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Section 5 discusses the complications arising from compositional zoning of zircons, and outlines a ‘pseudo depth profiling’ method to semi-quantitatively assess the extent to which a sample suffers from such problems.
This insight forms the basis of the simplified method, which does not require knowledge of any absolute abundances or concentrations, but instead uses the raw mass spectrometer measurements.
In its simplest form, assuming identical laser ablation rates in the standard and the unknown, the method works as follows: In practice, the standard and the unknown are combined on a pairwise basis.
Section 3 introduces a way to determine the Th/U ratio of the standard without the need for NIST glass as a reference material, by measuring the Pb ratio and assuming concordance of the U-Th-Pb system in the age standard.
Section 4 outlines a method to estimate relative differences in laser ablation rate without the need for absolute depth measurements, by tracking the signal strength of stoichiometric Si.
This is done by calculating the normalised U, Th, and He concentrations of the standard from its known age and measured Th/U ratio (Figure 1.a), and then using this normalised composition as a benchmark against which to compare the U, Th, and He signals of the unknown (Figure 1.b,c).
To correct for instrument drift and plasma-induced fluctuations in the ICP-MS, the U and Th signals should be measured relative to stoichiometric Si.
However, the two end segments, the Marble Canyon and the Westernmost Grand Canyon, are both young and were carved in the past 5–6 million years.
Thus, although parts of the canyon are old, we conclude that the integration of the Colorado River through older palaeocanyons carved the Grand Canyon, beginning 5–6 million years ago. CO;2&rft_id=info:pmid/&rft_val_fmt=info:ofi/fmt:kev:mtx:journal&rft.genre=article&rft.aulast=Dumitru&rft.aufirst=T. A.&rft.jtitle=Geology&rft.volume=22&rft.spage=499&rft.epage=502&rft.date=1994&rft.atitle=Mesozoic-Cenozoic burial, uplift, and erosion history of the west-central Colorado Plateau&rfr_id=info:sid/nature.com: Nature.com&id=doi:10.1130/0091-7613(1994)0222.3.
We reconstruct the thermal histories of samples taken from the modern canyon base and the adjacent canyon rim 1,500 m above, to constrain when the rocks cooled as a result of canyon incision.
We find that two of the three middle segments, the Hurricane segment and the Eastern Grand Canyon, formed between 70 and 50 million years ago and between 25 and 15 million years ago, respectively.
More fundamental to the physical processes of mountain building, however, is the motion of the Earth's surface itself, which remains elusive.Tags: Adult Dating, affair dating, sex dating