Research Article| May 01, 1970 Geochemistry of the Steens Mountain Basalts, Oregon BERNARD M GUNN; BERNARD M GUNN Department of Geology, Université de Montréal, Montreal 101, Province Quebec, Canada Search for other works by this author on: GSW Google Scholar NORMAN D WATKINS NORMAN D WATKINS Department of Geology, Florida State University, Tallahassee, Florida 32306 Search for other works by this author on: GSW Google Scholar Author and Article Information BERNARD M GUNN Department of Geology, Université de Montréal, Montreal 101, Province Quebec, Canada NORMAN D WATKINS Department of Geology, Florida State University, Tallahassee, Florida 32306 Publisher: Geological Society of America Received: 07 Jul 1969 Revision Received: 10 Nov 1969 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1970, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1970) 81 (5): 1497–1516. https://doi.org/10.1130/0016-7606(1970)81[1497:GOTSMB]2.0.CO;2 Article history Received: 07 Jul 1969 Revision Received: 10 Nov 1969 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation BERNARD M GUNN, NORMAN D WATKINS; Geochemistry of the Steens Mountain Basalts, Oregon. GSA Bulletin 1970;; 81 (5): 1497–1516. doi: https://doi.org/10.1130/0016-7606(1970)81[1497:GOTSMB]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search 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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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Gunn et al. (1970) studied this question.