Pre-eruptive conditions of Miocene and Pliocene nephelinites from Saghro

Although the bulk composition of the Miocene and Pliocene nephelinites that erupted at Saghro shows minimal compositional variations (Fig. 4.3–4.4), the proportion of mineral and their composition, as well as the occurrence of variable fluid inclusions record different magma environments. Temperature and pressure of clinopyroxene crystallization in nephelinites have been determined using the clinopyroxene-melt thermobarometer of Putirka et al. (2003). All analyzed clinopyroxenes were tested for equilibrium with their host lava using trace element composition (Fig. 4.5). Composition of melt in equilibrium with clinopyroxene was calculated using clinopyroxene-basanite partition coefficients from Adam and Green (2006) and Green et al.

(2000). These compositions were then plotted against the composition of the host lava (Fig. 4.5E- F) and the closest equilibrium composition for each type of nephelinite was used in the

Figure 4.11. (continued)

Chapitre 3 – Les inclusions fluides et les minéraux riches en volatils comme traceurs des fluides

thermobarometer of Putirka et al. (2003). This thermobarometer is based on jadeite- diopside/hedenbergite exchange equilibria, the standard errors for estimation of temperature and pressure are ±33 °C and ±170 MPa, respectively. The best fit was obtained for augite with high Mg# and Kd^Fe/Mg # 0.4 in both olivine (TLA6a*, Mg# = 84.4, Kd^Fe/Mg = 0.40) and Pliocene nephelinite (pinkish augite FK18e*, Mg# = 81.2, Kd^Fe/Mg = 0.44) (Table 4.3). Both samples yield high pressure and temperature with 2200 MPa and 1364 °C for miocene nephelinites, and 1760 MPa and 1320 °C for Pliocene nephelinites (Fig. 4.12). These values are close to the experimental equilibria of Bultitude and Green (1971) on natural miocene nephelinites with olivine Fo90 and clinopyroxene Mg# = 87 compositions (Fig. 4.12) but slightly higher than those determined by Berger et al. (2008) (up to 1400 MPa for Pliocene nephelinite), suggesting that the pressure of crystallization should have been close to their higher estimate.

Figure 4.12. P-T conditions from clinopyroxenes of both nephelinites, calculated using the thermobarometer of Putirka et al. (2003). The liquidus of miocene nephelinite and the 100*Mg/(Mg+Fetot) of clinopyroxenes and olivines in equilibrium with natural olivine nephelinite liquid are from Bultitude and Green (1971).

Although small variation of major and trace element composition of clinopyroxene (e.g.

Kd^Fe/Mg, REE) in Miocene and Pliocene nephelinite suggests deep crystallization and equilibrium with the whole rock, detailed petrographic observations indicate different magma differentiations.

In miocene nephelinites, the presence of normally zoned olivine (Fo91–79) and clinopyroxene (Mg# = 85–62) strongly suggests a simple magmatic history with little to no stagnation in a magma chamber before eruption. This is also well correlated with the low volumes of material erupted and the presence of peridotite xenoliths (Ibhi et al., 2002) indicating a rather high magma velocity during ascent, minimizing chances for reaction with wall rocks and magma mixing.

The occurrence of rare pyrrhotite (sulfides - S^2-) and S-bearing apatites (S⁶⁺) suggests that miocene nephelinites evolved under relatively oxidized conditions close to $log fO2 = NNO+1 (Jugo et al., 2010) and volatile elements are present at depth during magma crystallization. The amount of S, F, and Cl, in silicate liquid can be estimated using experimentally determined

partition coefficient for volatile elements between apatite and silicate melt (Mathez and Webster (2005); Parat et al. (2011); Fig. 4.13). In miocene nephelinites, apatite crystallized from a silicate liquid with high F and Cl content (0.60–1.42 wt.% and 0.11–0.38 wt.% in the melt, respectively), whereas the sulfate content is low (7–15 ppm S⁶⁺) (Fig. 4.13). It should be noted that pyrrhotites are liquidus phases present as inclusions in mafic minerals, whereas apatites are groundmass minerals. The sulfate content determined from apatite composition represents concentration in the melt after pyrrhotite crystallization and does not refer to the initial sulfur content in nephelinite.

However, the rare occurrence of pyrrhotite also suggests low initial S content. Opposite to S, the estimates of halogen element concentrations are relatively high compared to concentration measured in melt inclusions in alkali basalts (900–2000 ppm Cl, 500–4000 ppm F, e.g. Aiuppa et al., 2009; Bureau et al., 1998; Hudgins et al., 2015; Métrich et al., 1993, 2010) but the absence of a saline-component in fluid inclusions (see below) strongly suggests that Cl-bearing fluid or saline fluid was not present at depth (P = 590 MPa, see below). This is in agreement with high Cl solubility calculated for olivine nephelinite composition using a model computed from fluid- saturated experiments at high pressure (Cl solubility = 4.5–5.3 wt.% at 700 MPa and 1200°C, Webster et al., 2015). Despite Cl under-saturation in miocene nephelinite, the abundant CO2-fluid inclusions suggest however that a fluid phase was present and that exsolution of CO2 occurred during magma evolution at high pressure (see below). The solubility of CO2 strongly depends on the NBO/T ratio of the silicate liquid (e.g. Mysen et al., 1975; Papale et al., 1996; Brooker et al., 2001). For miocene nephelinites, the NBO/T ranges from 0.94 to 1.01 (Table 4.1). Assuming that a CO2-dominated fluid phase was present during olivine crystallization at depth, during or before clinopyroxene crystallization, solubility experiments predict solubility of 6 wt.% CO2 in H2O-free

Figure 4.13. F, Cl and S concentration in the liquids in equilibrium with apatites for Miocene and Pliocene nephelinite. These concentrations were calculated using partition coefficients from Mathez and Webster (2005) and Parat et al. (2011).

Chapitre 3 – Les inclusions fluides et les minéraux riches en volatils comme traceurs des fluides

nephelinite melt at 2 GPa and 1300 °C (clinopyroxene-melt equilibria) (Brooker et al., 2001).

This represents a minimum value as CO2 solubility increases with pressure and temperature (e.g.

Mysen et al., 1975).

In Pliocene nephelinites, the abundance of clinopyroxene and the occurrence of two populations (i.e. green and pinkish augites) suggest different magma history early during magma crystallization compared to Miocene nephelinites. The abrupt change in composition between core and rim (inverse zoning) of green augite crystals indicates that green augites have inherited cores (xenocryst with low Mg#) and early mantle or magmatic differentiation processes occurred at mantle conditions during Pliocene nephelinite evolution (e.g. Duda and Schminck, 1985;

Ulianov et al., 2007). The rims of green augites have the same composition as the pinkish augite phenocrysts (with high Mg#) and are in equilibrium with nephelinite liquid and crystallization occurred at 1760 MPa and 1320°C. The close trace element compositions of green and pinkish augites (Fig. 4.5) indicate however that mantle and/or magma processes and magma ascent were relatively fast, leaving little time for significant compositional equilibrium and further overprint by shallower level crystallization in agreement with detailed study of texture and composition of clinopyroxene from Pliocene nephelinite by Berger et al. (2008). The crystallization of augite at slightly lower pressure and temperature in Pliocene nephelinites (1760 MPa and 1320 °C) than in Miocene nephelinites may then be related to mantle/magmatic processes.

The volatile concentrations in Pliocene nephelinite determined from apatite during fractional crystallization indicate lower Cl and higher F and S⁶⁺ content (0.01–0.17 wt.% Cl, 1.09–1.70 wt.% F, and 7–156 ppm S in the silicate melt; Kd from Mathez and Webster (2005) and Parat et al. (2011); Fig. 4.13) than in Miocene nephelinite. The high proportion of pyrrhotite and the high S content of the bulk rock (300–1300 ppm S) in Pliocene nephelinite also suggest higher sulfur content in the silicate liquid. As discussed for Miocene nephelinite, the concentrations of halogens are lower than the solubility concentration in alkali basalt, suggesting that Cl-bearing fluid or saline fluid was not present at depth. Unlike in Miocene nephelinite, minerals in Pliocene nephelinite do not have primary fluid inclusions (Type 1B), suggesting that the fluid phase was absent at depth during phenocryst crystallization.

The bulk rock CO2 concentrations of nephelinite ranges from 0.07 to 0.36 wt.% and from 0.20 to 1.55 wt.% in Miocene and Pliocene nephelinite, respectively. Because no carbonated phases (e.g. magmatic or secondary calcite) have been observed in the thin sections, these values can be considered as the amount of CO2 trapped in fluid inclusions. However, higher CO2

concentrations in Pliocene nephelinite compared to Miocene nephelinite can result also from very small carbonate-bearing xenoliths reported by Ibhi et al. (2002) but not observed in our thin sections.