Abstract
A combined ultrasonic treatment, with de-ionized H2O, dilute HAc or dilute HCl, of three Mid-Miocene glauconite samples was applied to K–Ar date the different separates in order to compare the results with those obtained by the Rb–Sr method using the same three samples and that were analyzed strictly in the same way. Two aliquots yield opposite elemental and K–Ar trends, which suggests different initial mineral compositions for the various pellets. The K–Ar data of two untreated and leached L7 and L8 aliquots are almost within analytical uncertainty from 17.3 ± 0.6 Ma to 19.6 ± 0.7 Ma (2σ), while those of the third L10 sample are slightly higher at 22.1 ± 1.2 Ma (2σ). Comparatively, the earlier published Rb–Sr ages of the three untreated samples and of the leached aliquots gave similar data for the L7 aliquots by an isochron at 18.1 ± 3.1 (2σ) Ma and for the sample L8 by an isochron with an age of 19.6 ± 1.8 (2σ) Ma, while the untreated L10 aliquot yields a very high Rb–Sr date of 42.1 ± 1.6 (2σ) Ma. This untreated L10 glauconite fraction contains blödite, a Sr-rich carbonate that impacted the two isotopic systems differently. Generally, dilute HCl or HAc acids dissolve carbonates, sulfates, sulfites and oxides, while they do not affect the clay-type crystals such as glauconites. These soluble minerals can be identified indirectly, as here, by X-ray diffraction and the amounts of leached Na2O, CaO and Fe2O3 contents. Together with the leaching of some metallic trace elements, those of NaO confirm the leaching of metals and of blödite that are both hosted by the glauconite pellets. The occurrence of this Sr-enriched mineral explains the age differences of the non-treated aliquots and suggests a systematic leaching of any glauconite separate before isotope determination and, possibly, a comparison of the Rb–Sr and K–Ar results. Ultrasonic shaking appears appropriate for physical disaggregation of any contaminating grains that may remain hosted within the pellets, even after a preliminary H2O wash, which may dissolve and remove the soluble minerals but not the H2O-insoluble silicates. The K–Ar study completed here as a complement to a previous Rb–Sr study highlights, again, the importance of the preparation step in isotopic studies of glauconite-type and, by extension, of any clay material, as all occurring minerals can interfere in the final age determinations and, therefore, differently in the mineral assemblages. All those not in isotopic equilibrium need to be removed before analysis, including the soluble Sr or alkali-enriched ones.
Highlights
Because of their wide geographic and stratigraphic occurrence, authigenic glauconite minerals of various types of sediments have long been considered as reference materials for the calibration of time scale and were, studied in detail in many basic mineralogical and chemical studies and reviews e.g., [1,2,3]
The contents of the major elements of the three untreated glauconite samples were compared to the aliquots submitted to ultrasonic shaking in different reagents (Figure 1; Table 1A)
Some Na content could or have resulted as part of halite NaCl crystallized from the evaporation of seawater that could have diffused into the growing authigenic glauconite pellets during crystallization, while blödite is a reasonable alternative, contemporaneously or after the crystallization of the glauconite pellets
Summary
Because of their wide geographic and stratigraphic occurrence, authigenic glauconite minerals of various types of sediments have long been considered as reference materials for the calibration of time scale and were, studied in detail in many basic mineralogical and chemical studies and reviews e.g., [1,2,3]. Pasteels et al [6] and Keppens [14] chose dilute hydrochloric acid as the reagent, with an ultrasonic shaking, whereas other authors preferred ammonium-acetate leaching without shaking e.g., [7,15]. In this context, Clauer et al [16] examined isotopic data based on various preparation methods used by different investigators to focus on the most appropriate preparation. Analytical aspects are precisely addressed here again by focusing on the ultrasonic shaking of glauconite separates strictly using the same procedure as Pasteels et al [6] with the same reagents during the same shaking durations, and by comparing the K–Ar data obtained here with their Rb–Sr data published long ago
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