Other
Scientific paper
Mar 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.1441f&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 5, pp.1441-1462
Other
28
Scientific paper
The voluminous shields of Hawaiian volcanoes are dominantly composed of tholeiitic basalts, but there are important intershield geochemical differences. The subaerial lavas forming the ~2-3 Ma Koolau shield have several extreme characteristics: relatively high abundances of SiO 2 , low abundances of total iron and CaO, and high ratios of La / Nb and Sr / Nb . In addition, they range to near bulk-earth strontium, neodymium, and lead isotopic ratios. Although postmagmatic alteration has significantly affected the compositions of some Koolau lavas (decreases in SiO 2 , K 2 O, and rubidium contents, increases in total iron and in unusual cases, increases in yttrium and REE abundances), the geochemical charac teristics of unaltered Koolau lavas reflect a distinctive primary magma composition. Within a stratigraphic sequence of lavas, Koolau lavas vary significantly in incompatible element abundance and isotopic ratios, but these variations are not systematic with eruption age, and they are smaller than the differences between Hawaiian shields. Intershield differences in some incompatible element abundance ratios, La / Nb and Sr / Nb , are correlated with intershield differences in isotopic ratios, thereby indicating that each shield formed from a compositionally distinct source. However, other intershield compositional differences are not correlated with differences in radiogenic isotope ratios. Some of these compositional differences probably reflect variations in the melting process; e.g., inverse correlations between SiO 2 and total iron contents may reflect differences in the pressure of melt segregation, differences in abundances of incompatible elements may reflect variations in mean degree of melting, and variations in ratios like Sm / Nd may reflect the presence of residual garnet. Each shield appears to reflect a unique combination of source components and variables, such as extent of melting and pressure of melt segregation. Consequently, the intershield geochemical differences have important implications for plume structure. Either a relatively large plume has a spatially systematic distribution of geochemical heterogeneities which are sampled by the overlying shields, or each shield is derived from a small radius (<20 km) conduit composed of geochemically distinct diapirs or solitary waves.
Frey Frederick A.
Garcia Michael O.
Roden Michael F.
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