd isotopic characteristics of ODP Leg 192 basalts

Abstract

Age-corrected Pb, Sr and Nd isotope ratios for early Aptian basalt from four widely separated sites on the Ontong Java Plateau that were sampled during Ocean Drilling Program Leg 192 cluster within the small range reported for three earlier drill sites, for outcrops in the Solomon Islands, and for the Nauru and East Mariana basins. Hf isotope ratios also display only a small spread of values. A vitric tuff with eNd(t) = +4.5 that lies immediately above basement at Site 1183 represents the only probable example from Leg 192 of the Singgalo magma type, flows of which comprise the upper 46-750 m of sections in the Solomon Islands and at Leg 130 Site 807 on the northern flank of the plateau. All of the Leg 192 lavas, including the high-MgO (8-10 wt%) Kroenke-type basalts found at Sites 1185 and 1187, have eNd(t) between +5.8 and +6.5. They are isotopically indistinguishable from the abundant Kwaimbaita basalt type in the Solomon Islands, and at previous plateau, Nauru Basin and East Mariana Basin drill sites. The little-fractionated Kroenke-type flows thus indicate that the uniform isotopic signature of the more evolved Kwaimbaita-type basalt (with 5-8 wt% MgO) is not simply a result of homogenization of isotopically variable magmas in extensive magma chambers, but instead must reflect the signature of an inherently rather homogeneous (relative to the scale of melting) mantle source. In the context of a plume-head model, the Kwaimbaita-type magmas previously have been inferred to represent mantle derived largely from the plume source region. Our isotopic modelling suggests that such mantle could correspond to originally primitive mantle that experienced a rather minor fractionation event (e.g. a small amount of partial melting) approximately 3 Ga or earlier, and subsequently evolved in nearly closed-system fashion until being tapped by plateau magmatism in the early Aptian. These results are consistent with current models of a compositionally distinct lower mantle and a plume-head origin for the plateau. However, several other key aspects of the plateau are not easily explained by the plume-head model. The plateau also poses significant challenges for asteroid impact, Icelandic-type and plate separation (perisphere) models. At present, no simple model appears to account satisfactorily for all of the observed first-order features of the Ontong Java Plateau.Several massive volcanic plateaus appeared at equatorial to mid-southern latitudes in the Pacific Basin between the latest Jurassic and the middle Cretaceous. Of these, the Ontong Java Plateau (OJP; Fig. 1) in the western Pacific is the world's largest (the 'elephant' in our title), with

Phreatomagmatic eruptions on the Ontong Java Plateau: chemical and isotopic relationship to Ontong Java Plateau basalts

“…Four bulk tuff samples, representing the range of trace-element characteristics of the volcaniclastic rocks, were analysed for Sr-, Nd-and Pb-isotopic composition at the Scripps Institution of Oceanography using the procedure described in Tejada et al (2004;see also Janney & Castillo 1996. For detailed descriptions of the lithologies from which these samples were taken, see Mahoney et al (2001) and Thordarson (2004).…”

“…Powders of samples analysed for Sr-and Nd-isotopic ratios were first leached in 4.5 N HC1 to mitigate the effects of sea-water alteration (e.g. Mahoney 1987;Janney & Castillo 1996Tejada et al 2004); those for Pb-isotopic ratios were not leached because it was suspected that the majority of the Pb was in a leachable phase. SiO~ (wt.…”

“…In addition, a comparison of the major-element data for unaltered glass and altered bulk tuff samples, as well as isotopic data for altered volcaniclastic rocks and basalts from the OJP (cf. Mahoney et al 1993;Tejada et al 1996Tejada et al , 2002Tejada et al , 2004Neal et aL 1997), provide some constraints on the chemical and isotopic changes that occur during sedimentation and alteration, and may assist future studies of similarly altered volcaniclastic rocks. This work is complemented by a study of lithic clasts from the volcaniclastic rocks at Site 1184 (Sharer et al 2004).…”

“…This sample belongs to Subunit IIA, which has a Kroenke-type elemental signature according to glass clasts and bulk tuff analyses. Kroenke-type lavas have a Kwaimbaita-type lava isotopic signature (Tejada et al 2004). The Nd and Sr concentrations of sample 1184A-11R-3, 28-34 cm dropped dramatically after leaching (see table 4 of Fitton & Godard 2004 for unleached values), suggesting that most of its Sr and Nd contents are in the altered, leachable phases such as secondary phosphates and clay (e.g.…”

South China Sea Tectonics

“…Trace element chemical analyses and measurement of Sr, Nd, and Pb isotopic ratios will provide insights into the material influx and deep crustal and mantle processes (Castillo et al, 1991;Tejada et al, 2004). The opening mechanism of the SCS can be constrained by investigating the variation in these geochemical tracers in the igneous basement rock.…”