5.5.3.1 Crystal-melt constraints - a westward shift towards more evolved magmas?
The Fe=Mg relationships help us to constrain the composition of magmas and crystals involved in the formation and differentiation of the sills constituting the FCSC. Fig. 5.7 shows the average FCSC chill composition, together with analyses of DZ and OZ rocks from the FCSC sills we have sampled. We have also plotted the LLD pathway predicted from PELE (Boudreau 1999) for the average FCSC chill composition, and labelled the point where plagioclase and clinopyroxene join Fo84 olivine on the liquidus. This model
notion that these are cotectic precipitates derived from fractional crystallisation of a tholeiitic melt similar to the average FCSC chill composition.
Fig. 5.7 (page 82): The Fe=Mg systematics of the FCSC with bulk-rock compositional fields as well as the constraints provided by the olivine compositional data and their inverse melt compositions. The histogram attached to the olivine Fo-Fa join shows the distribution of primocrystic, chadacrystic and groundmass olivine compositions observed in the FCSC OZ from all three sills belonging to the FCSC. Also shown on the diagram is a melt loss trajectory with collinear L1, L2 and L3 poles (see text for details).
Although the petrography (Fig. 5.3) and MgO-enrichment trends of the FCSC OZ rocks (Fig. 5.4) imply olivine accumulation, the OZ arrays in the FCSC sills (WUS, PS and LPS) show distinct trends (Fig. 5.4). In principle, each individual OZ bulk rock composition represents a two-component mixture of cumulus olivine and trapped melt, now mostly represented by clinopyroxene and plagioclase. The modal proportion of olivine in these rocks ranges between 20-55% (Fig. 5.7), of which most is ‘cumulus’ textured. A least squares regression trend was fitted to each OZ array, defining both the composition of (bulk) olivine that accumulated from the melt (Cawthorn et al., 1992, Wilson, 2012) and the composition of the dominant entrapped basaltic melt (Fig. 5.7). The WUS OZ array intersects the average FCSC chill composition (10.3 wt% MgO) and projects towards an olivine composition of Fo87, which we will later show is not in equilibrium with this melt composition. The PS OZ array projects towards a more evolved basaltic melt composition (9 wt% MgO) and a more evolved olivine composition of Fo86. The LPS OZ array projects toward a still more evolved basaltic melt composition (8 wt% MgO) and a still more evolved olivine composition of Fo85. If we force the regression of the LPS OZ through the average FCSC chill composition (shown in Fig. 5.4), then the bulk-extract olivine
compositions that are required to satisfy the bulk-rock mass balance for the PS and LPS OZ rocks are even more Fe-rich (~Fo83) than the dominant primocrystic olivines (~Fo86) that these rocks contain. Although the bulk-rock Fe=Mg systematics imply that the olivine cumulates in the FCSC were derived from increasingly more Fe-rich melts towards the west, the observed compositions of olivine in these rocks only approximately matches the required crystal ‘cumulates’, and do not show systematic Fe-enrichment towards the west (Fig. 5.7 and appendix F2). The diversity of olivine compositions (histograms on the Fo-Fa join: Fig. 5.7) observed within each OZ does not fit an idealised equilibrium crystal-extract model (Cawthorn et al., 1992), and implies that the OZ cumulate arrays reflect an average cumulate formed from a heterogeneous olivine crystal cargo.
5.5.3.2 Determining OZ porosities using inverse melt modelling
The fossil porosity of each OZ sample from the FCSC is defined from its position on the Fe=Mg diagram (Fig. 5.7), with the proportion of olivine in each rock ranging between 20-55%. We tested these porosities using the equilibrium distribution method of Bédard (1994). The equilibrium distribution method uses the bulk-rock trace element contents of plutonic rocks to evaluate the relationships between the compositions of cumulate crystals and the amount and composition of its trapped melt fraction (TMF). It can be used to constrain the TMF if the melt composition is known. To begin, we assumed that all WUS OZ cumulates formed from a melt similar to the average FCSC chill composition
(supported by the position of the WUS OZ array least squares regression: Fig. 5.7). The model melts calculated from the WUS OZ rocks resemble the average FCSC chill
composition for TMFs ranging between 50-70%, for complementary olivine modes of 30-50% (results included in appendix F1). The WUS OZ model melt solutions are almost identical to the olivine modes defined on the Fe=Mg diagram (Fig. 5.7). We fitted the LPS OZ model melts to typical 8 wt% MgO Franklin suite melt (average of Type-1 chills between 7.5 and 8.5 wt% MgO) because the LPS OZ array least squares regression intersects a melt with this more evolved composition (Fig. 5.7). The model melts
calculated from the LPS OZ rocks yield close matches to this 8 wt% MgO melt with TMFs that are very similar to those defined on the Fe=Mg diagram (Fig. 5.7). The LPS OZ rocks range to slightly lower modal TMF (higher olivine modes), in comparison to those
calculated for the WUS OZ.
In summary, the porosities calculated using the equilibrium distribution method closely resemble the porosities defined on the Fe=Mg diagram, supporting the inference that OZ rocks are principally composed of olivine and trapped melt with compositions that match those defined by regression of OZ rocks on the Fe=Mg diagram (Fig. 5.7). Only 2-3%
fractional crystallisation is needed to explain the incompatible trace element variation between melts calculated to be in equilibrium with the WUS OZ and the LPS OZ (Fig.
5.7). PELE fractional crystallisation modelling implies that ~5% olivine-only fractionation is required to shift melt MgO from 10 to 8 wt%, consistent with what was calculated from the trace elements.
5.5.4 Can progressive magma fractionation explain the westward FCSC OZ