HABs, PICRITES AND MORE: EMERGING DETAILS ON MAGMATIC PROCESSES AT PRIMITIVE ARC VOLCANOES IN THE SOUTHERN MARIANAS

Primitive arc lavas provide critical data on subduction zone magmagenesis and sub-arc mantle compositions and volcanoes that erupt primitive lavas are present in many Western Pacific arcs. Among these are Chaife seamount and Mt. Manganese, a pair of submarine volcanoes in the Southern Mariana arc, NW of the island of Rota. Mt. Manganese is the larger and presumably older edifice, as it has been cut by a NNE trending graben that Chaife has erupted onto. Chaife lavas dredged sampled in 2001 have been documented as picrite and ankaramite derived from decompression melting of the mantle wedge. A more comprehensive set of samples was recovered in 2005 from both seamounts via ROV. Those from Chaife can be broadly divided into OL+CPX+chromite high-magnesia basalts (HMB) and Pl+CPX+ magnetite high-alumina basalts (HAB). An ol-rich picrite (with some reverse–zoned olivine) can be separated from the HMBs, and the HABs can be divided into low and high-K types. Typical HMB have Mg#s of ~61-65, 11.2-13.9 wt% MgO and 11-13 wt% Al₂O₃, while the picrites are more primitive with Mg#s of ~66-69, 9.5-10.1% Al₂O₃ and up to 374 ppm Ni. HABs have Mg#s in the mid-40s, Al₂O₃> 17 wt% and Ni contents <100 ppm; the high-K variant has 1.26-1.36 wt% K₂O compared to <0.53 wt% for the low-K HAB. In comparison, Mt. Manganese lavas are not as diverse and are less primitive basalts (8.2-9.3 wt% MgO) and low-K HABs only.

The data and modeling demonstrate that Chaife HMBs and Mt. Manganese basalts arise via decompression melting, with picrite being derived from HMB by olivine accumulation. Mt. Manganese HAB may be produced by ol fractionation from basalt, however this process will not derive the Chaife HABs from HMBs and more complex models involving AFC and addition of hydrous flux melts need to be invoked.