Depth versus age: new perspectives from the chemical compositions of ancient crust

Petrological data provide a new approach to an evaluation of the depth–age problem for ancient seafloor. The correlations among basalt chemical composition, axial depth and mantle temperature at current ocean ridges allow the determination of initial depth and mantle temperature for any portion of ancient seafloor that was created at a spreading center, provided the chemical composition of the ancient crust is determined. It is then possible to calculate a petrologically constrained depth at any age, which can be compared to observed depths and depths from the classical half space models. We evaluate data from DSDP and ODP drill holes on crust older than 80 Ma, considering chemical composition, back-tracked depth and crustal thickness. The data are complex, and interpretation of their chemical composition requires consideration of alteration, absence of glass compositions, data quality, and the influence of off-axis volcanism and near-ridge hot spots. To check and expand the data set, we develop and use trace element proxies for major element compositions, since many trace element ratios are less influenced by alteration and by variable proportions of phenocrysts. The twenty drill holes for which reliable data can be obtained are well distributed around the globe, and include multiple sites on old crust in the Atlantic, Pacific and Indian ocean basins. Comparison of the chemical and crustal distributions between ancient and current N-MORB show that the oceanic crust older than 80 Ma has significantly lower Na8.0, Zr=Y, Sm=YbN, and higher CaO=Al2O3, Fe8.0 and crustal thickness. Quantitative modeling of these results suggests that the mantle was hotter in this time period by about 50oC, that the crust was several hundred meters shallower and 1–2 km thicker. These observations show that half to two thirds of the observed flattening relative to a half space model is due to the change in mantle temperature and crustal composition. Thus, only a few hundred meters of flattening by plate reheating by hot spots or by other mechanisms is required. These results are consistent with the existence of abundant oceanic plateaus even at fast-spreading rates in the Mesozoic, and with the apparent thickening of ocean crust with time.  1999 Elsevier Science B.V. All rights reserved.