Glass in the submarine section of the HSDP2 drill core, Hilo, Hawaii

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. SiO 2 contents span a significant range but are bimodally distributed, leading to the identification of low‐SiO 2 and high‐SiO 2 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‐SiO 2 glasses, the submarine section of the core is divided into four zones. In zone 1 (1079–∼1950 mbsl), most samples are degassed high‐SiO 2 hyaloclastites and massive lavas, but there are narrow intervals of low‐SiO 2 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‐SiO 2 pillows. In zone 4 (2481–3098 mbsl), samples are mostly high‐SiO 2 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‐SiO 2 . Phase equilibrium data suggest that parental magmas of the low‐SiO 2 suite could be produced by partial melting of fertile peridotite at 30–40 kbar. Although the high‐SiO 2 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‐SiO 2 parents nor by mixing of known dacitic melts of pyroxenite/eclogite with the low‐SiO 2 parents. Our hypothesis for the relationship between these magma types is that as the low‐SiO 2 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‐SiO 2 magmas. There are glasses with CaO, Al 2 O 3 , and SiO 2 contents slightly elevated relative to most low‐SiO 2 samples; we suggest that these differences reflect involvement of pyroxene‐rich lithologies in the petrogenesis of the CaO‐Al 2 O 3 ‐enriched glasses. There is also a small group of low‐SiO 2 glasses distinguished by elevated K 2 O and CaO contents; the sources of these samples may have been enriched in slab‐derived fluid/melts. Low‐SiO 2 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.