DISTINCT MANTLE SOURCE FOR THE CARIBBEAN LARGE IGNEOUS PROVINCE ULTRAMAFIC LAVAS OF TORTUGAL, COSTA RICA

The Caribbean Large Igneous Province (CLIP) is the only known large igneous province in the world to have erupted komatiite lavas since the Archean. The mafic and ultramafic lavas of Gorgona Island are well documented but the age equivalent Tortugal lavas of western Costa Rica are not as well characterized. We present major element, trace element, and Sr-Nd-Pb-Os isotope data for the mafic and ultramafic Tortugal lavas and ⁴⁰Ar-³⁹Ar ages from the ultramafic lavas. Petrological and geochemical data indicate at least three rock types: 1) an alkali- and LREE-enriched (La/Yb_n 4.2-14.3) ultramafic series of flows with abundant olivine and occasional spinifex pyroxene; 2) a LREE-enriched (La/Yb_n 3.8-12.1) set of alkalic basalts and diabases that intrude the ultramafic lavas; and 3) tholeiitic basalts with flat REE patterns (La/Yb_n 0.92-1.01) similar to the nearby 88-90 Ma Nicoya Peninsula basalts. The plagioclase separates yield age plateaus of 88.5± 0.7 Ma and 85.4 ±1.4 Ma, consistent with an earlier published analysis. This confirms that the Tortugal ultramafic flows erupted during the main constructional phase of the CLIP, including the nearby Nicoya basalts. Measured ¹⁸⁷Os/¹⁸⁸Os values for ten of the ultramafic lavas are 0.122-0.124 with Os concentrations of 1.6-3.0 ppb. These values are lower than the published Gorgona komatiite ¹⁸⁷Os/¹⁸⁸Os values of 0.137-0.159 and are similar to an estimated value of 0.1246 ± 0.0014 for depleted upper mantle (Snow and Reisberg, 1995). While this suggests derivation from a previously depleted mantle source, the LREE enriched patterns , low ε_Nd (+2.2 - +5.1), and the high alkali content of the ultramafic lavas indicate that other mechanisms had modified the source mantle. Taken together with previously published isotopic data (Hauff et al., 2000), the Tortugal lavas represent a distinct and complex mantle source tapped during the formation of the Caribbean Large Igneous Province.