page_title: Hadean zircon
section_title: Mineral inclusions
hierarchical_section_title: Hadean zircon / Properties / Mineral inclusions
caption_attribution_description: Polarizing microscope picture of thin section taken in crossed polarized light. The green mineral is biotite and the coloured one is muscovite. The black inclusion in biotite is magnetite. Rock:Nepheline syenite gneiss Local:Rio de Janeiro, Brazil Author:Eurico Zimbres Free for all use Date: 1990
context_page_description: Hadean zircon is the oldest-surviving crustal material from the Earth's earliest geological time period, the Hadean eon, about 4 billion years ago. Zircon is a mineral that is commonly used for radiometric dating because it is highly resistant to chemical changes and appears in the form of small crystals or grains in most igneous and metamorphic host rocks.
  Hadean zircon has very low abundance around the globe because of recycling of material by plate tectonics. When the rock at the surface is buried deep in the Earth it is heated and can recrystallise or melt. In the Jack Hills, Australia, scientists obtained a relatively comprehensive record of Hadean zircon crystals in contrast to other locations. The Jack Hills rocks are from the Archean eon, about 3.6 billion years old. However, the zircon crystals there are older than the rocks that contain them. Many investigations have been carried out to find the absolute age and properties of zircon, for example the isotope ratios, mineral inclusions, and geochemistry of zircon. The characteristics of Hadean zircons show early Earth history and the mechanism of Earth's processes in the past.
context_section_description: The development of textural criteria for identifying primary inclusions opens up possibilities for recognising zircons' changing provenance with time and investigating their post-depositional alteration history. There are two common inclusion assemblages that are consistent with their forming in "I-type" (hornblende, quartz, biotite, plagioclase, apatite, ilmenite) and "S-type" (quartz, K-feldspar, muscovite, monazite) granitoids. Dominated by quartz with less abundant K-feldspar, plagioclase, muscovite, biotite, and phosphates, that are interpreted to have formed under relatively low geothermal gradient similar to that pertaining to modern subduction zones.
caption: Greenish brown biotite with opaque magnetite inclusions and yellow-violet muscovite (cross polarized microscopic view)
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