WITHIN-FLOW VARIABILITY OF THE SUGAR LOAF MELILITE NEPHELINITE FLOWS, HONOLULU VOLCANICS, HAWAII

The Sugar Loaf flow is one of the youngest in the Honolulu Volcanics. The cinder/ash and flow erupted 76±2 ka, based on new incremental Ar-Ar age data on nepheline separated from a pegmatoid vein in the flow. This age is consistent with its stratigraphic position above the Waimanalo stage 5e reef (~124 ka). Melilite nephelinite, the most silica-undersaturated lava in Hawaii (36% SiO₂), ponded in Manoa Valley forming a ~15m thick flow. The flow is exposed in a quarry face on the University of Hawaii campus. The Sugar Loaf and nearby Tantalus flow in Pauoa Valley have co-eruptive cinder and ash below and above the flows. The volatile-rich Tantalus ash includes evolved glass with 6.3% MgO compared to the Sugar Loaf flow average of 11.8%. We evaluated the compositional variability of 14 representative Sugar Loaf flow samples, including 10 from a continuously cored section. We found that major elements deviate from the mean by <4% except CaO (7%) Na₂O (9%), and K₂O (11%). Sr, Ba, Rb, Pb, and U deviate from the mean by 9-17%, whereas other elements deviate by 2-6%, with larger deviation for HREE (>5%) than LREE (<3%) or MREE (<4%). The flow contains light-colored nepheline-rich zones that are chemically similar to the other flow samples whereas darker clinopyroxene-rich zones have higher MgO and Ba and lower CaO, Na₂O, and Sr. The pegmatoid veins that crosscut the core of the flow are enriched in REE and all major elements except FeO* and MgO compared to the average flow. LREE and HREE are enriched by factors of 1.9 and 2.5, so that separation of pegmatoid reduces the HREE contents in the rest of the flow. The chemical variability within the flow reflects mineralogical variability, segregation of pegmatoid veins, and mobility of Na, K, Sr, Ba, Rb, Pb and U, most likely in a vapor phase. Mobility of these elements after eruption introduces scatter in element concentrations and ratios. Depletion of these elements relative to other highly incompatible elements, such as La, Th, and P, was interpreted to indicate that mantle residua contained phlogopite and amphibole after the small amounts of partial melting that produced these melts. The variability observed does not necessarily negate this conclusion, but makes quantitative modeling of mantle melting challenging for thick, strongly silica-undersaturated flows like the Sugar Loaf.