Geochemistry of Mauna Loa and Mauna Kea Lavas Sampled by the Hawaii
Scientific Drilling Project
J M Rhodes (413-545-2841; jmrhodes@geo.umass.edu)
University of Massachusetts, Amherst, MA 01003, USA
Based on geochemical criteria, the boundary between Mauna Loa lavas and
underlying Mauna Kea lavas occurs at a depth of 257 m. At a given MgO
content, the Mauna Kea lavas tend to be lower in SiO2 and higher in
total alkalis, TiO2, CaO, and most incompatible elements. Ratios
involving Nb (e.g., Zr/Nb, Nb/Y) are particularly good discriminants between Mauna Loa and Mauna Kea lavas. The Mauna Loa lavas have compositional characteristics similar to
historical (1843-1984) lavas, but with a greater range in MgO content
(7-29 %). The upper Mauna Kea lavas (258-381 m.) contain two alkali
basalt flows (units 42 and 48) interlayered with transitional
tholeiites. Below this depth, all the Mauna Kea lavas are tholeiitic.
Apart from large fluctuations in MgO content (6-29 %), the subaerial
section of the Mauna Kea lavas are relatively uniform in composition,
becoming slightly more tholeiitic with depth. Two distinct interlayered
lava types can be identified in the submarine Mauna Kea section: one is
similar to the overlying subaerial lavas, whereas the other is lower in
SiO2 at a given MgO content, with slightly higher alkalis,
TiO2 and Nb contents. These lavas also have distinctly lower Zr/Nb than
typical Mauna Kea tholeiites (about 10 versus 12-13). Chemically they are very
similar to Loihi tholeiites. Thus raising the intriguing questions as to whether they are late-stage lavas from some other volcano, or whether early shield stage Mauna Kea lavas have been sampled. The lack of associated alkali basalts would seem to favor the latter interpretation.
FIGURES:
FIG-1 - Zr/Nb versus depth for HSDP samples. Note that the Mauna Loa samples have higher Zr/Nb than those from Mauna Kea.. Also that the uppermost transitional tholeiitic and alkalic Mauna Kea lavas have lower Zr/Nb than the underlying tholeiitic subaerial lavas. Among the submarine Mauna Kea tholeiites there appear to be two inter-layered lava types, one similar to the subaerial tholeiites, and the other with lower ratios.
FIG-2 - Alkalinity Index versus depth for HSDP samples. Just below the transition from Mauna Loa lavas are two alkalic flows inter-layered with transitional tholeiites. The rest of the lavas are tholeiitic. The alkalinity Index is derived from the line separating tholeiitic and alkalic lavas in a total alkali vs, silica plot (Carmichael et al., 1974) and is given by:-
alkalinity index = total alkali - SiO2 * 0.37 - 14.43.
FIG-3 - SiO2 versus MgO for HSDP samples. In addition to the scatter created by altered samples and the alkalic and transitional tholeiitic lavas, there appear to be two Mauna Kea lava types with different SiO2 values at a given MgO content.
FIG-4 - Normalized SiO2 versus depth for HSDP samples. The two lava types are clearly brought out in this figure, where SiO2 has been normalized to 13 wt. percent MgO through the addition or subtraction of olivine (Fo89). The low SiO2 lavas have distinctly lower Zr/Nb than typical Mauna Kea tholeiites (about 10 versus 12-13) and are chemically very similar to Loihi samples.
