Wp. Leeman et al., GEOCHEMICAL VARIATIONS IN LAVAS FROM KAHOOLAWE VOLCANO, HAWAII - EVIDENCE FOR OPEN SYSTEM EVOLUTION OF PLUME-DERIVED MAGMAS, Contributions to Mineralogy and Petrology, 116(1-2), 1994, pp. 62-77
The Kahoolawe shield volcano produced precaldera and caldera-filling t
holeiites and mildly alkalic post-caldera lavas that petrographically
and compositionally resemble such lavas from other Hawaiian shield vol
canoes. However, Kahoolawe tholeiites display wide ranges in incompati
ble trace element ratios (e.g., Nb/Th = 9-24, Th/Ta = 0.6-1.3), Sr-87/
Sr-86 (0.70379-0.70440), Nd-143/Nd-144 (0.51273-0.51298), and Pb-206/P
b-204 (17.92-18.37). The isotopic variation exceeds that at any other
Hawaiian shield volcano, and spans about half the range for all Hawaii
an tholeiites. Quasi-cyclic temporal evolution of Kahoolawe tholeiites
is consistent with combined fractional crystallization and periodic r
echarge by primitive magmas. Ratios of highly incompatible trace eleme
nts and Sr, Nd, and Pb isotopic ratios form coherent sub-trends that r
eflect recurrent interactions between variably evolved magmas and two
other mantle components whose compositions are constrained by intersec
tions between these trends. The most MgO-rich Kahoolawe tholeiites are
partial melts of a high Nb/Th (approximately 23.5) ascending plume, p
ossibly comprising ancient subducted oceanic lithosphere. Slightly evo
lved tholeiites experienced combined crystal fractionation and assimil
ation (AFC) of material derived from a distinct-reservoir (Nb/Th appro
ximately 9) or asthenospheric derivation. The most evolved tholeiites
display compositional shifts toward a third component, having mid ocea
n ridge basalt-like isotopic ratios but enriched OIB-like trace elemen
t ratios, representing part of the lithospheric mantle (or melts there
of). Periodic recurrence of all three magma variants suggests that eru
ptions may have tapped coeval reservoirs distributed over a large dept
h range. Kahoolawe provides new evidence concerning the nature of the
Hawaiian plume, the distribution of compositional heterogeneities in t
he suboceanic mantle, and the processes by which Hawaiian tholeiites f
orm and evolve.