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U-series disequilibria provide important constraints on the processes and time scales of melt production, differentiation, and transport in subduction settings. Such constraints, which are essential for understanding the chemical evolution of the continental crust, are conventionally based on the assumption that the U-series disequilibria measured in mafic lavas are produced during mantle metasomatism and melting, and that intracrustal differentiation and assimilation have limited impacts. Here we show that mantle-derived U-series disequilibria in mafic lavas erupted at Volcán Llaima, Chile are significantly diminished by assimilation of plutonic rocks forming Llaima's subvolcanic basement. This contamination process is extremely subtle in terms of “classical” indicators of crustal assimilation like Sr, Nd or Pb isotopes because it is a manifestation of assimilative recycling of the plutonic roots of the arc. This process results in variations in U-series disequilibria and incompatible trace element ratios that are significant compared to regional and global variability in arc magmas. Furthermore, it yields linear correlations between U-series excesses and incompatible trace element ratios that are generally interpreted as slab-fluid indicators and chronometers, or tracers of sediment recycling in subduction zone. Cannibalization of ancestral magmas by ascending melts warrants careful evaluation when considering the components and chemical fluxes in subduction zones. Linear arrays defined by activity ratios of U-series nuclides with different half-lives may be the most reliable indicators of assimilative recycling of ancestral intrusive magmas., Earth and Planetary Sciences

High-pressure, low-temperature (HP–LT) rocks from a Cretaceous age subduction complex occur as tectonic blocks in serpentinite mélange along the Motagua Fault (MF) in central Guatemala. Eclogite and jadeitite among these are characterized by trace element patterns with enrichments in fluid mobile elements, similar to arc lavas. Eclogite is recrystallized from MORB-like altered oceanic crust, presumably at the boundary between the down-going plate and overlying mantle wedge. Eclogite geochemistry, mineralogy and petrography suggest a two step petrogenesis of (1) dehydration during prograde metamorphism at low temperatures (<500°C) followed by (2) partial rehydration/fertilization at even lower T during exhumation. In contrast, Guatemalan jadeitites are crystallized directly from low-T aqueous fluid as veins in serpentinizing mantle during both subduction and exhumation. The overall chemistry and mineralogy of Guatemalan eclogites are similar to those from the Franciscan Complex, California, implying similar P–T–x paths. Li concentrations \((\leq 90ppm)\) in mineral separates and whole rocks (WR) from Guatemalan and Franciscan HP–LT rocks are significantly higher than MORB (4–6ppm), but similar to HP–LT rocks globally. Li isotopic compositions range from −5‰ to +5‰ for Guatemalan HP–LT rocks, and −4‰ to +1‰ for Franciscan eclogites, overlapping previous findings for other HP–LT suites. The combination of Li concentrations greater than MORB, and Li isotopic values lighter than MORB are inconsistent with a simple dehydration model. We prefer a model in which Li systematics in Guatemalan and Franciscan eclogites reflect reequilibration with subduction fluids during exhumation. Roughly 5–10% of the Li in these fluids is derived from sediments. Model results predict that the dehydrated bulk ocean crust is isotopically lighter ( \(\sigma\)\(^{7}\)Li\(\leq\)+1±3‰) than the depleted mantle (∼+3.5±0.5‰), while the mantle wedge beneath the arc is the isotopic complement of the bulk crust. A subduction fluid with an AOC–GLOSS composition over the full range of model temperatures (50–600°C) gives an average fluid \(\sigma\)\(^{7}\) Li (∼+7±5‰ 1\(\sigma\)) that is isotopically heavier than the depleted mantle. If the lowest temperature steps are excluded (50–260°C) as too cold to participate in circulation of the mantle wedge, then the average subduction fluid ( \(\sigma\)\(^{7}\)Li=+4±2.3‰ 1 \(\sigma\)), is indistinguishable from depleted mantle. Because of the relatively compatible nature of Li in metamorphic minerals, the most altered part of the crust (uppermost extrusives), may retain a Li isotopic signature (∼+5±3‰) heavier than the bulk crust. The range of Li isotopic values for OIB, IAB and MORB overlap, making it is difficult to resolve which of these components may contribute to the recycled component in the mantle using \(\sigma\)\(^{7}\)Li alone., Earth and Planetary Sciences

An evaluation of the historical record of volcanic eruptions shows that subaerial volcanism increases globally by two to six times above background levels between 12 ka and 7 ka, during the last deglaciation. Increased volcanism occurs in deglaciating regions. Causal mechanisms could include an increase in magma production owing to the mantle decompression caused by ablation of glaciers and ice caps or a more general pacing of when eruptions occur by the glacial variability. A corollary is that ocean ridge volcanic production should decrease with the rising sea level during deglaciation, with the greatest effect at slow spreading ridges. CO2 output from the increased subaerial volcanism appears large enough to influence glacial/interglacial CO2 variations. We estimate subaerial emissions during deglaciation to be between 1000 and 5000 Gt of CO2 above the long term average background flux, assuming that emissions are proportional to the frequency of eruptions. After accounting for equilibration with the ocean, this additional CO2 flux is consistent in timing and magnitude with ice core observations of a 40 ppm increase in atmospheric CO2 concentration during the second half of the last deglaciation. Estimated decreases in CO2 output from ocean ridge volcanoes compensate for only 20% of the increased subaerial flux. If such a large volcanic output of CO2 occurs, then volcanism forges a positive feedback between glacial variability and atmospheric CO2 concentrations: deglaciation increases volcanic eruptions, raises atmospheric CO2, and causes more deglaciation. Such a positive feedback may contribute to the rapid passage from glacial to interglacial periods. Conversely, waning volcanic activity during an interglacial could lead to a reduction in CO2 and the onset of an ice age. Whereas glacial/interglacial variations in CO2 are generally attributed to oceanic mechanisms, it is suggested that the vast carbon reservoirs associated with the solid Earth may also play an important role., Earth and Planetary Sciences

Acute myeloid leukaemia (AML) is characterized by a block in differentiation and an unregulated proliferation of myeloid progenitor cells. While the cause of AML in children is unknown, risk factors that have been identified include exposure to toxins such as ethanol, pesticides and dietary topoisomerase II inhibitors, prior chemotherapy with alkylating agents or topoisomerase II inhibitors, constitutional disorders such as Down's syndrome and type I neurofibromatosis, and haematopoietic failure syndromes such as Fanconi anaemia and severe congenital neutropenia. With intensified chemotherapy including high-dose Ara-C, followed in many cases by bone marrow transplantation, and with improvements in supportive care, current survival rates approach 50%. Future advances in paediatric AML will include better risk stratification to determine optimal treatment and targeted cytotoxic therapy., (Copyright 2001 Harcourt Publishers Ltd.)

The ets-family transcription factor PU.1 is required for the proper development of both myeloid and lymphoid progenitors. We used PU. 1-deficient animals to examine the role of PU.1 during dendritic cell development. PU.1(-/-)animals produce lymphoid-derived dendritic cells (DC): low-density class II major histocompatibility complex [MHC-II(+)] CD11c(+) CD8alpha(+) DEC-205(+). But they lack myeloid-derived DC: low-density MHC-II(+) CD11c(+) CD8alpha(-) DEC-205(-). PU.1(-/-) embryos also lack progenitors capable of differentiating into myeloid DC in response to granulocyte-macrophage colony-stimulating factor plus interleukin-4. The appearance of lymphoid DC in developing PU.1(-/-)thymus was initially delayed, but this population recovered to wild type (WT) levels upon organ culture of isolated thymic lobes. PU. 1(-/-)lymphoid DC were functionally equivalent to WT DC for stimulating T-cell proliferation in mixed lymphocyte reactions. These results demonstrate that PU.1 is required for the development of myeloid DC but not lymphoid DC.