A tales Two specimens from a geologic sequence are isotopically dated-one from t
ID: 291514 • Letter: A
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A tales Two specimens from a geologic sequence are isotopically dated-one from the top of the sequence, and the other from the bottom of the sequence. The layers at the top of the nce are sandstones, and zircons within them produce an average radiometric age of 2.0 Ga. The layers at the bottom of the sequence are volcanic tuffs that yield an age of 250 Ma. Why would a geologist be suspicious of the validity of one of these dates, given the a. absence of structural deformation in the sequence. b. Which date would less suspect? Why?Explanation / Answer
a. The principle of superposition suggests that the rock at greater depth should be older than the rocks at shallower depths. However in the case of deformation, tilting or complete inversion is highly possible. But in our case, given the absence of deformation such cases are unlikely. Thus the age of the older shallower beds and younger deeper beds is highly suspicious.
b. Zircon is a mineral that forms in silicated melts with Hight Field Strength Elements like Hafnium. It is an accessory mineral in igneoud rocks formed as a primary crystallization product, in metamorphic as well as detrital grain in sedimentary rocks.
The their Uranoum and Thorium content they are used for geochronology by using the Uranium to Lead ratio.However, Detrital Zircons are derived from weathering and erosion of a pre-existing rock (mostly igneous rock). ZIrcon's are heavy and highly resistant and thus are transported, deposited and preserved, without losing much structural integrity in sedimentary rocks.
However, these Zircon's retain properties like age and mineral chemistry of their parent igneos rocks. Thus,Detrital ZIrcon age can be used to constrain the Maximum depositional age, determine the provenance and reconstructing the tectonic sttings at a regional scale.
Thus the age of the Zircons in the Sedimentary sandstone is highly midleading or suscpicous.
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