Investigating Mantle Sources of Basaltic Melts Using Olivine LA-ICPMS Analysis, Mount Taylor Volcanic Field, New Mexico

Abstract:

The Mount Taylor Volcanic Field (MTVF), New Mexico, is located along the Jemez Lineament, a major crustal feature and a focus of post-Laramide magmatism. The MTVF comprises at least three regions containing contemporaneous (~3.7 to 1.26 Ma) Ne-normative volcanic rocks. The intermediate Mount Taylor (MT) strata-volcano contains early central and flanking mantle xenolith-bearing alkali basalts and hawaiites. The Rio Puerco (RP) volcanic necks contain mantle xenolith-bearing basanites and alkali basalts and no evolved lavas. Mesa Chivato (MC) contains an alkaline mafic to felsic suite with geochemical similarities to RP and MT lavas but no known mantle-xenoliths.

The MTVF xenoliths are diverse (e.g., Thomas et al., 2012, AGU Fall Meeting, V43A-2825) and suggest varying degrees of melt enrichment/fertilization. By LA-ICPMS, we are characterizing olivine trace element chemistry from the mantle xenoliths and basaltic (sensu lato) phenocrysts to test how much can be determined about likely source rocks by phenocryst olivine alone. This is part of a continuing project to investigate spatial trends in Laramide mantle melt metasomatism and its relation to post-Laramide magma compositions.

We have analyzed samples from a RP neck (lherzolite in alkali basalt); flows from the MT flank (websterite in alkali basalt) and from the MT amphitheater (wehrlite in hawaiite). Additionally, we analyzed olivine phenocrysts from three xenolith-free lavas: a MT basanite and MC alkali basalt and hawaiite.

(1) As diverse as the xenoliths are, their olivine clusters together with regards to most trace elements, though the xenoliths can be discriminated from each other by Co abundances alone or in Cr-Al and Cr-Zn space.

(2) Phenocrysts from xenolith-bearing alkali basalts cluster with the xenolith olivine, suggesting the melt was in equilibrium with a lithology of a similar trace element budget.

(3) Phenocrysts from the xenolith-bearing MT hawaiite and most MC phenocrysts are depleted in incompatible elements (Zr, Ti, and Y) relative to the xenoliths and cluster together, suggesting another melt source (assuming the xenolith-bearing lava is near-primary). Some MC olivine rims and the MT basanite phenocrysts are enriched relative to the xenoliths and may represent fractionation or mixing.