First data on the distribution of platinum group elements (Ir, Os, Ru, Pt, and Pd) and Re in island-arc basalts of Kamchatka

Abstract

Researchers became interested in the distribution ofplatinum group elements (PGE) in mantle magmas fortwo reasons: (i) some mantle-derived magmas bearhigh PGE concentrations and, therefore, can be ore-bearing [2]; (ii) the PGE distribution pattern can yieldadditional information concerning processes of magmaformation [5, 9, 11, 15]. Petrogenetic information ofsuch a kind on basaltic rocks from various geodynamicsettings is limited to a small number of published PGEanalyses, because it is rather difficult to determine<1-ppm concentrations. We developed and tested amethod satisfying the constraints mentioned above forthe determination of PGE and Re [3]. This methodmade it possible to obtain novel data on the distributionof the entire PGE spectrum (except Rh) and Re in rep-resentative basalt samples from the Kamchatka island-arc system (Fig. 1).At the first stage of investigation of PGE distributionin basaltic rocks of Kamchatka, we analyzed six sam-ples of Pliocene–Quaternary basalts of the low- andmoderate-K types. As is known, the recent Pliocene–Quaternary development stage of the Kamchatkaisland-arc system in the course of the Pacific Plate sub-duction was marked by the formation and inheriteddevelopment of three major (South Kamchatka, EastKamchatka, and Sredinny Ridge) volcanic belts (Fig. 1).The South Kamchatka and East Kamchatka volcanicbelts are located at the front of the island-arc system;the Sredinny Ridge, in the rear zone. The East Kam-chatka volcanic belt is characterized by eruptions of thelow- and moderate-K island-arc basalt (IAB) magmas[4]. The South Kamchatka and Sredinny Ridge volca-nic belts are dominated by the moderate- and high-Krocks of the IAB type [14]. In the PM-normalized dia-gram, data points of basalts from the Sredinny Ridgeand East Kamchatka volcanic belts show the IAB-typedistribution patterns: relative depletion in HFSE (Nb,Ta, and Ti) and enrichment in Rb, Ba, K, Sr, and Pb(Fig. 2). At the same time, the low-K rocks are signifi-cantly depleted in most trace elements (Table 1).The chondrite-normalized patterns of PGE and Rein basalts of Kamchatka are shown in Fig. 3a; their con-centrations, in Table 2. Despite a sufficiently wide scat-ter of values, they reflect the relative enrichment ofbasalts in Pt, Pd, and Re relative to Os, Ir, and Ru. Sucha distribution of PGE and Re has also been recorded forthe island-arc basaltic rocks in other regions [11, 15](Fig. 3a). The distribution patterns revealed in our workexhibit the following essential feature: relative to themoderate-K basalts, the low-K basalts are commonlycharacterized by the highest concentrations of Pt, Pd,and Re, whereas the concentrations of Os, Ir, and Ru inboth varieties are similar.PGEs are divided into the iridium (Ir, Os, and Ru)and platinum (Pt, Pd, and Rh) subgroups with meltingabove and below 2000°C, respectively. As was men-tioned in [15], elements of these two groups are con-centrated in different mineral phases in mantle rocks.Elements of the iridium subgroup make up native min-