The most Mg-rich samples are from the Cone Mt. 1004 olivine-phyric lavas (Figures 2A,B), which are characterized by MgO contents lying within 9.2–9.5 wt.% (Figure 3). Two samples from the 1941 eruption (lava and scoria) are almost identical in MgO content (8.7 wt.%), as well as in all other chemical components. One sample from the GTFE and one from the Unnamed Prehistoric Eruption (UPE) also are high-Mg. Low-Mg lavas (Figures 2C,D) and scoria were collected from the GTFE and 2012–2013 eruption. Their MgO contents range from 4.0 to 5.1 wt.%, however they have significantly higher Al₂O₃, Na₂O, K₂O, and TiO₂ contents than the high-Mg basalts (Figure 3). One sample from each of the 1975 eruption scoria and UPE have K₂O contents significantly lower than those of equally primitive samples (Figures 3E,F,H). Iron contents remain almost constant across the whole compositional range, similar to the primitive melt composition calculated by Portnyagin et al. (2015). Mg# (MgO/(MgO + FeO), mol%) ranges from 0.58 to 0.63 for high-Mg basalts and from 0.42 to 0.55 for low-Mg lavas (Table 2).

The concentrations of platinum-group elements (PGE) in Tolbachik lavas are summarized in Table 2. Iridium is enriched in high-Mg basalts (Figure 4A). Similar behavior is observed for Ru (Figure 4B); however, the difference between high-Mg and low-Mg rocks is much less pronounced. Rhodium also defies this trend showing significantly higher concentrations in high-Mg rocks (Figure 4C).

Platinum is only slightly enriched in high-Mg basalts (with the exception of three anomalous samples; Figure 4D). These anomalous samples are high- and low-Mg basalt from the UPE (21.5 and 15 ppb Pt, respectively), and low-Mg basalt from the GTFE 1975-76 (17.1 and 21.5 ppb Pt for sample and duplicate).

Palladium is enriched in low-Mg rocks (Figure 4E). High-Mg samples from the 1941 eruption and one of the samples from GTFE have high Pd contents of 8.0–8.5 ppb, whereas other high-Mg basalts have moderate Pd contents (5.7–6.5 ppb). In general, low-Mg basalts are enriched in Pd relative to high-Mg basalts. Gold distribution closely mimics that of Pd (Figure 4F), with significantly higher concentrations in low-Mg (2.8–3.9 ppb) relative to high-Mg samples (1.8–2.6 ppb). Several samples, two from Cone Mt. 1004 and one of the 1975-76 eruption, are anomalously Au-rich and have not been considered in the calculations.

To determine whether PGE and Au contents are representative of the sampled lava flow, 13 samples and 3 duplicates from Cone Mt. 1004 were analyzed. The results are uniform and have relatively low standard deviations for all elements except Au (Table 3), indicating homogenous distribution of PGE-bearing phases.

Nevertheless, six samples in the full dataset (n = 32) showed anomalous enrichment in one noble metal. Gold-rich samples are exclusively from high-Mg basalts, either the 1941 eruption or GTFE, whereas Pt-rich samples are either from the UPE or GTFE. This can be attributed to the “nugget effect” (heterogeneous distribution of anomalously large grains of a particular metal) or occur as a result of a specific process that may have enriched the melt. The duplicate of the UPE Pt-rich sample also yielded anomalous Pt contents. Given the established homogenous Pt distribution in samples from Cone Mt. 1004, this suggests that, for some samples, high metal contents may be an intrinsic property of the magma and cannot be ascribed to the presence of nuggets (Figure 4D).

A plausible explanation for Pt enrichment is the presence of sulfide blebs which have been reported for high-Mg basalts of the 1941 eruption and Cone Mt. 1004 (Zelenski et al., 2017). However, these blebs show a perfect Pt-Pd correlation, whereas the studied samples are enriched exclusively in Pt (Figure 5). Alternatively, such enrichment could be attributed to the ability of Pt to form inclusions within Cr-spinel (Finnigan et al., 2008; Kamenetsky et al., 2015) or as separate nuggets (Park et al., 2013). Selective enrichment of Pt over Pd may be explained in terms of their relative solubilities in silicate melts, e.g. at a given oxygen fugacity, Pd solubility is more than two orders of magnitude higher than Pt (Borisov and Palme, 2000). Therefore, Pd is retained in the melt, whereas Pt precipitates as Pd-free alloys.

Despite working well for Pt, this phenomenon cannot explain the presence of gold nuggets. Gold solubility in basaltic melts is far too high, restricting its direct precipitation at a wide range of temperatures and fO₂. For example, the Au solubility estimated by Borisov and Palme (2000) ranges from 40 ppb for QFM-2 at 1,200°C to 5,000 ppb for QFM+2 at 1,350°C, or even higher if sulfide complexation is considered (Botcharnikov et al., 2010). The estimated fO₂ for Tolbachik lavas is QFM+1 to +2, based on chromian spinel compositions (Kamenetsky et al., 2017; Nekrylov et al., 2018) and olivine-sulfide pairs (Zelenski et al., 2018) At this fO₂, Au solubility might have approached 400 ppb. However, this is two orders of magnitude higher than the most enriched samples (13.3 ppb), making direct gold precipitation from Tolbachik magmas unrealistic. Therefore, the idea of Au enrichment as a result of crustal assimilation, discussed in Zelenski et al. (2016), appears to be the most plausible explanation.

Iridium, Ru and Rh show compatible behavior, reflected in their lower contents within more evolved Al-rich basalts. Similar trends have been observed in various volcanic arcs (Figures 6A,B), and can be explained by the capturing of these elements either as admixtures in Cr-spinel (Park et al., 2012; Park et al., 2017), or as nuggets (Kamenetsky et al., 2015). Their accumulation in Cr-spinel is attributed to a redox related response to mineral growth or re-equilibration with the melt surrounding Cr-spinel crystals (Finnigan et al., 2008). Local reduction within the mineral–melt interfacial region occurs as a consequence of the selective uptake of trivalent over divalent Cr and Fe from the melt by spinel (Finnigan et al., 2008).

Both Pt and Pd show no correlation with MgO from primitive compositions, studied here and from other magmatic arcs, down to ∼9 wt.% MgO (Figures 6C–F). Their behavior, however, becomes variable for evolved compositions from global arcs, where Pt and Pd contents often drop drastically, whereas, in Tolbachik, their concentrations remain stable as MgO decreases progressively, indicating a lack of sulfide saturation for the latter, at least up to 4 wt.% MgO. In contrast, Pd contents of MORB lavas with less than ∼9.5 wt.% MgO vary drastically (Dale et al., 2012; Hao et al., 2021). This suggests that MORB and some arc melts achieved sulfide saturation at ∼9–9.5 wt.% MgO, producing Pd and Pt depletion, and that other arc melts either did not reach sulfide saturation, or that sulfide saturation did not affect the metal budget.

However, the Tolbachik samples reveal that low-Mg lavas bear slightly higher amounts of Pd than high-Mg lavas (Figure 4E). This indicates incompatible Pd behavior and may be evidence against sulfide saturation during the Tolbachik magmas evolution. Although previous studies reported sulfide-silicate immiscibility in Tolbachik lavas (Zelenski et al., 2018), this process was limited to a small amount of melt and did not significantly affect the overall PGE budget. Moreover, sulfide precipitation itself is not sufficient to affect the PGE budget; rather, the extraction/removal of newly formed sulfide (liquid or crystalline) from a melt is more crucial.

Slight Pt enrichment in high-Mg lavas suggests it behaves compatibly during magma evolution from 10 down to 4 wt.% MgO (Figures 4D, 6C,D). This does not support sulfide saturation as the cause for Pt fractionation, because Pd contents would also be expected to decrease. Nevertheless, the lack of significant Pt–MgO correlations is evident across a range of compositions (9–37 wt.% MgO), i.e., rocks which are likely cumulates of olivine and chromian spinel. This implies that in spite of evidence for Pt-Fe alloy coprecipitation with chromian spinel (Finnigan et al., 2008; Kamenetsky et al., 2015; Arguin et al., 2016), their fractionation does not contribute significantly to the Pt budget of arc magmas in general.

The understanding of the PGE budget in parental melts is hindered by fractionation processes. The plurality of factors involved in the formation of particular lava makes precise evaluation of the primitive melt composition almost impossible. Nevertheless, a rough estimation is possible for at least some elements. Due to the lack of significant sulfide saturation, Pd contents show the following: 1) moderate enrichment from ∼9 to ∼5 wt.% MgO in Tolbachik lavas, and 2) no correlation across the range between 37 and 9 wt.% MgO (Figures 6E,F). This long flat trend suggests that during evolution from the assumed parental melt (∼14 wt.% MgO Portnyagin et al., 2015) to the high-Mg basalt, Pd contents did not change significantly, and thus the mean concentration of 6.5 ppb is close to that of primitive mantle-derived melts. For Pt, such an estimate is more complex as it behaves slightly incompatibly. However, other arc rocks show minimal effects Pt contents decrease towards Mg-poor compositions (Figure 4D). Therefore, mean Pt contents in high-Mg basalts (5.8 ppb, ignoring three anomalous samples) may be used as a lower estimate of initial Pt abundances in the Tolbachik primitive melt.

Iridium, unlike Pt and Pd, shows a broad correlation with MgO due to strong compatibility with chromian spinel (Figure 4A). The parental melt of the studied lavas should thus be significantly enriched in Ir. Statistical analysis of previously published data of arc rocks provides an adequate MgO–Ir linear correlation with R ² = 0.82. This allows for the extrapolation of mean Ir contents in low- and high-Mg lavas to 14 wt.% MgO, which results in 0.29 ppb Ir. Normalization of estimated Ir, Pt and Pd contents in the parental melt to primitive mantle values (Lyubetskaya and Korenaga, 2007) yields 0.1, 1.3 and 1.8 ppb, respectively, and a Pd_N/Ir_N (“_N” stands for primitive mantle normalization) ratio of 20. This indicates that the fractionated pattern of the Tolbachik lavas is not only the result of fractional crystallization, but also inherited from the mantle-derived melt.

Mechanisms of metal transfer from mantle to melt are variable in subduction and oceanic rift environments (e.g., Dale et al., 2012; Mungall and Brenan, 2014; Park et al., 2017). The main PGE hosts in the mantle are sulfides (Keays et al., 1981; Kepezhinskas et al., 2002; Barnes et al., 2015) and alloys (e.g., Rehkämper et al., 1999). Oxidizing conditions in the sub-arc mantle during magma generation should result in complete sulfide consumption. Therefore, Pd, and to a lesser extent Pt, partition into the melt, whereas Ir, Os and Ru are retained in the source residue (Dale et al., 2012). Thus, a comparison between PGE contents in primitive arc and MORB melts may provide insights into the role of the subduction flux on the PGE budget.

Palladium can fractionate from Ir during common magma differentiation processes, including the formation of immiscible sulfide melts and the partitioning of Ir in Cr-spinel (Barnes et al., 1985; Park et al., 2012; Barnes et al., 2015). It has been shown, however, that the fractionated patterns of arc rocks are partly inherited from their mantle source or produced during partial melting (Park et al., 2017). This is also supported by the estimated Pd_N/Ir_N value for the Tolbachik primitive magmas (20). The difference in Pd_N/Ir_N between MORB and arc rocks across the range of 8.5–12.0 wt.% MgO is not significant, yielding 35.8 and 53.4, respectively (Figures 7, 8A). This similarity is also observed for Pt_N/Pd_N (Figure 8B). The principal difference between arc and MORB is related to Ru. For example, ratios of Pt_N/Ru_N (Figure 8A) or Ir_N/Ru_N (Figure 8B) are much greater in arc rocks.

The differences in individual element contents are more prominent than their ratios (Figures 8C,D). For instance, the average Pd content in MORB samples containing MgO from 8.5 to 14 wt.% is 0.4 ppb, which is ∼15 times less than the corresponding arc values (6.4 ppb). Such a drastic difference evokes the idea that fractionation processes may in fact produce low Pd concentrations in MORB. However, Hao et al. (2021) calculated primitive MORB Pd content to be 1.5 ± 0.5 ppb, which is significantly lower than our estimates of Pd content in the primitive Tolbachik melt (∼6.5 ppb). This implies that unfractionated primitive arc melts are enriched in Pd relative to MORB.

The differences between the primitive or near-primitive arc and MORB melts are summarized as the following:

1. Palladium contents are significantly higher in primitive arc magmas than those reported for parental MORB melts.

Factors influencing PGE contents in primitive arc and MORB melts are summarized in Figure 9. Each process, from partial melting of a MORB-type mantle (depletion) to subsequent alteration in subduction zones (accompanied by potential refertilization), and melt-mantle reactions, precede fractionation processes en-route to, and following emplacement at, the Earth’s surface or crust.