Abstract

Major and trace element analyses have been made for 63 of 70 successive basaltic lavas from Steens Mountain in southeastern Oregon. The section was extruded 15.1 ± 0.3 m.y. ago, during a period when the geomagnetic field was changing from reversed to normal polarity. This indicates that the entire 1000-m section accumulated in no more than 50,000 years, and perhaps as little as 2000 years. Groups of 3 to 12 successive lavas possessing related within-group chemistry characterize the sequence. These major lava-groups are separable mainly on their pyroxene/plagioclase ratio which ranges from 46 to 76 percent by weight. Within certain lava groups, the TiO2 and total Fe contents systematically decrease with decreasing age by as much as 30 percent in sequences of as many as 12 successive lavas, with, in some cases, sharp reversals back to the composition of the parental magma type taking place at the boundaries of the major groups. Two geochemical trends are present, those dominated by plagioclase fractionation, and those due to olivine fractionation. This is expressed in the following facts:

(a) Elements virtually absent in plagioclase are diluted in concentration with increase in feldspar content, compared with the presumed parental magma. These include Fe, Ti, Mn, and Cu. Ratios between pairs of these elements remain essentially constant. Ratios involving these elements and elements that are included in plagioclase (such as Al/Ti or Ca/Fe) show, however, a great range of variation.

(b) Elements present in approximate equal amount in feldspar and parental magma (such as Si and Na) remain nearly constant in amount throughout.

(c) Homologous element pairs with markedly different distribution coefficients between plagioclase and magma show strong fractionation between high and low feldspar rocks; for example, K and Rb. The K/Rb ratio has a range of 500 to 750.

(d) Accumulation and subtraction of olivine has resulted in a Mg/Ni ratio increasing from 300 to 1000 with decreasing olivine content. Superimposed on this Mg/Ni distribution is the concentration-dilution trend due to plagioclase variation.

(e) Enrichment of K, Ba, Rb and P is found in lavas affected either by plagioclase or olivine removal.

The limitations which the paleomagnetic data place on the time span of the section suggests that systematic changes (as described above) occupying sequences of as many as 12 successive lavas took place in a time perhaps as short as 200 years. This provides an estimate of the maximum rate of change of TiO2 of the order of 1 percent absolute per thousand years for some of the sequences.

First Page Preview

First page PDF preview
You do not currently have access to this article.