The validity of two 40Ar/39Ar ages presented in our recent paper on the Manihiki Plateau (Ingle et al., 2007) is questioned by Baksi (2008). We welcome the opportunity to clarify the strengths and weaknesses of the data underpinning these ages.
Very little is known about the geochemistry, age, and origin of the Manihiki Plateau, one of several Early Cretaceous large igneous provinces (LIPs) present in the western Pacific Ocean. Several recent papers addressing the origin of the giant Ontong Java Plateau (e.g., Fitton et al., 2004; Ingle and Coffin, 2004), ~2500 km to the west of Manihiki, have stimulated renewed interest and debate about relationships among these LIPs (e.g., Taylor, 2006). The primary purpose of our paper was to analyze and interpret rocks of unusual composition that are not represented in rocks recovered thus far from the Ontong Java or Hikurangi Plateaus, or indeed from any other oceanic location.
A secondary purpose of our paper was to present two 40Ar/39Ar dates which we interpret to reflect crystallization ages. The original data are: D2–1, a tholeiitic basalt, yielded an age of 117.9 ± 3.5 Ma (2σ), and D3–1, a trachybasalt, gave an age of 99.5 ± 0.7 Ma (2σ). Baksi is correct that alteration of Cretaceous rocks on the seafloor is a pervasive problem that needs to be carefully evaluated when employing Ar-dating techniques (Koppers et al., 2000). As it was not possible to obtain mineral separates from the rocks we analyzed, we were required to use groundmass material for Ar isotope analysis. We took precautionary steps for groundmass sample preparation, and followed careful analytical methods (detailed in Data Repository item 2007150, published with our paper), but it is clear that alteration remains a problem in the published Ar data. As the alteration index described in Baksi (2007) was published several months after our paper, it was impossible to assess our data according to those criteria.
In favor of our data, we point out that each sample produced a plateau and isochron age that agreed within error; both isochrons also had intercepts within error of the atmospheric value. Although Baksi (2008) states that errors on the plateau steps of D2–1 are overestimated, analysis of low-K altered tholeiites is difficult, and large analytical uncertainties resulting from the low signal strength are not unexpected. Exclusion of the youngest age step (Baksi, 2008) for D3–1 has no effect on the plateau age for this sample, as the ages are weighted inversely by their errors; furthermore, the isotope composition of this age step lies on the same isochron produced by the other three steps.
Baksi states that 40Ar/39Ar results need to be evaluated for statistical validity and alteration extent; however, he neglects the importance of geological context. Regarding the older age for Manihiki, it coincides with a published Aptian age based on biostratigraphic markers determined from sediments immediately overlying basement rock drilled at Deep Sea Drilling Project Site 317 (Bukry, 1976) on the Manihiki Plateau. It is also contemporaneous with several published 40Ar/39Ar ages from the Ontong Java Plateau—in addition to the Mahoney et al. (1993) paper cited by Baksi (2008)—including those of Tejada et al. (2002). The significance of the younger 40Ar/39Ar age in regards to a relationship among the three LIPs is not clear. In our paper, we stated that late-stage volcanism affected all three LIPs; however, the relationship between (presumably) minor late-stage volcanic events and main construction events is commonly ambiguous.
The origin of LIPs remains enigmatic. We remain cautious in linking the Ontong Java, Hikurangi, and Manihiki plateaus. Any purported links cannot be made solely on the basis of geochronology. As we suggested in our paper, these three giant plateaus share several characteristics including (preliminary) age, geochemical, geophysical, and tectonic relationships. We agree fully with Baksi that much more supporting data are needed to substantiate a temporal connection among the Hikurangi, Manihiki, and Ontong Java plateaus. Recent cruises to the Hikurangi and Manihiki plateaus by the R/V Sonne recovered large numbers of basement samples (Hoernle et al., 2004;Werner et al., 2007, respectively). The analyses of these samples should dramatically improve our understanding of massive volcanism in early Cretaceous times.
The authors thank A. Koppers for helpful comments.
