Origin of chemical and isotopic heterogeneity in a mafic, monogenetic volcanic field: A case study of the Lunar Crater Volcanic Field, Nevada.

Major and trace element geochemistry and Sr, Nd, Pb, Hf and Os isotope signatures of basaltic lavas and tephra from volcanic centers in the northern Lunar Crater Volcanic Field (LCVF), Nevada, provide insight into the nature of their mantle sources and the role of lithospheric contamination versus source-related enrichment in producing compositional variations in basaltic monogenetic volcanic fields. Three of the studied eruptive centers (Hi Desert and Mizpah, ~620-740ka; and Giggle Springs, <80ka) are located within ~500m of each other; the Marcath volcano (~35-38ka) and Easy Chair (140ka), two of the youngest eruptive centers in the field, are located ~6 and 12km southwest of these cones, respectively. Isotopic studies of the volcanic rocks show a limited range in ¹⁴³Nd/ ¹⁴⁴Nd and ¹⁷⁶Hf/ ¹⁷⁷Hf, but significant heterogeneity in ⁸⁷Sr/ ⁸⁶Sr, ²⁰⁶Pb/ ²⁰⁴Pb and ¹⁸⁷Os/ ¹⁸⁸Os. The older (>140ka) Hi Desert, Mizpah, proto-Easy Chair and several unnamed flows exhibit Nb-Ta enrichment, Rb, Cs and K depletion, and high ²⁰⁶Pb/ ²⁰⁴Pb but low ⁸⁷Sr/ ⁸⁶Sr. In contrast, the younger (≤140ka) Giggle Springs, Easy Chair and Marcath lavas have high Ba, Rb and Cs and lower ²⁰⁶Pb/ ²⁰⁴Pb and higher ⁸⁷Sr/ ⁸⁶Sr. The lavas produce a well-defined negative correlation between Sr and Pb isotopes, attributed to mixing of heterogeneous mantle sources. The geochemical and isotopic signatures of the older Hi Desert, Mizpah, proto-Easy Chair and unnamed lavas are consistent with derivation from a mantle source with a component of ancient recycled oceanic crust. In contrast, the relatively high Ba, Rb and Cs coupled with lower ²⁰⁶Pb/ ²⁰⁴Pb and higher ⁸⁷Sr/ ⁸⁶Sr of the younger Giggle Springs, Easy Chair and Marcath lavas are consistent with derivation from a similar, but fluid-enriched, mantle source. Mixing calculations indicate that incorporation of ~18% of 0.8Ga recycled oceanic crust into depleted mantle can explain the trace element and isotopic signatures of the older group end member. Subsequent addition to this source of minor (<1%) hydrous fluid derived from subducted oceanic crust could account for the chemical and isotopic compositions of the younger group end member. Variable degrees of mixing between these two mantle end members can generate the full range of isotopic compositions observed in the northern LCVF sample suite, as well as within single eruptions. Our data indicate that the mantle source region in the LCVF is characterized by chemical and isotopic heterogeneity that manifests itself over a very small spatial scale (<500m) and within the time frame of a single monogenetic eruption. Similar processes may explain the geochemical and isotopic heterogeneities observed in other mafic monogenetic volcanic fields, the evidence for which may be preferentially preserved where small degrees of melting and rapid source to surface transport prevail.