This is represented by a trajectory that goes down and to the left, as shown above in the Granger example.
So samples that are “below the banana” have experienced both a period of exposure and a period of burial.
The basic concept here is that if your sample stays at the surface and experiences steady exposure with or without erosion, nuclide concentrations are confined to the “simple exposure region” highlighted with dark lines in the above figure.
In certain manifestations of this diagram (primarily when plotted with a log x-axis and a linear y-axis), the simple exposure region vaguely resembles a banana, for example: This resemblance, perhaps unfortunately, has resulted in the common use of the term “banana diagram.” Then the important aspect of this diagram is that if the sample gets buried after a period of surface exposure, both Al-26 and Be-10 concentrations decrease due to radioactive decay, and Al-26 decreases faster than Be-10.
Past stratovolcanoes of the High Cascades and across Oregon’s remote High Lava Plains lie three sunstone mines (figure 3).
All three produce labradorite feldspar ranging from near-colorless to pale yellow to red and green, including bicolor specimens.
This superb 2.85 ct sunstone from Sunstone Butte displays the gem’s most valued attributes: a blend of green and red bodycolor, with reflective spangles of native copper glittering in the interior. No discussion of this topic would be complete without mention of the controversy surrounding treated copper-bearing feldspar; Rossman (2011) provides a chronology.
So, basically, in the Al-26/Be-10 two-nuclide diagram (let’s not use “banana diagram” any more…historically, it probably should be called a “Lal-Klein-Nishiizumi diagram,” although that is a bit cumbersome), exposure goes to the right and burial goes down. The problem arises when other nuclides are involved.The sunstone often contains tiny reflective platelets of native copper, referred to as “schiller.” From Portland, we drove five hours southeast to Ponderosa, near the town of Burns.We spent three days there before driving three hours southwest to spend the next two days at the Dust Devil and Sunstone Butte mines. The sparsely populated high desert of eastern Oregon is home to three localities producing natural copper-bearing labradorite feldspar.In this view of Lake County’s Dust Devil mine, the local source of the sunstone-bearing lavas is the group of rounded hills (Dudeck Ridge) in the background. Over five days in late July 2013, we visited three important sources of gem-quality sunstone in eastern Oregon: the Ponderosa mine in Harney County, and the Dust Devil (figure 1) and Sunstone Butte mines, both of which are about 120 miles further south, in Lake County.