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We use a combination of graphic projections for exploratory interpretation of natural phase equilibria and liquid lines of descent. The data used are original shipboard X-ray fluorescence (XRF) analyses, supplemented by additional XRF analyses completed on shore, microprobe analyses of minerals in key samples, and microprobe data for glasses. Good agreement exists in the data groupings and trends between the projections used (CMAS, Al-Fe, and Pearce plots). The natural phase assemblages in the rock samples are consistent in most cases with those predicted by the graphically projected phase boundaries and liquid lines of descent, suggesting a major role for crystal-melt equilibria in the evolution of these rocks. This assumption is further tested with materials balance calculations and by analogy to experimental phase equilibria studies on a sample from Hole 839B. Shifts in graphic projections of inferred phase boundaries resemble those obtained experimentally at elevated pressure water-saturated, as detailed elsewhere in this volume. Materials-balance calculations yield excellent fits for most major element oxides. This provides strong support for inferred fractionation models, which imply "wet" equilibria for several subsets of the data. However, trace element variations in some data subsets are difficult to equate with the models, based on major element data, and appear to require heterogeneity in melt sources and/or in the trace element signatures of possible xenocrystal phases. It is also possible that the distribution coefficients usually assumed for "dry" systems are significantly altered under the high water contents inferred for these magmas. INTRODUCTION Lau Basin Sites 834, 836, and 839 each contain a range of compositions, from basalt or basaltic andesite to variously evolved andesitic rocks. This paper reports the results of exploratory interpretation of possible petrogenetic relations within and between drill sites. Specifically, we examine the extent to which these relations can be explained by crystal-melt equilibria. Bulk-rock and natural glass compositions from these sites follow systematic trends in several graphic analogs to phase diagrams that are highly suggestive of liquid lines of descent controlled by crystal-melt equilibria. Subtle differences between trends, and the offset of some trends from the typical 1-atm. phase boundaries established for mid-ocean-ridge basalt (MORB) and mafic island arc lavas suggests the possible influence of variations in pressure and/or volatile content on phase equilibria in these backarc lavas. An important role for volatiles, presumably largely water, is also suggested by the very high vesicularity of these rocks, which in some cases exceeds 30 vol% (Parson, Hawkins, Allan, et al., 1992). In this paper we present additional bulk-rock X-ray fluorescence (XRF) data and initial microprobe data on selected rock samples to test more rigorously these possible relationships, originally suggested by shipboard studies. We emphasize that these investigations are still exploratory in nature, and that our results require further integration with other trace element and isotope data. DATA SOURCES AND LIMITATIONS The 20 XRF analyses for major elements and selected trace elements conducted at Woods Hole are presented in Tables 1 and 2. Hawkins, J., Parson, L., Allan, J., et al., 1994. Proc. ODP, Sci. Results, 135: College Station, TX (Ocean Drilling Program). 2 Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A. 3 Department of Geology and Mineralogy, University of Queensland, St. Lucia, Brisbane 4067, Australia. 4 Department of Earth and Planetary Science, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. 5 Department of Geology, Queen's University, Kingston, Ontario, K7L 3N6, Canada. The methodology has been described by Schroeder et al. (1980). Our standards and correction procedures are generally similar to those used on shipboard (Sims and Hergt, this volume), so it is not surprising that the new analyses are very similar to shipboard analyses from the same lithologic units. This further confirms the consistency between lithology and chemistry noted in the Initial Reports volume (Parson, Hawkins, Allan, et al., 1992). These analyses were combined with those completed on board the JOIDES Resolution and were grouped and averaged by lithologic units. The correlations between lithology and chemistry have also been confirmed by microprobe analyses of natural rock glasses for those units for which glass was available (Hergt and Nilsson, this volume). The Woods Hole and shipboard XRF data have been used interchangeably for graphic interpretation and materials balance calculations. The glass data, as shown below, may plot in groups distinct from the bulk-rock data. In such cases, the glasses are always more evolved in composition (higher in SiO2, FeO, Na2O, and K2O) than the corresponding bulk rock. This relation is not unexpected, as it has also been observed in phyric MORB (Bryan et al., 1981;TormeyetaL, 1987). This generally precludes a direct comparison of XRF and glass data, although the agreement is generally good for most aphyric units. However, materials balance calculations that use both XRF and glass data very commonly show misfits for Na2O even where all other elements are well matched, suggesting a possible systematic difference between the XRF and microprobe Na2O data. Comparisons of glass to corresponding aphyric rock also show that the XRF data are consistently lower in Na2O than the glass. Electron microprobe analyses of selected minerals (Table 3) were conducted on the automated 4-spectrometer JEOL microprobe at the Massachusetts Institute of Technology. Analytical conditions are described by Gaetani et al. (this volume). We used backscatter images to check for heterogeneity (usually slight) and, in most cases, did short core-to-rim traverses to confirm the nature and extent of zoning in typical phenocrysts, xenocrysts, and microphenocrysts. The hindsight provided by subsequent materials-balance calculations suggests that the "representative" samples selected for analysis did not quite cover the range of phases and phase compositions likely to be involved in fractionation schemes in this large and rather complex data set. We comment on these cases at the appropriate point in the following discussions.

Natural Phase Equilibria and Petrologic Models: Lau Basin Sites 834, 836, and 839