Geochemistry of manganese nodules from offshore areas of Mariana Islands and Johnston Island

Abstract

The manganese nodules near the Mariana Islands generally range from 2 to 4 cm in diameter and some samples have porous surfaces. The nodules near Johnston Island are larger in size (5–8 cm in diameter) and more compact than the Mariana nodules. The major FeMn minerals found in Mariana Islands samples are todorokite, birnessite and akaganeite (β-FeOOH) while, in the Johnston Island samples, only birnessite and akaganeite are found. The Mariana Islands nodules are higher in Mn, Mg, Na, K, Co, Ni, Pb and Th but lower in Fe, Sr, Zn, Ba, Zr, Y and REEs than the Johnston Island samples. The (Cu + Ni + Co) contents of the Mariana Islands samples are generally higher than 15,000 ppm with Co Zn ratios varying from 10 to 15, while the Johnston Island samples generally have (Cu + Ni + Co) between 8000 and 10,000 ppm with Co Zn ratios varying from 5 to 7. Therefore these nodules may not be related to hydrothermal activity (Toth, Geol. Soc. Am. Bull. 91, 44–54, 1980). Both the Mariana Islands and Johnston Island nodules show similar LREE-enriched patterns with distinct positive Ce anomaly and negative Eu anomaly. The positive correlation between the Ce anomaly defined as log Ce 2 3 La+ 1 3 Nd and Mn Fe ratios found in the nodules studied suggest that a phosphorus-rich phase with REE pattern similar to that for biogenous apatite may be the third component in considering the source of REEs in the nodules. The growth rate determined by the excess 230Th method for Johnston Island nodule JA-1, from 0.2 to 0.45 mm depth, is 1.12±0.10 mm/Ma but a much higher rate (17.66 ± 7.61 mm/Ma) is observed from 0.45 to 1.8 mm depth. The growth rate of the nodule may be related to its Mn and Fe contents.