Trace element geochemistry of nyerereite and gregoryite phenocrysts from natrocarbonatite lava, Oldoinyo Lengai, Tanzania: Implications for magma mixing

The abundances of Li, P, Cl, V, Mn, Rb, Sr, Y, Cs, Ba, Pb, Th, U and REE, within and between, phenocrysts of nyerereite and gregoryite occurring in natrocarbonatite lavas erupted from the active volcano Oldoinyo Lengai (Tanzania) have been determined by electron microprobe, LA-ICP-MS and SIMS. These data show that, in general, nyerereite is enriched in Rb (71–137 ppm), Sr (14,485–23,240 ppm), Y (2.0–8.9 ppm), Cs (1.6–5.3 ppm), Ba (4000–11,510 ppm), but poorer in Li (21–91 ppm), P (820–1900 ppm) and V (5.1–47 ppm) relative to gregoryite (Rb = 43–106; Sr = 4255–7275; Y = 0.3–4.0; Cs = 0.6–5.1; Ba = 1125–7052; Li 84–489; P = 6790–15,860; V = 33–155 ppm). Nyerereite is highly enriched in REE (La = 236–973; Ce = 395–1044 ppm) relative to gregoryite (La = 59–309; Ce = 59–301 ppm). Chondrite normalized REE distribution patterns for nyerereite and gregoryite are parallel and linear with no Eu anomalies. They show extreme enrichment in light REE and depletion in heavy REE (nyerereite La/YbCN = 1759–7079; gregoryite La/YbCN = 1051–10,247). Significant differences exist in the abundances of trace elements within and between coexisting crystals occurring in diverse natrocarbonatite flows, although there do not appear to be any significant secular variations in phenocryst compositions in lavas erupted from a given vent. It is concluded that both major, minor and trace element compositional data for nyerereite and gregoryite phenocrysts occurring in natrocarbonatite lavas are derived by the crystallization of several different batches of magma in a continuously replenished fractionating magma chamber. Natrocarbonatite lavas are considered to be hybrids formed by the mixing of both crystals and melts formed from several batches of natrocarbonatite magma; thus bulk rock compositions cannot represent the compositions of the primary magma composition before the onset of fractionation. Differentiation of natrocarbonatite melts leads to enrichment of residua in Ba and Mg.