Vapour transport of rare earth elements (REE) in volcanic gas: Evidence from encrustations at Oldoinyo Lengai

Journal of Volcanology and Geothermal Research 176 (2008) 519–528

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Vapour transport of rare earth elements (REE) in volcanic gas: Evidence from encrustations at Oldoinyo Lengai C.D. Gilbert ⁎, A.E. Williams-Jones Department of Earth and Planetary Sciences, McGill University, 3450 University, Montréal, Quebec, Canada H3A 2A7

A R T I C L E

I N F O

Article history: Received 26 December 2007 Accepted 3 May 2008 Available online 20 May 2008 Keywords: Oldoinyo Lengai REE vapour-transport encrustation

A B S T R A C T Fumarolic encrustations and natrocarbonatite lava from the active crater of Oldoinyo Lengai volcano, Tanzania, were sampled and analysed. Two types of encrustation were distinguished on the basis of their REE content, enriched (~ 2800–5600 × [REEchondrite]) and depleted (~ 100–200 × [REEchondrite]) relative to natrocarbonatite (1700–1900 × [REEchondrite]. REE-enriched encrustations line the walls of actively degassing fumaroles, whereas REE-depleted encrustations occur mainly along cracks in and as crusts on cooling natrocarbonatite lava flows; one of the low REE encrustation samples was a stalactite from the wall of a possible fumarole. The encrustations are interpreted to have different origins, the former precipitating from volcanic gas and the latter from meteoric/ground water converted to steam by the heat of the overlying lava flow(s). REE-profiles of encrustations and natrocarbonatite are parallel, suggesting that there was no preferential mobilization of specific REE by either volcanic vapour or meteoric water vapour. The elevated REE-content of the first group of encrustations suggests that direct REE-transport from natrocarbonatite to volcanic vapour is possible. The REE trends observed in samples precipitating directly from the volcanic vapour cannot be explained by dry volatility based on the available data as there is no evidence in the encrustation compositions of the greatly enhanced volatility predicted for Yb and Eu. The observed extreme REE-fractionation with steep La/Sm slopes parallel to those of the natrocarbonatite reflects solvation and complexation reactions in the vapour phase that did not discriminate amongst the different REE or similar transport of REE in both the natrocarbonatite magma and its exsolving vapour. The low concentrations of REE in the encrustations produced by meteoric vapour suggest that the temperature was too low or that this vapour did not contain the ligands necessary to permit significant mobilization of the REE. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Until fairly recently, the vapour phase has been largely dismissed as a possible agent of metal transport in ore-forming hydrothermal systems on the basis of low metal dry-volatilities. However, fluid inclusion analyses by Heinrich et al. (1999) have documented metal concentrations in the vapour phase approaching and in some cases exceeding (Cu, Au) those required for an ore fluid. Moreover, experiments summarized in Williams-Jones et al. (2002) have helped reconcile observation and theory, by revealing that solvation, and complexation reactions involving ligands like chlorine can lead to concentrations of metals in water vapour orders of magnitude higher than predicted by dry volatility data. These findings, and the observation that the mass of vapour in many magmatic hydrothermal systems can exceed that of the liquid, provide considerable support for the idea that some magmatic

⁎ Corresponding author. Tel.: +1 450 424 6879. E-mail addresses: [email protected], [email protected] (C.D. Gilbert), [email protected] (A.E. Williams-Jones). 0377-0273/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2008.05.003

hydrothermal deposits may have formed as a result of the transport of the metals by the vapour (Williams-Jones and Heinrich, 2005). In contrast to most base and precious metals, the REE were long considered to be immobile in hydrothermal systems and were thought instead to concentrate primarily by magmatic processes, e.g., by crystallization from carbonatite magmas (see review in Mitchell (2005)). Although the formation of many rare earth element (REE)-deposits has since been attributed to hydrothermal processes (Chao et al., 1992; Williams-Jones and Wood, 1992; Williams-Jones et al., 2000; Gultekin et al., 2003; Samson and Wood, 2005; Salvi and Williams-Jones, 2005), the possibility of a role for the vapour phase in the formation of these deposits has not been considered. Nevertheless, several studies have reported significant concentrations of REE in the vapour phase (e.g., Michard, 1989; Moller et al., 2003), and relatively high vapour-liquid partition coefficients for the REE have been determined for the Lardarello–Travale geothermal field, Italy (Moller et al., 2003). Interestingly, the highest partition coefficients (0.3 to 0.4) are for waters sourced by limestones, suggesting a role in the vapour for complexation involving CO2. In view of this and of the close genetic association of hydrothermal REE deposits with carbonatites (Williams-Jones and Wood, 1992; Samson and Wood,

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2005), it is also interesting to note that in partitioning experiments involving carbonate melt and CO2 fluid, the REE prefer the latter by at least an order of magnitude (Wendlandt and Harrison, 1979). To shed further light on the capacity of H2O–CO2 vapours to transport the REE, we report here the results of a study of the recent encrustations forming around fumaroles and on natrocarbonatite lavas being erupted by Oldoinyo Lengai, an active carbonatite volcano in the African Rift Valley of Northern Tanzania. 2. Regional setting 2.1. Geology of Oldoinyo Lengai Rising steeply to an altitude of 2878 m, Oldoinyo Lengai is one of several alkaline volcanoes situated in the Gregory Rift Valley of Tanzania (2°75′ S, 35°90′ E; Fig. 1) and the only volcano on Earth currently erupting carbonatite (Dawson, 1962b). The volcanic edifice, which is capped by two craters, is composed dominantly (N90%) of phonolitic and nephelinitic tuffs (Dawson, 1989). Natrocarbonatite lavas began erupting in the early twentieth century (from the northern crater) and have scoured the flanks of the volcano, creating deep erosion gullies. Earlier silicate eruptions were restricted to the extinct southern crater, which is separated from the active northern crater by an E–W trending ridge that forms the summit of the volcano. Chimney-like structures referred to as hornitos, positioned on top of a broad dome, provide the locus of current volcanic activity, and, where breached reveal the presence of one or more lava lakes immediately below the surface (Pyle et al., 1995). Volcanic activity in the Gregory Rift started immediately after a major episode of normal faulting at 1.2 Ma BP, with pyroclastic eruptions of nephelinite building volcanic edifices at several centres, notably at Kerimasi, followed by extrusion of carbonatitic lavas (Dawson, 1962a). Construction of the volcanic edifice at Oldoinyo Lengai, occurred later, at 0.37 Ma, after the cessation of volcanic activity at Kerimasi, which is located a few km to the south of it (MacIntyre et al., 1974). The natrocarbonatites, which represent the current phase of volcanic activity at Oldoinyo Lengai, are characterized by extreme alkalinity (up to 33% wt.% Na2O and 5 wt.% K2O) and enrichment in F, Cl, incompatible elements, volatiles and rare earth elements (REE) (Keller and Spettel, 1995). These lavas have the lowest viscosity of any lava ever recorded (1–5 Pas) (Dawson et al., 1990), erupt at extremely-low temperature (b550 °C) (Keller and Krafft, 1990) and crystallize mainly to gregoryite [(Na,K,Ca)2CO3] and nyerereite [(Na,K)2Ca(CO3)2].

2.2. Previous studies of encrustations The first report of encrustations was by Keller and Krafft (1990), who observed that within a day or two of their extrusion, the lava flows ‘became covered with white and in part greenish and yellow incrustations [sic]’. Sulphur, nahcolite (NaHCO3), sylvite and trona were identified in these encrustations. Genge et al. (2001) subsequently described thermonatrite (Na2CO3·H2O), aphthitalite (K,Na)3Na (SO4)2, halite and sylvite occurring as ‘fringes and tubes’ lining cracks in lava flows. Thermonatrite comprised ~70 vol.% of the salt fringes and commonly contained inclusions of halite and sylvite. The latter minerals filled voids and occurred on the margins of the salt fringes and as overgrowths on aphthitalite needles (Genge et al., 2001). A similar description of these encrustations was provided more recently by Zaitsev and Keller (2006) who, in addition to the minerals mentioned above, also identified, kalicinite (KHCO3) and villiaumite (NaF). The general consensus is that these encrustations represent precipitates (sublimates) from vapours produced by the extrusion of lavas over a meteoric-water saturated substrate and/or alteration of the lavas by these vapours (Genge et al., 2001; Zaitsev and Keller, 2006). Encrustations composed of aggregates of sodian sylvite, potassian halite, trona, thermonatrite and a newly-discovered F-bearing sodium phosphate-carbonate (Na5–4.5PO4(CO3,F,Cl)), which is unstable under normal atmospheric conditions, were described by Mitchell (2006b). These encrustations were observed on natrocarbonatite lapilli ejected in July, 2000, and were shown by him to have developed by replacing phenocrystic and groundmass gregoryite. They are interpreted to have formed at low temperature (b50 °C) and to have been the products of reactions with volcanic gases, which added H2O, Cl, and F− to the lapilli. Zaitsev and Keller (2006) studied encrustations around three fumaroles near hornito T46 and identified native sulphur crystals 1–3 mm in diameter1. Based on X-ray diffraction analyses of these encrustations, they also identified calcite, gypsum, anhydrite, monohydrocalcite (CaCO3·H2O), barite, fluorite and celestine. The calcite, gypsum and anhydrite were intergrown in the encrustations and were interpreted to have formed as a result of reaction of the natrocarbonatite with the volcanic gases, whereas some of the other minerals, e.g., native sulphur, were clearly sublimates, i.e., precipitates from the volcanic gases. In June 2003, hornito T45 was breached to the northwest, producing an opening measuring 1 m2 (McFarlane et al., 2004). Stalactites inside the resulting cave reached 3 m in length and were shown to be composed mainly of trona with accessory apthitalite, kogarkoite (Na3SO4F), schairerite (Na21(SO4)7F6Cl), sylvite and halite (Mitchell, 2006a). The stalactites were interpreted to have formed as a result of the evaporation of brines dripping from the roofs of the hornitos which were produced by interaction of meteoric waters with the natrocarbonatite lavas (Mitchell, 2006a). 3. Materials and methods 3.1. Sampling Lavas and encrustations were sampled on February 25th–26th, 2006 in the active northern crater of Oldoinyo Lengai. During the visit, the odor of H2S was omnipresent. Summit hornitos T37B, T45, T46, and T51 could be seen intermittently degassing. The temperature of these gases, measured with a K-type Chromega®–Alomega® thermocouple supplied by Omega Engineering Inc., ranged between 402 °C and 419 °C. Hornito T56B was spattering and loud bangs could be heard from within the hornito (Fig. 1). Lava had likely been ejected from it a few hours prior to our arrival, as the adjacent flows were still

Fig. 1. Active crater of Oldoinyo Lengai volcano. Photograph of the location of the hornitos sampled.

1 The numbering system of hornitos used in the present study is that of the Smithsonian Global Volcanism program.

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together with their locations. All samples were wrapped in plastic and placed in sealed containers with NaOH desiccant to prevent alteration.

Table 1 Sampling locations/dates/descriptions Sample

Location

Date

Description

A1

25/02/06

Fresh pahoehoe, coarsely granular

25/02/06

Weathered flow assumed to be a few days old

25/02/06

Surficially altered flow; displays a fresh core

A4

Between T58C and T37B Between T58C and T37B Between T58C and T37B T58C

25/02/06

A5 & A7

T58C

26/02/06

A6

T56B

25/02/06

A8

S2

Between T56B 26/02/06 and T58C Depression in 26/02/06 crater floor T37B 26/02/06

Fresh lava, altered post-collection (not properly sealed) Lava from the flow on February 25th, collected when hot Lava collected from the lake inside the breached hornito Severely altered lava flow

S3

T56

26/02/06

S4

T37B

26/02/06

S5

T56B

26/02/06

S6 S7

T49B T56B

26/02/06 26/02/06

A2 A3

S1

521

3.2. Major and trace-element analyses Lava samples A1, A2, A3, A4, A5, A6, A7 and A8 as well as encrustations S1, S2, S3, S4, S5, S6 and S7 were analysed by Activation Laboratories in Ancaster, Ontario, for major and trace elements with Fusion ICP-MS. The instruments used for these analyses (Jarrell-Ash Enviro II for major element oxides and Perkin Elmer Sciex Elan 6000 for trace elements and REE) employed argon plasma as the ionization source and a quadruple mass spectrometer to detect the ions produced. The detection limit for major elements was 0.01 wt.% and for minor elements was 0.001 wt.%. Trace-element detection limits were variable and ranged from 0.05–0.1 ppm (REE) to 20–30 ppm (Cr, Ni, Zn). Lava samples A1–A8 were also analysed for Cl by Instrumental Neutron Activation Analysis (INAA), for F by Fusion Specific Ion Electrode-ISE, for FeO by Titration, for CO2 by coulometry and for S by IR. The detection limit for these methods is 0.01 wt.%. Owing to the small sample size, the encrustation samples could not be analysed for LOI and volatile species.

Stalactite-like sample forming from drips of fluid in the crater floor White-green encrustation lining an active vent adjacent to S4 White-grey encrustation on flanks of a breached hornito Dark encrustation superimposed on native sulfur lining a fumarole White and green phases co-precipitated as an encrustation White encrustation along crack in cooling lava Encrustation similar to S6 but separated on-site from fresh lava substrate

3.3. Mineralogical analyses Polished thin-sections were prepared at Vancouver Petrographics (Langley, BC) using methods that avoided exposure of the samples to water. Samples were stored with a NaOH desiccant in sealed containers to avoid possible alteration by atmospheric water vapour. The encrustation samples were extremely fragile and were thus embedded in epoxy resin prior to polishing. Thin-sections of samples S2 and S4 were carbon-coated and analysed quantitatively for REEphases using a JEOL 8900 instrument at the McGill Electron Microprobe Laboratory. The analyses reported are minimum estimates of REE contents due to the low resolution/beam size ratio employed to minimize volatilization. It was not possible to make X-ray maps of the samples due to the required long exposure to the electron beam and the strong likelihood that they would disintegrate during such exposure. X-ray diffractograms were prepared at the ‘laboratoire de radiocrystallographie’ at Université du Québec a Montréal, (Montreal, QC) and estimates were made of the modal mineralogy based on these diffractograms employing Jade software provided by Materials Data Inc. The instrument used is a Siemens D-5000 equipped with a Co tube, a Kevex-Si detector and continuous scanning capability. Encrustation samples were coated with Au-Pd or C (sample S4 only) and analysed using a Hitachi S-4700 Field Emission Gun Scanning Electron

black and hot. The hornito was open to the northwest and a bubbling lava lake was visible about 5 m below its base. A sample of this lava (A6) was collected by lowering a metal spoon into the hornito. At approximately 17:00 the base of hornito T58C (Fig. 1) broke open and lava flowed out rapidly to the southwest down the gentle slope of the dome for about five minutes, forming a pahoehoe textured sheet measuring about 10 m in width and 120 m in length On the night of February 25th, there was an intense rain storm, which produced extensive weathering of the fresh flows from the day before. Fresh samples (A1, A3, A5, A6, and A7) were collected on February 25th and weathered samples (A2, A4, A8) on February 26th, 2006. A variety of encrustation samples were also collected. Samples S2 and S4 were taken from the encrustations lining the walls of an active fumarole on the flank of hornito T37B (Fig. 1). The other encrustation samples (S1, S3, S5, S6 and S7) were found around inactive fumaroles, in depressions in the crater floor or between cracks in cooling natrocarbonatite lava. Table 1 presents a list of the samples collected,

Table 2 Major element compositions of natrocarbonatite (A1–A8) and encrustations (S1–S7) wt.%

dl1

FeO SiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O TiO2 P2O5 Cl F LOI Total

0.01 0.01 0.01 0.01 0.001 0.01 0.01 0.01 0.01 0.001 0.01 0.01 0.01

A1

A2

A3

A4

A5

A6

A7

0.44 0.47 0.1 0.03 0.43 0.29 18.4 36.2 8.00 0.002 1.28 2.59 3.58 33.6 99.3

0.26 0.42 0.13 0.11 0.33 0.21 19.0 32.9 6.62 0.005 1.43 0.82 2.20 38.3 99.7

0.43 1.23 0.43 0.01 0.35 0.21 16.4 35.0 7.11 0.008 1.42 1.96 2.48 37.0 99.5

0.09 0.31 0.03 0.32 0.30 0.19 18.9 36.9 7.70 0.001 1.38 1.83 2.06 32.9 99.1

0.18 0.28 0.05 0.17 0.33 0.21 18.4 36.2 7.61 0.002 1.34 2.05 2.73 34.1 98.9

0.53 0.29 0.04 b 0.01 0.41 0.25 18.9 34.1 7.98 0.002 1.26 2.45 3.26 34.6 98.2

0.36 0.27 0.03 b 0.01 0.33 0.22 19.3 34.3 7.41 0.002 1.35 2.34 2.67 35.6 99.2

A8

S1

S2

S3

S4

S5

S6

S7

– 0.20 0.02 0.43 0.33 0.17 21.0 32.5 2.42 b0.001 1.38 – – – 58.5

– 0.32 0.10 0.1 0.02 0.03 0.59 22.0 34.5 0.005 0.84 – – – 58.4

– 0.6 0.16 0.61 0.63 0.38 22.2 25.3 2.86 0.002 1.20 – – – 54.0

– 0.23 0.07 0.07 0.04 0.14 0.95 42.6 0.17 0.005 0.04 – – – 44.3

– 0.76 0.14 0.74 0.04 0.07 12.2 0.73 0.13 0.018 0.18 – – – 15.0

– 0.97 0.35 0.09 0.05 0.04 2.26 44.2 8.03 0.005 0.91 – – – 56.9

– 1.15 0.42 0.28 0.06 0.05 1.66 43.8 7.18 0.005 1.61 – – – 56.2

– 0.35 0.11 0.05 0.03 0.02 1.00 44.5 7.42 0.002 1.60 – – – 55.0

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Table 3a Trace element compositions of natrocarbonatite (A1–A8) and encrustations (S1–S7) ppm

dla

A1

A2

A3

A4

A5

A6

A7

A8

S1

S2

S3

S4

S5

S6

S7

Sc Be V Ba Sr Y Zr Cr Co Ni Cu Zn Ga Ge As Rb Nb Mo Ag In Sn Sb Cs Hf Ta W Tl Pb Bi Th U

1 1 5 3 2 2 4 20 1 20 10 30 1 1 5 2 1 2 0.5 0.2 1 0.5 0.5 0.2 0.1 1 0.1 5 0.4 0.1 0.1

b dl 9 130 9360 11,700 8 b dl b dl b dl b dl b dl 70 2 b dl 14 138 28 44 b dl b dl b dl 1.5 3.5 b dl 0.1 20 0.2 56 0.7 3.1 7.3

bdl 6 86 7420 11,900 7 bdl bdl bdl bdl bdl 70 2 bdl 12 103 29 18 bdl bdl bdl 1.5 2.7 bdl 0.1 8 0.5 69 1 3.2 7.4

bdl 7 106 7460 11,100 7 5 bdl bdl bdl bdl 70 2 bdl 13 130 29 30 bdl bdl bdl 1.2 3.2 bdl bdl 14 0.4 75 0.9 3.3 7.8

b dl 6 89 8360 11,600 6 b dl b dl b dl b dl b dl 70 2 b dl 14 138 30 46 b dl b dl b dl 0.8 3.7 b dl 0.1 22 0.3 70 0.7 3.3 7.8

b dl 7 108 8510 11,600 6 b dl b dl b dl b dl b dl 60 2 b dl 14 135 28 42 b dl b dl b dl 1.6 3.5 b dl 0.1 19 0.3 70 0.9 3.1 7.3

bdl 9 124 9300 11,900 7 bdl bdl bdl bdl bdl 80 3 bdl 15 151 32 56 bdl bdl bdl 1.1 4 bdl 0.1 26 0.3 76 1 3.5 8.4

bdl 7 109 8400 11,500 6 bdl bdl bdl bdl bdl 70 2 bdl 17 145 31 51 bdl bdl bdl 2.5 3.8 bdl 0.1 28 0.3 81 1 3.4 8

b dl 6 59 7090 12,400 7 b dl b dl b dl 80 b dl 100 2 b dl b dl 41 31 10 b dl b dl b dl b dl 0.8 b dl b dl 7 0.8 76 b dl 3.7 4.5

bdl bdl 92 428 409 bdl bdl bdl bdl bdl bdl bdl bdl bdl 113 408 5 N100 bdl bdl bdl bdl 7.2 bdl bdl 57 bdl bdl bdl 0.2 5.1

b dl 14 120 13,000 14,700 8 b dl b dl b dl b dl b dl 110 4 b dl 12 53 52 12 b dl b dl b dl b dl 1.9 b dl 0.2 14 0.4 72 0.6 4.5 10.6

bdl bdl 19 956 789 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 4 8 2 bdl bdl bdl 0.6 bdl bdl bdl 1 0.9 11 bdl 0.5 0.8

b dl 4 119 31,100 11,400 18 5 60 b dl b dl b dl 130 6 b dl 15 4 56 28 b dl b dl b dl b dl b dl 0.3 0.3 15 1.7 73 b dl 9.5 23.5

b dl 2 61 862 1260 b dl 4 b dl b dl b dl b dl b dl b dl b dl 6 95 14 9 b dl b dl b dl b dl 0.7 b dl b dl 6 b dl 7 b dl 0.7 2.9

b dl 1 69 781 1020 b dl b dl b dl b dl b dl 20 b dl b dl b dl 12 82 5 19 b dl b dl b dl b dl 0.8 b dl b dl 10 b dl b dl b dl 0.4 3.1

bdl bdl 151 513 680 bdl bdl bdl bdl bdl bdl bdl bdl bdl 16 125 5 49 bdl bdl bdl bdl 2.7 bdl bdl 23 bdl bdl bdl 0.3 6.4

a

Detection limit.

tions exceeding the REE-contents of natrocarbonatite (La~ 2800–5500 × [chondrite]) and samples S1, S3, S5 S6 and S7, which have substantially lower REE-contents (La~ 90–200 × [chondrite]) (Fig. 3). Fresh natrocarbonatite samples have a compositional range for La of [1652– 1936]× chondrite, are tightly grouped (as can be seen in Fig. 2) and lie between the two groups of encrustation samples for all REE. As is evident from Fig. 4, in which the REE contents of the encrustations are normalized to those of our freshest natrocarbonatite lava sample (A6), samples S2 and S4 are 50% and three times more enriched in REE, respectively, than sample A6 (the concentration of REE in encrustation S2 may be actually higher than reported due unavoidable contamination by the natrocarbonatite substrate). By contrast, samples from Group 2 have less than 10% of the REE content of the natrocarbonatite. In addition to their higher REE-content, group 1 samples have higher FeO and CaO contents and a lower Na2O content than group 2 samples. Sample S4 has

Microscope (FEGSEM) equipped with an EDS detector and Oxford INCA processing software for EDS-spectral analysis. The accelerating voltage was 15 kV and the beam current 20 μA. 4. Results 4.1. Bulk rock composition Tables 2, 3a and 3b report the major, trace and rare earth element compositions, respectively, of the natrocarbonatite and encrustation samples. The compositions of the natrocarbonatite samples are all quite similar, and chondrite-normalized REE-profiles for both fresh and weathered natrocarbonatite samples are indistinguishable (Fig. 2). Encrustation samples, however, form two distinct groups based on their REE contents: samples S2 and S4 (group 1) with REE-concentra-

Table 3b Rare Earth Element (REE) compositions of natrocarbonatite (A1–A8) and encrustations (S1–S7) ppm

dla

A1

A2

A3

A4

A5

A6

A7

A8

S1

S2

S3

S4

S5

S6

S7

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

0.1 0.1 0.05 0.1 0.1 0.05 0.1 0.1 0.1 0.1 0.1 0.05 0.1 0.04

401 479 35.7 88.5 6.6 1.19 1.6 0.2 0.8 0.1 0.4 bdl 0.2 bdl

405 488 35.5 89.9 6.8 1.31 1.9 0.2 0.8 0.1 0.4 b dl 0.2 b dl

404 493 35.5 87.3 6.8 1.27 1.4 0.2 0.8 0.1 0.4 b dl 0.2 b dl

435 515 37.2 92.9 6.8 1.25 1.6 0.2 0.9 0.1 0.4 bdl 0.2 bdl

408 497 36.7 89.6 6.8 1.24 1.4 0.2 0.9 0.1 0.4 b dl 0.2 b dl

454 539 38.8 96.9 7.1 1.34 1.5 0.2 0.9 0.1 0.4 bdl 0.2 bdl

444 529 38.8 94.2 6.9 1.37 1.6 0.2 0.9 0.1 0.5 bdl 0.2 bdl

427 517 41.2 101 7.2 1.41 2.8 0.2 0.9 0.1 0.5 b dl 0.2 b dl

21.1 23.1 1.6 3.7 0.3 bdl bdl bdl bdl bdl bdl bdl bdl bdl

668 747 57.6 139 9.5 1.86 3.5 0.3 1.2 0.2 0.6 bdl 0.3 bdl

45.4 58.5 3.41 7.8 0.6 0.11 bdl bdl bdl bdl bdl bdl bdl bdl

1320 1350 96.3 218 15.2 2.96 4.7 0.5 2.2 0.4 1 b dl 0.5 0.05

48.3 58 4.33 10.5 0.8 0.16 0.4 b dl 0.2 b dl b dl b dl b dl b dl

40.2 49.1 3.43 8.4 0.6 0.14 0.2 bdl 0.1 bdl bdl bdl bdl bdl

27.6 32.8 2.35 5.7 0.5 0.09 0.1 b dl b dl b dl b dl b dl b dl b dl

a

Detection limit.

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Fig. 4. REE compositions of selected encrustations normalized to those of fresh natrocarbonatite. It is evident that S2 and S4 are up to 1.5 times and 3 times more REEenriched than the natrocarbonatite, respectively. Sample S7 represents group 2 encrustations and is REE-depleted.

Fig. 2. Chondrite-normalised REE-profiles for fresh (A1,A3, A5, A6, A7) and weathered (A2, A4, A8) natrocarbonatite. The observation that the profiles are indistinguishable indicates that there was minimal mobility of the REE during weathering. The chondrite values are from McDonough and Sun (1995).

an exceptionally low Na2O content (0.73 wt.%). As is the case for REE, the two encrustation groups have contrasting distributions of the other trace elements (on a spider-diagram); where sample S4 (group 1) shows peaks in concentration, sample S7 (group 2) has low values and viceversa and natrocarbonatite generally lies between the two (Fig. 5). For example, S4 is characterized by high Ba and Th and low Cs, Rb and K, whereas S7 contains comparatively elevated Cs, Rb and K and low Ba and Th. 4.2. Encrustation mineralogy The modal mineralogy of samples S1, S2 and S4 was determined using X-ray diffraction analysis. Sample S1 consists predominantly of octahedral crystals of sylvite (75 vol.%), and contains subordinate trona (~15 vol.%) and aphthitalite (~5 vol.%). Halite (~ 3 vol.%) is also

Fig. 3. Chondrite-normalized REE distributions of encrustation samples. Two groups are clearly evident, those with REE-concentrations exceeding those of natrocarbonatite, and encrustations with REE-concentrations well below those of natrocarbonatite lava. The chondrite values are from McDonough and Sun (1995).

present. Fig. 6 a) and b) shows SEM images of sylvite and apthitalite in sample S1. The specimen was found in a depression in the crater floor, had the morphology of a stalactite, and appeared to be forming from drips of fluid circulating within the crater floor and crystallizing upon exposure to ambient temperature (Fig. 6). However, we cannot rule out the possibility that it is a natrocarbonatite dripstone, which was altered by these fluids and completely replaced by secondary minerals. The morphology of S1 is similar to that of the stalactites found in hornito T45 by McFarlane et al. (2004), however the composition of the two samples is different, as the latter consists predominantly of trona. Sample S2 is a white-green encrustation which formed near the outer edge of a blanket of encrustations around an active fumarole on the flank of hornito T37B (Fig. 1) on top of altered lava forming the hornito (Fig. 7). Fig. 8 shows the original sample, together with its substrate of altered lava, which was subsequently removed manually under a binocular microscope. The encrustation is only about 0.5 mm thick and is composed mainly of nahcolite (~ 14 vol.%), trona (~8 vol.%), gaylussite (Na2Ca(CO3)2·5H2O; 8 vol.%) and an amorphous substance (~ 50 vol.%), which we suspect is native sulphur that melted after being deposited. Minor phases include calcite (3 vol.%), anhydrite (1 vol.%) and native sulphur (crystalline) (b1 vol.%). Fig. 9 c) and d) shows SEM photographs of the nahcolite and trona. Nyerereite makes up about 7 vol.% of the sample. However, as observations made using a FEG–SEM indicate that some substrate remained in the encrustation after its manual separation from the rest of the sample, it is likely that the nyerereite is not part of the encrustation.

Fig. 5. Spidergram for fresh natrocarbonatite, and representatives of the two encrustation groups (S4-group 1 and S7-group 2). The contrasting profiles for S4 and S7 indicate that the two groups of encrustation had quite different origins, albeit related to natrocarbonatite, which has a profile generally intermediate between the two encrustation profiles. The chondrite values are from McDonough and Sun (1995).

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} Fig. 8. A photograph of sample S2 showing the encrustation (white) and its lava substrate.

Fig. 6. Stalactite-bearing cavity in the crater floor. Sample S1 was collected from this location; gases were slowly being emitted.

On the basis of electron microprobe analyses, the REE appear to be concentrated in two minerals. Unfortunately, because of the small size of their crystals, it was not possible to avoid including the surrounding material in the analysed volumes. Consequently, we were not able to confidently identify the minerals. However, in two of the analyses (1 and 2; Table 4), the atomic ratio of La + Ce to F is very close to unity, suggesting strongly that the REE phase is a fluorocarbonate (e.g. a member of the bastnäsite group of minerals). This is consistent with the fact that these analyses have low totals. In addition to the REE fluorcarbonate mineral, native sulphur, Ba- and Sr- bearing phases, possibly sulphates, and minor apatite or monazite (see below) were probably also present in the analysed volume. The other four analyses are characterised by high proportions of phosphorous, and in the case of analysis 6, this proportion is so high, that it cannot be explained by any known phosphorous-bearing phase other than the REE-phosphates monazite, rhabdophane (REEPO4·H2O) or churchite (REEPO4·2H2O). Although the ratio of La + Ce to P determined in our analyses (0.7 to 0.9) is a little below the stoichiometric REE:P ratio, it is a minimum value because it ignores possible contributions from other

Fig. 7. Photograph of the fumarole at the base of hornito T37B from which samples S2 and S4 were taken. As is evident in this image, the fumarole was degassing and the white and black precipitates forming encrustations S2 and S4, respectively, were being deposited from that vapour.

REE. Furthermore, the small excess phosphorous could easily be explained by the presence of minor apatite. We therefore conclude that the second REE mineral is probably monazite or possibly rhabdophane (the relatively high totals, particularly considering the nature of the material, make it less likely that it is a hydrated phase). As was the case for analyses 1 and 2, native sulphur and Ba/Sr phases were likely included in the analysed volume. The elevated concentrations of F suggest that fluorite, a mineral that is common in sample S4 (see below), was also present in this volume. Sample S4 comprises material taken from the throat of the fumarole on the flank of hornito T37B (Fig. 1) that furnished sample S2, and occurs on top of and adjacent to native sulphur encrustations (Fig. 7). It was therefore probably formed at higher temperature than S2 (located near the outer edge of the encrustation blanket), which could explain its higher concentration of REE. Sample S4 is black, relatively featureless and extremely fine-grained. On the basis of XRD analysis, it is composed mainly of native sulphur (~80 vol.%) and fluorite (~ 20 vol.%). The sample could not be imaged using the SEM because of excess charging effects resulting from the volatile composition of the sample. Furthermore, as was the case for sample S2, micro-chemical analysis using the electron microprobe was problematic due to the small grain size. In contrast to sample S2, sample S4 does not appear to contain a mineral in which the REE are major components. Indeed, the only mineral that could be confidently identified from its composition (Ca: F ratio of 1:2) was fluorite, which contains up to 0.93 wt.% La2O3 and 1.17 wt.% Ce2O3; the average concentrations are 0.32 and 0.34 wt.%, respectively (Table 5). Although these concentrations are quite low, they may be sufficient to account for a large part of the bulk REE content of sample S4 considering that fluorite comprises a relatively large proportion of this encrustation. Most analyses yielded significant concentrations of sulphur, due almost certainly to contamination with native sulphur, and a number of analyses indicated relatively high concentrations of barium and strontium. Based on an analysis of the data in Table 5, proportions of Ba and Sr up 0.2 and 0.4 at.%, respectively, are interpreted to result from dissolution of these elements in the structure of fluorite, and higher concentrations to indicate inclusion of a Ba- and Sr-bearing phase(s) in the analysed volume. The nature of this phase(s) is uncertain, but positive correlations of Ba and Sr with sulphur suggest that it is a sulphate. Interestingly, the analysis reporting the second highest concentration of these elements (1.39 and 1.42 at.%, respectively), has the highest concentrations of REE, suggesting that these elements are partly hosted by the Ba and/or Sr phase(s), albeit in low concentrations. Samples S6 and S7 formed fringes along cracks of fresh cooling natrocarbonatite at inactive venting sites T49B and T56B, respectively (Fig. 1). Sample S7 was peeled from the natrocarbonatite upon collection, whereas sample S6 was taken together with the lava (Fig. 10). These two samples appear to be quite similar to the ‘salt fringes and tubes’ described by Genge et al. (2001) and thus likely

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Fig. 9. a) Backscatter Electron Image of sylvite in apthitalite in sample S1. b) Scanning Electron Image of apthitalite in sample S1. c) Scanning Electron Image of nahcolite in sample S2. d) Scanning Electron Image of trona in sample S2.

have similar mineralogy. They were not analysed by FEG-SEM or electron microprobe. 5. Discussion 5.1. Encrustation origins The clear separation of encrustation compositions into REE-rich and REE-poor groupings and the lack of intermediate compositions support the idea that encrustation formation at Oldoinyo Lengai results from two entirely different processes. We propose that the encrustations of group 1 were produced by a high-temperature REEenriched vapour and those of group 2 by a low-temperature vapour poor in REE. The REE-enriched vapour proposed as the source for group 1 encrustations (samples S2 and S4) most likely represents gas exsolved from the natrocarbonatite magma in the lava lakes, which probably included remelted hydrated natrocarbonatite. Based on the analyses of Oppenheimer et al. (2002) and Fischer et al. (2006) this vapour contains ~ 75 wt.% H2O and ~25 wt.% CO2. This interpretation is consistent with the location of these samples around and in a fumarole on the flank of active hornito, T37B, and the high temperatures of the vapours (402 °C to 419 °C). We therefore propose that group 1 encrustations precipitated from volcanic gases and are thus sublimates. The REE-poor encrustations consist mostly of low-temperature minerals stable below 50 °C (Königsberger et al., 1999) (with the exception of sylvite which is stable to much higher temperatures) and likely precipitated from steam produced by the interaction of meteoric water with hot rock at the crater floor or below recent flows as observed on February 26th 2006. As noted above, a similar conclusion

was reached by Genge et al. (2001) and Zaitsev and Keller (2006). However, these studies did not address the REE content of the encrustations. We propose that the low REE content of these encrustations can be explained by the relative immobility of the REE during weathering (the concentration of REE in fresh and weathered natrocarbonatite is similar), i.e., meteoric water failed to dissolve significant concentrations of REE and consequently the resulting

Table 4 Compositions of REE-bearing phases in sample S2 Analysis # Weight % CaO BaO SrO La2O3 Ce2O3 F P2O5 SO3 Total Atomic % Ca Ba Sr La Ce F P S Total (La + Ce)/F (La + Ce)/P

1

2

3

4

5

6

7.36 25.00 4.53 15.60 17.70 3.92 2.34 6.39 81.30

14.00 0.42 16.80 4.41 3.62 0.88 0.24 32.40 72.50

2.96 34.90 6.80 6.80 7.27 0.56 6.70 28.30 94.00

4.29 27.00 8.85 7.94 8.16 0.85 10.02 25.20 92.00

6.41 25.90 8.42 6.03 9.81 1.99 8.65 25.40 91.70

6.80 5.98 4.39 20.00 20.30 2.10 23.80 6.59 89.10

15.20 18.90 5.08 11.10 12.53 24.00 3.83 9.27 100.00 0.99 6.18

27.20 0.30 17.70 2.95 2.40 5.05 0.37 44.10 100.00 1.06 14.50

5.80 25.00 7.22 4.59 4.87 3.24 10.40 38.90 100.00 2.92 0.91

8.16 18.80 9.11 5.20 5.31 4.77 15.10 33.60 100.00 2.20 0.70

11.40 16.80 8.08 3.68 5.95 10.40 12.10 31.60 100.00 0.92 0.79

12.40 3.99 4.34 12.60 12.70 11.30 34.30 8.42 100.00 2.23 0.74

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Table 5 Compositions of REE-bearing phases in sample S4 Analysis # Weight % CaO BaO SrO La2O3 Ce2O3 F P2O5 SO3 Total Atomic % Ca Ba Sr La Ce F P S Total Ca/F

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

45.40 11.60 4.19 0.42 0.39 30.70 2.19 11.50 93.40

50.40 0.45 0.97 0.66 0.66 36.60 1.97 4.33 80.60

51.80 0.42 1.13 0.65 0.67 37.50 2.15 4.30 82.90

35.90 0.23 0.38 0.45 0.51 26.30 1.33 4.50 58.60

62.70 0.60 2.02 0.32 0.32 42.50 0.96 2.82 94.30

62.00 0.75 2.49 0.22 0.11 43.50 0.51 3.01 94.30

62.80 0.72 2.11 0.16 0.04 43.10 0.64 2.26 93.70

61.50 0.72 2.74 0.23 0.31 42.90 0.83 3.03 94.20

61.40 1.19 2.36 0.17 0.18 42.30 0.93 2.86 93.60

63.00 0.64 1.76 0.14 0.12 43.60 0.58 2.44 94.00

61.40 0.06 2.13 0.10 0.00 42.70 1.14 2.69 92.20

60.70 0.90 2.54 0.09 0.18 42.60 0.92 3.11 93.10

60.50 1.22 2.44 0.14 0.14 42.80 0.50 3.80 93.50

60.90 0.42 2.56 0.23 0.42 43.20 0.88 3.35 93.80

64.50 0.25 1.67 0.08 0.18 44.10 0.57 2.08 94.80

62.80 0.73 1.60 0.12 0.20 42.20 0.59 2.75 93.20

61.50 1.33 2.50 0.25 0.11 41.90 0.68 2.77 93.40

62.20 2.64 2.21 0.08 0.02 42.20 1.02 3.51 96.10

58.60 2.44 1.15 0.10 0.15 39.30 1.93 9.07 96.20

49.80 5.91 4.55 0.86 1.08 34.70 0.92 9.12 92.30

29.80 2.78 1.49 0.09 0.09 59.40 1.13 5.28 100 0.50

30.70 0.10 0.32 0.14 0.14 65.80 0.95 1.85 100 0.47

30.80 0.09 0.36 0.13 0.14 65.70 1.01 1.79 100 0.47

30.30 0.07 0.17 0.13 0.15 65.60 0.89 2.66 100 0.46

32.60 0.11 0.57 0.06 0.06 65.20 0.39 1.03 100 0.50

31.90 0.14 0.69 0.04 0.02 66.00 0.21 1.08 100 0.48

32.40 0.14 0.59 0.03 0.01 65.70 0.26 0.82 100 0.49

31.90 0.14 0.77 0.04 0.05 65.70 0.34 1.10 100 0.49

32.20 0.23 0.67 0.03 0.03 65.40 0.39 1.05 100 0.49

32.30 0.12 0.49 0.02 0.02 65.90 0.23 0.88 100 0.49

32.10 0.01 0.60 0.02 0.00 65.80 0.47 0.98 100 0.49

31.80 0.17 0.72 0.02 0.03 65.80 0.38 1.14 100 0.48

31.60 0.23 0.69 0.03 0.02 65.90 0.21 1.39 100 0.48

31.50 0.08 0.72 0.04 0.07 66.00 0.36 1.21 100 0.48

32.60 0.05 0.46 0.01 0.03 65.90 0.23 0.74 100 0.50

32.90 0.14 0.45 0.02 0.04 65.20 0.24 1.01 100 0.50

32.40 0.26 0.71 0.05 0.02 65.20 0.28 1.02 100 0.50

32.40 0.50 0.62 0.01 0.00 64.80 0.42 1.28 100 0.50

31.80 0.48 0.34 0.02 0.03 63.00 0.83 3.45 100 0.51

30.30 1.31 1.50 0.18 0.22 62.20 0.44 3.88 100 0.49

precipitates are impoverished in these elements (their concentrations are at least an order of magnitude below those of the lava). Elevated concentrations of Cs, Rb and K, relative to natrocarbonatite, and the depletion of these elements in weathered natrocarbonatite (Tables 2 and 3a) support the idea that steam produced by the heating of meteoric waters circulating in natrocarbonatite leached these elements and then deposited them as sublimates to form the group 2 encrustations. Significantly, group 1 encrustations are depleted in Cs, Rb and K, suggesting that these elements did not partition strongly into the volcanic gas. 5.2. Volatility of REE and stability of REE gas complexes The strong enrichment of group 1 encrustation samples in REE provides clear evidence that these elements can be transported in significant concentrations by low density vapours. In principle, this could be accomplished by dry volatility, if the corresponding vapour pressures of the REE were high enough. In order to test this hypothesis, we calculated the combined flux of La and Ce (the two most abundant REE in the encrustation) from the volcano using the volatility data of Hultgren et al. (1973) and the total gas flux for the

Fig. 10. Photograph of the setting of sample S6. As is evident from this image, the encrustation forms along cracks in the cooling lava.

volcano estimated by Brantley and Koepenick (1995) and Koepenick et al. (1996) assuming a temperature of 544 °C (the temperature of the lava lake natrocarbonatite as recorded by Krafft and Keller, 1989). The resulting figure of 10− 16 kg/year clearly rules out simple volatility as the mechanism for REE transport, because the time required to concentrate the REE in the encrustations at hornito T37B would exceed the age of the Earth, even if the total gas flux from the volcano were channeled through the single fumarole sampled at this hornito. Moreover, the encrustations have REE-profiles parallel to those of the lava samples, implying that there was no selective transport of particular REE, whereas the data of Hultgren et al. (1973) predict enhanced volatility of Yb and Eu, and relatively low volatility of La and Ce (Fig. 11). It therefore seems to be reasonable to conclude that, as is the case for other metals (Williams-Jones et al., 2002), the principal controls of metal transport in the vapour phase are solvation and complexation.

Fig. 11. Vapour pressures of REE over the corresponding solids (dry volatility). The diagram was constructed from the data of Hultgren et al. (1973). Note that the strong positive anomalies in Yb and Eu and low volatility of La and Ce predicted by these data are not consistent with the compositions of encrustations S2 and S4.

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Recent experiments by Migdisov and Williams-Jones, (2002, 2006, 2007) and Migdisov et al., 2006 have shown that appreciable concentrations of REE can be dissolved in chloride, fluoride and sulfate-bearing aqueous liquids at elevated temperatures. Furthermore, Migdisov and Williams-Jones (2006) and Migdisov et al. (2008) have shown that there is only slight LREE-enrichment or no preference for particular REE in chloride and sulfate-bearing solutions, respectively. If, as seems reasonable, this behavior of the REE extends to lower density fluids, such as vapours, parallel REE-profiles for encrustation and natrocarbonatite samples would result, as is the case with our data. However, given that the REE were deposited in part in fluorocarbonate minerals, it might be more attractive to invoke complexes involving carbonate or fluoride, particularly considering that the lavas are composed dominantly of carbonate and that the volcanic gases are rich in CO2. Unfortunately, there are no high temperature experimental data for REE carbonate complexes in aqueous fluids and those for fluoride are restricted to a single REE, Nd (Migdisov and Williams-Jones, 2007). Furthermore, the experimental data at ambient temperature for these complexes predict a strong preference for the HREE (Samson and Wood, 2005), suggesting that species involving fluoride and carbonate in the vapour phase would selectively concentrate the HREE and potentially, fractionate the REE of the encrustations relative to those of the natrocarbonatites. In view of the above discussion there are several possible explanations for how the REE were concentrated in the volcanic vapour without fractionation. One of these is that solvation alone was sufficient to concentrate the REE by forming species like REE2O3·nH2O, as has been shown for other metals, e.g. Mo (Rempel et al., 2006). This process would not be expected to discriminate among the REE because of their very similar charge/radius ratios. Another possibility is that the REE were dissolved as REE-chloride complexes (e.g., REECl3·nH2O), which as discussed above, would also not be expected to discriminate among the REE, and is consistent with the presence of significant concentrations of HCl in the vapour (0.067 mol%; Koepenick et al., 1996). Finally, if, as seems likely, the REE were transported in the natrocarbonatite melt as carbonate complexes, it would be possible to call upon carbonate complexation to explain the transport of the REE in the vapour as any discrimination among the REE would be similar in both vapour and melt. 6. Conclusions The encrustation and lava samples collected from actively venting hornitos at Oldoinyo Lengai in Tanzania provide direct evidence for the transport of REE by a H2O–CO2 vapour exsolved from natrocarbonatite magma. Two distinct encrustation types with high and low REEcontents were observed, and are interpreted to have formed as a result of entirely different processes. High-REE encrustations formed from vapour exsolved directly from natrocarbonatite magma, whereas lowREE encrustations are the products of leaching of solidified natrocarbonatite by meteoric water vapour (steam). We propose that significant concentrations of REE in the volcanic vapour were achieved by a combination of solvation and complexation, and that the latter involved carbonate species as was also likely the case for REE dissolution in the carbonatite magma. However, we do not rule out the possibility that chloride species played a role, given the significant concentrations of HCl in the volcanic gas, or that solvation alone was sufficient to ensure the necessary concentration of REE. In contrast to the volcanic gases, the vapours of meteoric origin were either at too low a temperature or did not contain the ligands required for significant mobilization of the REE. Acknowledgements This research was funded by a NSERC Discovery grant to AEW-J and a NSERC-USRA award to CDG. The authors would like to thank Line

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Mongeon and Kelly Sears for help with the FEG–SEM, Shi Lang and Jim Clark for help with the electron microprobe, Olivier Nadeau for helping organize the field equipment, and the Masai porters of the village of Engare Sero for helping transport equipment and samples. The original manuscript was improved considerably by the insightful reviews of S. Salvi, S. Wood, and an anonymous referee.

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