Petrology of the hilina formation,Kilauea volcano,Hawaii

The Hilina Formation comprises the oldest sequence of lava flows and tuffs exposed on Kilauea Volcano. These rocks are only exposed in kipukas in younger Puna Formation lavas along cliffs on the south flank of Kilauea Volcano. Locally, tuffs and flows of the Pahala Formation separate the underlying Hilina Formation rocks rom the overlying Puna Formation rocks. Charcoal collected from the base of the Pahala Formation yielded a C¹⁴ age of 22.800±340 years B.P. which defines a minimum age for the Hilina Formation. Hilina Formation lavas crop out over a wide region and probably originated from the summit area and from both rift zones. The Hilina Formation contains both olivine-controlled and differentiated lavas (using the terminology ofWright, 1971). The olivine-controlled lavas of the Hilina Formation are distinguishable mineralogically and geochemically from younger olivine-controlled Kilauea lavas. The younger lavas generally contain discrete low-calcium pyroxene grains. greater glass contents, higher K₂O/P₂O₅ ratios and lower total iron contents. Similar geochemical trends prevail for Manuna Loa lavas, and may typify the early lavas of Hawaiian shield volcanoes. Despite these similarities, the Hilina Formation (and all Kilauea) lavas have higher TiO₂ and CaO, and lower SiO₂ and Al₂O₃ contents than Mauna Loa Lavas. These differences have existed for over 30,000 years. Therefore, it is unlikely that the older lavas of Kilauea are compositionally similar to recent Mauna Loa lavas as was previously suggested. K₂O, TiO₂, Na₂ and Zr contents of lavas from a stratigraphic sequence of Hilina Formation lavas are variable. These variations may be utilized to subdivide the sequence into geochemical groups. These groups are not magma batches. Rather, they represent lavas from batches whose compositions may have been modified by crystal fractionation and magma mixing.