U12A-01 INVITED
Mapping isotope data back into structures in the mantle
Mapping isotope data back into structure in the mantle seems to me one of the most exciting 'route ahead'. This is obviously challenging, since it requires a profound understanding of the Earth's differentiation processes and of the stirring efficiency in the convecting mantle. Although it is widely accepted that the mantle is chemically heterogeneous, the nature of such heterogeneities, the time evolution of their shape and size and their architecture inside the mantle are still quite elusive. Important progresses can be achieved in the near future by high resolution three dimensional numerical simulations of mantle stirring which need to include: first, more realistic physical properties of the compressible mantle. Second, the ability to keep track of compositionally distinct material with specific physical properties, namely density, compressibility and viscosity. Third, the ability to keep track of heterogeneous and time-varying concentrations of heat producing elements. In other words, we have to move beyond passive heterogeneities, and consider mantle stirring in the presence of active (i.e., able to affect the flow) heterogeneities. In the last years numerical simulations and laboratory experiments of thermo-chemical convection have shown an unexpected dynamics of upwelling deep mantle material and have produced large scale lower mantle structures that are quite consistent with seismological observations. The step ahead will be to include Lagrangian methods recently proposed to quantify mixing in chaotic and time- aperiodic geophysical flows into mantle convection models solving for stretching, dispersal and segregation of active heterogeneities.
U12A-02
Mantle Reservoirs From a Noble Gas Perspective
The noble gases provide unique insight into mantle structure and the origin of the different mantle reservoirs. Many OIBs, such as Hawaii and Iceland, have 3He/4He ratios that are a factor of 4 to 6 higher than the canonical MORB value of 8±1 RA. The high 3He/4He ratios in OIBs are conventionally viewed as evidence for the existence of a primitive mantle reservoir. Such a view, however, is frequently challenged on the grounds that noble gas abundances in OIBs are an order of magnitude lower than in MORBs, an observation that traditional models of magmatic degassing cannot explain. The apparent concentration paradox has been resolved by incorporating kinetic fractionation of the noble gases during magmatic degassing of the erupting magma and it can be shown that higher CO2 and H2O content of OIBs, compared to MORBs, leads to more extensive degassing of He in OIB magmas (Gonnermann and Mukhopadhyay, 2007). In contrast to Hawaii and Iceland, some ocean islands, such as the Cook-Austral Islands and Canary Islands (HIMU ocean islands) have 3He/4He ratios of 4-7 RA, lower than the MORB range. The low 3He/4He ratios are attributed to the addition of radiogenic 4He from recycled slabs. Surprisingly, recent high-precision neon isotopic measurements made at Harvard in olivine phenocrysts from the Cook-Austral Islands indicate that HIMU neon is less nucleogenic than the MORB source. The He and Ne systematics from the Cook-Austral's demonstrate that the noble gas signature of HIMU basalts cannot arise either from simple diffusive equilibration of a recycled slab with a MORB source, or result from mixing of melts that are derived from recycled slabs and the MORB mantle. The He-Ne systematics, however, can be quantitatively modeled as a mixture of recycled slab and a primitive mantle reservoir. The scenario is consistent with He-Os and He- Nd correlations seen in the Cook-Austral basalts. Thus, both low and high 3He/4He OIBs incorporate the same primitive mantle reservoir, although in varying proportions. The notion of a reservoir that is primitive in its volatile content and sampled at ocean islands is very much alive. In spite of whole mantle convection, it appears that part of the Earth's mantle has remained largely undegassed. While significant progress has been made with respect to understanding the geochemical implications of He and Ne isotopic composition measured in MORBs and OIBs, our knowledge of Xenon in the mantle remains poor. Since 129Xe and 136Xe have been produced by the now extinct nuclides, 129I and 244Pu respectively, Xe isotopic composition of the mantle can be used to test models of atmosphere formation and provide unique clues to the volatile history of the Earth's mantle. Some of the outstanding issues that still need to be resolved are whether the Earth's mantle has solar or chondritic heavy noble gases, whether OIBs and MORB have the same Xe isotopic composition, and what fraction of the 136Xe is from 244Pu vs. 238U fission. Addressing these issues will require not only high precision measurements but also innovative experimental techniques to reduce air contamination that is ubiquitous in mantle-derived samples. High precision Xe isotopic measurements made at Harvard indicates that Samoa (a high 3He/4He ocean island) and MORBs have exactly the same proportion of radiogenic 129Xe to 136Xe. Although this result needs to be verified from other OIBs, it suggests that a single mantle reservoir supplies the excess 129Xe and 136Xe to both the MORB and OIB mantle source. The primitive mantle reservoir is the most likely carrier of the xenon isotopic anomaly.
U12A-03
Plate tectonics, ancient crust, and the geochemical evolution of the mantle
Isotopic ratios measured in oceanic basalts indicate the presence of variably mixed depleted, enriched and pristine components in the convecting mantle. A persistent dense layer at the base of the mantle may modulate the isotopic composition of enriched compositional end members, particularly if this layer is at least partially composed of ancient oceanic crust. We model the generation of a dense layer from subducted crust in models with stiff mobile plates approximated by the force-balance method [1][2]. The inclusion of plates allows us to self- consistently model crust petrogenesis at divergent plate boundaries. An embedded geochemical model tracks the isotopic evolution of Pb,U,Th,Sm,Nd,Rb,Sr,Re,Os, and He within this model framework. By varying thermal convective vigor and chemical density within reasonable parameter values, we investigate the admissible range of model behavior. In all cases we find that the extraction of continental crust is essential to reproduce the spread of observed isotope ratios. The size and longevity of pools of ancient oceanic crust at the base of the mantle further modulate isotope systematics by delaying the remixing of ancient oceanic crust. With no chemical buoyancy the average age of melting forms a gradient between lower and upper mantle values of 1.5 and 2.75 Byr respectively. With dense oceanic crust the gradient becomes discontinuous, producing an older upper mantle age in excess of 3.0 Byr. This indicates an average residence time of pooled crust on the order of 250 to 500 Myr, and leads to more distinct separation between isotopic end-member compositions. Using these relationships, we construct a series of viable continental crust extraction scenarios and compare these with published models. Mass balance calculations are performed and the role of the pooled oceanic crust as a complimentary reservoir to the continental crust is evaluated. [1] Gable, C.W., R.J. O'Connell, B.J. Travis (1991) "Convection in 3 dimensions with surface plates; generation of a toroidal flow," J. Geophys. Res., 89, 8391--8405 [2] Brandenburg, J.P., P.E. van Keken (2007) "Methods for thermochemical convection in Earth's mantle with force-balanced plates," Revised version submitted to Geochem. Geophys. Geosyst.
U12A-04
Earth's Various Recipes for Making Lherzolites
Petrological and cosmochemical arguments suggest that the convecting upper mantle overall should have a lherzolitic composition, otherwise, continous production of MORB would not be feasible. The predominance of harzburgites among ocean floor peridotites fits this picture because harzburgites are commonly believed to be the residue of high degrees of partial melting at shallow depths, with fertile components lost during polybaric partial melting. Implicitly, it is commonly assumed that the deeper parts of the asthenosphere and new-formed lithosphere should be residues of low-degree partial melting. This view has been supported by the abundance of lherzolites among mantle xenoliths and orogenic peridotite massifs. But is this model really correct? Data and observations on oceanic and continental peridotites accumulated over recent years hint that reality is more complicated. On the basis of mineral and whole rock compositions, and isotopic data, it has long been suspected that many continental peridotites have undergone some form of pyroxene addition via percolating melts, yet the efficacy of these processes has been uncertain. Novel combination of structural and chemical work by Le Roux et al. (2007) indicates that melt influx may have converted deformed harzburgitic rocks of the Lherz peridotite massif into little-deformed spinel lherzolites. Refertilization by MORB-like sub-lithospheric melts, and marble cake style stretching of pyroxenites have been implicated as major processes that affected the composition of peridotites from the Totalp spinel lherzolite body, a fragment of Jurassic ultra-slow spreading Thetys ocean floor in the Swiss Alps (van Acken et al., 2007). Refertilization by melts has been associated with lherzolites from oceanic fracture zones (e. g., Seyler and Bonatti, 1997) and may be responsible for lherzolites alternating with harzburgitic domains at the Arctic Gakkel ridge (Liu et al. 2007). Evidence for compositional transformation of depleted peridotites into fertile rocks, both in young oceanic and in continental settings brings up questions that need to be addressed in the future: How common are truly residual lherzolites? Are lherzolites suitable to constrain the composition of the primitive mantle? How are fertile components in the asthenosphere distributed? Mantle rocks may have more surprises in stock.
U12A-05
Mantle Dynamics Inferred From the Samoan Hotspot: Implications for Recycling and the Preservation of Hemispheric-Scale Geochemical Anomalies.
The standard model for the geochemical evolution of the mantle assumes that much of the chemical diversity is a result of subduction, a tectonic process that introduces enriched oceanic crust and sediment into a primitive (or slightly depleted) mantle. Do surface materials survive the "blender" that we call the mantle, to be recycled and erupted as ocean island basalts (OIBs) that bear resemblance to the original subducted material? Do OIBs offer hints that there are regions isolated from the convective vigors of the mantle, where heterogeneities can be preserved for long periods of earth's history? Recently discovered lavas from Samoa show the strongest evidence yet for continental sediment recycling through the mantle, and exhibit 87Sr/86Sr > 0.7216 [Jackson et al., 2007a]. In spite of the large quantities of sediment having entered the mantle at subduction zones over geologic time, recycled sediment (EM2, enriched mantle 2) signatures are rare in OIBs [Hofmann, 1997]. If large quantities of sediment have survived subduction zones and now occupy the deep mantle, either 1.) the mantle is so dynamic that the recycled sediment signatures are diluted and difficult to discern in OIBs, or 2.) the sediments have accumulated and become isolated in the deep mantle and are rarely sampled at the surface in OIBs. The observation of ancient, large-scale isotope anomalies may offer an important insight. The DUPAL (of Dupré and Allègre fame) anomaly, a global-encircling feature of isotopic enrichment observed primarily in southern hemisphere OIBs, has survived mantle convection for as long as 3 Ga [Hart, 1984]. New high 3He/4He lavas (33.8 times atmospheric, Ra) from Samoa are consistent with a DUPAL signature in the high 3He/4He, or FOZO (Focus Zone [Hart, 1992]), reservoir [Jackson et al., 2007b]: The highest 3He/4He samples from each southern hemisphere high 3He/4He hotspot (i.e., > 11 Ra) exhibit lower 143Nd/144Nd ratios than their counterparts in the northern hemisphere (excluding lavas erupted in continental, back-arc, and submarine ridge environments). Like DUPAL, the FOZO reservoirs are ancient. The existence of these long-lived, large-scale anomalies suggests that the mantle does not always efficiently destroy heterogeneities. Where in the mantle are these long-lived geochemical anomalies preserved from convective mixing? Among geochemists, the deepest part of the lower mantle—the core mantle boundary—is a favorite location for the FOZO and DUPAL anomalies, and many geochemical models require that this region be isolated and immune to convective stirring. A recent discovery demonstrated that the 142Nd/144Nd ratios in all measured terrestrial mantle rocks are 20 ppm higher than chondrites, suggesting that the mantle hosted a superchondritic Sm/Nd reservoir during the short lifetime of 146Sm [Boyet and Carlson, 2005]; if bulk silicate earth is chondritic, then a complementary "hidden enriched reservoir" (HER) with subchondritic 142Nd/144Nd must also exist in the deep earth. The implication is that the DUPAL, FOZO, EM2 and HER reservoirs are all suggested to occupy the same location in the mantle. Surely, therefore, they must interact: If the signature of one deep reservoir escapes to the surface in plumes, the signatures of the others will be detectable as well? Further 142Nd/144Nd measurements on OIBs from plume-fed hotspots will provide an important test of this hypothesis, as mixing with the REE-rich EER may perturb the 142Nd/144Nd of the other deep mantle reservoirs. Additionally, coupling observations of surface geochemical anomalies with increasingly higher resolution seismological techniques may provide insights into the home of these (deep?) reservoirs.
U12A-06 INVITED
Geochemical Consequence of Extraction of Incipient CO2-rich melts from Earth's Deep Upper Mantle
The initiation of partial melting beneath mid-oceanic ridges and ocean islands likely produces incipient carbonatitic melts. These highly mobile melts generated at great depth may not only affect geophysical properties of deep upper mantle but can also influence Earth's geochemical evolution by releasing incompatible parent and/or daughter elements, heat producing elements, and volatiles. But constraints on the fractionation of the key trace elements between the peridotitic residue and carbonatitic melts are lacking at conditions of initiation of partial melting beneath ridges.Experiments at 6.6-8.6 GPa and 1265-1470 °C on carbonated peridotite doped with a blend of trace elements produced cpx + garnet + magnesite ± opx ± olivine + carbonatitic melt (cbL) similar in composition to that expected at the solidus of carbonated peridotite (Ca# = 0.52 at 6.6 GPa and 0.45 at 8.6 GPa; Na2O = ~4 wt.% at 6.6 GPa and ~2.5 wt.% at 8.6 GPa). Compared to previous studies at lower pressures (2.0-4.6 GPa), Dcpx/cbL from the present study are smaller for elements substituting into the cpx M2 site, especially trivalent cations ( DLu = 0.17, DLa = 0.006). Dcpx/cbL for U (0.001) and Th (0.002) are also noticeably lower than the previous estimates. In contrast, Dgarnet/cbL values are higher for REEs ( DLu = 4.6, DLa = 0.039) and HFSEs, U (0.023) and Th (0.017).Our estimate of Dperidotite/cbL indicates that incipient carbonatite extraction from the deep upper mantle will produce a residue with a more depleted Rb/Sr, U/Pb, Th/U, and enriched Sm/Nd, which may evolve to produce the most common of the mantle end member components, PREMA. Metasomatic implantation of deep carbonatitic melt into the lithosphere can generate a high-μ (238U/204Pb) signature, whereas mixing of carbonatite with elevated Th/U can contribute to the observed 230Th-excesses in MORBs. However, carbonatites derived by incipient melting of depleted (DMM) mantle have limited trace element enrichments, ~10-100×primitive mantle (PM), compared to natural magnesio-carbonatites (~100-1000×PM). Thus, natural carbonatites either derive from highly enriched sources or become enriched in trace elements by extensive melt/rock reaction during ascent. Owing to much deeper intersection of carbonated peridotite solidus with solid mantle adiabat, the volume of the mantle contributing to the highly incompatible elements and volatiles is likely much larger than the volume that supplies the major elements of basalts, thus the abundance of various incompatible trace elements might be overestimated for the upper mantle basalt source regions.
U12A-07
Changes in Crystal Structure and Thermo-Elastic Properties of (Mg,Fe)SiO3 across the Post-Perovskite Transition
Recent studies have proposed intriguing changes in physical properties across the post-perovskite transition which may explain some enigmatic seismic observations at the lowermost mantle. However, most of these predictions are based on first-principles calculations and high-quality measurements have been difficult due to the extreme P-T stability of post-perovskite. We measured diffraction patterns of (Mg0.91Fe0.09)SiO3 post-perovskite over wide P-T conditions (37-126 GPa at 300 K and 135 GPa at 2300-2700 K), some of which are directly relevant to the conditions expected for the D" layer, under an argon pressure medium and the gold pressure scale. Through Rietveld refinement, we confirmed the proposed crystal structure of post-perovskite, which provides a critical test for the first-principles predictions. Combined with our measurements on perovskite, we found that density increases by 1.8± 0.1% and bulk modulus decreases by 6.5± 2.0%, resulting in a 3.9± 1.5% decrease in bulk sound speed across the post- perovskite transition, which is consistent with first-principles predictions. This implies that variations in mineralogy between perovskite and post-perovskite may result in anti-correlation between the bulk sound speed and shear wave velocity anomalies combined with the predicted shear wave velocity increase. Our high- temperature data suggest that Grüneisen parameter decreases by 21± 15% across the post-perovskite transition, which would influence the dynamic stability of the post-perovskite patches reported by recent seismic studies.