Importance of zircon in uranium-lead dating
The mineral zircon adds three more fundamental advantages to uranium–lead dating. First, its crystal structure allows a small amount of tetravalent uranium to substitute for zirconium but excludes with great efficiency the incorporation of lead. (It might be said that one begins with an empty box.) Second, zircon, once formed, is highly resistant to change and has the highest blocking temperature ever observed. Finally, with few predictable exceptions, zircon grows or regrows only in liquid rock or in solid rock reheated to approach its melting point. Combining all of these attributes, it is often possible to measure both the time of crystallization and the time of second melting in different parts of the same grain or in different selected grains from the same rock. Of course, such a high blocking temperature can have its disadvantages. Inherited cores may give a mixed false age when the age of crystallization is sought. For this reason, three or more grain types or parts of a grain are analyzed to establish that material of only one age is present.
Experience with the results of the uranium–lead method for zircons has demonstrated an interesting paradox. If left at low surface temperatures for a geologically long time, the radioactivity within the crystal can destroy the crystal lattice structure, whereas at higher temperatures this process is self-annealing. In fact, when examined by X-ray methods, some zircons have no detectable structure, indicating that at least 25 percent of the initial atoms have been displaced by radiation damage. Under these conditions a low-temperature event insufficient to even reset the potassium–argon system (see below Potassium–argon methods) in biotite can cause lead to be lost in some grains. It is no coincidence that, when criteria were finally found to locate concordant grains, these grains were also found to be those with the lowest uranium content and the lowest related radiation damage.
Given the two related uranium–lead parent–daughter systems, it is possible to determine both the time of the initial, or primary, rock-forming event and the time of a major reheating, or secondary, event. The uranium–lead isotopes in the mineral titanite (CaTiSiO5) from a series of rocks that have a common geologic history can be plotted on a concordia diagram. New titanite, distinguishable on the basis of colour, may form in the same rock, while older, partly reset titanite is still present. Geochronologists can separate recent lead loss due to some disturbance event, such as the reheating of the rock, from the normal rate of lead loss by plotting the ratio of lead to uranium in the sample. A new line, the discordia, will plot along a different trajectory, but it will intercept the concordia in two places. The upper intercept will denote the timing of the primary rock-forming event, while the lower intercept will denote the timing of the reheating event.
Uranium–lead dating relies on the isolation of very high-quality grains or parts of mineral grains that are extremely rare but nevertheless present in most igneous, metamorphic, and sedimentary rock units. Samples weighing 10 to 50 kg (22 to 110 pounds) are collected, crushed, and ground into a fine sand, and the various minerals are isolated on the basis of specific gravity, grain size, and magnetic properties. The minerals used are not visible in the field, but their presence can be inferred from the easily identified major minerals present.
One of the most interesting applications of the improved uranium–lead zircon technique has to do with its ability to achieve nearly concordant results from single grains extracted from sandstone. This is possible because zircon is chemically inert and is not disturbed during weathering and because single grains with a diameter about the thickness of a human hair contain sufficient uranium and lead for analysis in the most advanced laboratories. In one sample it was determined that a sandstone that underlies most of the province of Nova Scotia in Canada was probably originally deposited off the coast of North Africa and thrust over the continent before the opening of the Atlantic Ocean. This follows because the ages observed occur in North Africa, whereas those common in North America are absent.
Another sample, this one from sandstone deposited by a large river in northern Scotland, must have been derived from continental rocks whose ages are represented by those determined for the individually dated sand grains. In this case, the continent from which the sand was derived has moved away as a result of continental drift, but it can be identified by the ages measured.
Rubidium–strontium method
The radioactive decay of rubidium-87 (87Rb) to strontium-87 (87Sr) was the first widely used dating system that utilized the isochron method. Rubidium is a relatively abundant trace element in Earth’s crust and can be found in many common rock-forming minerals in which it substitutes for the major element potassium. Because rubidium is concentrated in crustal rocks, the continents have a much higher abundance of the daughter isotope strontium-87 compared with the stable isotopes. This relative abundance is expressed as the 87Sr/86Sr ratio, where strontium-86 is chosen to represent the stable isotopes strontium-88, strontium-86, and strontium-84, which occur in constant proportions in natural materials. Thus, a precise measurement of the 87Sr/86Sr ratio in a modern volcano can be used to determine age if recycled older crust is present. A ratio for average continental crust of about 0.72 has been determined by measuring strontium from clamshells from the major river systems. In contrast, Earth’s most abundant lava rocks, which represent the mantle and make up the major oceanic ridges, have values between 0.703 and 0.705. This difference may appear small, but, considering that modern instruments can make the determination to a few parts in 70,000, it is quite significant. Dissolved strontium in the oceans today has a value of 0.709 that is dependent on the relative input from the continents and the ridges. In the geologic past, changes in the activity of these two sources produced varying 87Sr/86Sr ratios over time. Thus, if well-dated, unaltered fossil shells containing strontium from ancient seawater are analyzed, changes in this ratio with time can be observed and applied in reverse to estimate the time when fossils of unknown age were deposited.
Dating simple igneous rocks
The rubidium–strontium pair is ideally suited for the isochron dating of igneous rocks. As a liquid rock cools, first one mineral and then another achieves saturation and precipitates, each extracting specific elements in the process. Strontium is extracted in many minerals that are formed early, whereas rubidium is gradually concentrated in the final liquid phase. At the time of crystallization, this produces a wide range in the Rb/Sr ratio in rocks that have identical 87Sr/86Sr ratios. On the isochron diagram shown in the figure above, the samples would plot initially at points R1 to R3 along a line representing the initial ratio designated (87Sr/86Sr)0. Over geologic time, this ratio is increased in proportion to the 87Rb/86Sr ratio, as discussed earlier, and the line rotates with a slope equal to (eλt − 1) that represents the time elapsed; thus, the present-day ratio (87Sr/86Sr)p equals the initial ratio (87Sr/86Sr)0 plus radiogenic additions, or (87Sr/86Sr)p = (87Sr/86Sr)0 + 87Rb/86Sr (eλt − 1). This equation is that of a straight line of the form y = b + xm, where y = (87Sr/86Sr)p, the value measured today; b represents (87Sr/86Sr)0, the value initially present; x stands for the 87Rb/86Sr ratio; and m is the slope of the line (eλt − 1).
In practice, rock samples weighing several kilograms each are collected from a suite of rocks that are believed to have been part of a single homogeneous liquid prior to solidification. The samples are crushed and homogenized to produce a fine representative rock powder from which a fraction of a gram is withdrawn and dissolved in the presence of appropriate isotopic traces, or spikes. Strontium and rubidium are extracted and loaded into the mass spectrometer, and the values appropriate to the x and y coordinates are calculated from the isotopic ratios measured. Once plotted as R1p (i.e., rock 1 present values), R2p, and R3p, the data are examined to assess how well they fit the required straight line. Using estimates of measurement precision, the crucial question of whether or not scatter outside of measurement error exists is addressed. Such scatter would constitute a geologic component, indicating that one or more of the underlying assumptions has been violated and that the age indicated is probably not valid. For an isochron to be valid, each sample tested must (1) have had the same initial ratio, (2) have been a closed system over geologic time, and (3) have the same age.
Well-preserved, unweathered rocks that crystallized rapidly and have not been subjected to major reheating events are most likely to give valid isochrons. Weathering is a disturbing influence, as is leaching or exchange by hot crustal fluids, since many secondary minerals contain rubidium. Volcanic rocks are most susceptible to such changes because their minerals are fine-grained and unstable glass may be present. On the other hand, meteorites that have spent most of their time in the deep freeze of outer space can provide ideal samples.
Dating minerals
Potassium-bearing minerals including several varieties of mica, are ideal for rubidium–strontium dating as they have abundant parent rubidium and a low abundance of initial strontium. In most cases, the changes in the 87Sr/86Sr ratio are so large that an initial value can be assumed without jeopardizing the accuracy of the results. When minerals with a low-rubidium or a high-strontium content are analyzed, the isochron-diagram approach can be used to provide an evaluation of the data. As discussed above, rubidium–strontium mineral ages need not be identical in a rock with a complex thermal history, so that results may be meaningful in terms of dating the last heating event but not in terms of the actual age of a rock.
