Mathematics – Logic
Scientific paper
Dec 1969
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1969gecoa..33.1485m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 33, Issue 12, pp.1485-1520
Mathematics
Logic
28
Scientific paper
In the Auckland volcanic field numerous, small volcanoes were built by basalt lavas and tephra in very recent times, as shown by geomorphology and stratigraphy, and confirmed by radiocarbon dating carried out on wood and peat buried by the eruptions. Searle suggested that activity was restricted to about the last 60,000 years. Because of the good age control, this area was chosen for a detailed study to test whether the K-Ar dating method could be used for dating such young basaltic rocks. Whole rock samples from 16 volcanoes were measured by the K-Ar method, and direct or indirect radiocarbon dating control was available for 11 of them. With few exceptions, anomalously old but often internally consistent, K-Ar dates were found for the lavas. Additional radiocarbon dating was then carried out on several wood samples; with one notable exception, the new results were consistent with the earlier determinations. For the volcanic island of Rangitoto, the radiocarbon, geological and botanical evidence unequivocally shows that it was active and was probably built during the last 1000 years. The K-Ar dates on basalts from this volcano range from 145,000 to 465,000 years. These and other results conclusively demonstrate that excess radiogenic argon is present in virtually all the lavas of the Auckland volcanic field in amounts between about 1 and 5×10 -13 moles/g. An isochron type of plot was developed to examine the K-Ar data in more detail. Together with independent geological evidence, this approach shows that lavas from an individual volcano commonly contain an essentially constant amount of excess Ar. This suggests that the excess Ar is not carried in the liquid at the time of eruption, but in a crystal phase or phases present in the magma. The possibility exists that the excess Ar is environmental and occurs in the very rare fluid inclusions found in olivine phenocrysts, but this idea is difficult to evaluate. The almost ubiquitous occurrence in the lavas of extremely small amounts of xenolithic quartz and of pyroxene aggregates after xenoliths leads us to postulate that the excess Ar is inherited and was derived from contamination of the magmas by older country rock, probably the underlying Mesozoic greywackes and argillites. The excess Ar may be carried in these pyroxene aggregates, but in the case of the Rangitoto lavas, some evidence was adduced that suggests the excess Ar may occur in inclusions of glass within augite phenocrysts. The relatively constant excess Ar in lavas from several of the volcanoes implies that eruption occurred from some form of underlying magma chamber in which the excess radiogenic Ar was uniformly distributed. For Pupuke volcano, a direct relation exists between K content of the lavas and apparent K-Ar age; variable contamination by older country rock is indicated in this case. These results show that even greater caution is now necessary in the interpretation of K-Ar dates on very young rocks, and that internal consistency may not always be a sufficient criterion by which to judge reliability. The problem of excess Ar is likely to be more severe in continental regions, where magmas must traverse a great thickness of older relatively K-rich rocks, than in oceanic regions. When dating very young rocks, those most likely to yield reliable K-Ar dates are well crystallized lavas that show no evidence of contamination by foreign material of any kind.
McDougall Ian
Polach H. A.
Stipp Jerry J.
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