40Ar/39Ar ages of Precambrian manganese ore minerals from Sweden, India and Morocco

• Verfasst von: Lippolt, Hans J. [VerfasserIn]
• Verfasst von: Hautmann, Siegfried [VerfasserIn]
• Titel: 40Ar/39Ar ages of Precambrian manganese ore minerals from Sweden, India and Morocco
• Verf.angabe: H.J. Lippolt, S. Hautmann
• Jahr: 1995
• Umfang: 11 S.
• Fussnoten: Im Titel sind die Zahlen 40 und 39 jeweils hochgestellt ; Gesehen am 18.01.2023
• Titel Quelle: Enthalten in: Mineralium deposita
• Ort Quelle: Berlin : Springer, 1966
• Jahr Quelle: 1995
• Band/Heft Quelle: 30(1995), 3, Seite 246-256
• ISSN Quelle: 1432-1866
• DOI: doi:10.1007/BF00196360
• Datenträger: Online-Ressource
• Sprache: eng
• Sach-SW: Alteration Process
• Sach-SW: Manganese Deposit
• Sach-SW: Manganese Oxide
• Sach-SW: Step Heating
• Sach-SW: Vein Deposit
• K10plus-PPN: 1831411768

Abstract

In order to test their chronometric potential, 40Ar/39Ar stepheating- (and 4He-) analyses have been carried out on five manganese ore minerals of the hollandite-cryptomelane series from three Precambrian manganese deposits (Ultevis/Sweden, Sitapar/India, Bachkoun/Morocco). Samples from the metamorphic occurrences Ultevis and Sitapar yielded Ar ages of 1.8 Ga and 0.95 Ga, interpreted as the age of postmetamorphic cooling (Hollandites/Ultevis) and of an early, K-introducing alteration process subsequent to amphibolite facies metamorphism (cryptomelanes/Sitapar). Both data are consistent with known chronologies of the “Svecokarelian” and “Satpura” orogenic cycles. A date of 670 Ma obtained for a hollandite from a volcanogenic vein deposit (Bachkoun), however, contrasts with published extrusion ages of 580-560 Ma for the volcanic host rocks (“Ouarzazate Series”), probably due to incorporation of excess argon. The use of the 40Ar/39Ar technique, together with multiple isotope systematics, made it nevertheless possible to establish a reasonable estimate of a mineralization age close to 580 Ma. Measurement of fractional Ar losses during vacuum step heating (500-1600 °C), although indicating good Ar retentivities, failed to define model diffusion parameters because of non-linear Arrhenius arrays. Helium diffusion results (200-1200 °C) indicated retention of radiogenic 4He by the samples, corroborated by U/He mineral dates between 0.96 and 0.31 Ga. Potassium-bearing manganese oxides are therefore able to retain argon (possibly also 4He) through geological times and may thus provide ages of ore-forming processes (and perhaps later cooling and alteration stages).