Timescales of rhyolite formation at a mid-ocean ridge: Alarcon Rise segment of the northern East Pacific Rise.

Abstract:

Alarcon Rise is a ~50km long spreading segment and the northernmost of the East Pacific Rise. A near-continuous spectrum of lava compositions from basalt to rhyolite occur in the northern section of the Rise along a ~8km fault-lineament, with a rhyolite dome (~2340 m below sea level, ~0.01 km³) near its center. Lava flow mapping with~1m resolution AUV reveals that individual flows are compositionally variable. Two distinct trends are apparent in glass major elements. The dominant trend is fractional crystallization melt evolution from basalt to rhyolite. A subordinate trend from basalt to dacite offset to lower FeO_T, TiO₂, S0₃, and P₂O₅is interpreted as mixing between mafic and felsic magmas. Evidence for mixing is also preserved in mineral compositions that are commonly out of equilibrium with their host melts. SHRIMP U-Th zircon ages from 4 rhyolites yielded a pooled weighted mean age of 23.4 ± 4.5 (2σ) ka and a small population at 45 ± 9ka, likely inherited. Preliminary Ar-Ar incremental heating of 4 splits of oligoclase from one rhyolite sample yielded an identical age of 23.7 ± 4.3 ka. Marine-calibrated radiocarbon ages of foraminifera collected in pushcores atop separate andesite and dacite flows west of the rhyolite dome support minimum eruption ages >11.7 ka and >2.1- 3.4 ka, and geological relationships indicate they erupted both before and after the dome. Thus, the generation of differentiated magmas was protracted and their eruptions were repeated. Pending age data on older basalts to the west and interspersed intermediate lavas will clarify the minimum timescales of basalt-to-rhyolite differentiation.

Intra-flow and closely-spaced inter-flow compositional variability recorded in relatively few eruptions means that the magma reservoir was stratified, also observed in many much larger terrestrial silicic eruptions, and that an eruptive episode(s) may evacuate a significant volume that was stored in the reservoir, partially mixed with and expelled by newly injected melt. These processes may be ubiquitous along ridges, but it is more evident here because the stored, differentiated melts and the newly input mafic melts are so dissimilar chemically and mineralogically.