Formation of mantle lithosphere beneath northern Mexico: Chemical and Sr‐Nd‐Pb isotopic systematics of peridotite xenoliths from La Olivina
Authors: Nimz Gregory J. , Cameron Kenneth L. , Niemeyer Sidney 
Published: February 2004

Peridotite and orthopyroxenite xenoliths of the Cr‐Diopside Group are found in ∼2 Ma basanitic cinder at the La Olivina locality in north central Mexico. The samples form continuous arrays on chemical and isotopic covariation diagrams and have similar coarse‐granular textures and equilibration temperatures (∼950°–1050°C). Sr and Nd isotopic compositions of acid‐washed diopsides (εNd= +1.9 to +13.3;87Sr/86Sr = 0.70220 to 0.70328) indicate long‐term depletion of Rb relative to Sr and Nd relative to Sm, similar to asthenospheric (mid‐ocean ridge basalt (MORB)) mantle. Lead isotopic compositions for all but one of the samples are generally higher in206Pb and207Pb, relative to204Pb, than is typical of MORB (206Pb/204Pb = 18.53 to 19.40;207pb/204pb = 15.57 to 15.62). The peridotite suite as a whole is significantly depleted in both clinopyroxene and incompatible trace element concentrations (Sm/Nd = 0.5 to 0.8; [Nd] = 1.19–2.69 μg/g). Isotopic and chemical systematics require that more than one depletion episode occurred in the history of the suite. Assuming that the La Olivina mantle had CHUR‐like (chrondrite uniform reservoir) isotopic compositions prior to any of the depletion events, the initial event must have occurred at least ∼735 Ma ago. The final depletion event could have occurred no more than ∼280 Ma ago and involved a melt extraction of about 5–10% of the existing mass. Nd isotopic similarities between the peridotites and late Paleozoic arc‐related basaltic rocks occurring in the Coahuila terrane to the southeast of La Olivina suggest that the final melting event may have produced this magmatism. The calculated magnitude of the presumed late Paleozoic melting and melt extraction implies that the La Olivina mantle was asthenospheric at that time. The peridotites contain veins of websterite that have resulted from mid‐Cenozoic magmatic activity in the La Olivina region, indicating that the peridotites were in place and attached to the continent prior to the mid‐Cenozoic. Increased buoyancy of the newly depleted asthenosphere coupled with a delamination of previously existing lithosphere, suggested by other studies, may have led to the accretion of the La Olivina mantle to the crust by the early Mesozoic (end of the Ouachita orogeny). Several La Olivina peridotites that have Sr‐Nd isotopic compositions that indicate long‐term depletion of incompatible elements have subsequently been enriched in these elements. These peridotites contain hydrous minerals and have rare earth element patterns that show enrichment of the light REE relative to the heavy REE. The Sr‐Nd‐Pb isotopic compositions of these samples indicate that the enriching agent contained isotopic components also present in the two episodes of Cenozoic magmatism in the La Olivina region. However, no satisfactory combination of agents directly related to the two magmatic events can account for the observed isotopic compositions of the enriched samples. This suggests that the enriching agent was not related to either of these magmatic episodes and that the isotopic components are not necessarily coupled together in the mantle regions producing the Cenozoic magmas.