Diversidad biológica

The chemical variability in the UTC can not be the result of the mixing of two different primary magmas because of the continuity of the trend of depletion, followed by enrichment, of incompatible trace elements (Fig. 4.3). It also can not be the result of mixing between the tuff primary magma and the lava shield primary magma; this is clear because the evolutionary trend from the most primitive sample of the tuff cone stage (U1-23) does not extend towards the LS stage composition. Trace element trends, such as decreasing Ni, Cr, V, Sc with increasing magmatic evolution, suggest fractionation of olivine + clinopyroxene ± spinel ± orthopyroxene. Spinel appears a more likely fractionating aluminous phase than plagioclase, because Sr is enriched with evolution in this suite. Orthopyroxene is present only in trace amounts as a phenocryst phase in the UTC samples and is generally rimmed by clinopyroxene, suggesting that it was not an equilibrium phase in the fractionating assemblage. TiO2 is not depleted with evolution, indicating the absence of a Fe-Ti oxide phase in the fractionation assemblage.

Crystal fractionation within the tuff cone compositions is modelled, with the aim of investigating the fractionating assemblages introduced above, using U1-23 as the most primitive sample and U1-11 as the most evolved one based on MgO and Zr concentrations (Table 4.1). We used the least squares mass balance (Stormer & Nicholls, 1978) option in the software PETROGRAPH (Petrelli et al., 2005) with ten components (SiO2, TiO2, Al2O3, FeOtot, MnO, MgO, CaO, Na2O, K2O and P2O5) and three to four fractionating phases (olivine + clinopyroxene ± aluminous spinel ± orthopyroxene) with compositions reported in Table 4.2. Representative olivine and clinopyroxene cores in sample U1-23 were chosen based on calculated partition of FeO and MgO between crystal and liquid equal to c. 0.30 and c. 0.32 respectively, in agreement with Roeder & Emslie (1970) and Takahashi & Kushiro (1983), indicative of them being in equilibrium with the most primitive erupted magma composition at Udo. Crystal core compositions were used to approximate deep fractionating composition, in order to distinguish it from late crystallizing rims, as discussed later. The composition of olivine, clinopyroxene and orthopyroxene is from microprobe

61 data on the tuff stage lapilli, whereas the spinel composition is from aluminous spinel in spinel peridotite xenoliths found in lavas in the northeastern part of Jeju Island (Kil

et al., 2008) and is used to approximate fractionating spinel in upper mantle

conditions beneath Jeju.

The modelling assumes constant composition of the fractionating phases and is hence independent of factors such as temperature, pressure and oxygen fugacity. Although these calculations do not give a unique solution, they nevertheless allow semi- quantitative evaluation of the fractionation process, especially when the sum of the residual squared is <2 (Stormer & Nicholls, 1978). The mass balance calculations result in three assemblages with the sum of the residuals squared <1 (Table 4.3). These are assemblages of ol + cpx + opx, ol + cpx + sp and ol + cpx + opx +sp and these will be discussed further below.

Table 4.2: Mineral compositions used for mass balance calculations. Spinel composition is from Kil et al. (2008). Recalculated to 100% totals.

olivine clinopyroxene spinel orthopyroxene

SiO2 40.06 51.67 0.06 53.73 TiO2 0 1.03 0.1 0.3 Al2O3 0 2.99 63.73 3.01 FeOtot 14.94 5.75 12.63 14.06 MnO 0.17 0.04 0.01 0.2 MgO 44.55 16.11 23.4 26.81 CaO 0.25 21.63 0.03 1.71 Na2O 0.01 0.53 0.01 0.1 K2O 0.03 0.12 0.02 0.08 P2O5 0 0.13 0 0 total 100 100 100 100

Table 4.3: Results of the mass balance calculation for investigation of the fractionating assemblages. See text for discussion.

Assemblage subtracted amount as wt% of initial magma

added amount as wt% of initial magma

sum of the squares of the residuals

ol + cpx 5.0 ol, 5.8 cpx 2.4

ol + cpx + opx 11.7 ol, 7.2 cpx 13.2 opx 0.4 ol + cpx + sp 3.6 ol, 10.6 cpx, 2.2 sp 0.8 ol + cpx +opx + sp 11.2 ol, 7.4 cpx, 0.1 sp 12.5 opx 0.4

Modelling of the behaviour of Cr suggests that no, or only minor spinel was involved in the fractionation process contrasting to the model above. By using the variation of

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P as an indicator of evolution by assuming a bulk PKD of 0 (that is, P is not partitioned in any crystallizing phases), the Cr depletion trend can be modelled with a bulk CrKD of c. 2.5 (Fig. 4.7), which is too low if chromian spinel is involved other than in trace amounts (McKenzie & O'Nions, 1991). This CrKD value is more consistent with clinopyroxene + olivine fractionation with c. 24% crystals removal. Al2O3 is enriched with evolution, suggesting the absence of a strongly aluminous phase (such as aluminous spinel) in the fractionation assemblage other than in trace amounts. Magmatic aluminous spinel is also not observed petrographically.

Figure 4.7: Modelling fractionation of Cr (ppm) using PKD = 0 results in bulk CrKD

= c. 2.5 and removal of c. 24 % crystal fraction. Symbols as in Figure

4.4. P2O5 in wt% oxide.

Ni is less well correlated with P, however, modelling suggests a bulk NiKD of c. 2.5. Comparing this to the value calculated by Smith et al. (2008), Ni appears slightly more compatible in the fractionating assemblage, suggesting the presence of olivine rather than orthopyroxene. Further discrimination between olivine and orthopyroxene is difficult. However, SiO2 is enriched with evolution (decreasing MgO). Microprobe data (Table 4.2), as well as representative analyses (Deer et al., 1992), indicate that both clinopyroxene and orthopyroxene have SiO2 contents similar to or higher than the starting composition (U1-23). As spinel was fractionating in very limited amounts (if at all), olivine is the only suitable phase that could generate SiO2 enrichment with fractionation.

The preferred fractionating assemblage is therefore clinopyroxene + olivine ± spinel. Orthopyroxene is present as phenocrysts, however, it does not fit crystal fractionation

63 models and has calculated partition of FeO and MgO between crystal and liquid >0.4, which is too high for equilibrium orthopyroxene (Beattie et al., 1991). We suggest that orthopyroxene is xenocrystic, being derived from the mantle and entrained in various amounts in the fractionating magma. The presence of mainly olivine phenocrysts and plagioclase microphenocrysts in the eruptive products can be explained as in the case of Crater Hill (Smith et al., 2008), with these phases undergoing low pressure crystallization during magma ascent in the upper plumbing system. The relatively greater abundance of plagioclase microphenocrysts observed in the LTC compared to the UTC suggests heterogeneous nucleation of this phase in the rising magma column. Plagioclase was however not being fractionated given that Al2O3 and Sr are not depleted with evolution, and hence all crystallizing plagioclase was retained in the rising magma. The presence of intergrown chromian spinel in some olivine crystals suggests that these are residual cores from upper-mantle olivine, with overgrown magmatic rims with lower MgO/FeO ratios. Clinopyroxene is present as a phenocryst phase just in the UTC. The absence of clinopyroxene in the LTC suggests that this phase had settled out of the upper part of the fractionating magma column, but was carried to the surface by the ascent of the lower column.

Figure 4.8: Calculated REE bulk partition coefficients into the fractionating assemblage in the tuff sequence of Udo. Garnet and clinopyroxene fields are after Smith et al. (2008) and references therein and the solid line is their calculation for Crater Hill. Diamonds are the Udo data.

By determining partition coefficients of REE by comparing them to that of P, with P

KD assumed to be 0, the plotted pattern (Fig. 4.8) resembles that determined by Smith et al. (2008) using the same method. This can be attributed to the fractionation

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of clinopyroxene. However, the LREEs appear to have been more compatible in the Udo fractionating assemblage, compared to a distribution coefficient involving just clinopyroxene. This may be due to buffering by residual amphibole in the upper mantle at the site of fractionation. Slightly higher SmKD compared to NdKD and EuKD also supports the presence of amphibole (Rollinson, 1993 and references therein). Presence of amphibole in the upper mantle below Jeju has been suggested by Tatsumi

et al. (2005) and resorbed kaersutite was described as a xenocryst in Jeju basalts by

Eom et al. (2007). This may have crystallized following metasomatism by silicic, low

Mg# fluids (Tiepolo et al., 2001), which affected the mantle beneath Jeju, as found in mantle xenolith inclusions (Yu et al., 2009).

Older basaltic lava flows underlie Udo Volcano (Sohn & Chough, 1993), and the chemical variation of samples with intermediate composition may be due to contamination from such rocks. Alternatively, given that the intermediate samples always plot between the tuff cone stage and the lava shield stage compositions (Fig. 4.4, 4.5), mixing of approximately equal amounts of the two magma batches could also produce these intermediate compositions.