No abstract

Plate tectonic processes introduce basaltic crust (as eclogite) into the peridotitic mantle. The proportions of these two sources in mantle melts are poorly understood. Silica-rich melts formed from eclogite react with peridotite, converting it to olivine-free pyroxenite. Partial melts of this hybrid pyroxenite are higher in nickel and silicon but poorer in manganese, calcium, and magnesium than melts of peridotite. Olivine phenocrysts' compositions record these differences and were used to quantify the contributions of pyroxenite-derived melts in mid-ocean ridge basalts (10 to 30%), ocean island and continental basalts (many >60%), and komatiites (20 to 30%). These results imply involvement of 2 to 20% (up to 28%) of recycled crust in mantle melting.

The Hawaii Scientific Drilling Project recovered ∼3 km of basalt by coring into the flank of Mauna Kea volcano at Hilo, Hawaii. Rocks recovered from deeper than ∼1 km were deposited below sea level and contain considerable fresh glass. We report electron microprobe analyses of 531 glasses from the submarine section of the core, providing a high‐resolution record of petrogenesis over ca. 200 Kyr of shield building of a Hawaiian volcano. Nearly all the submarine glasses are tholeiitic. SiO2 contents span a significant range but are bimodally distributed, leading to the identification of low‐SiO2 and high‐SiO2 magma series that encompass most samples. The two groups are also generally distinguishable using other major and minor elements and certain isotopic and incompatible trace element ratios. On the basis of distributions of high‐ and low‐SiO2 glasses, the submarine section of the core is divided into four zones. In zone 1 (1079–∼1950 mbsl), most samples are degassed high‐SiO2 hyaloclastites and massive lavas, but there are narrow intervals of low‐SiO2 hyaloclastites. Zone 2 (∼1950–2233 mbsl), a zone of degassed pillows and hyaloclastites, displays a continuous decrease in silica content from bottom to top. In zone 3 (2233–2481 mbsl), nearly all samples are undegassed low‐SiO2 pillows. In zone 4 (2481–3098 mbsl), samples are mostly high‐SiO2 undegassed pillows and degassed hyaloclastites. This zone also contains most of the intrusive units in the core, all of which are undegassed and most of which are low‐SiO2. Phase equilibrium data suggest that parental magmas of the low‐SiO2 suite could be produced by partial melting of fertile peridotite at 30–40 kbar. Although the high‐SiO2 parents could have equilibrated with harzburgite at 15–20 kbar, they could have been produced neither simply by higher degrees of melting of the sources of the low‐SiO2 parents nor by mixing of known dacitic melts of pyroxenite/eclogite with the low‐SiO2 parents. Our hypothesis for the relationship between these magma types is that as the low‐SiO2 magmas ascended from their sources, they interacted chemically and thermally with overlying peridotites, resulting in dissolution of orthopyroxene and clinopyroxene and precipitation of olivine, thereby generating high‐SiO2 magmas. There are glasses with CaO, Al2O3, and SiO2 contents slightly elevated relative to most low‐SiO2 samples; we suggest that these differences reflect involvement of pyroxene‐rich lithologies in the petrogenesis of the CaO‐Al2O3‐enriched glasses. There is also a small group of low‐SiO2 glasses distinguished by elevated K2O and CaO contents; the sources of these samples may have been enriched in slab‐derived fluid/melts. Low‐SiO2 glasses from the top of zone 3 (2233–2280 mbsl) are more alkaline, more fractionated, and incompatible‐element‐enriched relative to other glasses from zone 3. This excursion at the top of zone 3, which is abruptly overlain by more silica‐rich tholeiitic magmas, is reminiscent of the end of Mauna Kea shield building higher in the core.

No abstract

No 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.

No abstract

No abstract