To date, geologists have thoroughly documented the influence of oceanic sediments—even specific river basins—on the isotope geochemistry of volcanic rocks around the world.
The most relevant conclusion to this blog/discussion is that the subduction of oceanic crust was accompanied by the slow accumulation of sediment in the deep ocean over tens of millions of years.
He writes: "...radioisotopes in [historic/recent] lavas reflect the isotopic compositions of the mantle sources of these lavas, and of any crustal contamination the magmas may have incorporated during ascent and extrusion." (emphasis mine) This should sound familiar to you by now.
It appears that Snelling generally understands that most radiogenic isotope systems in volcanic arcs are not hypothesized to reflect the age of the actual eruption. Snelling continues as though geologists believe these isotopes should reflect the age of eruption and admits—albeit slyly—that he has wasted the grant money from the RATE project: In other words, after referencing a couple of 15-25 year-old geochemistry textbooks (which all specify that Rb-Sr, Sm-Nd, and Pb-Pb isotopes in volcanic arcs are related to mixing between various mantle and crustal sources), Snelling decided to spend thousands of dollars on a handful of useless isotope data.
I say 'useless' because Snelling had no meaningful hypothesis that could be tested by these data.
In actuality, Snelling's examples of 'unreliable ages' primarily derive from K-Ar determinations that were made before geochronologists knew how to correct for excess argon and xenoliths (bits of old minerals in young volcanic rocks).
His stated agenda is therefore unwarranted, since more recent analyses of very young, historical volcanic rocks using the Argon-Argon technique are spot on.