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Isotopic data are reported from carbonatite, syenite, and galena. The average initial values for four carbonate samples from Magnet Cove, Hot Springs County, Arkansas, give ∈ (Nd) = +3.8 ± 0.3 and 87S r/86Sr = 0.70363, based on an age of 100 m.y. for the complexes. Two carbonates from Potash Sulfur Springs yield lower ∈ (Nd) values of +2.4 and 3.0. Lead in the carbonate samples plots within the field defined by midocean-ridge basalts (MORB) but is not isotopically uniform. The least radiogenic sample yields 206Pb/204Pb = (18.98,207Pb/204Pb = 15.53, and 208Pb/204Pb = 38.90. A galena sample at Magnet Cove has similar, but slightly more radiogenic, ratios. The Pb, Sr, and Nd data for the Magnet Cove samples indicate an origin in large ion-lithophile (LIL) element-depleted mantle, with little or no crustal interaction with the magmas.

Whole-rock samples of three syenites from the Little Rock district have average ∈ (Nd) = +2.6 at 100 m.y., but have indeterminate initial 87Sr/86Sr ratios due to postem-placement alteration. Lead ratios from seven of nine syenite feldspars are similar to the carbonatite leads, averaging 206Pb/204Pb = 19.20, 207Pb/104Pb = 15.59, and 208Pb/204Pb = 38.96, but two samples exhibit extensive contamination from crustal lead, as defined by galena samples from central Arkansas. Variations in Pb isotopic composition are documented for samples from a single syenite quarry.

The carbonatite data generally agree with parallel data from the Oka, Quebec, carbonatite complex, although Nd is somewhat less radiogenic and Sr is somewhat more radiogenic than the Oka values. The 206Pb/204Pb ratio at Magnet Cove is lower than the lowest value at Oka (19.65), although the 207Pb/204Pb ratios are identical within error limits. All the isotope data indicate that the source for the Magnet Cove carbonatite is slightly less depleted in LIL elements than is the Oka source, probably illustrating mantle compositional heterogeneity. The Arkansas data provide additional evidence that alkaline complexes can yield information on the geochemical character of subcontinental mantle.

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