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Mantle peridotite xenoliths from Archean cratons generally have high molar Mg=(Mg C Fe), or Mg#. The best known suites, from the Kapvaal and Siberian cratons, have high modal orthopyroxene (Opx). These high Opx compositions are probably not residues of partial melting. Less well known cratonic xenolith suites from Greenland and North America include high Mg# peridotites with much lower modal Opx. Such low Opx compositions could be residual from high degrees of polybaric, decompression melting, ending in the spinel lherzolite stability field at pressures of 30 to 20 kbar. This paper presents additional evidence that the great majority of both spinel-and garnet-bearing xenoliths are also residues of polybaric melting that ended at pressures Ä30 kbar. Where xenoliths record equilibration pressures > 30 kbar, this must result from tectonic transport of peridotites to greater depth after melting. Proposed mechanisms for producing the high Mg#, high Opx compositions include metamorphic differentiation of high pressure residues, mixtures of residual peridotites and high pressure igneous cumulates from ultramafic magmas, and addition of SiO 2 to low Opx peridotites via melt=rock reaction. This paper focuses on a positive correlation between Ni contents of olivine and modal proportions of Opx in mantle xenoliths, and uses this correlation to constrain the processes that produced high Mg#, high Opx cratonic mantle compositions. The observed correlation is probably not produced by partial melting, metamorphic differentiation, or formation of igneous cumulates. It can be produced by reaction between SiO 2 -rich liquids (e.g., small degree melts of subducted eclogite) and previously depleted, low Opx peridotites. We propose a two step process. First, high Mg#, low Opx peridotites were created by large degrees of polybaric melting ending at pressures < 30 kbar. Later, these depleted residues were enriched in Opx by interaction with SiO 2 -rich melts generated mainly by partial melting of eclogitic basalt and sediment in a subduction zone. Magmas modified by such a process could have formed a major component of the continental crust. Thus, this hypothesis provides a genetic link between cratonic upper mantle and continental crust.

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Shallow (garnet-free), depleted cratonic mantle, occurring as xenoliths in kimberlites and alkaline basaltic lavas, has a high Mg# (100 × Mg/(Mg + Fe) > 92) and is poor in Al and Ca compared to off-cratonic mantle. Here we compile data for many suites of shallow cratonic mantle xenoliths worldwide, and demonstrate a remarkably small range in their olivine Mg#, with an average of ~92.8. Via comparison with data for experimental melting of mantle peridotite compositions, we explain consistent olivine Mg# as the result of mantle melting and melt extraction to the point of orthopyroxene exhaustion, leaving a nearly monomineralic olivine residue.

Artículo de publicación ISIWe produce the first icefield-wide volume change rate and glacier velocity estimates for the Cordillera Darwin Icefield (CDI), a 2605 km2 temperate icefield in southern Chile (69.6 W, 54.6 S). Velocities are measured from optical and radar imagery between 2001–2011. Thirty-six digital elevation models (DEMs) from ASTER and the SRTM DEM are stacked and a weighted linear regression is applied to elevations on a pixel-by-pixel basis to estimate volume change rates. The CDI lost mass at an average rate of −3.9±1.5 Gt yr−1 between 2000 and 2011, equivalent to a sea level rise (SLR) of 0.01±0.004mmyr−1 and an area-averaged thinning rate of −1.5±0.6mw.e.(water equivalent) yr−1. Thinning is widespread, with concentrations near the front of two northern glaciers (Marinelli, Darwin) and one western (CDI-08) glacier. Thickening is apparent in the south, most notably over the advancing Garibaldi Glacier. The northeastern part of the CDI has an average thinning rate of −1.9±0.7mw.e. yr−1, while the southwestern part has an average thinning rate of −1.0±0.4mw.e. yr−1. Velocities are obtained over many of the CDI glaciers for the first time. We provide a repeat speed time series at the Marinelli Glacier. There we measure maximum front speeds of 7.5±0.2mday−1 in 2001, 9.5±0.6mday−1 in 2003 and 10±0.3mday−1 in 2011. The maintenance of high front speeds from 2001 to 2011 supports the hypothesis that Marinelli is in the retreat phase of the tidewater cycle, with dynamic thinning governed by the fjord bathymetry

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