The petrology and geochemistry of Nyiragongo lavas of 2002, 2016, 1977 and 2017 AD, and the trace element partitioning between melilitite glass and melilite, nepheline, leucite, clinopyroxene, apatite, olivine and Fe-Ti oxides: a unique scenario

Abstract

The Nyiragongo lava of 2002, and the new one flowing from the spatter cone grown up in 2016 close to the lava lake, are melilitites (melilite-nephelinites). They have low MgO (4 wt%), are chemically almost indistinguishable from the lavas of 1977 and from that erupted in 2017, are multiply saturated in melilite, nepheline, leucite, clinopyroxene, olivine, apatite and magnetite, and are rich in fresh glass having significant concentration of SO3, F and Cl. Glasses and bulk-rocks plot close to one atmosphere cotectics in pertinent phase diagrams. The LA-ICP-MS analyses of the observed phases and mineral/glass partition coefficients indicate that REE and Y are moderately incompatible in melilite (e.g., DLa = 0.46–0.48, DEu = 0.56–0.57; DLu = 0.18–0.25; DY = 0.25–0.27), whereas Sr is compatible (DSr = 1.6–1.8); clinopyroxene/melt partition coefficients are similar to melilite for LREE (e.g., DLa = 0.36), and significantly higher for HREE (DLu = 0.74), Y (DY = 0.7), Zr (≈1) and Hf (DHf = 1.6), while lower for Sr (DSr = 0.38). Apatites have high concentration ratios for REE, particularly MREE (DLa = 4.6–6.4, DNd = 5.8–7.7; DEu = 4.9–6.9; DLu = 1.3–1.6; DY = 3.1–4.4), and high DSr (1.8–2.2). Nepheline and leucite are highly and differently selective for Rb, Cs, Ba and Sr; both phases show negligible concentration of REE (hence Eu, +Y), Th, U and first row transition elements. Olivine and magnetite have the expected high concentration of transition elements, with small preference of olivine for divalent cations and magnetite for trivalent cations. The distribution of trace elements in melilite compared with clinopyroxene, nepheline -or even feldspar- clearly indicate that melilite is not an equivalent of any of these phases, and that melilite removal cannot be considered the main cause of Eu troughs or of decreasing fractionation between light and heavy REE in more evolved melts, being these elements still significantly incompatible. At the same time, removal of the observed phenocrysts at this stage of crystallization cannot be the cause of anomalous fractionation of elements with similar geochemical behaviour found in the Nyiragongo lavas. These new data indicate that the magmatic system of the Nyiragongo is in a roughly steady state at least since the eruption of 1977 to the activity of 2017, with periodic eruptions, withdrawal or feeding from the uppermost magma reservoir by broadly the same “cotectic” magma compositions, from which phases of the same composition nucleated in a very low-pressure regime.