Distribution of rubidium, strontium, and zirconium in tuff from two deep coreholes at Yucca Mountain, Nevada Page: 3 of 7
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chowpjI lilitA (pco nme 33Figure 2. Distribution of (A) clinop tolite/mordenite
and analcime in G-1u, (B) clinoptilolite and analcime
in G-315, (C) smectite in -1 and G-315. Dashed lines
denote approximate boundaries between zones of
diagenetic alteration I, II, III, and IV14.
in G-1. For example, Rb concentrations, averaging 159
7 ppm (n=number of samples, n=15) are tightly
clustered within the 130-m-thick moderately to densely
welded zone of the Bullfrog Member of the Crater Flat
Tuff in G-3, which is directly above the water table.
The relative uniformiy of Rb concentrations in the
Bullfrog amd Topop Spring Members suggests a
strong resistance of thick moderately to densely welded
zones that are above the present-day water table to
trace-element redistribution resulting from
devitrification and(or) secondary alteration.
Strontium
Sr content varies significantly with depth. Sr
concentration increases from an average value of 31 and
a range of 21 to 55 ppm (n=23) within the Topopah
Sprng Member to an average of 166 ppm (n=4) in the
unts a and b of the older tuffs (fig. 3 ). In addition to
this gradual increase with depth, intervals that show
anomalously high Sr concentrations are present in (1)
the lower part of the Tram Member and the dacite lava
in G-1, (2) unit c of the older tuffs in G-1, and (3) the
-densely welded zone of the Bullfrog Member in G-3.
The highest Sr concentration, 869 ppin, is near the
middle of the dacite lava in G-1 (fig. 3C), which
corresponds to a clay-rich interval in diagenetic zone III
where% smectite content is more than 60 percent (fig.
2C). Unlike stratigraphic units directly above and
below, the dacite lava is autobrecciated near its upper
and lower contacts ind contains an abundance of
calcite-filled fractures.
Alternatively, hilh concentrations of Sr may be
an artifact of a change in rock type from the high-silica
rhyolitic tuffs to lava of dacitic composition and trace-
element contamination from abundant lithic inclusions.C)
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Although the highest Sr concentration is in the dacite
lava, anomalously high concentrations are also present
in the lower half of the overlying Tram Member. These
high Sr values correlate with abundant lithic inclusions
of intermediate composition in the lower half of the
Tram Member, providing a potential source for
elemental contamination of bulk-rock samples from this
interval. In G-1, within clay-rich unit c of the older
tuffs, Sr increases abruptly from about 340 ppm to a
maximum of 594 ppm over an interval of about 100 m
(fig. 2C, 3C).
In G-3, Sr concentration generally increases with
depth, except from 674 m to 685 in, where Sr
concentrations are as high as 459 ppm within the upper
part of the moderately to densely welded zone of the
Bullfrog Member. Although the upper limit of the
anomaly is poorly constrained, Sr values abruptly
decline below a depth of 685m, forming a tight cluster
of values that averages 127 ppm and ranges between
102 and 157 ppm (n=12) within the thick densely
welded zone of the Bullfrog Member (fig 3C). This
anomaly is well above the present-day water table, does
not appear associated with abrupt changes in welding or
zeolite and smectite content, and, therefore, may be
related to primary composition.G-7
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Spengler, R.W. & Peterman, Z.E. Distribution of rubidium, strontium, and zirconium in tuff from two deep coreholes at Yucca Mountain, Nevada, article, May 1, 1991; Denver, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc620863/m1/3/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.