GEOCHEMICAL CHARACTERISTICS OF BASALTIC GLASSES FROM THE AMAR AND FAMOUS AXIAL VALLEYS, MID-ATLANTIC RIDGE (36-DEGREES-37-DEGREES-N) - PETROGENETIC IMPLICATIONS

被引:62
作者
FREY, FA
WALKER, N
STAKES, D
HART, SR
NIELSEN, R
机构
[1] UNIV TEXAS,DEPT GEOL SCI,AUSTIN,TX 78712
[2] UNIV TEXAS,DEPT GEOL SCI,AUSTIN,TX 78712
[3] OREGON STATE UNIV,COLL OCEANOG,CORVALLIS,OR 97331
[4] WOODS HOLE OCEANOG INST,WOODS HOLE,MA 02543
关键词
D O I
10.1016/0012-821X(93)90217-W
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
The axial valley of the Mid-Atlantic Ridge from 36-degrees to 37-degrees-N was intensively sampled by submersible during the FAMOUS and AMAR projects. Our research focussed on the compositional and isotopic characteristics of basaltic glasses from the AMAR valley and the NARROWGATE region of the FAMOUS valley. These basaltic glasses are characterized by: (1) major element abundance trends that are consistent with control by multiphase fractionation (olivine, plagioclase and clinopyroxene) and magma mixing, (2) near isotopic homogeneity deltaO-18 = 5.2 to 6.4, Sr-87/Sr-86 = 0.70288 to 0.70299 and Pb-206/Pb-204 = 18.57 to 18.84, and (3) a wide range of incompatible element abundance ratios; e.g., within the AMAR valley chondrite-normalized La/Sm ranges from 0.7 to 1.5 and La/Yb from 0.6 to 1.6. These ratios increase with decreasing MgO content. Because of the limited variations in isotopic ratios of Sr, Nd and Pb, it is plausible that these compositional variations reflect post-melting magmatic processes. However, it is not possible to explain the correlated variation in MgO content and incompatible element abundance ratios, such as La/Sm and Zr/Nb, by fractional crystallization or more complex processes such as boundary layer fractionation. These geochemical trends can be explained by mixing of parental magmas that formed by very different extents of melting. In particular, the factor of three variation in Ce content in samples with approximately 2.1% Na2O and 8% MgO requires a component derived by < 1% melting. If the large variations in abundance ratios of incompatible elements reflect the melting process, a large, long-lived magma chamber was not present during eruption of these AMAR lavas. The geological characteristics of the AMAR valley and the compositions Of AMAR lavas are consistent with episodic volcanism; i.e., periods of magma eruption were followed by extensive periods of tectonism with little or no magmatism.
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页码:117 / 136
页数:20
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