GEOCHEMISTRY OF MANGANESE NODULES FROM OFFSHORE AREAS OF MARIANA ISLANDS AND JOHNSTON ISLAND

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 Fe-Mn minerals found in M ariana Islands samples are todorokite, birnessite and akaganeite (beta -FeOOH) while, in the Johnston Island samples, only birnessite and aka ganeite are found. The Mariana Islands nodules are higher in Mn, Mg, N a, K, Co, Ni, Pb and Th but lower in Fe, Sr, Zn, Ba, Zr, Y and REEs th an the Johnston Island samples. The (Cu + Ni + Co) contents of the Mar iana Islands samples are generally higher than 15,000 ppm with Co/Zn r atios varying from 10 to 15, while the Johnston Island samples general ly have (Cu + Ni + Co) between 8000 and 10,000 ppm with Co/Zn ratios v arying from 5 to 7. Therefore these nodules may not be related to hydr othermal activity (Toth, Geol. Sec. Am. Bull. 91, 44-54, 1980). Both t he Mariana Islands and Johnston Island nodules show similar LREE-enric hed 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 t hat a phosphorus-rich phase with REE pattern similar to that for bioge nous apatite may be the third component in considering the source of R EEs in the nodules. The growth rate determined by the excess Th-230 me thod 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 obs erved from 0.45 to 1.8 mm depth. The growth rate of the nodule may be related to its Mn and Fe contents.