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U-Pb petrochronology of deep crustal xenoliths and outcrops across northeastern Tanzania track the thermal evolution of the Mozambique Belt and Tanzanian Craton following the Neoproterozoic East African Orogeny (EAO) and subsequent Neogene rifting. At the craton margin, the upper-middle crust record thermal quiescence since the Archean (2.8-2.5 Ga zircon, rutile, and apatite in granite and amphibolite xenoliths). The lower crust of the craton documents thermal pulses associated with Neoarchean ultra-high temperature metamorphism (ca. 2.64 Ga, > 900 °C zircon), the EAO (600-500 Ma rutile), and fluid influx during rifting ( 650 °C (above Pb closure of rutile and apatite) at the time of eruption. Zoned titanite records growth during cooling of the lower crust at 550 Ma, followed by fluid influx during slow cooling and exhumation (0.1-1 °C/Myr after 450 Ma). Permissible lower-crustal temperatures for the craton and orogen suggest variable mantle heat flow through the crust and reflect differences in mantle lithosphere thickness rather than advective heating from rifting., Author(s): Francisco E. Apen [sup.1] [sup.2], Roberta L. Rudnick [sup.1] [sup.2], John M. Cottle [sup.1] [sup.2], Andrew R. C. Kylander-Clark [sup.1] [sup.2], Madalyn S. Blondes [sup.3], Philip M. Piccoli [sup.4], [...]

The petrology, mineral major and trace-element concentrations, and garnet oxygen isotopic composition of low-MgO (11-16 wt%) eclogites from the Obnazhennaya kimberlite, Siberian craton, are used to infer their petrogenesis. These eclogites contain two types of compositionally distinct garnet: granular coarse garnet, and garnet exsolution (lamellae and fine-grained garnet) in clinopyroxene. The former record higher temperatures at lower pressures than the latter, which record the last stage of equilibrium at moderate pressure-temperature conditions 2.3-3.7 GPa and 855-1095 °C in the upper mantle at the time of entrainment. Although derived from the garnet stability field, these rocks have low-pressure cumulate protoliths containing plagioclase, olivine, and clinopyroxene as reflected by pronounced positive Eu and Sr anomalies in all eclogites, and low heavy rare earth element (HREE) contents in both minerals and reconstructed bulk rocks for a number of samples. Major elements, transition metals, and the HREE compositions of the reconstructed whole rocks are analogous to modern oceanic gabbro cumulates. Despite geochemical signatures supporting an oceanic crust origin, mantle-like [delta].sup.18O of the garnets (5.07-5.62â°) for most samples indicates that the protoliths either did not interact with seawater or have coincidently approximately normal igneous values. Some of the eclogite xenoliths have lower SiO.sub.2 contents and depleted light REE ((Nd/Yb).sub.N < 1) compared to modern oceanic gabbros, suggesting that they experienced partial melting. Positively inclined middle to heavy REE patterns ((Dy/Yb).sub.N < 1) of the reconstructed bulk rocks mostly result from repeated partial melting in the eclogite stability field, based on melting model calculations. We, therefore, suggest that the Obnazhennaya low-MgO eclogites may represent the gabbroic section of subducted or foundered basaltic crust that underwent continued partial melting processes at high pressures, where garnet was the main residual phase., Author(s): Jing Sun [sup.1] [sup.2], Roberta L. Rudnick [sup.3], Sergey Kostrovitsky [sup.4] [sup.5], Tatiana Kalashnikova [sup.4], Kouki Kitajima [sup.6], Ranpeng Li [sup.3], Qiao Shu [sup.7] [sup.8] Author Affiliations: (1) State [...]

Byline: Ralf Halama (1,3), Ivan P. Savov (2,4), Roberta L. Rudnick (1), William F. McDonough (1) Keywords: Lithium; Li isotopes; Sub-arc mantle; Mantle xenoliths; Metasomatism; Kamchatka Abstract: Harzburgitic xenoliths cut by pyroxenite veins from Avachinsky volcano, Kamchatka, are derived from the sub-arc mantle and record element transfer from the slab to the arc. Olivine and orthopyroxene in the harzburgites have Li isotopic compositions ([delt[a].sup.7]Li = +2.8 to +5.6) comparable to estimates of the upper mantle ([delt[a].sup.7]Li ~ +4 +- 2). The pyroxenite veins, which represent modal metasomatism and may therefore provide information about the metasomatic agent, have mantle-normalized trace element characteristics that suggest overprinting of their mantle source by an aqueous, slab-derived fluid. These include relative enrichments of Pb over Ce, U over Th and Sr over Nd. Li is enriched relative to the HREE, and ortho- and clinopyroxene from the veins are in Li elemental and isotopic equilibrium with each other and the surrounding harzburgite. Vein samples ([delt[a].sup.7]Li = +3.0 to +5.0) do not record a significant slab-derived [delt[a].sup.7]Li signature. These observations can be reconciled if slab Li diffusively re-equilibrates in the mantle wedge. Modeling demonstrates that Li equilibration of small (1--2 cm width) veins or melt conduits is achieved at mantle wedge temperatures within 10.sup.1--10.sup.5 years. We conclude that strongly fractionated Li isotopic signatures cannot be sustained for long periods in the sub-arc mantle, at least at shallow ( Author Affiliation: (1) Department of Geology, University of Maryland, College Park, MD, 27042, USA (2) Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC, 20015, USA (3) Institut fur Geowissenschaften and SFB 574, Universitat Kiel, 24098, Kiel, Germany (4) School of Earth and Environment, Institute of Geophysics and Tectonics, The University of Leeds, Leeds, LS2 9JT, UK Article History: Registration Date: 07/01/2009 Received Date: 03/07/2008 Accepted Date: 07/01/2009 Online Date: 30/01/2009 Article note: Communicated by J. Hoefs. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-009-0378-5) contains supplementary material, which is available to authorized users.

Byline: Sonja Aulbach (1,2), Roberta L. Rudnick (1), William F. McDonough (1) Keywords: Lithium isotopes; Strontium isotopes; Neodymium isotopes; East African Rift; Plume; Cratonic lithospheric mantle; Tanzania Abstract: Lithium concentrations and isotopic compositions of olivine and 87.sup.Sr/ [.sup.86]Sr and 143.sup.Nd/ [.sup.144]Nd of coexisting clinopyroxene from peridotite xenoliths from the Quaternary Labait volcano, Tanzania, document the influence of rift-related metasomatism on the ancient cratonic mantle. Olivines show negative correlations between Fo content and both [delt[a].sup.7]Li and Li concentrations. Olivines in iron-rich peridotites (Fo.sub.85--87) have high Li concentrations (3.2--4.8 ppm) and heavy [delt[a].sup.7]Li (+5.2 to +6.6). In contrast, olivines in ancient, refractory peridotites have lower Li concentrations ([proportional to]2 ppm) and relatively light [delt[a].sup.7]Li (+2.6 to +3.5). This reflects mixing between ancient, refractory cratonic lithosphere and asthenosphere-derived rift magmas. A uniquely fertile, deformed, high-temperature garnet lherzolite, interpreted to be from the base of the lithosphere, has a 87.sup.Sr/ [.sup.86]Sr of 0.7029 and 143.sup.Nd/ [.sup.144]Nd of 0.51286, similar to HIMU oceanic basalts. It provides the best estimate of the Sr--Nd isotope composition of the upwelling mantle (i.e., plume, sensu lato) underlying this portion of the East African Rift, and is slightly less radiogenic compared to previous estimates of the plume that were based on rift basalts. Although elevated [delt[a].sup.7]Li are not exclusive to HIMU source regions, the data collectively indicate that the plume beneath Labait has HIMU characteristics in Sr, Nd and Li isotope composition. Author Affiliation: (1) Geochemistry Laboratory, Department of Geology, University of Maryland, College Park, MD, 20742, USA (2) Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, T6G 2E3, Canada Article History: Registration Date: 29/05/2007 Received Date: 10/10/2006 Accepted Date: 29/05/2007 Online Date: 17/07/2007 Article note: Communicated by J. Hoefs. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-007-0226-4) contains supplementary material, which is available to authorized users.

To study Li isotopic fractionation during granite differentiation and late-stage pegmatite evolution, Li isotopic compositions and concentrations have been measured for the S-type Harney Peak Granite, the spatially associated Tin Mountain pegmatite, and possible metasedimentary source rocks in the Black Hills, South Dakota. The Harney Peak Granite is isotopically heterogeneous, with [[delta].sup.7]Li varying from -3.1 to +6.6. The [[delta].sup.7]Li values of Proterozoic metasedimentary rocks that are possible sources of the Harney Peak Granite range from -3.1 to +2.5 and overlap with post-Archean shales and the Harney Peak Granite. For the granite suite, there is no correlation between [[delta].sup.7]Li and elements indicative of degrees of granite differentiation (Si[O.sub.2], Li, Rb, etc.). The Li isotopic composition of the Harney Peak Granite, therefore, appears to reflect the source composition. Minerals from the zoned Tin Mountain pegmatite have extremely high Li contents and heavier Li isotopic compositions than the granite or surrounding Black Hills metasedimentary rocks. The heavier compositions may reflect Li isotopic fractionation resulting from extensive crystal-melt fractionation. Lithium concentrations decrease in the order: spodumene (~3.7 wt%), muscovite (0.2 to 2.0 wt%), plagioclase (100-1100 ppm), quartz (30-140 ppm). Plagioclase, muscovite, and spodumene in all zones display a relatively narrow range in [[delta].sup.7]Li of +7.9 to +11.4. In contrast, quartz is isotopically heavier and more variable (+14.7 to + 21.3), with [[delta].sup.7]Li showing an inverse correlation with Li concentration. This correlation reflects the mixing of isotopically heavy Li in quartz and lighter Li in fluid inclusions, as documented by fluid inclusion compositions ([[delta].sup.7]Li = +8.1 to +13.4 and Li of 280 to 3960 ppm). Extrapolation of this trend to an estimated intrinsic Li concentration in quartz of +21. The large difference in [[delta].sup.7]Li between quartz and other minerals may reflect [sup.7]Li preference for less highly coordinated sites, which have higher bond-energies (i.e., the two- or fourfold site in quartz vs. higher coordination number sites in other minerals). Comparison of the Li isotopic composition of fluid inclusions with that of the wall zone of the Tin Mountain pegmatite suggests ~4[per thousand] isotopic fractionation during fluid exsolution, which agrees with the results derived from studies of hydrothermal alteration of basalts. Keywords: Stable isotopes, igneous petrology, pegmatites, fluid phase, lithium, Harney Peak Granite, Tin Montain pegmatite, isotope fractionation

Author(s): Shan Gao (corresponding author) [1, 2]; Roberta L. Rudnick (corresponding author) [3]; Hong-Ling Yuan [1]; Xiao-Ming Liu [1]; Yong-Sheng Liu [2]; Wen-Liang Xu [4]; Wen-Li Ling [2]; John Ayers [...]

Author(s): Cin-Ty Lee (corresponding author) [1, 2]; Qingzhu Yin [1]; Roberta L. Rudnick [1, 2]; Stein B. Jacobsen [1] Stable continental regions, free from tectonic activity, are generally found only [...]

Primary carbonates in peridotite xenoliths from the East African Rift in northeastern Tanzania occur as intergranular patches with accessory minerals (olivine and spinel), as patches with accessory magmatic minerals (nepheline), and as round monomineralic inclusions in primary olivine grains. All are characterized by calcitic compositions (Ca/Ca + Mg + Fe from 0.83 to 0.99), extremely low Si[O.sub.2] + [Al.sub.2][O.sub.3] + [Na.sub.2]O + [K.sub.2]O, low trace element abundance [total rare-earth element (REE) abundance

The Earth's continental crust is a thick layer consisting mainly of andesitic rocks and contains a significant fraction of incompatible trace elements. Several models have been proposed to describe the composition, growth processes and mode of formation of the continental crust, particularly in the Archean period. These models have also been used as basis for describing the production of the present continental crust. A review of the processes that shaped the earth's crust and the methods used to analyze these processes are presented.

In a continuing effort to understand crust-mantle dynamics, we have determined the S and Pb isotopic compositions of mantle sulfides encapsulated within diamonds from under the Siberian craton and compared these results to those of previously investigated African counterparts. Because diamond inclusions are isolated from exchange with surrounding mantle, they may preserve the history of diamond growth and act as direct tracers of the origins of mantle materials. Study of these inclusions may thus offer the best chance of recognizing global-scale interaction between Earth's crust and mantle. Although delta(sup 34)S values of the Siberian sulfides do not deviate significantly from the mantle value of 0 0/00 plus or minus 3 0/00, Pb isotopic compositions are highly variable. Pb isotopic compositions of sulfides from peridotitic suite diamonds generally plot near the terrestrial Pb growth curve, with model ages ranging between 0 and 2 Ga, whereas sulfides from eclogitic suite diamonds have radiogenic compositions, plotting beyond the growth curve. These results, which are similar to those for sulfides in African diamonds, suggest that the sulfides from eclogitic suite diamonds were derived from a source with an unusually high U/Pb ratio and may indicate a common process (such as subduction of crustal materials into the mantle) operating beneath Africa and Siberia. The absence of extremely radiogenic Pb in sulfides from eclogite xenoliths suggests that the radiogenic material from which eclogitic suite diamonds grew was a transient feature of the mantle, associated with diamond growth. The ultimate origin of this high U/Pb signature, however, remains enigmatic. Large variations in Pb isotopic composition of sulfides from different zones in a single peridotitic suite diamond document (1) crystallization of the diamond's core near 2.0 Ga, (2) growth of its outer zone in an environment with a high U/Pb ratio similar to the growth environment of eclogitic suite diamonds, and (3) growth of the diamond rim near the time of kimberlite emplacement.

Zircons extracted from five chemically diverse granulite-facies crustal xenoliths from La Olivina cinder cone, Chihuahua, Mexico, were analyzed by SHRIMP ion microprobe in order to determine the age of the deep crust in northern Mexico. The results demonstrate the extreme age diversity of the deep crust in this region: zircon populations of ca. 1,25-37, ca. 200, 350, 1100, and 1370 Ma are present. Nearly all samples contain 0-20 Ma zircons, reflecting recent granulite-facies conditions in the deep crust. In addition, high-grade metamorphism may also have occurred at 200 Ma and 1.10 Ga (Grenville). The paucity of Carboniferous-Permian ages in the xenoliths suggests that the Ouachita collision event provided little thermal input to the deep crust beneath La Olivina.

Lithospheric removal, in the form of detachment, foundering, or peeling away of the lithospheric mantle with or without lower crust into the convecting mantle, has been predicted by various geodynamic [...], Thermobarometric and Os isotopic data for peridotite xenoliths from late Miocene and younger lavas in the Sierra Nevada reveal that the lithospheric mantle is vertically stratified: the shallowest portions ([is less than] 45 to 60 kilometers) are cold (670 ° to 740 ° C) and show evidence for heating and yield Proterozoic Os model ages, whereas the deeper portions (45 to 100 kilometers) yield Phanerozoic Os model ages and show evidence for extensive cooling from temperatures [is greater than] 1100 ° C to 750 ° C. Because a variety of isotopic evidence suggests that the Sierran batholith formed on preexisting Proterozoic lithosphere, most of the original lithospheric mantle appears to have been removed before the late Miocene, leaving only a sliver of ancient mantle beneath the crust.

The elements Ti, Zr, Nb, Ta, and rare earth elements (REE) are refractory and lithophile and therefore should exist in chondritic relative abundances in the silicate Earth. Continental crust and [...], A mass imbalance exists in Earth for Nb, Ta, and possibly Ti: continental crust and depleted mantle both have subchondritic Nb/Ta, Nb/La, and Ti/Zr, which requires the existence of an additional reservoir with superchondritic ratios, such as refractory eclogite produced by slab melting. Trace element compositions of minerals in xenolithic eclogites derived from cratonic lithospheric mantle show that rutile dominates the budget of Nb and Ta in the eclogites and imparts a superchondritic Nb/Ta, Nb/La, and Ti/Zr to the whole rocks. About 1 to 6 percent by weight of eclogite is required to solve the mass imbalance in the silicate Earth, and this reservoir must have an Nb concentration [is greater than or equal to] 2 parts per million, Nb/La [is greater than or equal to] 1.2, and Nb/Ta between 19 and 37--values that overlap those of the xenolithic eclogites. As the mass of eclogite in the continental lithosphere is significantly lower than this, much of this material may reside in the lower mantle, perhaps as deep as the core-mantle boundary.