Ultramafic and basaltic rocks dredged from the nearshore flank of the Tonga Trench

Deep dredging in the Tonga Trench (Southwest Pacific Ocean) at a depth of 9150 to 9400 m yielded fresh to granulated and serpentinized peridotite and dunite. Other rocks recovered there and at three stations deeper than 7000 m include basalts, tuffs, and tuffaceous agglomerates.

Chemical analyses of the fresh peridotite, with combined H₂O < 0.10 weight percent, indicate that the rock consists of Si, Mg, Fe (6 percent), and Cr + Ni about 0.7 percent. Mineralogically, the peridotite contains forsteritic olivine and enstatite with minor spinels.

The ultramafic mass exposed at 9400 m probably is an accumulate exposed by faulting.