Incompatible element

About: Incompatible element is a research topic. Over the lifetime, 2420 publications have been published within this topic receiving 154052 citations.

The tholeiite to alkalic basalt transition at Haleakala Volcano, Maui, Hawaii

Abstract: Previous studies of alkalic lavas erupted during the waning growth stages (<0.9 Ma to present) of Haleakala volcano identified systematic temporal changes in isotopic and incompatible element abundance ratios. These geochemical trends reflect a mantle mixing process with a systematic change in the proportions of mixing components. We studied lavas from a 250-m-thick stratigraphic sequence in Honomanu Gulch that includes the oldest (∼1.1 Ma) subaerial basalts exposed at Haleakaka. The lower 200 m of section is intercalated tholeiitic and alkalic basalt with similar isotopic (Sr, Nd, Pb) and incompatible element abundance ratios (e.g., Nb/La, La/Ce, La/Sr, Hf/Sm, Ti/Eu). These lava compositions are consistent with derivation of alkalic and tholeiitic basalt by partial melting of a compositionally homogeneous, clinopyroxene-rich, garnet lherzolite source. The intercalated tholeiitic and alkalic Honomanu lavas may reflect a process which tapped melts generated in different portions of a rising plume, and we infer that the tholeiitic lavas reflect a melting range of ∼10% to 15%, while the intercalated alkalic lavas reflect a range of ∼6.5% to 8% melting. However, within the uppermost 50 m of section. 87Sr/86Sr decreases from 0.70371 to 0.70328 as eruption age decreased from ∼0.97 Ma to 0.78 Ma. We infer that as lava compositions changed from intercalated tholeiitic and alkalic lavas to only alkalic lavas at ∼0.93 Ma, the mixing proportions of source components changed with a MORB-related mantle component becoming increasingly important as eruption age decreased.

The fidelity of melt inclusions as records of melt composition

Abstract: A series of experiments created melt inclusions in plagioclase and pyroxene crystals grown from a basaltic melt at 1,150°C, 1.0 GPa to investigate diffusive fractionation during melt inclusion formation; additionally, P diffusion in a basaltic melt was measured at 1.0 GPa. Melt inclusions and melts within a few 100 microns of plagioclase–melt interfaces were analyzed for comparison with melt compositions far from the crystals. Melt inclusions and melt compositions in the boundary layer close to the crystal–melt interface were similar, but both differ significantly in incompatible element concentrations from melt found greater than approximately 200 microns away from the crystals. The compositional profiles of S, Cl, P, Fe, and Al in the boundary layers were successfully reproduced by a two-step model of rapid crystal growth followed by diffusive relaxation toward equilibrium after termination of crystal growth. Applying this model to investigate possible incompatible element enrichment in natural melt inclusions demonstrated that at growth rates high enough to create the conditions for melt inclusion formation, ∼10−9–10−8 m s−1, the concentration of water in the boundary layer near the crystal was similar to that of the bulk melt because of its high diffusion coefficient, but sulfur, with a diffusivity similar to major elements and CO2, was somewhat enriched in the boundary layer melt, and phosphorus, with its low diffusion coefficient similar to other high-field strength elements and rare earth elements, was significantly enriched. Thus, the concentrations of sulfur and phosphorus in melt inclusions may over-estimate their values in the bulk melt, and other elements with similar diffusion coefficients may also be enriched in melt inclusions relative to the bulk melt.

Chemical composition of the mantle

Abstract: The composition of primitive mantle (54 elements) is estimated by a mass balance approach that does not make a priori assignments of basalt:peridotite ratios or LIL contents of these components. It is also not necessary to assume that such ratios as Rb/Sr and K/U are the same as in the crust. Primitive upper mantle is treated as a four-component system: crust, peridotite, LIL-depleted basalt (MORB), and an LIL-enriched component. These are combined to give chondritic ratios of the oxyphile refractory trace elements. The composition of the whole mantle is estimated by requiring chondritic ratios of the major elements as well. In this way one can estimate the volatile and siderophile content of the mantle. The primitive mantle has K = 152 ppm, U = 0.020 ppm, Th = 0.078 ppm, K/U = 7724, and Rb/Sr = 0.025. The ratios are significantly less than previous estimates. The inferred steady state heat flow, 0.9 μ cal/cm^2s, implies a substantial contribution of cooling to the observed heat flow. The crust and upper mantle may contain most of the terrestrial inventory of the incompatible elements, including K, U, and Th. There is no evidence that the chalcophiles are strongly partitioned into the core.