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 th
Trang 1banded ironstones) of south-west Greenland The
evi-dence derived from the Mount Narryer zircons
(in-cluding 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
over-growths on zircon, a mineral that is formed at high
temperature and is insoluble in groundwater, have
long attracted attention They are composed of
xeno-time, essentially yttrium phosphate (Figure 11) Such
overgrowths occur in the Millstone Grit
(Carbonifer-ous, Namurian) of northern England Though
diage-netic glauconite has been radiometrically dated, it
suffers from the defect that it is prone to
postdeposi-tional alteration The SHRIMP technique has recently
been applied to such xenotime outgrowths (xenotime
contains traces of radioactive elements) The
out-growths have been shown to precede all other
authi-genic minerals such as quartz, and are thus thought to
be very early diagenetic deposits The xenotime is
precipitated from seawater with other biogenic
phos-phates shortly after burial, in a zone of high bacterial
activity and sulphate reduction Xenotime
out-growths 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
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 En-vironments: Alluvial Fans, Alluvial Sediments and Set-tings; 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 con 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
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.
608 MINERALS/Zircons