Age determinations and growth rates of Pacific ferromanganese deposits using strontium isotopes

⁸⁷Sr⁸⁶Sr ratios, trace element and REE compositions, and textural characteristics were determined for three hydrogenetic Fe-Mn crusts, one hydrothermal deposit, and two mixed hydrothermalhydrogenetic crusts from the Pacific. The Sr isotope data are compared to the Sr seawater curve for the Cenozoic to determine the ages and growth rates of the crusts. The ⁸⁷Sr⁸⁶Sr in the crusts does not increase monotonically with depth as expected if the Sr were solely derived from seawater and perfectly preserved since deposition. This indicates post-depositional exchange of Sr or heterogeneous sources for the Sr originally contained in the crusts.

Textures of hydrogenetic crusts generally correlate with Sr isotopic variations. The highest porosity intervals commonly exhibit the highest ⁸⁷Sr⁸⁶Sr ratios, indicating exchange with younger seawater. Intervals with the lowest porosity commonly have lower ⁸⁷Sr⁸⁶Sr and may preserve the original Sr isotopic ratios. Minimum ages of crust growth inception were calculated from dense, low porosity intervals. Growth of the hydrogenetic crusts began at or after 23 Ma, although their substrates are Cretaceous. Estimated average growth rates of the three hydrogenetic crusts vary between 0.9 and 2.7 mm/Ma, consistent with published rates determined by other techniques. Within the Marshall Islands crust, growth rates for individual layers varied greatly between 1.0 and 5.4 mm/Ma. For one crust, very low ⁸⁷Sr⁸⁶Sr ratios occurred in detrital-rich intervals.

Hydrothermal Fe-Mn oxide from the active Lau Basin back-arc spreading axis (Valu Fa Ridge) has an ⁸⁷Sr⁸⁶Sr ratio with a predominantly seawater signature (⁸⁷Sr⁸⁶Sr87Sr86Sr0.709196">0.709196), indicating a maximum age of 0.9 Ma.

One crust from an off-axis seamount west of Gorda Ridge may have begun precipitating hydrogenetically at 0.5 Ma (0.709211), and had increasing hydrothermal or volcanic input in the top half of the crust, indicated by a significantly lower ⁸⁷Sr⁸⁶Sr ratio (0.709052).