Chemistry of Kilauea and Mauna Loa lava in space and time

Abstract

Kilauea and Mauna Loa, Hawaii's two active shield volcanoes, are composed of tholeiitic basalt having MgO contents ranging from more than 20 percent to less than 4 percent. Most eruptive vents are located either within the central caldera or on two rift zones extending to the east and southwest from each volcano's summit. Mauna Loa also has a few isolated vents on its northwest slope that are apparently unrelated to any rift zone. The chemical variability of Mauna Loa and Kilauea lava having more than about 6.8 percent MgO is principally explained by addition or removal of olivine (olivine control), but other minerals are involved to a lesser degree. The chemical variability of these lavas is described and interpreted in this paper. Lavas having less than 6.8 percent MgO are fractionated by separation of pyroxene, plagioclase, and Fe-Ti oxides in addition to olivine. Fractionated basalt from Kilauea is confined to the rift zones. The rare fractionated lavas from Mauna Loa are also confined to the rift zones and are less fractionated (higher MgO) than those from Kilauea. The chemistry of nonfractionated lava from single eruptions of Kilauea may be explained by olivine control alone, and this process is consistent with the observation that olivine is the only true phenocryst in unfractionated Kilauea lavas. The chemistry of nonfractionated lava from single eruptions of Mauna Loa can be explained by a more complex mineral control involving olivine, hypersthene, augite, and plagioclase, all being commonly present as phenocrysts. The addition or removal of these minerals is inferred to take place at pressures not exceeding 2 kilobars in shallow magma reservoirs located beneath the two volcanoes. The chemistry of Mauna Loa lavas shows no correlation with either the time of eruption or location of the eruptive vents. By contrast the lavas erupted at Kilauea summit may be subdivided into three groups pre-1750, 18th-19th century, and 20th century on the basis of their chemical composition compared at the same MgO content. The younger lavas are richer in lower melting constituents; for example, K2O, PaOs, and TiOa. The average composition of Mauna Loa lavas is similar to what one would obtain by extrapolating the Kilauea chemical variation backwards in time. It is possible that the oldest unexposed Kilauea lavas would be similar in composition to Mauna Loa lava. The similarity in the ratio of K20:P2Os both within the lavas of each volcano and between the two volcanoes suggests that Kilauea and Mauna Loa originated by partial melting in a mantle of uniform composition and that no processes other than either melting or crystal-liquid fractionation are needed to explain the chemical variations. cent into shallow crustal reservoirs at 2-4 km depth. 4. Settling of olivine during storage at 2-4 km and periodic eruption of the upper parts of these reservoirs, or rarely, the entire reservoir including settled olivine.

Kilauea Volcano, Hawaii: chronology and morphology of the surficial lava flows

“…37). Lavas erupted from this strand have olivine-controlled chemical compositions similar to lavas erupted at the summit (Wright, 1971). (Wright, 1971;Wright and Fiske, 1971) and morphology (Fig.…”

“…At least one ancient filled caldera has been identified and the relationship between caldera collapse and long-term variations in eruptive behavior discussed (R. T. Holcomb, 1976bHolcomb, , 1978. Temporal variations in magma chemistry have also received recent study (Wright, 1971). Though Stearns* idealized life history has held up well through several decades of critical evaluation, some of the recent contributions regarding large-scale slumping, phreatomagmatic eruptions, and caldera development have suggested revisions to the general model.…”

Temperatures in ambient mantle and plumes: Constraints from basalts, picrites, and komatiites

“…In contrast, the Ontong Java Plateau provides a good example of primary magma modification in magma chambers or mantle conduit walls (Figure 1). [Clague et al, 1991;Norman and Garcia, 1999;Murata and Richter, 1966;Wright, 1972Wright, , 1973Wright and Fiske, 1971;Wright et al, 1975], and Theistareykir, Iceland [Slater et al, 2001;Maclennan et al, 2003]. Crosses indicate Archean Al-undepleted komatiites [Bickle et al, 1975Nisbet et al, 1977Nisbet et al, , 1987Lesher and Arndt, 1995;Lahaye and Arndt, 1996].…”

On some fundamentals of igneous petrology

“…Thus, the large variation in MgO does not represent actual melt compositions, but generally reflects the overall large crystal content, mainly olivine, most of which are socalled 'Tramp Crystals' (Wright and Fiske 1971;Wright 1971). These are highly forsteritic (often Fo 85-92 ) olivine crystals entrained during the eruption process.…”