Mathematics – Logic
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.v62b1400t&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #V62B-1400
Mathematics
Logic
1010 Chemical Evolution, 1020 Composition Of The Crust, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 1094 Instruments And Techniques, 3640 Igneous Petrology
Scientific paper
The IGBA igneous databank, compiled about 20 years ago by Dr. Felix Chayes, contained at least 19,000 petrological analyses of igneous and plutonic rocks from throughout the world and spanning time from the Archean to the present. It exists in two forms, one being the original ASCII format as entered on 80-character-per-line punch cards, and the other in a format accessible by Microsoft Access 97. This databank, though in need of updating, contains a wealth of data that may offer clues to the Earth's chemical and thermal evolution. It differs from others in that it includes, in addition to location and chemical compositions, petrographic and mineralogical details, and stratigraphic and radiometric ages. We are currently bringing the file up to date by entering data from more recent publications. Our initial investigations show that the file can be used to explore long-term trends in the compositions of common igneous rocks. For example, we find that the K/Rb ratio of flood basalts declines steadily from the earliest Archean until about the end of the Paleozoic, then increases at least until the Miocene. The same trend is seen in rhyolites, but data on younger rocks show that the trend reversed again around mid-Miocene time and has been declining until the present. Since K and Rb concentrations are governed mainly by the stability of amphibole and phlogopite in the source region, the temporal trends we observe could reflect global changes in the geothermal gradient and depths of melting. We see a somewhat similar effect in ocean island basalts from Paleocene time to the present. The K2O/(K2O + Na2O) ratio of flood basalts and rhyolites also shows similar behavior, that is, a decline from the Archean to Mesozoic and increase until Miocene time, though the pattern is less clear than that of K/Rb. These preliminary results show that databank can provide valuable information on the chemical and thermal evolution of the Earth.
McBirney Alexander R.
Torley Roy F.
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