Laurie Reisberg

Os isotopic systematics of the MORB mantle: results from altered abyssal peridotites

Abyssal peridotites are fragments of the oceanic upper mantle. Previous studies have indicated that their Os isotopic compositions span a wide range, including values more radiogenic than most estimates of the average bulk Earth 187Os188Os ratio. This is difficult to reconcile with their derivation as residues of MORB partial melting. We present results that suggest that some of this variation, particularly the higher values, may be of secondary origin. Comparison of altered rims and unaltered cores of two mylonitic peridotites reveal that submarine weathering can increase 187Os188Os ratios by more than 5%. Leaching experiments demonstrate the presence of a radiogenic, easily leachable phase that is most easily explained as a seawater-derived component. Thus, Os isotopic results from abyssal peridotites must be interpreted with some caution unless it can be shown that the effects of seawater alteration were minor. We develop criteria based on Sr and Nd isotopic measurements, as well as major element compositional changes, that allow strongly altered samples to be identified. Applying these criteria, we find 10 samples, including literature data as well as new analyses, that have apparently suffered only minimal alteration. These samples range in 187Os188Os from 0.1221 to 0.1270, with a mean of 0.1246 and a standard deviation of 0.0014. Thus, we see no evidence for depleted mantle rocks more enriched in 187Os188Os than the preferred value of ∼ 0.127. Finally, if Os is lost from peridotites during weathering, it may provide a significant source of mantle Os to the worlds oceans.

The ReOs systematics of the Ronda Ultramafic Complex of southern Spain

The Os isotopic compositions of twelve ultramafic and six mafic layer samples from the Ronda Ultramafic Complex of southern Spain have been determined. Among the ultramafic rocks, 187Os/186Os varies from 0.98 to 1.12. A weak correlation is observed between 187Os/186Os and Re/Os. A much stronger correlation exists between Os isotopic ratio and Mg#, suggesting that the Re/Os ratios have been perturbed to some extent. Two alternatives are proposed to explain the relationship between Os composition and Mg#: (1) Continuous processes in the convecting mantle; (2) Radiogenic ingrowth since an ancient melt depletion event. No relationship is observed between 187Os/186Os and 143Nd/144Nd. This is probably because the Nd systematics were strongly affected by a recent metasomatic event, which apparently had little effect on the Os isotopic compositions. The Os isotopic ratios of the mafic layers range from 1.7 to 47.9. Within a single thick layer, the ratios vary from 16.5 to 47.9. These high ratios demonstrate that the layers are ancient features. Among the mafic samples, Os isotopic ratio is found to decrease strongly with increasing Os concentration, which ranges from 0.009 ppb to 1.16 ppb. One layer, which had a SmNd model age of less than 200 Ma, yielded a ReOs model age of about 2 Ga. This implies that neither system can be trusted to give accurate information about the time of mafic layer formation.

Erratum of “Os isotopic systematics of the MORB mantle: results from altered abyssal peridotites” [Earth Planet. Sci. Lett. 133 (1995) 411–421]

Abstract Abyssal peridotites are fragments of the oceanic upper mantle. Previous studies have indicated that their Os isotopic compositions span a wide range, including values more radiogenic than most estimates of the average bulk Earth 187 Os/ 188 Os ratio. This is difficult to reconcile with their derivation as residues of MORB partial melting. We present results that suggest that some of this variation, particularly the higher values, may be of secondary origin. Comparison of altered rims and unaltered cores of two mylonitic peridotites reveal that submarine weathering can increase 187 Os/ 188 Os ratios by more than 5%. Leaching experiments demonstrate the presence of a radiogenic, easily leachable phase that is most easily explained as a seawater-derived component. Thus, Os isotopic results from abyssal peridotites must be interpreted with some caution unless it can be shown that the effects of seawater alteration were minor. We develop criteria based on Sr and Nd isotopic measurements, as well as major element compositional changes, that allow strongly altered samples to be identified. Applying these criteria, we find 10 samples, including literature data as well as new analyses, that have apparently suffered only minimal alteration. These samples range in 187 Os/ 188 Os from 0.1221 to 0.1270, with a mean of 0.1246 and a standard deviation of 0.0014. Thus, we see no evidence for depleted mantle rocks more enriched in 187 Os/ 188 Os than the preferred value of ∼ 0.127. Finally, if Os is lost from peridotites during weathering, it may provide a significant source of mantle Os to the worlds oceans.

Behavior of Li and its isotopes during serpentinization of oceanic peridotites

Analyses of Li and Li isotopes in serpentinized peridotites have been performed using Thermo‐Ionisation Mass Spectrometry (TIMS) and Secondary Ion Mass Spectrometry (SIMS) techniques on samples collected from the southwest Indian Ridge (SWIR). In the bulk samples, Li concentrations range from 0.6 to 8.2 ppm, while whole rock δ6Li values range from −2.9 to −14‰. In situ analyses display a greater range in both Li concentration (0.1–19.5 ppm) and Li isotopic composition (−27 to +19‰), with the serpentinized portions having higher Li concentrations than the associated relict phases. These variations may reflect changes in Li partitioning and isotopic fractionation between serpentine and fluid with temperature and water/rock ratio. They may also be explained by changes in the composition of the serpentinizing fluid over the course of serpentinization. As the serpentine forms by interaction with a circulating fluid, it preferentially removes 6Li, causing the Li in the fluid to become isotopically heavier. The isotopic composition of the initial hydrothermal fluid is dominated by basalt‐derived Li, which easily overwhelms the very low Li content originally present in seawater. As this fluid circulates through ultramafic rocks, it induces the formation of serpentine that incorporates this mantle‐derived Li. Hence, Li in serpentine is mainly derived from oceanic crust rather than from seawater and serpentinization involves Li recycling within this crust. Consequently, Li isotopes are good tracers of the hydrothermal contribution in serpentinizing fluid. These results imply that serpentinized peridotites are probably only a minor sink of oceanic Li.

Source, genesis, and timing of giant ignimbrite deposits associated with Ethiopian continental flood basalts

Abstract The Ethiopian continental flood basalt (CFB) province (∼30 Ma, > 3 × 105 km3) was formed as the result of the impingement of the Afar mantle plume beneath the Ethiopian lithosphere. This province includes major sequences of rhyolitic ignimbrites generally found on top of the flood basalt sequence. Their volume is estimated to be at least 6 × 104km3, which represents 20% of that of the trap basalts. Their phenocryst assemblage (alkali feldspar, quartz, aegyrine-augite, ilmenite ± Ti-magnetite, richterite, and eckermanite) suggests temperatures in the range of 740 to 900°C. Four units were recognized in the field (Wegel Tena, Jima, Lima Limo, and Debre Birhan areas), each with its own geochemical specificity. Zr/Nb ratios remain constant between basalt and rhyolite in each area, and rhyolites associated with high-Ti or low-Ti basalts are, respectively, enriched or depleted in titanium. Their trace element and isotope (Sr, Nd, O) signatures (high 143Nd/144Nd and low 87Sr/86Sr ratios, compared to those of rhyolites from other CFB provinces) are clearly different from those of typical crustal melts and indicate that the Ethiopian rhyolites are among the most isotopically primitive rhyolites. Their major and trace element patterns suggest that they are likely to be derived from fractional crystallization of basaltic magmas similar in composition to the exposed flood basalts with only limited crustal contribution. Since Ethiopian high-Ti basalts have been shown to form from melting of a mantle plume, it is likely that Ethiopian ignimbrites, at least those that are Ti-rich, also incorporated material from the deep mantle. Rb-Sr isochrons on whole rocks and mineral separates (30.1 ± 0.4 Ma for Wegel Tena and 30.5 ± 0.4 Ma for Jima ignimbrites) show that most of the silicic volcanism occurred within 1.4 × 1015 mol) and Cl (6.4 × 1015 mol) into the atmosphere over a short time span, with the global cooling event at 30.3 Ma suggests that this volcanism might have accelerated the climate change that was already underway.

Os, Sr, Nd, Pb, O isotope and trace element data from the Ferrar flood basalts, antarctica: evidence for an enriched subcontinental lithospheric source

Os, Sr, Nd, Pb and O isotopes and trace element data are reported for basaltic andesite and andesite whole rocks and, in part, for selected mineral separates from the Jurassic Ferrar flood basalt province. Radiogenic Sr (> 0.709), unradiogenic Nd (eNd= −3 to −5), and radiogenic Pb isotopes, as well as low Nb/La ratios of 0.4 – 0.6 and Nb/La ratios between 0.45 and 0.6 are found for all rocks including our most primitive sample (Mg# = 71.9). This indicates involvement of either continental crust or enriched lithospheric mantle in magma genesis. 187Re/188Os correlates strongly with 187Os/188Os, with an age of 172 ± 5 Ma, in agreement with published ArAr data. Initial 187Os/188Os of 0.194 ± 0.023 is close to the range of typical mantle values for MORB, OIB and lithospheric mantle and much lower than that of continental crust. δ18O values between 5‰ and 7‰ were obtained on fresh bulk samples, separated plagioclases and clinopyroxenes. SrO and SrOs isotope mixing calculations between depleted mantle peridotite or mantle melts and crustal material rule out assimilation involving basalts with low Os concentrations, and simple binary mixing or pure AFC processes involving picrites. AFC processes, combined with continuous replenishment of picritic magmas, can explain the isotopic data, provided the crustal end-member has high 87Sr/86Sr and low δ18O values. However, lower crustal samples displaying these characteristics are absent in the Ferrar region, and are also unlikely to impart the sediment-like trace element patterns observed in the Ferrar data. A more likely explanation is a lithospheric source enriched by subducted sediments. A contribution to Ferrar magmatism from a plume cannot be distinguished.

Further Sr and Nd isotopic results from peridotites of the Ronda Ultramafic Complex

Abstract Clinopyroxenes derived from peridotites of the spinel and garnet facies of the Ronda Ultramafic Complex yield Sr and Nd isotopic ratios which extend the range of compositions found in the massif to values as depleted as 0.70205 for Sr and 0.51363 for Nd. Large-amplitude, short-wavelength isotopic variations are found to be ubiquitous throughout the massif. In the garnet facies, some of these variations are shown to be produced by the tectonic disaggregation of mafic layers in an isotopically depleted peridotite matrix. Ages obtained from garnet-clinopyroxene Sm Nd isochrons (about 22 m.y.) agree with previous determinations of the time of crustal emplacement. In the plagioclase facies, where the Sr and Nd isotopic compositions have been very strongly affected by recent cryptic metasomatism, detailed study of one sample reveals that intermineral Nd isotopic equilibrium exists between clinopyroxene, orthopyroxene, and plagioclase. This indicates that the metasomatism occurred at high temperatures, and thus probably within the mantle. A rough correlation between 143 Nd/ 144 Nd and 147 Sm/ 144 N , with an apparent “age” of 1.3 b.y. and an initial e Nd (0) value of +6.0, is observed among clinopyroxenes derived from river sediments from throughout the massif. This age is interpreted as the time that the massif left the convecting mantle and became incorporated into the sub-continental lithosphere.

Re–Os systematics of UB-N, a serpentinized peridotite reference material

The reference material (RM) UB-N is a typical representative of earths upper mantle. It is a serpentinized garnet and spinel-bearing peridotite (a metamorphosed lherzolite) from the Vosges mountains, France, that is well characterized for major and many trace elements. In order to test whether UB-N is a suitable Re–Os reference material, 32 digestions in three different laboratories (CRPG/CNRS, MPI (Mainz) and University of Leoben) with four different digestion techniques (low-temperature acid attack, Carius tube dissolution, high-pressure asher (HPA-S) acid attack and alkali fusion) were performed. The results show that the low-temperature acid attack is unsuitable for the study of the Re–Os systematics of UB-N. Surprisingly, the well-established Carius tube acid digestion technique also fails to completely digest all Os-bearing mineral phases. Only alkali fusion and HPA-S acid attack yield the highest Os concentrations. Though sample inhomogeneity has been recognized (approximately 6% RSD for 2-g sample aliquots), it is possible to determine a well-defined average Os concentration of 3.85±0.13 ng g−1 (95% confidence; 19 digestions, fusion and HPA-S only). Rhenium-bearing minerals are very homogeneously distributed and replicates within each laboratory yield highly reproducible results independent of the digestion technique. A value of 0.2095±0.0040 ng g−1 (95% confidence; n=24) is assigned to the Re concentration. The best estimate for the whole-rock 187Os/188Os is 0.1278±0.0002 (95% confidence; n=12). The UB-N reference material now has well-understood Re–Os systematics that are typical of fertile upper mantle rocks. Analysis of this standard is, thus, highly recommended for the validation of Re–Os analytical procedures.

Platinum-group elements and melt percolation processes in Sidamo spinel peridotite xenoliths, Ethiopia, East African Rift

Platinum-group elements (Os, Ir, Ru, Rh, Pt, Pd) and gold contents have been determined along with S and modal sulfide abundances in 15 well-characterized spinel peridotite xenoliths from the Sidamo area (Gregory Rift, Ethiopia). In this xenolith suite, a group of deformed peridotites (refractory lherzolites to harzburgites) has been metasomatized by volatile-rich small melt-fractions while a group of granular peridotites (harzburgites to cpx-rich, fertile lherzolites) indicate extensive re-equilibration with high fractions of OIB-like melts at the bottom of the sub-continental lithosphere. The PGE data can be explained by a model combining a recent porous flow melt percolation event at variable melt–rock ratios superimposed on a protolith variably depleted by an old partial melting event. All PGE spectra have sub-chondritic Pt/Ir and Pt/Ru, decreasing with decreasing bulk-rock Al2O3 contents, and thus inherited from the partial melting event; likewise, whether deformed or granular, all the harzburgites have PGE and S abundances characteristic of residues from high degree partial melting, i.e. almost no base metal sulfides (especially Mss) and Pd- and Au-depleted CI-normalized PGE patterns. The two textural groups of lherzolites display contrasting PGE abundances and sulfide modal compositions, both reflecting the differing recent magmatic histories of the two groups. In the deformed lherzolites, Pd and Au abundances increase relative to Pt and Ir in parallel with the amount of Al-poor metasomatic diopside and intergranular Cu–Ni-rich sulfides, also of indisputable metasomatic origin. For this textural group, transport and precipitation of S, Cu, Pd and Au from the volatile-rich small-melt fraction is likely. Despite their fertile bulk-rock compositions (up to 3.65 wt.% Al2O3 contents) and evidence of metasomatic Fe–Ni-rich sulfides precipitated along with cpx, the granular lherzolites show surprisingly low total PGE contents (11.9–13.3 ppb) as well as sub-chondritic Pd/Ir ratios. Such a PGE budget can be explained by extensive re-equilibration between the OIB-like melts and Mss of residual origin, coupled with a mobility of sulfide melts in high-porosity zones of the lithospheric mantle. Our data indicate that porous flow melt percolation events may significantly alter the PGE budget of the sub-continental lithospheric mantle with respect to PUM estimates.

Os isotopic systematics in mantle xenoliths; age constraints on the Canadian Cordillera lithosphere

Abstract The 187 Os / 188 Os ratios of lherzolites from eight xenolith suites from the Canadian Cordillera show a correlation with Al2O3 and heavy rare earth elements (HREE). The best interpretation of these correlations appears to be ancient melt depletion followed by a long period of radiogenic ingrowth. The 187 Os / 188 Os–Lu correlation is used to calculate an Os model age of 1.12±0.26 Ga for the lithospheric mantle throughout the Canadian Cordillera. This single melting age suggests that the mantle lithosphere now underlying the entire Canadian Cordillera may have formed by melting events closely spaced in time. This is consistent with seismic evidence of the extension of crustal basement under much of the Canadian Cordillera that is independent of the upper-crustal terranes overlying it. Indeed, this Proterozoic Os model age for the mantle contrasts with the younger formation ages (Nd model ages and U–Pb ages of zircons) of most crustal terranes of the region which are around 0.5 Ga. Early Proterozoic basement is exposed only in southeastern British Columbia and has the same age (1.9 to 2.3 Ga) as the ancestral North American crust, but is older than the Os model age of the mantle lithosphere underlying the Canadian Cordillera. The Canadian Cordilleran mantle is thus probably not a simple extension of the North American cratonic lithosphere beneath the adjacent mobile orogenic belt of the Canadian Cordillera. The difference in age between the formation of the Canadian Cordillera upper-crust and the formation of the underlying mantle suggests that this mantle lithosphere does not represent the mantle roots of the crustal terranes overlying it. Instead, these crustal terranes were thrust onto the mantle lithosphere during Canadian Cordillera orogeny. This contrasts strongly with Archean cratonic zones and Early Proterozoic belts where oldest crustal rocks and mantle may have the same formation age.