RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES4001, doi:10.2205/2005ES000181, 2005
[43] The concentrations of major components in the rocks (Table 10) were
determined at the Geological Institute of the Russian Academy of
Sciences by conventional techniques. The rocks were analyzed for
trace elements at the Department of Mineralogy and Petrology of the
University of Granada, Spain. Zr was determined by XRF, using the
technique that involves fusing with Li tetraborates (the accuracy of
the method is
5% per 100 ppm Zr). Other trace elements were analyzed
by ICP-MS after decomposing 0.1000 g of the powdered material in an
HNO3
+ HF mixture in a teflon vessel at 180o C and a pressure of 200
pounds per square inch for 30 min and subsequent dissolving in 100 ml
of 4% HNO3. The accuracy of the analysis was close to
5% at an
analytical concentration of 10 ppm. The results are summarized in Table 11.
[44] Our analyses of the basalts and plutonic rocks from the study area for major and trace elements and for REE indicates that, having many common features, these rocks nevertheless show broadly varying geochemistry. The example of sites I1060 and I1063 demonstrates that samples from a single dredge are characterized by similar fractionation patterns of elements, but these patterns for analogous rocks vary from site to site. This suggests that the dredge captured rock fragments mostly from a single locality but not from the whole dredging course.
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Figure 16 |
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Figure 17 |
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Figure 18 |
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Figure 19 |
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Figure 20 |
[49] Most of the rocks are characterized by positive Pb anomalies and, often, also U anomalies; the opposite situation is typical only of the diorites, granodiorites, and plagiogranites. Most of the gabbroids have positive Sr and Eu anomalies, whereas the intermediate and acid rocks have negative anomalies at these elements. The only exceptions are the harzburgites, which have negative Sr and Eu anomalies. The overwhelming majority of the rocks is characterized by negative anomalies at Zr, Hf (the only exceptions are basalts I1027/5 and I1026/21), and Th (except only for the granites, which have small positive anomalies). Rocks from site I1063, particularly the Fe-Ti-oxide gabbroids, have strong positive anomalies at Ta and Nb and negative anomalies at Hf. Along with basalts from sites I1026 and I1027, these rocks are characterized by clearly pronounced positive anomalies at Cs and Rb, whereas the diorites and plagiogranites from site I1060 have negative anomalies at Rb.
[50] The concentrations of incompatible elements in our sample of the cumulus harzburgite (sample I1063/2) are generally notably higher than in the primitive mantle (Figure 18) at a generally similar character of fractionation, which is different from those in other rocks from the same site. These concentrations are only insignificantly lower than in E-MORB (Figure 20). According to the geochemical criteria in [Reverdatto et al., 2005], this harzburgite, which contains 1.35 ppm Sm, 1.64 ppm La, and 1.46 ppm Yb at a sum of REE of more than 20 ppm (Table 11), belongs to peridotites of crustal provenance. It should be stressed that some cumulate ultramafics from this site contain rare grains of kaersutite and ilmenite. This suggests that the ultramafic cumulates are the highest temperature members of the Fe-Ti-oxide association. The assemblages of trace elements contained in these rocks are generally the same as in other derivatives, but their concentrations are much lower.
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Figure 21 |
[52] Most of the rocks, except only the late derivatives and coarse-grained gabbro I1060/10, display positive Cu anomalies. Rocks from site I1060, particularly the trondhjemites, are depleted in Cu, whereas rocks from site I1063 (except sample I1063/6) are, conversely, slightly enriched in this element. All of the basalts are characterized by elevated contents of Cu, whose highest concentrations were detected in olivine gabbro from site I1032 and troctolites from site I1069. The Zn concentrations are generally higher than the normal value, particularly in the Fe-Ti-oxide gabbroids. Most of the samples display a negative correlation between their Cu and Zn concentrations.
[53] Similarly to the trace-element spidergrams of the primitive gabbroids and rocks of the Fe-Ti-oxide association, the configurations of ore-element plots of these rocks are also notably different. This pertains, first of all, to the Sn and Mo concentrations, which are notably higher in the latter rocks at low Ni and, particularly, Cr concentrations. The ore-element plots for the basalts are, again, much closer to the plots of rocks of the Fe-Ti-oxide association than to those of the primitive troctolites and gabbroids. Along with elevated concentrations of Cs and Pb in these rocks, this suggests that these rocks were produced by the mixing of two magma types or were derivatives of the magmas of the second association.
[54] Thus, the geochemistry of the seafloor rocks from the Sierra Leone MAR segment indicates that practically all of them, except only the diorites and trondhjemites, are enriched in Pb, U, Rb, Ta, Nb, Cs, and Rb relative to the primitive mantle and are depleted in Zr, Th, and Hf. The minimum concentrations of incompatible elements are typical of the primitive troctolites and gabbro, intermediate contents were detected in the hornblende Fe-Ti-oxide gabbronorites, and the maximum ones occur in the diorites and plagiogranites. Only some of the anomalies, such as those at Sr and Eu, were obviously controlled by crystallization differentiation, whereas this is not as evident for other elements: the rocks can be either depleted or enriched in components mobile in aqueous fluids. This also pertains to the peridotite with a relict cumulate texture.
[55] It follows that, according to their geochemistry, these rocks should be attributed to the spreading association. The diorites and trondhjemites are the closest to the plume association, although this is hardly significant because these associations are distinguished using basalts but not late derivatives. The rocks most different from the plume association are the gabbroids, including those belonging to the individual group of Fe-Ti-oxide gabbronorites, in spite of their anomalously high concentrations of Ti, Ta, and Nb, as is typical of plume-related melts. The basalts are somewhat different and are characterized, in contrast to the gabbroids, by flat chondrite-normalized REE patterns without significant depletion in LREE. However, these basalts are also heterogeneous: these rocks from sites I1026 and I1027 have positive Rb and, particularly, Cs anomalies, whereas basalt I1072/1 displays negative Rb and Cs anomalies.
[56] Our materials indicate that the rocks include no varieties that can be related (on the strength of geochemical evidence) to within-plate magmatism. The presence of weak positive Eu anomalies in the basalts suggests that they are not primary partial melts but underwent differentiation in intermediate chambers, in which cumulates of gabbro composition precipitated from the parental melt. An analogous negative anomaly in the diorites and granites suggests that they are late derivatives of the melts during the solidification of these chambers (intrusions). The virtually flat configurations of the HREE patterns indicate that the magmas were generated at moderate depths, and their petrogenesis did not involve garnet.
Citation: 2005), Silicic Fe-Ti-oxide series of slow-spreading ridges: petrology, geochemistry, and genesis with reference to the Sierra Leone segment of the Mid-Atlantic Ridge axial zone at 6° N, Russ. J. Earth Sci., 7, ES4001, doi:10.2205/2005ES000181.
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