Image: Quartz-pebble metaconglomerate (Jack Hills Quartzite, Archean, 2.65 to 3.05 Ga; Jack Hills, Western Australia) 2

Description: Polished cut of specimen of Archaean (i.e. ~3 Billion years old) quartz-pebble metaconglomerate from the Jack Hills, Western Australia. These metaconglomerates are renowned for containing the oldest mineral grains (so-called detrital zircons) of the earth (~4.4 Billion years old). Original caption on flickr.com (slightly modified): Quartz-pebble metaconglomerate from the Precambrian of Australia. (Cranbrook Institute of Science collection, Bloomfield Hills, Michigan, USA) Very old rocks are common on the Moon and in the Asteroid Belt. Very old rocks are scarce on Earth - this is the result of erosion by running water and plate tectonic recycling and deformation. The oldest reported Earth rocks are all Canadian - the Acasta Gneiss (Eoarchean, 4.03 Ga) and rocks in the Nuvvuagittuq Greenstone Belt (Eoarchean, 4.28 Ga). The rock shown above is a quartz-pebble metaconglomerate from the famous Jack Hills Quartzite (Jack Hills Formation). Microscopic detrital grains of the mineral zircon (ZrSiO4 - zirconium silicate) extracted from Jack Hills Quartzite rock samples are the oldest directly observable Earth materials. Jack Hills detrital zircons range in age from 3.05 Ga to 4.404 Ga. The latter date is early Eoarchean and very close to the age of the Earth. Many refer to this early time interval as "Hadean", but that term lacks a fixed definition and is rejected here. The depositional age of Jack Hills Quartzite sediments is not well constrained. They were deposited after the youngest known detrital zircons (3.05 Ga) and before low-grade metamorphism of the rocks (2.655 Ga, based on dating of metamorphic monazite crystals). Stratigraphy: Jack Hills Quartzite, Neoarchean to mid-Mesoarchean, ~2.65 to ~3.05 Ga Locality: unrecorded/undislcosed site in the Jack Hills, Western Australia ------------------ Some info. from: Tarduno & Cottrell (2013) - Signals from the ancient geodynamo: a paleomagnetic field test on the Jack Hills metaconglomerate. Earth and Planetary Science Letters 367: 123-132.
Title: Quartz-pebble metaconglomerate (Jack Hills Quartzite, Archean, 2.65 to 3.05 Ga; Jack Hills, Western Australia) 2
Credit: Quartz-pebble metaconglomerate (Jack Hills Quartzite, Archean, 2.65 to 3.05 Ga; Jack Hills, Western Australia) 2
Author: James St. John
Usage Terms: Creative Commons Attribution 2.0
License: CC BY 2.0
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