A slice of the planet's interior
The eastern arm of Halmahera island is a rare geological site: a massive slab of oceanic crust and the Earth's upper mantle, known as an ophiolite, thrust onto the land surface. This ophiolite is a relic of ancient seafloor, showing rocks that are normally inaccessible miles below the ocean. The Halmahera ophiolite consists of a characteristic sequence of rocks, from deep mantle peridotites like harzburgite and lherzolite, to gabbros and basalts that once formed the ocean floor.
This immense block of planetary interior was emplaced through a complex tectonic process. Halmahera is in the Molucca Sea Collision Zone, a unique area where an oceanic plate has been consumed by subduction on both its east and west sides. This intense arc-arc collision is responsible for lifting, or obducting, the dense oceanic and mantle rocks of the Halmahera ophiolite up and onto the continental margin. The basement rocks of eastern Halmahera are primarily this Mesozoic ophiolitic complex, tectonically mixed with ancient deep-water sediments.
Minerals from extreme depths
Within the mantle peridotites of the ophiolite are lenses and pods of chromitite, a rock containing the mineral chromite. These chromitites are remarkable because they contain inclusions of minerals that signify formation under ultra-high pressures (UHP). Geologists have identified minerals within ophiolitic chromitites worldwide that could only form at depths between 150 and 300 kilometers, reaching into the mantle transition zone. While some researchers propose alternative formation mechanisms like lightning strikes, the presence of these UHP minerals is strong evidence for the transport of materials from deep within the Earth.
The chromitites are also a repository for rare and valuable platinum-group minerals (PGMs). These typically occur as tiny grains, often less than 10 micrometers in size, enclosed within the more robust chromite crystals. Minerals such as laurite (a ruthenium sulfide) and various Osmium-Iridium alloys are found here. These PGMs crystallize at very high temperatures and are preserved by their inclusion in the chromite. The specific composition of the chromite and its associated minerals indicates the ophiolite formed in a supra-subduction zone setting, similar to the modern Mariana forearc. The Halmahera ophiolite is a natural laboratory for studying deep mantle processes and the formation of oceanic crust.