Variations in Post-Glacial Erupted Volume and Lava Composition Along the Western Volcanic Zone, Iceland: Implications for Mantle Heterogeneity and Melting Processes

Recent mapping has defined the outcrop areas, estimated total erupted volumes, ages and major and trace element geochemical variations within the 170 km-long plate boundary zone of western Iceland known as the Western Volcanic Zone (WVZ). Excluding the Grímsnes flank field, 35 total eruptions have occurred along the WVZ in post-glacial time. However, 57 % of these eruptions, accounting for 63 % of the total volume were erupted within the first 2-3 ky (25 %) of post-glacial time. Generally similar production variations have been documented everywhere else in Iceland where sufficient data exist, and have been related to the effects of glacial unloading. However, there is debate concerning the extent to which the volume maxima reflect increases in mantle melting or crustal controls on eruptive behavior. The variations in erupted volume along the WVZ approximately correlate with variations in incompatible element ratios that are best explained by melting processes. The lowest Zr/Y ratios coincide with the production maximum in early post-glacial time, suggesting higher total melting in the period immediately following deglaciation. However, production rates have continued to decline and Zr/Y ratios have continued to increase in the period from 9 Ka to present, features not predicted by current models of deglaciation effects. Superposed on the temporal evolution are spatial variations; all lavas from the northern third of the WVZ have lower Zr/Y at all ages than those from farther south. Although a range of complicated melting processes can produce this result without invoking significant mantle heterogeneity, the most likely interpretation, based on preliminary geochemical modeling is that the mantle underlying the northern WVZ is generally more depleted in incompatible elements than that farther south, despite lying slightly closer to the center of the Iceland hotspot. Our results support the general model for melting increases in the period immediately following deglaciation, but for the WVZ we also require significant along-axis mantle heterogeneity and a long-term melting decline that may be unrelated to deglaciation effects. Data from the Reykjanes Peninsula indicate similar long-term chemical trends, suggesting that overall decline in melt production characterizes all of western Iceland, and possibly of the Iceland hotspot generally over the last 9,000 years.