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.

[1] H 2 O, CO 2 , S, Cl, and F concentrations are reported for 556 glasses from the submarine section of the 1999 phase of HSDP drilling in Hilo, Hawaii, providing a high-resolution record of magmatic volatiles over $200 kyr of a Hawaiian volcano's lifetime. Glasses range from undegassed to having lost significant volatiles at near-atmospheric pressure. Nearly all hyaloclastite glasses are degassed, compatible with formation from subaerial lavas that fragmented on entering the ocean and were transported by gravity flows down the volcano flank. Most pillows are undegassed, indicating submarine eruption. The shallowest pillows and most massive lavas are degassed, suggesting formation by subaerial flows that penetrated the shoreline and flowed some distance under water. Some pillow rim glasses have H 2 O and S contents indicating degassing but elevated CO 2 contents that correlate with depth in the core; these tend to be more fractionated and could have formed by mixing of degassed, fractionated magmas with undegassed magmas during magma chamber overturn or by resorption of rising CO 2 -rich bubbles by degassed magmas. Intrusive glasses are undegassed and have CO 2 contents similar to adjacent pillows, indicating intrusion shallow in the volcanic edifice. Cl correlates weakly with H 2 O and S, suggesting loss during low-pressure degassing, although most samples appear contaminated by seawater-derived components. F behaves as an involatile incompatible element. Fractionation trends were modeled using MELTS. Degassed glasses require fractionation at p H 2 O % 5-10 bars. Undegassed low-SiO 2 glasses require fractionation at p H 2 O % 50 bars. Undegassed and partially degassed high-SiO 2 glasses can be modeled by coupled crystallization and degassing. Eruption depths of undegassed pillows can be calculated from their volatile contents assuming vapor saturation. The amount of subsidence can be determined from the difference between this depth and the sample's depth in the core. Assuming subsidence at 2.5 mm/y, the amount of subsidence suggests ages of $500 ka for samples from the lower 750 m of the core, consistent with radiometric ages. H 2 O contents of undegassed low-SiO 2 HSDP2 glasses are systematically higher than those of high-SiO 2 glasses, and their H 2 O/K 2 O and H 2 O/Ce ratios are higher than typical tholeiitic pillow rim glasses from Hawaiian volcanoes.