Metasomatism in oceanic and continental lithospheric mantle
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Abstract Petrological studies of spinel peridotite xenoliths provide information on the nature and physico-chemical evolution of the upper mantle and its variability on a regional scale, in both oceanic and continental environments. The xenolith populations studied in this work - from Mt. Lessini and Sardinia Italy , Mt. Accordingly, the incompatible element abundances of unmetasomatized samples vary from 1 -3 times chondritic in Iherzolites to about 0.
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Reractoy lithologies characterize suboceanic mantle. Thermobarometric estimates based on phase equilibria and CO2 inclusions indicate different pressure - temperature equilibration histories for xenoliths from different tectonic settings: I in both continental Sardinia and oceanic Cny and Cape Verde Islands stable within-plate settings, complete phase equilibration is generally reached at the P-T conditions of the regional geothermal gradient; 2 in continental rift settings Mt.
However, Mg in olivine in several Fish River peridotites extends to as low as The low orthopyroxene Al 2 O 3 in part stems from the occurrence of abundant minute exsolved rods of spinel that were too fine to analyze by microprobe. The high spinel Cr of single grains occurring in the matrix through the peridotites shows a significant departure from those in East Otago, North Otago, and the Auckland Islands Figure 3 b.
Olivine Mg tends to decrease toward these veinlets. The apatite grains are packed full of unidentified minute inclusions.
Most specimens from this location are greater than 10 cm in diameter, with the largest up to 40 cm in diameter. The Auckland Islands samples show the least chemical variation in southern Zealandia Figure 3. Orthopyroxene has Mg between Clinopyroxene Mg in all samples varies between Clinopyroxene is the principal repository of trace elements in the peridotites examined, with exceptions of amphibole in several of the West Otago samples and three North Otago samples, and apatite in several East Otago and West Otago samples.
Average normalized trace element values are illustrated in Figures 4 - 7. Trace element plots with all analyses are presented in supporting information Table 4. Averaged clinopyroxene trace element patterns for East Otago peridotites normalized to C1 Chondrite a, c, and e and Primitive Mantle b, d, and f values of Sun and McDonough [ ]. Sample locations are shown alphabetically.
Plots of individual clinopyroxene analyses can be found in supporting information File 3, and representative data in supporting information Table 4. East Otago clinopyroxene shows a complete spectrum of trace element patterns.
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Most of these samples also display the distinctive signs of having experienced metasomatic addition of LREE, as illustrated by upward inflections in normalized LREE e. This sample also has extreme depletions in Hf, Zr, and Nb Figure 4 f. There are some subtle differences between the clinopyroxene trace elements across the three North Otago locations.
Averaged clinopyroxene trace element patterns for North Otago a and b , West Otago c and d , and the Auckland Islands e and f. Plots of individual clinopyroxene analyses can be found in the supporting information File 3, and representative data in supporting information Table 4.
Most clinopyroxene grains also show very prominent Nb, Pb, and Ti negative anomalies Figure 5 d. The high and low temperature samples from this location have distinct trace element profiles. Data are tabulated in supporting information Table 2 and the methodology discussed in supporting information file 4. The isotope ratios overlap with the unradiogenic end of the intraplate basalt Sr isotope spectrum but extend to even less radiogenic compositions.
Selected isotope plots for Zealandia peridotites clinopyroxene; this study and Zealandia intraplate basalts whole rock; from the compilation of Timm et al. Note the apparent divergence in intraplate basalt and peridotite trends. See supporting information Table 4 for analytical methods and discussion of age corrections. Further details of the modeling and parameters are given in supporting information File 5.
Garnet peridotites reveal spatial and temporal changes in the oxidation potential of subduction
Most samples plot along the mantle reference line of Vervoort et al. To examine the origin and evolution of the peridotites, an attempt is made to model the clinopyroxene REE and isotope data. Following similar investigations [e. REE were modeled by adding various metasomatic components to variably depleted residues by bulk mixing Figure 7. The effects of depletion and metasomatism are most evident in the LREE because these elements are preferentially removed during melt depletion and common metasomatizing agents are LREE enriched.
HREE concentrations are less affected by metasomatism and can be used as proxies for melt depletion.
Metasomatism in Oceanic and Continental Lithospheric Mantle | NHBS Academic & Professional Books
The choice of metasomatizing agent has important implications. Our modeled carbonatitic compositions come from carbonatitic dikes that occur in West Otago [ Cooper and Paterson , ] and a published average carbonatitic composition from Bizimis et al. Following Stracke et al. The second scenario also involves source depletion in the past, but metasomatism occurs at the modern day immediately prior to xenolith entrainment. Innumerable intermediate scenarios are possible and the isotopic consequences of some of these are discussed in supporting information File 5.
Solid lines represent the effects of addition of a small amount 0. The silicate melt is a 0. Figure 8 b as for Figure 8 a but metasomatism is modern. In both models, the isotopic composition of the metasomatizing melt is assumed to be the same as the depleted source at the appropriate time. Figures 8 c and 8 d as for Figures 8 a and 8b, respectively, but the metasomatizing agent is an average Otago carbonatite from Cooper and Paterson [ ]. Figure 8 e a model using the same parameters as Figure 8 a illustrates the effect of increasing percentages of silicate metasomatizing component red lines to sources variably depleted at 1 Ga blue lines , immediately after the depletion event.
Figure 8 f as for Figure 8 e but source depletion occurred at 0. Analytical reproducibility for Hf and Nd isotopes is smaller than the symbol size. The mantle reference line in all diagrams is from Vervoort et al. Full details of the model parameters are presented in supporting information File 5. Ancient depletion followed by isolation and ingrowth will result in depleted lithologies with present day isotopic compositions significantly more radiogenic than modern depleted mantle. Even small degrees of ancient carbonatitic enrichment to a slightly depleted mantle source results in the trace element budget being dominated by the LREE of the carbonatitic component.
Metasomatism of ancient depleted residues with either silicate or carbonatitic components can thus decouple Nd and Hf isotopes, with the magnitude of the decoupling dependent upon the composition of the metasomatizing agent and the relative timing of depletion and enrichment Figures 8 e and 8 f. However, our goal is to use the results to demonstrate that the measured REE concentrations and isotope compositions are broadly consistent with processes of melt depletion and metasomatism and to allow estimation of the amounts of the metasomatic agent required.
Combining the clinopyroxene REE systematics and Hf and Nd isotopic compositions permits identification of three groups Groups 1—3 in the data set Figure 7. Such extreme decoupling of Hf from Nd has also been observed in some oceanic mantle samples [e. No indication of such a source component has been encountered in any Zealandia rock. Addition of a carbonatitic component to an ancient depleted source e. If these rocks were formed Ma by the degrees of depletion suggested by REE modeling, then they should now have much more radiogenic compositions e.
The models more closely replicate the isotopic composition of the peridotites when a metasomatic component that is more enriched than depleted mantle is used e. Carbonatitic enrichment is only consistent with the Group 3 isotopic compositions if depletion and metasomatism occurred relatively recently, such that there was insufficient time to allow for radiogenic ingrowth models not shown.
If source depletion was ancient 0. Therefore, depletion and replenishment of this sample must have been relatively recent and there has been insufficient time for ingrowth to change the isotopic compositions.
go site Before we can address the role of the Zealandia SCLM in intraplate basalt genesis, we must synthesize its composition, as this has not been done before. The peridotite major and trace element data reveal the unexpected result that moderately fertile SCLM domains beneath East Otago, North Otago, and the Auckland Islands reside adjacent to a highly refractory domain beneath West Otago.
It is striking that the some West Otago olivine grains have Mg that overlap spinel facies cratonic lithosphere Figure 9. An olivine Mg of Olivine Mg versus modal olivine diagram showing the variation in the four studied Zealandia peridotite suites. The gray arrow indicates the general melt depletion trend [ Boyd , ], with data falling off this trend caused by enrichment processes.
A number of the West Otago samples plot within the cratonic field for spinel facies depletion [ Bernstein et al. Olivine Mg can be found in supporting information Table 1, as can the modal counts. Therefore, the similar ranges of equilibration temperature imply that the depths of peridotite extraction between the different domains overlap. This is an important conclusion because it means that the broad compositional differences between the highly refractory West Otago and more fertile East Otago, North Otago and the Auckland Islands xenolith suites is most likely due to lateral SCLM variation.
Unlike some other situations where adjacent compositionally contrasting SCLM domains have been identified e. In fact, the continental crust of Otago is dominated by Carboniferous and younger quartzofeldspathic metasedimentary rocks [ Turnbull , ; Forsyth et al. Therefore, the West Otago lithosphere evidently has an early extensive depletion history that predates juxtaposition with the overlying continental crust.
However, there is increasing evidence that the oceanic lithosphere can contain ancient Hf and Os isotopic compositions that represent portions of ancient lithosphere that have been preserved within convecting asthenosphere for long periods of time [e. This is a preview of subscription content, log in to check access. Ackerman, L. CrossRef Google Scholar. Al-Fugha, H. Al-Malabeh, A. Barth, T.