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[1] This paper presents major and trace element compositions of lavas from the entire 3098 m stratigraphic section sampled by phase-2 of the Hawaii Scientific Drilling Project. The upper 245 m are lavas from Mauna Loa volcano, and the lower 2853 m are lavas and volcanoclastic rocks from Mauna Kea volcano. These intervals are inferred to represent about 100 ka and 400 ka respectively of the eruptive history of the two volcanoes. The Mauna Loa tholeiites tend to be higher in SiO 2 and lower in total iron, TiO 2 , alkalis, and incompatible elements at a given MgO content than Mauna Kea lavas. The transition from Mauna Loa to Mauna Kea lavas is all the more pronounced because the Mauna Loa tholeiites overlie a thin sequence of postshield Mauna Kea alkalic to transitional tholeiitic lavas. The Mauna Loa tholeiites display welldeveloped coherent trends with MgO that are indistinguishable in most respects from modern lavas. With depth, however, there is a slight decline in incompatible element abundances, and small shifts to depleted isotopic ratios. These characteristics suggest small changes in melt production and source components over time, superimposed on shallow melt segregation. The Mauna Kea section is subdivided into a thin, upper 107 m sequence of postshield tholeiites, transitional tholeiites and alkali basalts of the Hamakua volcanics, overlying four tholeiitic magma types that are intercalated throughout the rest of the core. These four magma types are recognized on the basis of MgO-normalized SiO 2 and Zr/Nb values. Type-1 lavas (high SiO 2 and Zr/Nb) are ubiquitous below the postshield lavas and are the dominant magma type on Mauna Kea. They are inter-layered with the other three lava types. Type-2 lavas (low SiO 2 but high Zr/Nb) are found only in the upper core, and especially above 850 m. Type-3 lavas (low SiO 2 and Zr/Nb) are very similar to tholeiites from Loihi volcano and are present only below 1974 m. There are only 3 discrete samples of type-4 lavas (high SiO 2 and low Zr/Nb), which are present in the upper and lower core. The differences between these magma types are inferred to reflect changes in melt production, depth of melt segregation, and differences in plume source components over about 400 ka of Mauna Kea's eruptive history. At the start of this record, eruption rates were high, and two distinct tholeiitic magmas (type-1 and 3) were erupting concurrently. These two magmas require two distinct source components, one similar to that of modern Loihi tholeiites and the other close to that of Kilauea magmas. Subsequently, the Loihilike source of the type-3 magmas was exhausted, and these lavas are absent from the remainder of the core. For the next 200 ka or so, the eruptive sequence consists of inter-layered type-1 and -2 lavas that are derived from a common Mauna Kea source, the major difference between the two being the depth at which the melts segregated from the source. At around 440 ka (corresponding with the transition in the core from submarine to subaerial lavas) eruption rates bega...

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Geochemical discriminants are used to place the boundary between Mauna Loa flows and underlying Mauna Kea flows at a depth of about 280 m. At a given MgO content the Mauna Kea flows are lower in SiO2 and total iron and higher in total alkali, TiO2, and incompatible elements than the Mauna Loa lavas. The uppermost Mauna Kea lavas (280 to 340 m) contain alkali basalts interlayered with tholeiites and correlate with the postshield Hamakua Volcanics. In addition to total alkalis, the alkali basalts have higher TiO2, P2O5, Sr, Ba, Ce, La, Zr, Nb, Y, and V relative to the tholeiites and lower Zr/Nb and Sr/Nb ratios. Some of the alkali basalts are extensively differentiated. Below 340 m all the flows are tholeiitic, with compositions broadly similar to the few “fresh” subaerial shield‐building Mauna Kea tholeiites studied to date. High‐MgO lavas are unusually abundant, although there is a wide range (7–28%) in MgO content reflecting olivine control. FeO/MgO relationships are used to infer parental picritic magmas with about 15 wt % MgO. Lavas with more MgO than this have accumulated olivine. The Mauna Loa lavas have compositional trends that are controlled by olivine crystallization and accumulation. They compare closely with trends for historical (1843–1984) flows, tending toward the depleted end of the spectrum. They are, though, much more MgO‐rich (9–30%) than is typical for most historical and young (<30 ka) prehistoric lavas. The unusual abundance of high‐MgO and picritic lavas is attributed to the likelihood that only large‐volume, hot, mobile flows will reach Hilo Bay from the northeast rift zone. FeO/MgO relationships are used to infer parental picritic magmas with about 17 wt % MgO. Again, lavas with more MgO than this have accumulated olivine. Systematic changes in incompatible element ratios are used to argue that the magma supply rate has diminished over time. On the other hand, the relatively constant Zr/Nb and Sr/Nb ratios that compare closely with historical and young (<30 kyr) prehistoric flows are used to argue that the source components for these lavas in the Hawaiian plume have remained relatively uniform over the last 100 kyr.

Lunar crustal rocks can be divided into two groups: the terra, or highland, types and the mare basalts. Interpretation of the highland samples is complicated by their derivative nature, which resulted from a series of crystallization, shock, and brecciation events. In contrast, mare basalts appear to be much less complicated and to have been rather uncompromised since their arrival at the lunar surface; thus a synthesis appears possible at this time. Although the mare basalts comprise less than 1% of the lunar crust, they contain much information about the thermal history of the moon and the nature of the lunar interior. It is now known that a complete suite of basalts, sampling all of the chemically and temporally distinct units, was not sampled by the Apollo and Luna missions. The mare basalts that have been studied have ages between 3.15 and 3.96 Gy. However, photogeologic evidence (crater counts and crater degradation studies) indicates that basalts as young as 2.5 Gy exist on the moon and were not sampled. The returned samples can be divided into two broad groups: the older, high‐titanium group (ages, ∼3.55–3.85 Gy; TiO2, 9–14 wt %) and the younger, low‐titanium group (ages, 3.15–3.45 Gy; TiO2, 1–5 wt %). Basalts from Apollo 11 and 17 fall into the older, high‐titanium group; basalts from Apollo 12 and 15 and Luna 16 fall into the younger, low‐titanium group. The two major groups of basalts can be further subdivided on the basis of major‐ and minor‐element chemistry. Within each of these subgroups a variety of grain sizes and textures, which result from different cooling histories, are present. Near‐surface fractionation of these basalts involved mainly olivine in the low‐titanium basalts and olivine plus iron‐titanium oxides in the high‐titanium basalts. The alkali‐depleted mare basalts evolved by rapid cooling at the lunar surface under extremely reducing conditions (∼10−13 atm at 1150°C). This low oxygen fugacity resulted in reduced valence states for Ti (Ti4+ → Ti3+) and Cr (Cr3+ → Cr2+), which in turn affected both the chemistry and the stability of the mare basalt minerals. The most important mineralogical species in these rocks are the silicates (pyroxene, feldspar, and olivine) and the Fe‐Ti oxides (ilmenite, spinel, and armalcolite). Models for the source regions of the mare basalts remain controversial. Three basic models for mare basalt source regions have been advanced. These include the cumulate source model (remelting of cumulates resulting from early lunar differentiation), the primitive source model (melting of deep undifferentiated mantle), and the assimilation model (primary melts are contaminated by assimilation). All of these models have problems. If one assumes that at least some of the lunar basalt samples arrived at the surface with unaltered chemistry, the high‐pressure experimental phase equilibria approach can provide constraints on the nature of the source regions for these rocks. Results of these studies indicate that the low‐ and high‐Ti mare basalt groups were derived from miner...

Major and trace element data for a suite of lavas from fifty-six dredges and ALVIN dives on the ridge axis and adjacent abyssal hills have been used to investigate the geometry and evolution of magmafie systems beneath the Endcavour Segment, Juan de Fuca Ridge. The morphology of the Endeavour Segment between the northward propagating Cobb Offset and the recently formed (<0.2 m.y.) Endcavour Offset is dominated by a shallow, rifted, elongate crestal volcano (Endcavour Ridge) that deepens along-strike into a broad, deep basin at each offset. A set of ridges, interpreted to be previous crestal volcanoes rifted apart by spreading, flank the Endcavour Ridge and chronicle the "dueling" propagator history of the Cobb Offset. The tectonic evidence strongly suggests that a large portion of the Endcavour Segment may be a failing rift segment at this time. Lavas from the current axis of the Endearour Segment are moderately fracfionated (MgO: 6-8.5 wt %) and have genera!ly higher SiO2, A1203, Na20, and K20, and lower FeO* than lavas from south of the Cobb Offset (SOCO lavas). Incompatible trace element abundances and ratios indicate the Endcavour lavas are primarily enriched E-MORBs and T-MORBs (e•g., ZrfNb: 7-24; Zr/Y: 2.5-5.9; and Ba/TiO2: 6-64), in contrast with the SOCO lavas, which are more depleted in character. Thus, the 30-1cm wide Cobb Offset appears to mark a major geochemical boundary beneath the Juan de Fuca Ridge. In contrast with the Endearour Segment axial lavas, samples frcnn adjacent abyssal hills are more similar to the SOCO lavas in their major and trace element character/sties. These observations suggest that the parental magmas of the Endearour Segment exhibit temporal variability, with more enriched material arriving only recently beneath the ridge axis. Pronounced compositional variability is observed at small spatial scales within the Endcavour Segment axial lavas, which does not correlate with axial morphology. This variability is interpreted to reflect ubiquitous small scale manfie heterogeneity and poor mixing of multiple parental magmas during migration from the melt region or within axial magma chambers. Highly enriched samples (Zr/Nb<10) are localized near the summit region of Endcavour Ridge, whereas slighfiy less enriched samples occur along the length of the axis and on the older flanking ridges. Recent enrichment may result from diminishing extents of partial melting of a heterogeneous source in response to tectonic reconfiguration, causing more fusible enriched domains to dominate the chemical signature of melts produced. Small scale heterogeneity along-strike seems incompatible with models of centralizexl upwelling of melts beneath the summit region of the ridge axis, with shallow lateral injection of melts to distal ends of the segment, unless these spatial vadafions actually reflect temporal variations in the source composition and collapse of the shallow magmatic systems toward the sum-m_it region as rift failure has progressed. LNTRODUCTION linearity (devals [lxmg•r et al., 1986]), and...

Samples of basalt were collected during the Rapid Response cruise to Loihi seamount from a breccia that was probably created by the July to August 1996 Loihi earthquake swarm, the largest swarm ever recorded from a Hawaiian volcano. 210 Po-210 Pb dating of two fresh lava blocks from this breccia indicates that they were erupted during the first half of 1996, making this the first documented historical eruption of Loihi. Sonobuoys deployed during the August 1996 cruise recorded popping noises north of the breccia site, indicating that the eruption may have been continuing during the swarm. All of the breccia lava fragments are tholeiitic, like the vast majority of Loihi's most recent lavas. Reverse zoning at the rim of clinopyroxene phenocrysts, and the presence of two chemically distinct olivine phenocryst populations, indicate that the magma for the lavas was mixed just prior to eruption. The trace element geochemistry of these lavas indicates there has been a reversal in Loihi's temporal geochemical trend. Although the new Loihi lavas are similar isotopically and geochemically to recent Kilauea lavas and the mantle conduits for these two volcanoes appear to converge at depth, distinct trace element ratios for their recent lavas preclude common parental magmas for these two active volcanoes. The mineralogy of Loihi's recent tholeiitic lavas signify that they crystallized at moderate depths (F8-9 km) within the volcano, which is approximately 1 km below the hypocenters for earthquakes from the 1996 swarm. Taken together, the petrological and seismic evidence indicates that Loihi's current magma chamber is considerably deeper than the shallow magma chamber (F3-4 km) in the adjoining active shield volcanoes.