Die Verteilung der Lanthaniden in basaltischen Gesteinen

Several types of basaltic and related rocks from NW Germany have been analysed for 14 lanthanides and yttrium. Alkali olivine basalts (13 samples) are the most common products of the late Tertiary volcanism in Northern Hessia and Lower Saxony. One basalt intermediate in composition between alkali olivine basalts and tholeiitic basalts has been investigated (intermediate basalt) beside 3 samples of the tholeiitic type. Several rare effusive rock species occur in this area. The number of samples is indicated in brackets: nepheline basanite (1), olivine nephelinites (5), peridotite inclusions (2) from one of the above mentioned alkali olivine basalts. Trachytes (3) and phonolite (1) from the Westerwald area, also Tertiary in age. Three nepheline leucite tephrites from the Eifel area, Pleistocene in age, and pyroxenes from the Recent Stromboli (Italy) have been included in this investigation.The lanthanides and yttrium are analysed after chemical preconcentration controlled by the use of spikes. La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb have been determined with good accuracy and precision by X-ray fluorescence, Eu, Tb, Ho, Tm and Lu by optical emission spectrography.Following earlier suggestions the distribution pattern of the lanthanides in basalts has been compared with that of chondrites. Tholeiitic basalts of the area under investigation show only a slight deviation from the relative distribution of the lanthanides in chondrites. The latter contain a twentieth of the absolute concentration in tholeiitic basalts. All other effusive rocks of this volcanic province have higher Y and La-Lu abundances and increasing ratios of La-Eu/Y, Gd-Lu (in brackets) in the following sequence: intermediate basalt (3.7); alkali olivine basalts (7.6); nepheline leucite tephrites (8.8); nepheline basanite (9.1); olivine nephelinites (10.2); phonolite (11.1); trachytes (11.6). The highest concentration of yttrium and of the lanthanides is observed in olivine nephelinites (up to 860 ppm Y, La-Lu).The observed increase in absolute concentration of the lanthanides and in relative accumulation of the light lanthanide elements from chondrites to tholeiitic basalts, to intermediate basalt, to alkali olivine basalts and to nepheline basanite makes a genetic interrelation in this sequence of rock types probable. Chondrites resemble garnet peridotites as potential main constituents of the upper earth's mantle. The pattern of the distribution of the lanthanides confirms a hypothesis that some tholeiitic basalts represent the most primitive of all basaltic magmas. Several models on the origin of both tholeiitic and alkali olivine basalts from potential source rocks or melts in the mantle have been checked with the data on the abundances of the lanthanides. There is still a lack of information on rare earths distribution in abundant rock forming minerals to completely exclude crystal fractionation under different pressures in the mantle as the origin of the different tholeiitic and alkali olivine basalt magmas. Alkali and gas accumulation (including the light lanthanides) in the upper parts of deep seated magma reservoirs should be considered as a potential source of the different alkali basalts. This is a process which has been observed by Richter and Moore (1966) in Hawaiian lava pools.The concentration of all and accumulation of the light lanthanides in the olivine nephelinites of our area is much too high to be explained by assuming an assimilation of sedimentary carbonate rocks in alkali olivine basalt melts.