U-Th-Pb “Dating”: An Example of False “Isochrons”
by Andrew A. Snelling, Ph.D., Answers in Genesis
December 9, 2009
As with other isochron methods, the U-Pb isochron method has been questioned in the open literature, because often an excellent line of best fit between ratios obtained from a set of good cogenetic samples gives a resultant “isochron” and yields a derived “age” that has no distinct geological meaning. At Koongarra, Australia, U-Th-Pb isotopic studies of uranium ore, host rocks, and soils have produced an array of false “isochrons” that yield “ages” that are geologically meaningless. Even a claimed near-concordant U-Pb ‘age’ of 862 Ma on one uraninite grain is identical to a false Pb-Pb isochron “age,” but neither can be connected to any geological event. Open system behavior of the U-Th-Pb system is clearly the norm, as is the resultant mixing of radiogenic Pb with common or background Pb, even in soils in the surrounding region. Because no geologically meaningful results can be interpreted from the U-Th-Pb data at Koongarra (three uraninite grains even yield a 232Th/208Pb “age” of 0 Ma), serious questions must be asked about the validity of the fundamental/foundational basis of the U-Th-Pb “dating” method. This makes the task of creationists building their model for the geological record much easier, since claims of U-Th-Pb radiometric “dating” having “proven” the claimed great antiquity of the earth, its strata and fossils can be safely side-stepped.
Keywords: geochronology, U-Th-Pb isotopes, isochrons, uranium ore, soils
This paper was originally published in the Proceedings of the Third International Conference on Creationism, pp. 497–504 (1994) and is reproduced here with the permission of the Creation Science Fellowship of Pittsburgh.
Radiometric dating has now been used for almost 50 years to establish “beyond doubt” the earth’s multibillion year geological column. Although this column and its “age” was firmly settled well before the advent of radiometric dating, the latter has been successfully used to help quantify the “ages” of the strata and the fossils in the column, so that in many people’s minds today radiometric dating has “proved” the presumed antiquity of the earth. Of the various methods, uranium-thorium-lead (U-Th-Pb) was the first used and it is still widely employed today, particularly when zircons are present in the rocks to be dated. But the method does not always give the “expected” results, leading to fundamental questions about its validity.
In his conclusion in a recent paper exposing shortcomings and criticizing the validity of the popular rubidium-strontium (Rb-Sr) isochron method, Zheng wrote:
. . . some of the basic assumptions of the conventional Rb-Sr isochron method have to be modified and an observed isochron does not certainly define a valid age information for a geological system, even if a goodness of fit of the experimental data points is obtained in plotting 87Sr/86Sr vs. 87Rb/86Sr. This problem cannot be overlooked, especially in evaluating the numerical timescale. Similar questions can also arise in applying Sm-Nd and U-Pb isochron methods.1
Amongst the concerns voiced by Zheng were the problems being found with anomalous isochrons, that is, where there is an apparent linear relationship between 87Sr/86Sr and 87Rb/86Sr ratios, even an excellent line of best fit between ratios obtained from good cogenetic samples, and yet the resultant isochron and derived “age” have no distinct geological meaning. Zheng documented the copious reporting of this problem in the literature where various names had been given to these anomalous isochrons, such as apparent isochron, mantle isochron and pseudoisochron, secondary isochron, source isochron, erupted isochron, mixing line, and mixing isochron.
Similar anomalous or false isochrons are commonly obtained from U-Th-Pb data, which is hardly surprising given the common open system behavior of the U-Th-Pb system. Yet in the literature these problems are commonly glossed over or pushed aside, but their increasing occurrence from a variety of geological settings does seriously raise the question as to whether U-Th-Pb data ever yields any valid “age” information. One such geological setting that yields these false U-Th-Pb isochrons is the Koongarra uranium deposit and the surrounding area (Northern Territory, Australia).
The Koongarra Area
The Koongarra area is 250 km east of Darwin (Northern Territory, Australia) at latitude 12°52'S and longitude 132°50'E. The regional geology has been described in detail by Needham and Stuart-Smith2 and by Needham,3, 4 while Snelling5 describes the Koongarra uranium deposit and the area’s local geology.
The Koongarra uranium deposit occurs in a metamorphic terrain that has an Archean basement consisting of domes of granitoids and granitic gneisses (the Nanambu Complex), the nearest outcrop being 5 km to the north. Some of the lowermost overlying Lower Proterozoic metasediments were accreted to these domes during amphibolite grade regional metamorphism (estimated to represent conditions of 5–8 kb and 550–630°C) at 1800–1870 Ma. Multiple isoclinal recumbent folding accompanied metamorphism. The Lower Proterozoic Cahill Formation flanking the Nanambu Complex has been divided into two members. The lower member is dominated by a thick basal dolomite and passes transitionally upwards into the psammitic upper member, which is largely feldspathic schist and quartzite. The uranium mineralization at Koongarra is associated with graphitic horizons within chloritized quartz-mica (±feldspar ±garnet) schists overlying the basal dolomite in the lower member. A 150 Ma period of weathering and erosion followed metamorphism. A thick sequence of essentially flat-lying sandstones (the Middle Proterozoic Kombolgie Formation) was then deposited unconformably on the Archean- Lower Proterozoic basement and metasediments. At Koongarra subsequent reverse faulting has juxtaposed the lower Cahill Formation schists and Kombolgie Formation sandstone.
Owing to the isoclinal recumbent folding of metasedimentary units of the Cahill Formation, the typical rock sequence encountered at Koongarra is probably a tectono-stratigraphy (from youngest to oldest.)
* muscovite-biotite-quartz-feldspar schist (at least 180 m thick)
* garnet-muscovite-biotite-quartz schist (90–100 m thick)
* sulphide-rich graphite-mica-quartz schist (±garnet) (about 25 m thick)
* distinctive graphite-quartz-chlorite schist marker unit (5–8 m thick)
* quartz-chlorite schist (±illite, garnet, sillimanite, muscovite) (50 m thick)—the mineralized zone
* reverse fault breccia (5–7 m thick)
* sandstone of the Kombolgie Formation
Polyphase deformation accompanied metamorphism of the original sediments, that were probably dolomite, shales and siltstones. Johnston6 identified a D2 event as responsible for the dominant S2 foliation of the schist sequence, which at Koongarra dips at 55° to the south-east. The dominant structural feature, however, is the reverse fault system that dips at about 60° to the south-east, sub-parallel to the dominant S2 foliation and lithological boundaries, just below the mineralized zone.
The Uranium Deposit
There are two discrete uranium orebodies at Koongarra, separated by a 100 m wide barren zone. The main (No. 1) orebody has a strike length of 450 m and persists to 100 m depth. Secondary uranium mineralization is present in the weathered schists, from below the surficial sand cover to the base of weathering at depths varying between 25 and 30 m. This secondary mineralization has been derived from decomposition and leaching of the primary mineralized zone, and forms a tongue-like fan of ore-grade material dispersed down-slope for about 80 m to the south-east. The primary uranium mineralized zone in cross-section is a series of partially coalescing lenses, which together form an elongated wedge dipping at 55° to the south-east within the host quartz-chlorite schist unit, subparallel to the reverse fault. True widths average 30 m at the top of the primary mineralized zone but taper out at about 100 m below surface and along strike.
Superimposed on the primary prograde metamorphic mineral assemblages of the host schist units is a distinct and extensive primary alteration halo associated, and cogenetic, with the uranium mineralization. This alteration extends for up to 1.5 km from the ore in a direction perpendicular to the host quartz-chlorite schist unit, because the mineralization is essentially stratabound. The outer zone of the alteration halo is most extensively developed in the semi-pelitic schists, and is manifested by the pseudomorphous replacement of biotite by chlorite, rutile and quartz, and feldspar by sericite. Silicification has also occurred in fault planes and within the Kombolgie Formation sandstone beneath the mineralization, particularly adjacent to the reverse fault.
Association of this outer halo alteration with the mineralization is demonstrated by the apparent symmetrical distribution of this alteration about the orebody. In the inner alteration zone, less than 50 m from ore, the metamorphic rock fabric is disrupted, and quartz is replaced by pervasive chlorite and phengitic mica, and garnet by chlorite. Uranium mineralization is only present where this alteration has taken place.
The primary ore consists of uraninite veins and veinlets (1–10 mm thick) that cross-cut the S2 foliation of the brecciated and hydrothermally altered quartz-chlorite schist host. Groups of uraninite veinlets are intimately intergrown with chlorite, which forms the matrix to the host breccias. Small (10–100 µm) euhedral and subhedral uraninite grains are finely disseminated in the chloritic alteration adjacent to veins, but these grains may coalesce to form clusters, strings, and massive uraninite. Coarse colloform and botryoidal uraninite masses and uraninite spherules with internal lacework textures have also been noted, but the bulk of the ore appears to be of the disseminated type, with thin (
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