Which Statement Accurately Describes Radioactive Dating

8.4 Isotopic Dating Methods

Originally fossils only provided us with relative ages because, although early paleontologists understood biological succession, they did not know the absolute ages of the unlike organisms. It was only in the early part of the 20th century, when isotopic dating methods were first applied, that information technology became possible to discover the absolute ages of the rocks containing fossils. In most cases, we cannot use isotopic techniques to directly date fossils or the sedimentary rocks they are found in, but we can constrain their ages by dating igneous rocks that cut across sedimentary rocks, or volcanic layers that lie within sedimentary layers.

Isotopic dating of rocks, or the minerals in them, is based on the fact that we know the disuse rates of certain unstable

of elements and that these rates have been constant over geological time. It is likewise based on the premise that when the atoms of an chemical element decay within a mineral or a stone, they stay there and don’t escape to the surrounding rock, water, or air. One of the isotope pairs widely used in geology is the decay of
⁴⁰Thou to
⁴⁰Ar (potassium-40 to argon-forty).
⁴⁰One thousand is a radioactive isotope of potassium that is nowadays in very small amounts in all minerals that accept potassium in them. It has a half-life of 1.3 billion years, meaning that over a period of ane.three Ga one-half of the
⁴⁰M atoms in a mineral or rock volition decay to
^fortyAr, and over the adjacent 1.3 Ga one-half of the remaining atoms will decay, and so on (Figure viii.four.1).

In club to use the K-Ar dating technique, we need to have an igneous or metamorphic rock that includes a potassium-bearing mineral. One adept instance is granite, which commonly has some potassium feldspar (Figure 8.4.2). Feldspar does not have whatever argon in information technology when it forms. Over time, the
⁴⁰K in the feldspar decays to
⁴⁰Ar. Argon is a gas and the atoms of
⁴⁰Ar remain embedded inside the crystal, unless the stone is subjected to high temperatures afterward it forms. The sample must be analyzed using a very sensitive mass-spectrometer, which can detect the differences between the masses of atoms, and can therefore distinguish between
⁴⁰K and the much more abundant
³⁹G. Biotite and hornblende are also usually used for K-Ar dating.

An of import assumption that nosotros have to be able to make when using isotopic dating is that when the rock formed none of the girl isotope was present (e.g.,
⁴⁰Ar in the example of the K-Ar method). A clastic sedimentary rock is made upwards of older stone and mineral fragments, and when the stone forms it is almost certain that all of the fragments already take daughter isotopes in them. Furthermore, in almost all cases, the fragments have come from a range of source rocks that all formed at dissimilar times. If nosotros dated a number of individual grains in the sedimentary rock, we would likely get a range of different dates, all older than the age of the rock.  That could be useful information, but it would not provide an accurate date for the stone in question.

Information technology might be possible to direct engagement some chemical sedimentary rocks isotopically, but at that place are no useful isotopes that can be used on old chemical sedimentary rocks. Radiocarbon dating can exist used on sediments or sedimentary rocks that contain carbon, but information technology cannot be used on materials older than most 60 ka.

Chiliad-Ar is simply ane of many isotope-pairs that are useful for dating geological materials. Some of the other important pairs are listed in Table 8.2, along with the historic period ranges that they apply to and some comments on their applications. When radiometric techniques are practical to metamorphic rocks, the results commonly tell us the appointment of metamorphism, not the date when the parent stone formed.

Popular:   To Prevent Collisions and Violations at Intersections Use the

Presume that a feldspar crystal from the granite shown in Effigy viii.4.2 was analyzed for
⁴⁰G and
⁴⁰Ar. The proportion of
⁴⁰K remaining is 0.91. Using the disuse bend shown on the graph below, estimate the age of the rock.

An instance is provided (in blueish) for a
^xlYard proportion of 0.95, which is equivalent to an age of approximately 96 Ma. This is determined by drawing a horizontal line from 0.95 to the decay curve line, and then a vertical line from there to the time centrality.Come across Appendix three for Exercise 8.iii answers.

Radiocarbon dating (using
^fourteenC) can be applied to many geological materials, including sediments and sedimentary rocks, but the materials in question must be younger than 60 ka. Fragments of wood incorporated into young sediments are practiced candidates for carbon dating, and this technique has been used widely in studies involving tardily Pleistocene glaciers and glacial sediments. An instance is shown in Figure viii.4.5; radiocarbon dates from wood fragments in glacial sediments have been used to estimate the timing of the final glacial advance along the Strait of Georgia.  It is axiomatic that the ice-front of the major glacier that occupied the Strait of Georgia was near to Campbell River at effectually 35 ka, virtually to Nanaimo and Vancouver at about 25 ka, and had reached the Victoria area by around 22 ka.

Over the past decade there has been increasing use of U-Pb dating to study sedimentary rocks, not necessarily to find out the age of the rock, but to find something virtually its history and origins.  All clastic sedimentary rocks contain some tiny clasts of the silicate mineral zircon (ZrSiO₄), derived from the weathering of the sediment parent rocks.  Zircon ever has some uranium in information technology (but no lead) so it is a expert candidate for U-Atomic number 82 dating, and it isn’t too difficult to separate the grains of zircon from the other grains in a sandstone.  The procedure is to isolate a few hundred tiny zircons from a stone sample, and then carry out U-Lead dating on each ane of them.  An example of the types of results obtained are shown on Figure eight.5.six.  All of the samples are from Nanaimo Gp. rocks on Vancouver Isle and nearby Salt Spring Island.

The three samples from Vancouver Island have zircons aged around ninety Ma, 118 Ma and 150 Ma.  The Common salt Spring Island sample has some zircons anile effectually 150 Ma, just most are much older, at 200 Ma and 340 to 360 Ma.  It is interpreted that the younger zircons (ninety to 150 Ma) are mostly derived from granitic rocks in the Coast Range, while the older ones (>200 Ma) are from older rocks on Vancouver Island (Huang, 2018).

Epitome Descriptions

[Return to Effigy 8.iv.one]

[Render to Figure 8.iv.4]

Media Attributions

• Figures 8.four.1, eight.4.2, 8.4.3, 8.four.4: © Steven Earle. CC Past.
• Figure 8.4.five: © Steven Earle. CC By. From J. Clague, 1976, Quadra Sand and its relation to late Wisconsin glaciation of southeast British Columbia, Can. J. Globe Sciences, V. 13, p. 803-815.
• Figure eight.4.six (left): “Zircon microscope” © Chd. CC By-SA.
• Figure 8.4.6b: © Steven Earle. CC BY. From data in Huang, C, 2018, Refining the chronostratigraphy of the lower Nanaimo Group, Vancouver Island, Canada, using Detrital Zircon Geochronology, MSc thesis, Department of Earth Science, Simon Fraser Academy, 74 p.