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

Abstract

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/Yb_CN = 1759–7079; gregoryite La/Yb_CN = 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.

Introduction

Oldoinyo Lengai, the world's only active carbonatite volcano, lies in the Gregory Rift Valley in northern Tanzania, Eastern Africa (2.764° S, 35.914° E). Various aspects of the geology, petrology, and mineralogy of the natrocarbonatite lavas have been described by Church and Jones (1995), Dawson et al., 1995, Dawson et al., 1996, Keller and Krafft (1990), Mitchell, 1997, Mitchell, 2006, Mitchell and Belton (2004), and Peterson (1990). The lavas exhibit a wide variety of textures reflecting the conditions of eruption and crystal–liquid fractionation during sub-aerial flow (Keller and Krafft, 1990). These can range from phenocryst-dominated vesicular spatter to aphyric cryptocrystalline filter-pressed residua containing skeletal nyerereite. The natrocarbonatites are composed dominantly of gregoryite [(Na,K,Ca)₂CO₃] and nyerereite [(Na,K)₂Ca(CO₃)₂] phenocrysts set in a matrix of a Na-rich gregoryite-like phase, barian nyerereite, sodian sylvite , fluorite, potassian neighborite, and a Ba–Sr–Mg–Na–Cl–F-rich carbonate formerly known as phase X (Dawson et al., 1995), but now considered to be khanneshite (Mitchell, 2006, Zaitsev et al., 2008).

As of September 2007, the style of activity changed from the quiet effusive eruption of natrocarbonatite lavas to violent pyroclastic eruptions composed initially of hybrid peralkaline nepheline–melilite–combeite–(Na–Ca–phosphate–carbonate) ash and lapilli (Mitchell and Dawson, 2007) and ultimately to ash consisting of diverse combeite wollastonite nephelinites (Keller et al., 2010, Kervyn et al., 2010, Mattsson and Reusser, 2010, Mitchell and Dawson, 2011). Subsequent to the last major pyroclastic silicate eruption in 1967 the volcano remained dormant until 1983. Since July 2008 the volcano has exhibited intermittent minor activity confined to small natrocarbonatite flows at the bottom of an inaccessible deep pit crater. As a consequence of this change in volcanic style, samples of fresh natrocarbonatite lava can no longer be obtained. Hence, conservation and investigations of existing material becomes imperative.

Although the major element compositional variation of the minerals forming natrocarbonatite is relatively well known (Keller and Krafft, 1990, Mitchell and Belton, 2004, Peterson, 1990), there are few data on their minor and trace element contents (Zaitsev et al., 2009). These data are important in understanding the differentiation of carbonate melts, as trace elements which are typically incompatible with respect to phenocrysts in common silicate magmas, are compatible in the phenocrysts of natrocarbonatite. These magmas also are unusual in that differentiation leads to the concentration of Mg in residual melts, resulting in the formation of potassian neighborite and a Na–Mg-carbonate, possibly an eitelite-like phase (Gittins and Jago, 1998, Mitchell and Kjarsgaard, 2011).

An initial investigation by Keller and Spettel (1995) of bulk single samples of each of nyerereite and gregoryite demonstrated that the former is richer in rare earth elements (REE) than the latter. Bulk analysis of gregoryite is not desirable as this mineral typically contains inclusions of nyerereite, ferroan alabandite, and quenched groundmass material. Recently, Mitchell and Kamenetsky (2008) and Zaitsev et al. (2009) using laser ablation ICP-MS have both demonstrated significant differences in the contents of Li, P, V, Mn, Rb, Sr, Y, Cs, Ba and REE and other trace elements within and between nyerereite and gregoryite phenocrysts. Zaitsev et al. (2009) provided only 12 compositions for REE from one sample collected in 2000. In this work we significantly extend these initial data by presenting trace element data obtained on phenocrysts of nyerereite and gregoryite occurring in several natrocarbonatite samples collected between 1996 and 2005. Major element data are also given for Ba-rich nyerereite and khanneshite occurring in the groundmass of the natrocarbonatites that erupted in July 2000.