TEMPORAL AND SPATIAL VARIATIONS IN THE MANTLE SOURCE REGIONS FOR BASALTIC MAGMATISM IN AN ACTIVE RIFT SYSTEM: EVIDENCE FROM MELT INCLUSIONS IN THE RIO GRANDE RIFT

In the Western United States, the Rio Grande Rift, extending from southern New Mexico to northwest Colorado, presents an opportunity to examine temporal and spatial variability in mantle sources for magmatism in an active rift setting with both variable rates and total amounts of extension, where the amount of extension decreases northward. Prior studies into mantle rheology have indicated that metasomatism and hydration may have a significant effect on the strength of the lithospheric mantle. Melt inclusions trapped within primitive phenocryst phases (e.g. olivine) provide a unique opportunity to examine variations in pre-eruptive volatile concentrations in basaltic magmas- providing greater insight into the nature of the hydration and metasomatism of the sub-rift mantle. To address the issues of temporal and spatial variability in mantle sources we have examined melt inclusions from basalts collected along the Rio Grande Rift (RGR), divided into southern, central and northern rift segments, with erupted ages spanning from ~37 million years to recent.

Sulfur and Cl (in addition to major elements) have been measured in melt inclusions from both tholeiitic and alkali basalts by electron microprobe analysis. A subset of inclusions has also been analyzed by secondary ion mass spectrometry for trace element and H₂O abundances. Water concentrations are generally below 0.6 wt% H₂O, with only one sample preserving concentrations above 1 wt%. Enrichment in Cl is monitored relative to Nb in basaltic inclusions. The highest Cl/Nb ratios are observed in ~37Ma basalts from the southern RGR. Similarly the southern RGR has the overall greatest temporal variability in chlorine enrichment, ranging from an avg. Cl/Nb of 150 (~37 Ma) to 10 (<50ka). Preliminary data suggests H₂O concentrations also decrease with time in the southern RGR. This temporal variation appears to be correlated to a transition from lithospheric to asthenospheric melting beneath the southern RGR. There is no systematic temporal variation in either the central or northern segments of the RGR. Chlorine enrichment also appears to decrease northward with a maximum avg. Cl/Nb of ~150 in the southern RGR to ~30 in the central RGR and ~19 in the northern RGR, coinciding with the decreasing amount of extension northward along the rift.