Geochemistry of Ultramafic Xenoliths from San Quintin, Baja California
- F.R. Boyd and
- Henry O.A. Meyer
Published Online: 19 MAR 2013
Copyright © 1979 by the American Geophysical Union.
The Mantle Sample: Inclusion in Kimberlites and Other Volcanics
How to Cite
Basu, A. R. (1979) Geochemistry of Ultramafic Xenoliths from San Quintin, Baja California, in The Mantle Sample: Inclusion in Kimberlites and Other Volcanics (eds F.R. Boyd and H. O.A. Meyer), American Geophysical Union, Washington, D.C.. doi: 10.1029/SP016p0391
- Published Online: 19 MAR 2013
- Published Print: 1 JAN 1979
Print ISBN: 9780875902135
Online ISBN: 9781118664858
- Basaltic liquids;
- Mafic and ultramafic inclusions;
- Plagioclase megacryst;
- San Quintin volcanic field
The maior element chemistry the abundances of K, Rb, Sr, Ba, and the 87Sr/86Sr ratios of separated silicate minerals in a suite of ultramafic xenoliths and plagioclase megacryst in alkalic basalts from San Quintin, Baja California, are reported. Data also include the trace elements and 87Sr/86Sr ratios in two host alkalic basalts. Although the xenoliths are enriched in modal diopsides, the trace element abundances are extremely low in all the silicate phases. For example, the diopsides contain as low as 10.7 ppm K, 0.01 ppm Rb, 1.7 ppm Sr and 0.6 ppm Ba. The diopsides from the pyroxenite layers usually show slightly higher abundances, such as 107 ppm K, 0.07 ppm Rb, 22 ppm Sr and 33 ppm Ba.
In four lherzolites, the coexisting olivines, orthopyroxenes, and elinopyroxenes were analyzed after separation by hand-picking. In three of these samples, each mineral fraction was analyzed twice - without washing and after washing in 2N cold HCl for three minutes. In the acid washed minerals (AW), K, Rb, Sr, and Ba abundances are reduced by about half from the unwashed abundances. In addition, 87Sr/86Sr ratios are also lowered by the acid washing. In one of these samples, the acid washed minerals and the acid washed whole rock reveal an isochron of T = 3.41±0.3 (2σ) and I = 0.70057±0.0004 (2σ). In the other three samples the coexisting silicates show clear disequilibrium in their 87Sr/86Sr ratios.
The clinopyroxenes from the different xenoliths show the lowest and the most variable 87Sr/86Sr ratios of all the silicates, from .70196±11 to .70445±5. Thus the data indicate that at least part of the vertical mantle profile beneath San Quintin, represented by the xenoliths studied here, is heterogeneous in its Sr-isotopic relies. This heterogeneity must reflect complex processes in the mantle, such as partial melting, removal of melt, cumulus processes, recrystalization, etc.
The extreme depletion of the trace elements in the silicate phases of the lherzolites, as reported here, contrasts remarkablv with the diopside-rich nature of xenoliths. No simple scheme of partial melting of these xenoliths can produce any normal basalt with its appropriate trace elemental abundances and ratios.