Noble metals in arc basaltic magmas worldwide: A case study of modern and pre-historic lavas of the Tolbachik volcano, Kamchatka

Platinum-group elements (PGE) and gold are a promising tool to assess the processes of
mantle melting beneath the subduction zones. However, fractionation processes in
magmas inevitably overwrite the initial metal budgets of magmas, making constraints
on the melting processes inconclusive. Moreover, little is still known about the geochemical
behavior of a particular metal in a single arc magmatic system, from mantle melting
towards magma solidification. Here we compare noble metals in lavas from several
eruptions of the Tolbachik volcano (Kamchatka arc) to better understand the effects of
magma differentiation, estimate primary melt compositions and make constraints on the
mantle melting. We show that Ir, Ru, Rh and, to a lesser extent, Pt are compatible during
magmatic differentiation. The pronounced incompatible behavior of Cu and Pd, observed
in Tolbachik magmas, rules out the significant influence of sulfide melts on the early
magmatic evolution in this particular case. Gold is also incompatible during magmatic
differentiation; however, its systematics can be affected by the inferred gold recycling in the
plumbing system of Tolbachik. Although the Tolbachik lavas show only slightly higher PGE
fractionation than in MORB, a notable negative Ru anomaly (higher Pt/Ru and Ir/Ru) is
observed. We attribute this to be a result of greater oxidation in the subarc mantle (by 1–4
log units), which promotes crystallization of Ru-bearing phases such as Fe3+-rich Cr-spinel
and laurite. The estimated Pd contents for the parental melt of the Tolbachik lavas
approaches 6.5 ppb. This is several times higher than reported MORB values (1.5 ±
0.5 ppb), suggesting the enrichment of Pd in the mantle wedge. Our results highlight
the influence of the subduction-related processes and mantle wedge refertilization on the
noble metal budgets of arc magmas.