Calcium and neodymium radiogenic isotopes of igneous rocks: Tracing crustal contributions in felsic magmas related to super-eruptions and continental rifting

Abstract

Radioactive decay of 40K within the continental crust produces a unique Ca isotopic reservoir, with measurable radiogenic 40Ca excesses compared to Earth's mantle (εCa = 0). Thus, igneous rocks with values of εCa > 1 unambiguously indicate a significant old, crustal contribution to their source magma. At our current level of analytical precision, values of εCa < 0.5 are indistinguishable from mantle-like Ca isotope compositions. So, whereas 40Ca excesses clearly define crustal contributions, the source contributions of igneous rocks with mantle-like Ca isotopic composition are less certain. The calcium in these rocks could be derived from partial melting of: young crust, crust with mantle-like K/Ca compositions, or the mantle itself. Here we present Ca isotopic measurements of intermediate to felsic igneous rocks from the western United States, and two crustal xenoliths found within the Fish Canyon Tuff (FCT) of the southern Rocky Mountain volcanic field (SRMVF), USA. Their isotope geochemistry is used to explore their source compositions and to help distinguish new mantle-derived additions to the crust from reworked older crust.Irrespective of age or tectonic setting a majority of the intermediate to silicic igneous rocks studied exhibit mantle-like Ca isotope compositions. Mantle-like Ca isotopic data for leucogranites associated with the beginning of Rio Grande rifting in Colorado indicate that felsic melts were generated from newly formed lower crust related to earlier calc-alkaline magmatism. These data also indicate that the Nd isotopic signature in early rift magmas is controlled by the lithospheric mantle, even if the major mantle source reservoir is the asthenospheric mantle.The two crustal xenoliths found within the 28.2 Ma FCT yield εCa values of 3.6 and 7.0, respectively. The 40Ca excesses of these Precambrian source rocks are supported by K–Ca geochronology. However, like several other ignimbrites from the SRMVF and from Yellowstone, USA, the FCT (εCa ∼ 0.3) has a Ca isotope composition that is indistinguishable from the mantle. Nd isotopic analyses of the FCT imply that it was generated from 10–75% of an enriched component, and so the Ca isotopic data appear to restrict that component to newly formed lower crust, low K/Ca crust, or enriched mantle. Contrary to these findings, several large ignimbrites and one granitoid from the SRMVF show significant 40Ca excesses. These tuffs (Wall Mountain, Blue Mesa, and Grizzly Peak) and one granitoid (Mt. Princeton) are sourced from near, or within the Colorado Mineral Belt. Collectively, these data indicate that felsic, Precambrian crust likely contributed less than 50% of the material to the petrogenesis of many of the large ignimbrites that have erupted across the western United States. However, the crustal components that contributed to magmas in the Colorado Mineral Belt have 40Ca excesses; consistent with felsic, Precambrian crust.