RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES4001, doi:10.2205/2005ES000181, 2005

Rock Chemistries

[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 pm 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 pm 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.

Major Elements

[45]  As follows from Table 10 and Figure 11, all of the rocks affiliate with the tholeiitic series. The composition points of the basalts and hornblende dolerite lie practically exactly on the boundary line separating the tholeiitic and calc-alkaline series, and sample I1072/1 is closer to MORB. All other rocks can be subdivided into two groups according to their major-element compositions: magnesian and ferrous. The rocks of the first group are primitive gabbroids and troctolites from sites I1032, I1060 (samples 10 and 12), and I1069. They are compositionally close to N-MORB and were, perhaps, produced by such melts. The rocks of the second group are Fe-Ti-oxide gabbronorites, gabbronorite-diorites, and diorites from sites I1060 and I1063. They are characterized by high contents of TiO2 (up to 4.47 wt %) and elevated concentrations of P2O5 (up to 1.44 wt % in gabbronorite I1063/5). It is interesting that the K2O concentrations of the rocks remain low and reach a maximum of 0.40 wt % in the same gabbrodiorite, whereas this concentration in the trondhjemites is as low as 0.16 wt %. The trondhjemites and harzburgites are quite different and have the lowest contents of these elements.

2005ES000181-fig16
Figure 16
[46]  As is seen in Figure 16, all analyses of the rocks can be approximated by a single line, i.e., judging from their major-element compositions, all of these rocks can be regarded as derivatives of a single parental melt like MORB. However, the analysis of the concentrations of trace elements points to a amore complicated situation (see below).

Trace and Rare-Earth Elements

2005ES000181-fig17
Figure 17
[47]  The chondrite-normalized REE patterns of the rocks are presented in Figure 17. The basalts and dolerites are characterized by practically unfractionated REE patterns with unclearly pronounced negative Eu anomalies, as is also typical of the cumulate harzburgites (sample I1063/2). In contrast to them, all gabbroids are depleted in LREE and have positive Eu anomalies. The primitive troctolites and gabbro bear the lowest concentrations of REE, particularly LREE, which are close to chondritic ones. The ilmenite-hornblende (Fe-Ti-oxide) varieties of the rocks are notably different from other varieties and have higher REE concentrations, which are close to those in the basalts; but these rocks differ from the basalts in having lower LREE contents and well pronounced positive Eu anomalies. The diorites and granites have REE concentrations by one order of magnitude higher than in the other rocks and generally flat REE patterns with clearly pronounced negative Eu anomalies. Slight LREE enrichment was detected only in the plagiogranites.

2005ES000181-fig18
Figure 18
2005ES000181-fig19
Figure 19
2005ES000181-fig20
Figure 20
[48]  The spidergrams of the rocks normalized to the primitive mantle (PM), N-MORB, and E-MORB are only insignificantly different from one another (Figures 18, 19, and 20) and, thus, are considered collectively. As can be seen from these patterns, the contents of incompatible elements in basalts and plutonic rocks from site I1063 are the closest to the analogous values of E-MORB, whereas gabbroids from other sites are close to N-MORB. The simplest fractionation patterns of these elements are characteristic of the basalts, and the most complicated ones are typical of the plutonic rocks. As could be expected, the highest concentrations of these elements typically occur in the diorites and granites, intermediate values are most common in the Fe-Ti-oxide gabbroids, and the minimum ones were detected in the troctolites and gabbro.

[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.

2005ES000181-fig21
Figure 21
[51]  The ore-element compositions of the samples were normalized to the primitive mantle (Figure 21). Note that none of the rocks, including fresh basalts containing volcanic glass, shows elemental patterns notably different from that of PM. As was mentioned above, all of the rocks are enriched in Pb, with its highest contents detected in basalts I1072/1, Fe-Ti-oxide gabbroids I1063/1 and I1063/6, diorites I1060/18, and trondhjemites. Practically all of the rocks display positive Ga anomalies. The primitive gabbroids are characterized by negative Sn and Mo anomalies, whereas most of the Fe-Ti-oxide gabbroids and, particularly, the late derivatives have positive anomalies at these elements. Analogous patterns are also shown by the basalts, whose concentrations are nearly normal. The Li contents of the rocks are close to or higher than those in PM. This also pertains to the basalts and peridotites. Moreover, the peridotite is enriched in Mo and Pb and depleted in Cu, Sn, and, to a lesser degree, V compared to PM.

[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.

Spreading versus plume association.
According to REE ratios, many researchers classify MAR basalts with two types of associations: related to spreading (normal) or plume (anomalous) [Dmitriev et al., 1999; Douglass et al., 1999; Schilling et al., 1983; Silantyev et al., 2002]. Following Schilling et al. [1983], the distinction is commonly drawn using the (La/Sm)N and (Ce/Yb)N ratios. According to their (La/Sm)N ratio equal to 0.70-0.83, all samples of the basalts and dolerites plot near the boundary between the plume and spreading associations. Although this systematics was proposed for basalts, we also applied it to the intrusive rocks, because their geochemistry retains many features of the parental melts. All of the gabbroids, including their Fe-Ti-oxide varieties plot within the field of normal (spreading) associations (La/Sm)N<0.7. The diorites belong to intermediate varieties with (La/Sm)N = 0.74 and 0.99, and the trondhjemites have this ratio equal to 2.01 (if this criterion is applicable to them) and fall into the field of plume associations. The (Ce/Yb)N ratios of the trondhjemites is at a maximum and reaches 2.22. (Ce/Yb)N ratios intermediate between those of these rocks and gabbroids are typical of the diorites, and the gabbroids are characterized by the minimum values (similarly to the (Ce/Yb)N ratio). As in the previous case, the harzburgites and basalts also exhibit intermediate values of this ratio.

[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.


RJES

Citation: Sharkov, E. V., N. S. Bortnikov, T. F. Zinger, and A. V. Chistyakov (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.

Copyright 2005 by the Russian Journal of Earth Sciences

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