Jason M. Huberty

Silician magnetite from the Dales Gorge Member of the Brockman Iron Formation, Hamersley Group, Western Australia

Abstract We report silician magnetite from banded iron formation (BIF) in the Dales Gorge Member of the Brockman Iron Formation, Hamersley Group, Western Australia. Magnetite mesobands typically consisting of individual ~100 μm microlaminae are revealed to be composed of silician magnetite overgrowths on magnetite. Silician magnetite overgrowths contain from 1 to 3 wt% SiO2, whereas (low-Si) magnetite domains contain less than 1 wt% SiO2. Silicon solid solution is present in the magnetite crystal lattice as determined by in situ micro-X-ray diffraction and high-resolution transmission electron microscopy. Three textures are distinguished in magnetite mesobands: (1) magnetite sub-microlaminae with silician magnetite overgrowths, (2) recrystallized magnetite fragments with silician magnetite overgrowths, and (3) a complex intergrowth of magnetite and silician magnetite. All three textures are found in magnetite mesobands from the BIF4-5 and BIF12-16 macrobands of the Dales Gorge type-section drill core DDH-47A from Wittenoom, Western Australia. Magnetite domains contain numerous submicrometer-to-micrometer inclusions of quartz, carbonate, stilpnomelane, and apatite, whereas silician magnetite overgrowths are devoid of mineral inclusions. The presence of mineral inclusions in magnetite indicates the BIF oxide precipitate was not chemically pure iron oxyhydroxide/oxide. Magnetite domains display textures formed during soft sediment deformation that are the earliest and best preserved relict sedimentary structures in this BIF. Silician magnetite is the dominant iron oxide in the Dales Gorge BIF and is present in many other sub-greenschist facies BIFs worldwide. We suggest the former presence of organic matter creates reducing conditions necessary to stabilize silician magnetite. Thus, silician magnetite is a potential biosignature in BIFs.

Origin and Metamorphic Evolution of Garnet Clinopyroxenite from the Sulu UHP Terrane, China: Evidence from Mineral Chemistry and Microstructures

Publisher Summary Garnet clinopyroxenite lenses or layers are common in orogenic peridotites, and also found as xenoliths in volcanic rocks and kimberlites. Pyroxenites have multiple origins, such as crystal liquid accumulation, refertilization of peridotite due to melt infiltration, subduction of oceanic crust and recycling of lower crystal cumulate and restites, and metasomatic process. In the DabieSulu ultrahigh-pressure (UHP) terrane, garnet pyroxenite can be divided into two types: as lenses or thin layers in mantle-derived peridotite, such as in the Xugou and Yangkou garnet peridotites and as a member of crust-hosted mafic and ultramafic cumulate complex, such as Bixiling body. Type (1) pyroxenite has a mantle origin. Type (2) pyroxenite is a cumulate resulting from differentiation of a basic magma. In addition, some pyroxene layers have been attributed to metasomatism. The Hujialing garnet clinopyroxenite occurs as lenses within a dunite body that is located in the southeastern Rizhao city (RZ) of Shandong Province of eastern China. The unique Hujialing Grt clinopyroxenite provides an opportunity to study origin of lamellar phases, mineral behavior, and rock textural and structural readjustment during subduction-zone metamorphism; this in turn provides insights into the juxtaposition processes of mantle and crust during the collision of the Yangtze and Sino-Korean cratons.