Detailed study of the Holocene and latest Pleistocene silicic
tephras
of Newberry volcano is in progress. To date, over 14 different silicic
tephras and 7 caldera obsidian flows have been analyzed for major
elements
by electron microprobe. These include the Newberry Pumice, Paulina Lake
ash-flow, Central Pumice Cone, East Lake tephra, several unnamed
tephras,
Big Obsidian Flow, East Lake obsidian flows, Interlake Obsidian flow,
Central
Pumice Cone obsidian flows, South obsidian flow, and Northeast obsidian
flow. A subset of these units has been analyzed for major and trace
elements
by XRF. Electron microprobe analysis of glass shards clearly
distinguishes
the Newberry Pumice, Paulina Lake ash-flow, and Big Obsidian Flow (all
73.37 wt% SiO2, 0.23 wt% TiO2, 14.24 wt% Al2O3, 2.16 wt% Fe2O3, 0.17
wt%
MgO, 0.85 wt% CaO, 4.86 wt% Na2O, 4.00 wt% K2O, 0.12 wt% Cl) from all
other
analyzed Newberry tephras and obsidians. A charcoal sample found below
the Newberry Pumice has been dated at 1510+/-80 14C years. The
excellent
condition of the sample, which represents the outermost part of a small
tree, suggests that the 200 year difference in published ages for the
Newberry
Pumice and Paulina Lake ash-flow (published average of 1310+/-40 14C
years)
may be real. Previously, this age difference had been attributed to
differences
in material sampled for dating. A 135 cm thick deposit of
compositionally
variable pumice found 12 km from the caldera on the lower northwest
flank
has been correlated to a 2 cm thick ash dated at 3135+/-100 14C years
found
at the southeast caldera rim. Ash possibly related to the East Lake
obsidian
flows overlies this compositionally variable ash and is found below
charcoal
dated at 2210+/-140 14C years. As much as 175 cm of pre-10190+/-240 14C
year tephra (75.43 wt% SiO2, 0.17 wt% TiO2, 13.27 wt% Al2O3, 1.71 wt%
Fe2O3,
0.11 wt% MgO, 0.52 wt% CaO, 4.47 wt% Na2O, 4.17 wt% K2O, 0.15 wt% Cl)
has
been found in auger holes near the east caldera rim and on the upper
east
flank. The same tephra has been found on the west caldera rim and as
far
as 15 km west of the caldera.