P. W. Gast

Trace element fractionation and the origin of tholeiitic and alkaline magma types

Abstract Oceanic volcanic rocks can roughly be divided into two major chemical types: (1) tholeiitic or abyssal basalts and (2) alkaline or undersaturated basalts. Trace element abundances in these two types are compared. It is shown that the high abundance of large ion elements in alkaline basalts cannot be consistently explained by fractional crystallization processes. Both the absolute abundance and relative abundance of many large ion elements in alkaline magmas are entirely consistent with the hypothesis that these liquids are produced by partial melting (3–7 %) of accepted upper mantle mineral assemblages. It is postulated that the high K/Rb and low Ba/Sr and La/Yb ratios in abyssal basalts are characteristics of the upper mantle region that produced these basalts and that these characteristics are produced by previous partial melting episodes.

Lead isotope compositions and uranium decay series disequilibrium in recent volcanic rocks.

Concentrations of uranium, thorium, radium, total lead, lead-210 and lead isotopic composition are given for recent volcanic rocks from Tristan da Cunha, Faial, Azores and Mt. Vesuvius. Disequilibrium observed among uranium daughter products is interpreted in terms of chemical fractionations occurring during magmatic processes. In all cases, the fractionation pattern Ra > Th > U was observed. The fractionation indicated by the 230Th/238U ratio implies that removal of liquids from mantle sources lowers the Th/U ratio of the source region. This is consistent with the fractionation postulated to explain observed lead isotopic compositions in many oceanic volcanic provinces. The U-Pb fractionation pattern is more difficult to interpret than the Th-U and Ra-U fractionations.

Effects of weathering on the RbSr and KAr ages of biotite from the Morton Gneiss, Minnesota

Abstract Weathering has drastically reduced the Rb Sr and, to a lesser extent, the K Ar age of biotite from the Morton Gneiss of southwestern Minnesota. The ages are approximately 75% and 25% lower than the corresponding ages for biotite from the fresh gneiss. The effects of even incipient weathering cannot be neglected in Rb Sr dating of biotite and, by analogy, of feldspar and whole-rock samples.

Isotopic composition of lead in pegmatitic feldspars

Abstract An ion-exchange method was developed for extracting microgram quantities of lead from pegmatitic feldspars. The isotopic composition of lead in feldspars ranging from 350 to 2750 million years (m.y.) in age was determined. Model ages were calculated and were generally found to be in good agreement with independent age determinations. The results are compatible with a lead model stipulating relatively short periods of time in a crustal environment for the lead. In one instance where there was a large discrepancy between the model age and the known age, because of excess radiogenic lead in the feldspar, the anomaly was resolved by acid leach experiments which preferentially removed the excess radiogenic lead. The lead content of twenty-five feldspars was measured and found to range from

The isotopic composition of lead from St. Helena and ascension islands

The isotopic rempositions of lead from 7 volcanic rocks from St. Helena and 7 volcanic rocks from Ascension Island are presented. The St. Helena leads are the most radiogenic volcanic lead samples analyzed to date. A graphical presentation relating the parameters μ2, μ2 and T1 for a two-stage lead evolution model is given. It is suggested that both the U/Pb ratio and Th/U ratio of the upper mantle source of the St. Helena voleanic rocks were fractionated prior to several hundred m.y. ago.

Rare earth, alkaline and alkali metal and 87/86Sr data for subsamples of lunar sample 12013

Abstract Four subsamples of lunar rock 12013 have been analyzed. A “whole rock” sample has high rare earth and very high Ba concentrations with the chondrite normalized abundance pattern showing a slight enrichment in Ba and Ce and the usual large negative Eu anomaly. The light and dark components of this rock have similar REE. The light colored materials are commonly quite enriched in radiogenic 87 Sr and the Rb-Sr system appears to have lost Rb.

RARE EARTH ABUNDANCES IN SOIL AND ROCKS FROM THE OCEAN OF STORMS.

Abstract The rare earth abundance patterns in Apollo 12 basalts have much lower concentrations and smaller Eu anomalies than Apollo 12 soil and all analyzed Apollo 11 samples. The rare earth abundance patterns of Apollo 12 basalts are similar to and overlap those of achondritic meteorites. A partial melting model can explain the rare earth data for lunar samples.