Evolution of Mauna Kea Volcano, Hawaii: Petrologic and geochemical constraints on postshield volcanism

Abstract

All subaerial lavas at Mauna Kea Volcano, Hawaii, belong to the postshield stage of volcano construction. This stage formed as the magma supply rate from the mantle decreased. It can be divided into two substages: basaltic (∼240–70 ka) and hawaiitic (∼66–4 ka). The basaltic substage (Hamakua Volcanics) contains a diverse array of lava types including picrites, ankaramites, alkalic and tholeiitic basalt, and high Fe‐Ti basalt. In contrast, the hawaiitic substage (Laupahoehoe Volcanics) contains only evolved alkalic lavas, hawaiite, and mugearite; basalts are absent. Sr and Nd isotopic ratios for lavas from the two substages are similar, but there is a distinct compositional gap between the substages. Lavas of the hawaiitic substage can not be related to the older basalts by shallow pressure fractionation, but they may be related to these basalts by fractionation at moderate pressures of a clinopyroxene‐dominated assemblage. We conclude that the petrogenetic processes forming the postshield lavas at Mauna Kea and other Hawaiian volcanoes reflect movement of the volcano away from the hotspot. Specifically, we postulate the following sequence of events for postshield volcanism at Mauna Kea: (1) As the magma supply rate from the mantle decreased, major changes in volcanic plumbing occurred. The shallow magma chamber present during shield construction cooled and crystallized, and the fractures enabling magma ascent to the magma chamber closed. (2) Therefore subsequent basaltic magma ascending from the mantle stagnated within the lower crust, or perhaps at the crust‐mantle boundary. Eruptions of basaltic magma ceased. (3) Continued volcanism was inhibited until basaltic magma in the lower crust cooled sufficiently to create relatively low‐density, residual hawaiitic melts. Minor assimilation of MORB‐related wall rocks, reflected by a trend toward lower 206Pb/204Pb in evolved postshield lavas, may have occurred at this time. A compositional gap developed because magma ascent was not possible until a low‐density hawaiitic melt could escape from a largely crystalline mush. Eruption of this melt created aphyric hawaiite and mugearite lavas which incorporated cumulate gabbro, wherlite, and dunite xenoliths during ascent.

Petrological Systematics of Mid-Ocean Ridge Basalts: Constraints on Melt Generation Beneath Ocean Ridges

“…Na,, is analogous to Na8.0 , where Na m is the Na content projected to 10% MgO. Diamonds are samples from Mauna Kea [Frey et al, 1990]; small solid squares from the Honolulu volcanics [Clague and Frey, 1981]; open squares from Loihi seamount. Samples were screened to have 7.5-12% MgO, > 41% Si02 , and a sum of > 98% for the major element oxides.…”

Geology, geochemistry and earthquake history of Lṑihi Seamount, Hawaìi's youngest volcano

“…At this stage, as the source once again samples cooler temperatures, magma compositions gradually switch back to alkalic during the post-shield stage (Feigenson et al, 1983;Frey et al, 1990). A rapid decline in eruption rate occurs over the next 250,000 years, which is accompanied by an abrupt shift to more fractionated lava compositions (hawaiites to trachytes, on some volcanoes such as Kohala; Spengler and Garcia, 1988), as magmas pond at greater depths (~30 km) before eruption (Frey et al, 1991).…”

Models of Hawaiian volcano growth and plume structure: Implications of results from the Hawaii Scientific Drilling Project

“…The integrated amount of melt produced per unit cross section of the plume is described approximately by a Gaussian distribution as a function of radial distance r from the plume axis [Watson and McKenzie, 1991]. As a volcanic center moves across the top of the plume the supply of magma to the volcano can be assumed to track the magma production rate in the plume, so that the eruption rate of the volcano starts out small, gradually increases to a peak value and then gradually decreases again to zero [e.g., Frey et al, 1990;Lipman, 1995]. For the purposes of constructing a model, we assume that the magma production rate per unit area of the plume is described by the following function of r' G(r) = G(0) exp (-r2/a2R 2) cos (rrr/2R),…”