Publication Date:
2019-07-13
Description:
Over time high K/Ca continental crust produces a unique Ca isotopic reservoir, with measurable 40Ca excesses compared to Earth's mantle (Ca=0). Thus, values of Cai 〉 1 indicate a significant crustal contribution to a magma. Values of Cai (〈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 Ca 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 (Cai ~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 Cai 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 Cai from 0 to ~3. In these samples Cai is generally correlated with Sri and is broadly negatively correlated with Ndi. However, for granites with similar Ndi at a given general location Cai can vary significantly (1 to 2 epsilon units). In rocks where low Ndi could also be due to melting from enriched reservoirs in the mantle lithosphere, the combination of high Cai with low Ndi clearly identifies crustal melts.
Keywords:
Lunar and Planetary Science and Exploration
Type:
JSC-CN-29420
,
2013 Geological Society of America Annual Meeting; Oct 27, 2013 - Oct 30, 2013; Denver, CO; United States
Format:
application/pdf
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