Anton P. le Roex Department of Geological Sciences, University of Cape Town, Cape Town, South Africa
Frederick A. Frey Department of Earth, Planetary and Atmospheric Sciences,
Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA
Stephen H. Richardson Department of Geological Sciences, University of Cape Town, Cape Town, South Africa
In this study a comprehensive dataset, comprising major elements, a wide variety of trace elements including rare earth elements (REE) and Sr and Nd isotopes, for crystalline rock samples, is used to develop a petrogenetic model for lavas from the AMAR Valley and Narrowgate region of the FAMOUS Valley at ~36°-37°N on the Mid-Atlantic Ridge (MAR). In contrast to early studies (e.g., Stakes et al., 1984), but in agreement with Frey et al. (1993), we find that the geochemical diversity of AMAR/Narrowgate lavas is not adequately explained by crustal processes. The upwelling mantle beneath this region is isotopically heterogeneous, but the full range in major and trace element variations occurs within isotopically homogeneous lavas. A complex model of incremental melting of progressively depleted spinel-lherzolite, followed by polybaric crystallisation over the depth interval 8-6 kbar, followed by rapid transport to, and eruption on, the sea-floor is required to explain the major and trace element abundance variations in the lavas.
Bulk Rock Geochemistry
Lavas from the AMAR/Narrowgate region of the MAR range in texture from aphyric to highly plagioclase phyric (>25% large plagioclase phenocrysts). Characteristic geochemical features are their relatively primitive character (Mg#=60 to 72), the wide range in incompatible element abundances, significant enrichment in highly incompatible elements (e.g., La, Nb, Ba) compared to slightly less incompatible elements (e.g., Zr and Sm) in many of the lavas, and the elevated Al2O3 content of the highly plagioclase phyric lavas. Absolute abundances of the incompatible elements in the most primitive lavas are low (e.g., Zr = 32-45 ppm) compared to most Mid-Ocean Ridge basalts (MORB). Aphyric lavas (and quench glasses) have remarkably constant CaO/Al2O3 ratios (0.80±0.02) and Sc abundances (40±1.5 ppm) over a considerable range in differentiation (Mg#=72-60; Fig 1). Incompatible elements with similar bulk distribution coefficients describe excellent correlations with constant inter-element ratios, whereas ratios between elements having slightly different bulk D (e.g., Zr and Nb, La and Sm) are quite variable (e.g., Zr/Nb=6-29; La/Smn=0.60-1.7) and show a well defined inverse correlation (Fig. 2).
Fig. 1: Variation in CaO/Al2O3 ratio and Sc abundance with respect to Mg# in aphyric AMAR lavas. Open symbols = CaO/Al2O3; solid symbols = Sc abundance.
Fig. 2: Covariation between (La/Sm)n and Zr/Nb ratio in AMAR/Narrowgate lavas. Field for FAMOUS lavas is shown for comparison.
Sr and Nd isotope data show that the lavas can be subdivided into two, individually homogeneous, groups; Group I lavas have lower 87Sr/86Sr ratios with an average value of 0.70288±1 (1<MATH> σ </MATH>; with corresponding 143Nd/144Nd = 0.51312±1), whereas Group II lavas have 87Sr/86Sr ratios with an average value of 0.70296±1 (with corresponding 143Nd/144Nd = 0.51309±2). With the exception of two samples, the two groups show a systematic spatial distribution within the rift valley, with the low 87Sr/86Sr group being confined to the rift valley floor, and the high 87Sr/86Sr group to the eastern and western flanks. Of significance is that the full compositional range in terms of major and trace element abundances, including incompatible elements ratios (e.g., Zr/Nb, La/Sm), occurs within isotopically homogeneous samples (e.g., Fig. 2).
The most striking characteristics of the AMAR/Narrowgate lavas are the wide range in incompatible element abundance ratios, the excellent correlation between such ratios (Fig. 2), and the near constant CaO/Al2O3 ratio and Sc abundance of aphyric lavas with decreasing Mg# (Fig. 1). Quantitative modelling assuming simple incremental batch melting with subsequent pooling of melts is unable to reproduce accurately the incompatible element correlations using either a spinel or garnet-lherzolite source. Furthermore, neither the low absolute abundances of incompatible elements nor the constant and high CaO/Al2O3 ratios of these lavas support a model requiring low degrees of melting to achieve the necessary fractionation in incompatible element ratios. The geochemical data are, however, consistent with a discontinuous melting process in which upwelling of an enriched mantle source with Zr/Nb ~8 and (La/Sm)n ~1.3 experiences moderate degrees of incremental batch melting (10-20%) to create the most LREE enriched and low Zr/Nb lavas, whereas those lavas with higher Zr/Nb ratios and LREE depletion are derived by similar high degrees of melting of the residue of an initially similar source, after prior extraction of a low (2-5%) degree melt fraction that does not contribute to the pooled melt.
Variations in CaO/Al2O3 ratio and Sc abundances are sensitive indicators of the relative ratio of plagioclase to clinopyroxene fractionation in MORB magmas. Calculated 'liquid-lines-of-descent' for primitive AMAR magmas predicts early and prolonged fractionation of plagioclase and olivine at low pressures, which causes a very rapid and strong increase in Sc abundance and CaO/Al2O3 ratio; a trend that differs markedly from that described by the aphyric AMAR lavas. The suppression of olivine crystallisation at high pressure in preference to clinopyroxene and plagioclase causes too rapid a decrease in Sc abundance and CaO/Al2O3 ratio. In contrast, polybaric crystallisation over the pressure interval from 8 to 6 kbar maintains a constant Sc abundance and CaO/Al2O3 ratio. The model predicts a liquidus temperature of 1280°C (at 8 kbar), at which point the magma is multiply saturated in clinopyroxene (Wo46En49Fs9; Al2O3=4.0%) and plagioclase (An86), with olivine (Fo88) joining the crystallisation sequence after 12% crystallisation (when P=6.8 kbar). The high pressure mineral compositions predicted by this model are remarkably similar to the observed compositions of megacrysts reported in the AMAR lavas by Stakes et al. (1984). The trend of constant Sc content and CaO/Al2O3 ratio with decreasing Mg# in aphyric AMAR lavas is attributed to superimposed polybaric crystallisation paths of distinct batches of primary magma (each sample forming the trends in Fig. 1 has a distinct Zr/Nb and La/Sm ratio).
The local variability in abundance ratios of incompatible elements and the spatial and temporal variability in the AMAR/Narrowgate region of the MAR are not expected from steady-state decompression melting of ascending mantle. The episodic melting process that we propose, coupled with subsequent high pressure polybaric crystallisation, is encompassed in the discontinuous, multi-stage melt extraction model of Bideau and Hekinian (1995).
Bideau, D. & Hekinian, R., J. Geophys. Res. 100, 10141-10162 (1995).
Frey, F.A., Walker, N., Stakes, D.S., Hart, S.R. & Nielson, R., Earth Planet. Sci. Lett. 115, 117-136 (1993).
Stakes, D.S., Shervais, J.W. & Hopson, C.A., J. Geophys. Res. 89, 6995-7028 (1984).
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