608 MINERALS/Zircons banded ironstones) of south-west Greenland The evidence derived from the Mount Narryer zircons (including oxygen isotope ratios) strongly implies that water existed on Earth at 4.4 Ga Life may even have developed by this time, within the Hadean Overgrowths on Zircons Dating of sedimentary rocks by radiometric methods has long been a difficult matter and has been achieved only to broad limits by dating igneous intrusions into them or concordant lava flows or tuffs close to the sedimentary stratum concerned Authigenic overgrowths on zircon, a mineral that is formed at high temperature and is insoluble in groundwater, have long attracted attention They are composed of xenotime, essentially yttrium phosphate (Figure 11) Such overgrowths occur in the Millstone Grit (Carboniferous, Namurian) of northern England Though diagenetic glauconite has been radiometrically dated, it suffers from the defect that it is prone to postdepositional alteration The SHRIMP technique has recently been applied to such xenotime outgrowths (xenotime contains traces of radioactive elements) The outgrowths have been shown to precede all other authigenic minerals such as quartz, and are thus thought to be very early diagenetic deposits The xenotime is precipitated from seawater with other biogenic phosphates shortly after burial, in a zone of high bacterial activity and sulphate reduction Xenotime outgrowths thus provide a method of obtaining what is virtually the age of deposition of a sedimentary rock, by radiometric methods The method can be applied to rocks of any geological age, to date strata directly Figure 11 Two xenotime outgrowths on a zircon crystal Re produced from Glover J (2003) Geological Journeys from Artifacts to Zircon, pp 105 106 Perth: Geological Society of Australia, Western Australian Division This new method is especially important in the case of sedimentary rocks devoid of fossils See Also Analytical Methods: Geochronological Techniques Mineral Deposits and Their Genesis Sedimentary Environments: Alluvial Fans, Alluvial Sediments and Settings; Shoreline and Shoreface Deposits Further Reading Amelin Y, Stern R, Wiechert U, Davis D, Lee D, Halliday A, and Pidgeon R (2001) Combined U Pb, trace element, oxygen, Zr and Lu Hf isotopic systematics of 3.90 4.25 Ga detrital zircons from Jack Hills, Western Australia In: Cassidy KF, Dunphy JM, and Van Kranendonk MJ (eds.) 4th International Archaean Symposium Extended Ab stracts AGSO Geoscience Australia Record 2001/37, pp 33 34 Perth, WA: Geoscience Australia Bauer J and Bouska V (1983) A Guide in Colour to Precious and Semi precious Stones, pp 108 109 London: Octo pus Books Berry LG and Mason B (1959) Mineralogy Concepts, Descriptions and Determinations San Francisco: WH Freeman Cary, DD (ed.) (1994) Industrial Minerals and Rocks, 6th edn Littleton, CO: Society for Mining, Metallurgy and Exploration Correia Neves JM, Lopez Nunes JE, and Sahama ThG (1974) High hafnium members of the zircon hafnon series from the granite pegmatites of Zambezia, Mozam bique Contributions to Mineralogy and Petrology 48: 73 80 Deer WA, Howie RA, and Zussman J (1982) Rock Forming Minerals, 2nd edn, vol 1A, pp 418 442 London: Long mans Evans AM (1993) Ore Geology and Industrial Minerals, 3rd edn Oxford: Blackwells Science Publications Glover J (2003) Geological Journeys from Artifacts to Zircon, pp 105 106 Perth: Geological Society of Australia, Western Australian Division Harben PW and Bates BC (1984) Geology of the Non metallics New York: Metal Bulletin Inc Mitchell RS (1973) Metamict minerals: a review Mineral Record 4: 177 182 Wilde SA, Valley JW, Peck WH, and Graham CM (2001) Evidence from detrital zircons for the existence of tinental crust and oceans on the Earth 4.4 Ga ago Nature 409: 175 178 Wilde SA, Valley JW, Peck WH, and Graham CM (2001) Evidence for the early growth of continents and oceans from >4 Ga detrital zircons In: Cassidy KF, Dunphy JM, and Van Kranendonk MJ (eds.) 4th International Ar chaean Symposium Extended Abstracts AGSO Geo science Australia Record 2001/37, pp 106 108 Perth, WA: Geoscience Australia