The Proceedings of the Eighth International Conference on Creationism (2018)

used geochronological methods for weathering geochronology are 40 Ar/ 39 Ar laser incremental heating analysis of K-bearing supergene minerals (particularly the hollandite-group K–Mn-oxides and alunite group sulfates) and (U–Th)/He analysis of supergene oxides and hydroxides (hematite and goethite). Some of the minerals found in the supergene weathering zones of ore deposits can be dated using the 40 Ar/ 39 Ar radioisotope method (Vasconcelos 1999a, b). It could be argued that method is unreliable and so cannot provide absolute dates (Snelling 2016a, 2017a). However, when it is carefully used it can still provide relative dates, anchored to the biblical parameters for subdividing and dating the rock record (Vardiman, Snelling and Chaffin 2005). So supergene weathering minerals such as alunite [KAl 3 (SO 4 ) 2 (OH) 6 ], jarosite [KFe 3+ 3 (OH) 6 (SO 4 ) 2 ], and hollandite [Ba, K(Mn 4+ 6 Mn 3+ 2 )O 16 ] have proven to be good candidates for 40 Ar/ 39 Ar dating of the time they formed. That potentially becomes the relative times these minerals formed after the Flood waters receded and the land was exposed to weathering. The 40 Ar/ 39 Ar method is most widely applied because many minerals precipitated by weathering reactions contain K (as detailed in Table 1 in the Appendix), and many of these minerals are relatively stable once formed. If these phases retain 40 K and 40 Ar quantitatively, then nuclide abundances can be used to determine when the mineral formed (details in Vasconcelos 1999b). The analysis of a representative suite of K-bearing supergene minerals from a vertical section through a weathering profile may be used to estimate weathering rates and to infer paleoclimatic conditions (de Oliveira Carmo and Vasconcelos 2006). A probability density distribution of mineral precipitation ages identifies times in the past when climatic conditions favored mineral dissolution and reprecipitation. Chemical reactions recorded by mineral precipitation require water as a reactant; therefore, the frequency distribution of ages through time permits one to identify periods in the geological past that were relatively wet (Vasconcelos 1999a, b). As with K-bearing minerals, the Fe-bearing goethites and hematites generated by water–rock interactions during the formation of supergene ore bodies can be dated. The decay of trace amounts of U and Th in goethite and hematite (Lippolt et al. 1998) results in 4 He by-products that can be used for dating mineral precipitation, as long as the 4 He, U, and Th are retained. But properly quantifying 4 He retention in goethite and hematite was not possible until scientists combined the (U–Th)/He and 4 He/ 3 He methods (Shuster et al. 2005; Heim et al. 2006; Monteiro et al. 2014). The interpretation of geochronological results for samples from weathering/supergene profiles of ore deposits requires the identification of the dated reactions (Vasconcelos 1999a). For this purpose the dated mineral samples must be characterized petrographically, and the reactions must be identified from paragenetic relationships. METHODS Results from applying this methodology are already available in the relevant literature (Vasconcelos et al. 2015). Geochronological data was compiled from a number of studies – South American supergene copper deposits (Alpers and Brimhall 1988; Arancibia et al. 2006; Mote et al. 2001; Sillitoe and McKee 1996; Vasconcelos 1999a); SouthAmerican supergenemanganese deposits (deOliveira Carmo and Vasconcelos 2006; Spier et al. 2006; Vasconcelos 1999b); other supergene manganese deposits in Africa (Beauvais et al. 2008), Australia (Dammer et al. 1999; Feng and Vasconcelos 2007; Li and Vasconcelos 2002; Vasconcelos 2002; Vasconcelos et al. 2013), China (Deng et al. 2014; Li et al. 2007), India (Bonnet et al. 2014), and Europe (Hautmann and Lippolt 2000); and supergene iron deposits in Australia (Heim et al. 2006; Vasconcelos 1999b; Vasconcelos et al. 2013), and Brazil (Monteiro et al. 2014). The frequency of mineral precipitation, determined by dating a representative number of samples of a particular mineral collected from distinct parts of the supergene orebody, reflects times in the geological past when weathering conditions were conducive to water–rock interaction. The frequency of mineral precipitation through time permits identifying periods in the geological past when climatic conditions were most conducive to chemical weathering and supergene ore genesis. RESULTS The available data were compiled in three histograms in Fig. 1 according to the supergene minerals dated (Mn oxides, goethite and hematite, or alunite-jarosite) and the dating methods used [ 40 Ar/ 39 Ar or (U–Th)/He], showing the frequencies of conventional ages in the range 0-70 Ma in 1 Ma increments. The distribution of these supergene minerals through time appears to help to identify periods in the geological past conducive to the dissolution and reprecipitation of ore elements in the weathering environment. Each mineral species records slightly different conditions. For example, Mn oxides record wetter conditions needed for the reduction–dissolution processes needed to dissolve and reprecipitate Mn oxides in the weathering environment. In contrast, the formation and preservation of supergene alunite and jarosite required relatively dry conditions after mineral precipitation, typically achieved by drawdown of the water table during a transition from humid or semi-arid to hyper-arid conditions. DISCUSSION There are several issues that must be carefully considered before conclusions can be drawn from these data in Fig. 1. 1. The Usefulness of Radioisotope Ages as Relative Ages It is first necessary to establish that it is still potentially valid to use radioisotope ages in a relative sense, even though it is well established that there are significant problems with the radioisotope dating methods to render the resultant ages as not absolute (Faure and Mensing 2005; Snelling 2000). All the radioisotope dating methods are ultimately calibrated against the U-Pb and Pb-Pb methods. However, Snelling (2017a) has documented from the literature the residual uncertainties in determinations of the 238 U and 235 U decay rates, especially the latter, which is somewhat dependent on the determinations of the 238 U decay rate. Recently, copious measurements of the 238 U/ 235 U ratio in a very wide variety of rocks, ores, minerals and meteorites has revealed that values vary widely. Indeed, the 238 U/ 235 U ratio value of the same mineral can be very different in different rocks, including zircons, which are so often used in geochronology. Snelling ◀ Flood/post-Flood boundary ▶ 2018 ICC 555