J. E. Snow

MPI‐DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios

We present new analytical data of major and trace elements for the geological MPI-DING glasses KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, BM90/21-G, T1-G, and ATHO-G. Different analytical methods were used to obtain a large spectrum of major and trace element data, in particular, EPMA, SIMS, LA-ICPMS, and isotope dilution by TIMS and ICPMS. Altogether, more than 60 qualified geochemical laboratories worldwide contributed to the analyses, allowing us to present new reference and information values and their uncertainties (at 95% confidence level) for up to 74 elements. We complied with the recommendations for the certification of geological reference materials by the International Association of Geoanalysts (IAG). The reference values were derived from the results of 16 independent techniques, including definitive (isotope dilution) and comparative bulk (e.g., INAA, ICPMS, SSMS) and microanalytical (e.g., LA-ICPMS, SIMS, EPMA) methods. Agreement between two or more independent methods and the use of definitive methods provided traceability to the fullest extent possible. We also present new and recently published data for the isotopic compositions of H, B, Li, O, Ca, Sr, Nd, Hf, and Pb. The results were mainly obtained by high-precision bulk techniques, such as TIMS and MC-ICPMS. In addition, LA-ICPMS and SIMS isotope data of B, Li, and Pb are presented.

Coupled major and trace elements as indicators of the extent of melting in mid-ocean-ridge peridotites

Rocks in the Earths uppermost sub-oceanic mantle, known as abyssal peridotites, have lost variable but generally large amounts of basaltic melt, which subsequently forms the oceanic crust. This process preferentially removes from the peridotite some major constituents such as aluminium, as well as trace elements that are incompatible in mantle minerals (that is, prefer to enter the basaltic melt), such as the rare-earth elements. A quantitative understanding of this important differentiation process has been hampered by the lack of correlation generally observed between major- and trace-element depletions in such peridotites. Here we show that the heavy rare-earth elements in abyssal clinopyroxenes that are moderately incompatible are highly correlated with the Cr/(Cr + Al) ratios of coexisting spinels. This correlation deteriorates only for the most highly incompatible elements—probably owing to late metasomatic processes. Using electron- and ion-microprobe data from residual abyssal peridotites collected on the central Indian ridge, along with previously published data, we develop a quantitative melting indicator for mantle residues. This procedure should prove useful for relating partial melting in peridotites to geodynamic variables such as spreading rate and mantle temperature.

A long in situ section of the lower ocean crust: results of ODP Leg 176 drilling at the Southwest Indian Ridge

Ocean Drilling Program Leg 176 deepened Hole 735B in gabbroic lower ocean crust by 1 km to 1.5 km. The section has the physical properties of seismic layer 3, and a total magnetization sufficient by itself to account for the overlying lineated sea-surface magnetic anomaly. The rocks from Hole 735B are principally olivine gabbro, with evidence for two principal and many secondary intrusive events. There are innumerable late small ferrogabbro intrusions, often associated with shear zones that cross-cut the olivine gabbros. The ferrogabbros dramatically increase upward in the section. Whereas there are many small patches of ferrogabbro representing late iron- and titanium-rich melt trapped intragranularly in olivine gabbro, most late melt was redistributed prior to complete solidification by compaction and deformation. This, rather than in situ upward differentiation of a large magma body, produced the principal igneous stratigraphy. The computed bulk composition of the hole is too evolved to mass balance mid-ocean ridge basalt back to a primary magma, and there must be a significant mass of missing primitive cumulates. These could lie either below the hole or out of the section. Possibly the gabbros were emplaced by along-axis intrusion of moderately differentiated melts into the near-transform environment. Alteration occurred in three stages. High-temperature granulite- to amphibolite-facies alteration is most important, coinciding with brittle^ductile deformation beneath the ridge. Minor greenschist-facies alteration occurred under largely static conditions, likely during block uplift at the ridge transform intersection. Late post-uplift low-temperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high- and low-temperature alteration are their respective

Magmatic and amagmatic seafloor generation at the ultraslow-spreading Gakkel ridge, Arctic Ocean

A high-resolution mapping and sampling study of the Gakkel ridge was accomplished during an international ice-breaker expedition to the high Arctic and North Pole in summer 2001. For this slowest-spreading endmember of the global mid-ocean-ridge system, predictions were that magmatism should progressively diminish as the spreading rate decreases along the ridge, and that hydrothermal activity should be rare. Instead, it was found that magmatic variations are irregular, and that hydrothermal activity is abundant. A 300-kilometre-long central amagmatic zone, where mantle peridotites are emplaced directly in the ridge axis, lies between abundant, continuous volcanism in the west, and large, widely spaced volcanic centres in the east. These observations demonstrate that the extent of mantle melting is not a simple function of spreading rate: mantle temperatures at depth or mantle chemistry (or both) must vary significantly along-axis. Highly punctuated volcanism in the absence of ridge offsets suggests that first-order ridge segmentation is controlled by mantle processes of melting and melt segregation. The strong focusing of magmatic activity coupled with faulting may account for the unexpectedly high levels of hydrothermal activity observed.

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.

190Pt–186Os and 187Re–187Os systematics of abyssal peridotites

Abstract Abyssal peridotites are normally thought to be residues of melting of the mid-ocean ridge basalt (MORB) source and are presumably a record of processes affecting the upper mantle. Samples from a single section of abyssal peridotite from the Kane Transform area in the Atlantic Ocean were examined for 190Pt–186Os and 187Re–187Os systematics. They have uniform 186Os/188Os ratios with a mean of 0.1198353±7, identical to the mean of 0.1198340±12 for Os–Ir alloys and chromitites believed to be representative of the upper mantle. While the Pt/Os ratios of the upper mantle may be affected locally by magmatic processes, these data show that the Pt/Os ratio for the bulk upper mantle has not deviated by more than about ±30% from a chondritic Pt/Os ratio over 4.5 billion years. These observations are consistent with the addition of a chondritic late veneer after core separation as the primary control on the highly siderophile element budget of the terrestrial upper mantle. The 187Os/188Os of the samples range from 0.12267 to 0.12760 and correlate well with Pt and Pt/Os, but not Re/Os. These relationships may be explained by variable amounts of partial melting with changing DRe, reflecting in part garnet in the residue, with a model-dependent melting age between about 600 and 1700 Ma. A model where the correlation between Pt/Os and 187Os/188Os results from multiple ancient melting events, in mantle peridotites that were later juxtaposed by convection, is also consistent with these data. This melting event or events are evidently unrelated to recent melting under mid-ocean ridges, because recent melting would have disturbed the relationship between Pt/Os and 187Os/188Os. Instead, this section of abyssal peridotite may be a block of refractory mantle that remained isolated from the convecting portions of the upper mantle for 600 Ma to >1 Ga. Alternatively, Pt and Os may have been sequestered during more recent melting and possibly melt/rock reaction processes, thereby preserving an ancient melting history. If representative of other abyssal peridotites, then the rocks from this suite with subchondritic 187Os/188Os are not simple residues of recent MORB source melting at ridges, but instead have a more complex history. This suite of variably depleted samples projects to an undepleted present-day Pt/Os of about 2.2 and 187Os/188Os of about 0.128–0.129, consistent with estimates for the primitive upper mantle.

Pervasive magnesium loss by marine weathering of peridotite

Bulk abyssal peridotites have lower MgO and higher SiO2 than comparable continental peridotites. If these low MgOSiO2 ratios were a primary feature of the oceanic mantle, a major re-evaluation of the concentrations of these elements in the bulk silicate Earth would be necessary. Alteration mechanisms documented until now in abyssal peridotites (amphibolite facies metamorphism and serpentinization) are nearly isochemical (besides water), and thus do not explain such widespread compositional differences. We investigate here the possibility that the low MgOSiO2 ratios are due to a different type of alteration. We have measured bulk compositions in a suite of abyssal peridotites and compared these to their primary compositions, measured where possible or estimated. It is clear that the compositional differences between continental and abyssal peridotites are an alteration phenomenon. It is likely that they are not a result of serpentinization, but of pervasive weathering, below about 150°C (the temperature below which seawater is undersaturated with Mg-rich minerals), and at water/rock ratios between 103 and 105. The average dredged abyssal peridotite appears to have lost about 5 wt% MgO. Peridotite is an important component of the ocean crust. Its chemical behavior during alteration thus has a large impact on global mass budgets for many elements. A preliminary estimate of the magnitude of Mg loss from the ultramafic part of the ocean crust shows the maximum contribution to the oceans (1012 moles Mg/y) to be significant, nearly 85% of the yearly Mg flux from rivers. Such an input of Mg to the oceans from peridotites would require that Mg-based estimates of the total hydrothermal flux be revised upward.

Ancient, highly heterogeneous mantle beneath Gakkel ridge, Arctic Ocean

The Earth’s mantle beneath ocean ridges is widely thought to be depleted by previous melt extraction, but well homogenized by convective stirring. This inference of homogeneity has been complicated by the occurrence of portions enriched in incompatible elements. Here we show that some refractory abyssal peridotites from the ultraslow-spreading Gakkel ridge (Arctic Ocean) have very depleted 187Os/188Os ratios with model ages up to 2 billion years, implying the long-term preservation of refractory domains in the asthenospheric mantle rather than their erasure by mantle convection. The refractory domains would not be sampled by mid-ocean-ridge basalts because they contribute little to the genesis of magmas. We thus suggest that the upwelling mantle beneath mid-ocean ridges is highly heterogeneous, which makes it difficult to constrain its composition by mid-ocean-ridge basalts alone. Furthermore, the existence of ancient domains in oceanic mantle suggests that using osmium model ages to constrain the evolution of continental lithosphere should be approached with caution.

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.

Discovery of abundant hydrothermal venting on the ultraslow-spreading Gakkel Ridge in the Arctic Ocean

Submarine hydrothermal venting along mid-ocean ridges is an important contributor to ridge thermal structure, and the global distribution of such vents has implications for heat and mass fluxes from the Earths crust and mantle and for the biogeography of vent-endemic organisms. Previous studies have predicted that the incidence of hydrothermal venting would be extremely low on ultraslow-spreading ridges (ridges with full spreading rates <2 cm yr-1—which make up 25 per cent of the global ridge length), and that such vent systems would be hosted in ultramafic in addition to volcanic rocks. Here we present evidence for active hydrothermal venting on the Gakkel ridge, which is the slowest spreading (0.6–1.3 cm yr-1) and least explored mid-ocean ridge. On the basis of water column profiles of light scattering, temperature and manganese concentration along 1,100 km of the rift valley, we identify hydrothermal plumes dispersing from at least nine to twelve discrete vent sites. Our discovery of such abundant venting, and its apparent localization near volcanic centres, requires a reassessment of the geologic conditions that control hydrothermal circulation on ultraslow-spreading ridges.