THE SCALE OF CHEMICAL HETEROGENEITY WITHIN INDIVIDUAL BASALT ERUPTIVE UNITS

The Jordan Valley Volcanic Field (JVVF) is a monogenetic basalt field in southeastern Oregon that displays heterogeneity within and between individual vents and lava flows, with particular complexities at ~1.5 to 2 Ma. A focus of ongoing research is to combine geochemical and paleomagnetic information as a means to understand the origin of these complexities.

We report here the results of a petrographic and chemical study focusing on sets of paleomagnetic cores taken from multiple JVVF basalt flows. A key question is when does a basalt sample become too small to be geochemically representative of its eruptive unit? To process a typical 2-3 kg hand sample for geochemical analysis a representative portion of the whole rock (~500 g) is reduced to a fine powder through a series of crushing and milling steps. Finally, 0.2 to 8 gram aliquots of powder are used for analytical work. For core samples, the entire core remaining after paleomagnetic work (~25-50 g) is processed, powdered, and analyzed using identical techniques. Core samples from an equigranular and fine grained basalt yield homogeneous bulk major and trace element chemistry comparable to analyses from hand specimens. In contrast, for a porphyritic to glomeroporphyritic basalt, core-to-core reproducibility is less precise particularly for those elements (e.g., Mg, Al, Ni) concentrated in the dominant constituent minerals. In this case the abundance of olivine phenocrysts and olivine + plagioclase clusters and a coarser more variable texture leads to core-to-core chemical heterogeneity. Thus, a single 25-50 gram sample of this type of basalt cannot be relied upon to adequately represent the geochemistry of the flow from which the core was taken. Therefore, we conclude that for the coarser grained basalts, an average of multiple separate core analyses or the processing of multiple cores (~125-300 g) as a single sample is required and should provide the same information as that derived from larger hand specimens. In contrast, more homogenous, fine-grained basalts can be accurately represented geochemically by analyzing individual small volume samples. These results serve as a basis for continuing investigations of remaining uncorrelated vents and flows in the JVVF and provide guidance in the combined use of core and hand samples for petrogenetic interpretations.