The results of the melting experiments are given in Table 2. Section 152-917A-27R-2 shows the melting order of low-Ca pyroxene at 1142° ± 6ºC and plagioclase and olivine at 1173° ± 5ºC. Section 152-918D-108R-1 melts in the order of low-Ca pyroxene at 1153° ± 5ºC, augite at 1182° ± 5ºC, and olivine and plagioclase at 1192° ± 6ºC. Section 163-989B-10R-7 melts in the order of low-Ca pyroxene at 1113° ± 12°C, augite at 1167° ± 5°C, plagioclase at 1177° ± 5°C, and olivine at 1184° ± 2°C. Fe-Ti oxide minerals were not detected in any of the melting experiments.

There is a remarkably good correlation between melting temperature, phase proportions, and compositional variables of the experimental liquids. The variation of melting temperature and liquid fraction (Fig. 1) allows a minimum estimate of the solidus temperature by extrapolation to zero melt fraction and gives solidus temperatures of ~1110°C for the most magnesian-rich lavas (Sections 152-917A-11R-4 and 152-917A-86R-7) and ~1050°C for the most magnesian-poor lava (Section 152-917A-27R-2).

The compositional liquid lines of descent for the investigated samples (Table 3) (Thy et al., 1998) are represented in Figure 2 as a function of MgO content (wt%). The main variation is caused by cosaturation of olivine, plagioclase, and augite as reflected in the decreasing Al₂O₃ and CaO with decreasing MgO.

Olivine is on the liquidus for all samples and present throughout the melting intervals. The composition of olivine correlates with melting temperature and liquid Mg/(Mg + Fe²⁺) ratio (Table 4). The distribution coefficient for the moles of MgO and FeO between olivine and liquid (K_D^FeO/MgO [ol/liq] = [FeO/MgO]_ol/[FeO/MgO]_liq) is 0.32 ± 0.03 for 49 determinations (Fig. 3A) and is consistent with previous determinations (Roeder and Emslie, 1970; Ulmer, 1989). These results indicate that olivine and liquid are well equilibrated for most experiments.

Plagioclase composition (Table 5) shows wide and irregular variations with melting variables and the composition of the coexisting liquid. The relationship between plagioclase and liquid composition can be expressed in terms of the exchange of Na and Ca (K_D^Na/Ca [pl/liq] = [Na/Ca]_pl/[Na/Ca]_liq) (Fig. 4A). The calculated K_D values range between 0.6 and 1.6 and show a general increase with decreasing temperature (and liquid fraction) (Fig. 4B). The results indicate that the K_D for the basaltic andesite (Section 152-917A-27R-2) is lower than for any of the other samples. These values are largely in accord with previous experimental results (Grove et al., 1982; Mahood and Baker, 1986; Tormey et al., 1987; Ussler and Glazner, 1989).

The covariation between the An content (mol%) of plagioclase and Fo content (mol%) of olivine shows systematic differences between the low-Na₂O (Sections 152-918D-108R-1 and 163-989B-10R-7) and the high-Na₂O basaltic samples (Sections 152-917A-11R-4 and 152-917A-86R-7) as a steepening in the slopes of the Fo-An covariation for the former group (Fig. 5), likely to be related to the depletion rates of Na₂O of the coexisting liquids. The errors associated with plagioclase compositions are especially large for the low-temperature runs because of incomplete equilibration between plagioclase and liquid.

The pyroxenes form two compositional groups, a high-Ca group and a low-Ca group. The high-Ca group are salites to augites, while the low-Ca group are magnesian to intermediate pigeonites (Poldervaart and Hess, 1951; Deer et al., 1978). Both augite and pigeonite show increasing ferrosalite content with decreasing melting temperature (Table 6). The exchange of Fe and Mg between pyroxene and liquid can be calculated as for olivine, except that all iron in the liquid is calculated as Fe²⁺. The calculated K_D^FeO/MgO (aug/liq) vary between 0.22 ± 0.02 (Sections 152-917A-11R-4 and 152-917A-86R-7; n = 15) and 0.26 ± 0.02 (Sections 152-918D-108R-1 and 163-989B-10R-7; n = 8) (Fig. 3B) and are consistent with other experimental results as well as natural pyroxene-liquid pairs (Grove and Bryan, 1983; Perfit and Fornari, 1983; Toplis and Carroll, 1995). The pigeonites indicate a K_D^FeO/MgO (pig/liq) of 0.23 ± 0.01 (Section 152-917A-27R-2; n = 5), 0.25 (Section 152-918D-108R-1; n = 2), and 0.23 (Section 163-989B-10R-7, n = 1). However, the very low K_D for one of the pigeon-ite and liquid pairs (Section 152-918D-108R-1) suggests disequilibrium (Table 6; Fig. 3B).

The experimentally determined phase relations are portrayed graphically in terms of the normative components quartz (q), plagioclase (pl), olivine (ol), and diopside (di) in Figure 6. Figure 6A shows the multiply-saturated liquids and coexisting pyroxenes for projections from pl to the plane of ol-di-q. Figure 6B shows the same data set as projected from di to the plane ol-pl-q. In contrast to olivine and plagioclase that plot near their ideal normative compositions (not shown), the experimental augites and pigeonites plot significantly away from their ideal normative compositions and form an array between normative hypersthene (hy) and a point off normative diopside (Fig. 6B). Augite becomes increasingly subcalcic in the sequence Sections 152-917A-11R-4, 152-917A-86R-7, 163-989B-10R-7, and 152-918D-108R-1.

The experimental liquids for Section 152-918D-108R-1 and 163-989B-10R-7 show a systematic increase in normative quartz with decreasing temperature and move away from a fairly constant volume defined by coexisting olivine, plagioclase, and augite. The displacement of the multiply-saturated cotectics for Sections 163-989B-10R7 and 152-918D-108R-1 toward lower normative plagioclase (Fig. 6) are principally caused by the variation in composition of the experimental augite not fully accounted for in projecting from normative diopside. However, the differences are small and the near linear variation in the liquid trends in these projections indicate that the cotectic proportions of olivine, plagioclase, and augite are nearly constant. Sections 152-917A-11R-4, 152-917A-86R-7, and 163-989B-10R-7 show early crystallization of olivine and/or plagioclase (cotectic proportion: 29% ol and 71% pl), followed by augite. The olivine, plagioclase, and augite cotectic corresponds to 7% olivine, 52% plagioclase, and 41% augite. Of these three sections, only Section 163-989B-10R-7 reaches low-Ca pyroxene saturation.

The liquid line of descent for the basaltic andesite sample (Section 152-917A-27R-4) is controlled by olivine, plagioclase, and pigeonite. This multiply-saturated relation is a reaction boundary along which olivine reacts with the liquid to form pigeonite (Grove et al., 1982, 1983; Grove and Juster, 1989; Juster et al., 1989). The result is that the liquid compositions lie outside the triangle defined by equilibrium plagioclase, pigeonite, and olivine. The results for Section 152-917A-27R-4 define a point on the pigeonite-saturated reaction curve. By extrapolating pigeonite-olivine-plagioclase relations to intersect with the olivine-plagioclase-augite cotectic as defined by Sections 152-917A-11R-4 and 152-917A-86R-7, we can estimate an approximate location of the pseudoinvariant point in Figure 6B, where olivine + liquid plagioclase + augite + pigeonite.

Section 152-918D-108R-1 shows an extended interval of olivine, plagioclase, augite, and pigeonite saturation, but defines the low-variance pigeonite-saturated conditions at a relatively higher temperature (1153ºC) and lower silica saturation compared to typical MORB compositions (Fig. 6B) (Juster et al., 1989; Grove and Juster, 1989). Section 163-989B-10R-7 is saturated in pigeonite at the lowest melting temperature (1113°C) and slightly higher normative quartz composition than 152-918D-108R-1. Sections 152-917A-86R-7 and 152-917A-11R-4 were not saturated in low-Ca pyroxenes, and low-Ca saturation likely will be below the lowest temperature melted (1111º-1120ºC) and the highest normative quartz of any of the investigated sections. The present results extend the stability of low-Ca pyroxenes to lower normative quartz than obtained by Juster et al. (1989) and Grove and Juster (1989) for basalts and basaltic andesite lavas (Fig. 6).