Mineral Compositions from the Hawaii Scientific Drilling Project (HSDP): Preliminary Results Part I - Clinopyroxene

Drill core samples recovered from the HSDP have sampled Mauna Kea volcano, Hawaii, to depths of over 3000 meters. These samples provide a temporal view of the evolution of the Mauna Kea magma plumbing system not easily discerned from surface sampling. We utilize mineral compositions from the HSDP as a monitor of the depths at which magmas stagnated and partially crystallized. Mineral compositions provide a record of magma stagnation, and due to their resistance to re-equilibration and mixing, provide an archive of the range of liquid compositions that entered the magma plumbing system. Most interesting is the question as to whether the Mauna Kea magma plumbing system changed during the construction of the volcanic edifice, or during the waxing and waning of magma supply as Mauna Kea passed over the Hawaiian mantle hot spot. Here we use clinopyroxenes to examine magma transport, and in two accompanying abstracts we compare these results to those obtained from the analysis of plagioclase and olivine compositions. We utilized the models of Putirka (1999) to test clinopyroxene-melt equilibria and the models of Putirka et al (2003) to calculate the P-T conditions of crystallization of clinopyroxene, where appropriate. Equilibrium tests suggest that clinopyroxene phenocrysts are out of equilibrium with whole rock compositions. As an attempt to recover an equilibrium liquid, we adjusted whole rock compositions by adding or subtracting olivine so as to achieve Fe-Mg exchange equilibrium between (calculated) residual liquid and average olivine phenocryst compositions. This strategy required the subtraction of large amounts of olivine, 20% on average. Approximately 68% of cpx phenocrysts were calculated to approach equilibrium with these corrected liquid compositions; only these phenocrysts are used for the calculation of P and T. The distribution of cpx phenocryst depth estimates is not unlike those found for Mauna Kea in earlier studies (Putirka, 1997; Yang et al., 1999). There is no evidence for ponding at any particular depth, such as the Moho; instead, partial crystallization appears to occur throughout a conduit that extends to depths well into the mantle (see Ryan, 1988). However, these estimates lose precision as our corrections for olivine accumulation cause our `liquid compositions' to migrate from an observable quantity; each individual depth estimate is imprecisely located. While the P-T record of HSDP clinopyroxenes is imprecise, clinopyroxenes record no striking temporal trends when composition is compared to core depth. Taken at face value, this lack of temporal variation may indicate that clinopyroxenes crystallized shortly after magma mixing (after olivine crystallization) and prior to plagioclase saturation.