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the crust surface. Growth rates were first measured using either uranium-series or
beryllium isotopes, which give reliable ages for the outermost 2 and 20 mm of crusts,
respectively, or by radiometric or paleontological dating of the substrate rock and
assuming that the substrate age is equivalent to the age of the base of the crust. With
both methods, the growth rates and ages of the crusts are extrapolated and do not take
into account changes in growth rates, growth hiatuses, or in the later method, the time
between formation of the substrate rock and the beginning of growth of the crusts. For
equatorial Pacific Cretaceous seamounts, the age of substrate rocks and crusts can vary
by as much as 60 Ma, although in rare circumstances the ages of very thick crusts may
approach those of the Cretaceous substrates. Beryllium isotope dating is the most
reliable and widely used technique today. However, using beryllium isotope techniques
requires that the age of the base of crusts thicker than about 20 mm be determined by
extrapolation using the growth rate(s) determined for the outer 20 mm.
Ratios of osmium isotopes may provide a reliable dating tool for crusts as old as
65 Ma by comparing the ratios in various crust layers with ratios that define the Cenozoic
seawater curve 52. However, additional data are required on osmium isotopes in the
oceans before that technique can be applied to age date crusts. Nannofossil
biostratigraphy has been used to date crusts from impressions and molds of nannofossils
left in crust layers after replacement of the carbonate by iron-manganese
oxyhydroxides 53. That technique, although reliable, is time consuming to perform and
consequently has not been widely used. Recent nannofossil biostratigraphy confirms the
crust ages determined from the extrapolation of beryllium isotope-determined growth
rates (Puyaeva and Hein, unpublished data, 2000). Finally, empirical equations have
been developed to date Fe-Mn crusts 54. Those equations usually give minimum ages for
the base of crusts and produce growth rates that are generally faster than those
determined from isotopic techniques, although the Manheim and Lane-Bostwick 55
equation does generally produce rates more in line with those determined by isotopic
methods 56. It is clear that additional techniques are needed to accurately date thick crusts
and the best opportunity may be development of osmium isotope stratigraphy, although
argon isotopes, potassium-argon, and paleomagnetic reversal stratigraphy should also be
studied. A significant problem with thick crusts is that the inner layers were
phosphatised by a diagenetic process that promoted the mobilization of many elements 57.
However, the remobilisation of elements apparently did not affect neodymium and lead
isotopic ratios 58 and also may not have affected osmium isotopic ratios. No correlation
exists in this compiled data set between growth rates of the outermost layer of the crusts
and water depth of occurrence.
54 International Seabed Authority