RADIOGENIC INGROWTH OF 40CA FROM DECAY OF 40K PROVIDES A POWERFUL TRACER FOR UNDERSTANDING THE ORIGINS OF FELSIC MAGMAS

Over time high K/Ca continental crust produces a unique Ca isotopic reservoir, with measurable ⁴⁰Ca excesses compared to Earth’s mantle (ɛCa=0). Thus, values of ɛCa_i > 1 indicate a significant crustal contribution to a magma. Values of ɛCa_i (<1) indistinguishable from mantle Ca indicate that the Ca in those magmas is either directly from the mantle, or is from partial melting of newly formed crust. So, whereas ⁴⁰Ca excesses clearly define crustal contributions, mantle-like ⁴⁰Ca/⁴⁴Ca ratios are not as definitive. 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).

The two crustal xenoliths found within the 28.2 Ma FCT of the southern Rocky Mountain volcanic field (SRMVF) yield ɛCa values of ~4 and ~7.5, respectively. The ⁴⁰Ca excesses of these possible source rocks are due to long-term in situ ⁴⁰K decay and suggest that they are Precambrian in age. However, the FCT (ɛCa_i ~0.3) is within uncertainty of the mantle ⁴⁰Ca/⁴⁴Ca. Together, these data indicate that little Precambrian crust was involved in the petrogenesis of the FCT. Nd isotopic analyses of the FCT imply that it was generated from 10-75% of an enriched component, and the Ca isotopic data appear to restrict that component to newly formed lower crust, or enriched mantle. However, the Ca isotopic data do permit assimilation of some crust with low Ca/Nd; decreasing the ¹⁴³Nd/¹⁴⁴Nd without adding much excess ⁴⁰Ca to the FCT. Several other large tuffs from the SRMVF and from Yellowstone have ɛCa_i indistinguishable from the mantle. However, a few large tuffs from the SRMVF show significant ⁴⁰Ca excesses. These tuffs (Wall Mountain, Blue Mesa, and Grizzly Peak) are likely sourced from near, or within the Colorado Mineral Belt.

New isotopic measurements of Mesozoic and Tertiary granites from across the northern Great Basin show a range of ɛCa_i from 0 to ~3. In these samples ɛCa_i is generally correlated with ɛSr_i and is broadly negatively correlated with ɛNd_i. However, for granites with similar ɛNd_i at a given general location ɛCa_i can vary significantly (1 to 2 epsilon units). In rocks where low ɛNd_i could also be due to melting from enriched reservoirs in the mantle lithosphere, the combination of high ɛCa_i with low ɛNd_i clearly identifies crustal melts.