Diese Webseite setzt Cookies für folgende Funktionen ein: Anmeldung, Suche, personalisierte Inhalte, Seitenanalyse, Facebook likes.

Auf Wunsch der EU muss explizit darauf hingewiesen werden. Durch Nutzung unserer Website erklären Sie sich damit einverstanden.

Volcanic biotite-sanidine 40Ar/39Ar age discordances reflect Ar partitioning and pre-eruption closure in biotite

Hits: 791
Year:
2010
Type of Publication:
Artikel
Autoren:
Hora, John M.; Singer, Brad S.; Jicha, Brian R.; Beard, Brian L.; Johnson, Clark M.; de Silva, Shan; Salisbury, Morgan
Journal:
Geology
Volume:
38
Nummer:
10
Seiten:
923-926
BibTex:
Kurzzusammenfassung:
The 40Ar/39Ar radioisotope system is widely used to date eruption and cooling of volcanic tephra–marker horizons that commonly provide the only means of correlating and assigning numerical ages to stratigraphy in which they are contained. This chronometer bridges the gap between 14C and longer-lived isotopic systems that are too imprecise for dating young samples. However, 40Ar/39Ar ages obtained from coevally erupted biotite and sanidine do not always match. Here, we use an independent chronometer, 238U-230Th disequilibrium, to demonstrate that 40Ar/39Ar age disparity is not caused by differences in pre-eruption crystallization times. Our findings indicate that the presence of extraneous 40Ar in biotite, and its absence in sanidine, may result from violations of two assumptions implicit in 40Ar/39Ar geochronology on volcanic samples: (1) Prior to eruption, minerals are devoid of 40Ar due to rapid loss to an “infinite reservoir” such as the atmosphere, and (2) closure to volume diffusion is geologically instantaneous and coincident with eruption. We propose a mechanism whereby the presence of extraneous Ar in certain minerals is explained by the relative sequence of four events in a magmatic system: (1) crystallization, (2) mineral closure with respect to Ar diffusion, (3) isotopic equilibration of magmatic and atmospheric Ar, and (4) quenching of the system by eruption. These data have potentially far-reaching implications for studies that depend on geochronological data, necessitating re-evaluation of interpretations based solely on biotite with no independent age control, particularly in young samples where the effects are most pronounced.
Free business joomla templates