Geochemical fingerprinting of Wilson Creek Formation tephra layers (Mono Basin, CA) using titanomagnetite compositions

The Wilson Creek Formation (WCF) near Mono Lake in eastern California is a 6-15 m thick sequence of lake sediments interbedded with 18 rhyolitic tephra layers attributed to eruptions from nearby Mono Craters, and 2 basaltic tephra layers erupted from Black Point and June Lake (not included in study). These tephra layers are numbered 1 to 19 (June Lake tephra designated as Ash 13*) from top to bottom (Lajoie, 1968), and range in age from ~14 to 67 ka (Zimmerman et al., 2006). The WCF tephras are important to understanding volcanism in the Long Valley region, and they provide age constraints for paleomagnetic, paleoclimatic, and paleoecologic studies. Major-oxide compositions of glass for the rhyolitic tephras are similar, with no reported compositional variations between layers. In this study, we show that titanomagnetite compositions can be used to fingerprint specific WCF tephra layers, thus enhancing their use as time-stratigraphic markers. We performed electron microprobe analyses of >700 titanomagnetite crystals and minor ilmenite. Compositional ranges of titanomagnetite are ~83-89 wt% FeO, and ~8-14 wt% TiO2, with MgO+MnO+Al2O3 <3 wt%. Major oxide concentrations trend linearly, with ratios of FeO/TiO2 generally increasing with age. There is a slight trend offset between the older (~57-67 ka) tephras (Ashes 16, 17, and 19) and younger (~14-40 ka) tephras (Ashes 1-15), and a compositional gap between Ash 8 and the rest of the WCF tephras. Compositional variability within individual tephra beds is minor (FeO <0.4 wt%; TiO2 <0.3 wt%). Older WCF tephra titanomagnetites (Ashes 16, 17, 19) are distinct and characterized by high FeO/TiO2 (~9-9.5). Ashes 7 and 8 have relatively high-TiO2 compositions (>12 wt%), although Ash 8 has the lowest FeO/TiO2 (~6). Titanomagnetite crystals in the other WCF tephra layers are not as compositionally distinct (FeO/TiO2 = ~7.5-8), although the youngest tephras (Ashes 1- 4) can be distinguished on the basis of MgO content. Ash 15 is unique and exhibits a bimodal population. The composition of Ash 18 titanomagnetites are completely off-trend, with MgO+MnO+Al2O3 ~6 wt% (a factor of 2 difference). Results show that (1) the WCF tephras are compositionally related, but some individual tephras can be distinguished using titanomagnetite compositions. The oldest WCF tephra (Ash 19) had an eruption temperature of ~715°C (Fe-Ti oxide geothermometry of Ghiorso and Evans, 2008), and the general trend of decreasing FeO/TiO2 suggests magmatic temperatures increased for successively erupted Mono Craters rhyolites. Also, uniform compositions for some of the WCF tephra titanomagnetites may reflect similar eruption temperatures. (2) Ash 18 is not compositionally related to the other WCF tephras. Its eruption temperature of ~962°C suggests it is a distal fall-out tephra of a dacitic eruption from elsewhere in the region, rather than a product of one of the Mono Craters eruptions, as was commonly assumed.