Evolution of Low-Al Orthopyroxene in the Horoman Peridotite, Japan: an Unusual Indicator of Metasomatizing Fluids

Unusually alumina-poor orthopyroxene is found in a spinel peridotitefrom the Horoman Peridotite Complex, Japan. Al₂O₃, Cr₂O₃ andCaO contents in the low-Al orthopyroxene (named Low-Al OPX hereafter)are <0·25 wt %, <0·04 wt % and <0·3wt %, respectively, and are distinctively lower than those inorthopyroxene porphyroclasts. The Low-Al OPX occurs in two modes,both at the margin of olivine. The first mode of occurrenceis as the rim of a large orthopyroxene porphyroclast in contactwith olivine. This type of Low-Al OPX occurs only locally (15µm x 45 µm), and the orthopyroxene rim in contactwith olivine more commonly has normal Al₂O₃ contents (>2wt %). In the second mode of occurrence, the Low-Al OPX occursas a thin film, 5 µm x 50 µm in dimension, at agrain boundary between olivine and clinopyroxene. Trace elementcompositions of porphyroclast clinopyroxene in the sample indicatethat the sample having the Low-Al OPX underwent metasomatismalthough there are no hydrous minerals around the Low-Al OPX.Petrographic observations and trace element compositions ofclinopyroxene combined with an inferred P–T history ofthe Horoman peridotite suggest that the Low-Al OPX was formedthrough a very local reaction between peridotite and invasivefluids, probably formed by dehydration of a subducted slab,in a late stage of the history of the Horoman peridotite. Crystallizationof orthopyroxene, representing addition of silica to mantlelherzolite via a CO₂ + H₂O-bearing fluid phase, is a mechanismfor metasomatic alteration of mantle wedge peridotite. KEY WORDS: Horoman Peridotite Complex; low-Al orthopyroxene; metasomatism; mantle wedge     

The resuspension of underwater sediment by rain

The rain-induced resuspension of bottom sediment in shallow water is measured in a simple laboratory experiment, and found to increase significantly with increasing drop size and rain intensity, and decreasing water depth.     

Migration of Entrepreneurs from Hong Kong: Background, Trendsand Impacts

This paper critically examines the movement of entrepreneurs from Hong Kong within the context of whether Chinese culture predisposes them to become entrepreneurs: that the type of business relations and networks that the Chinese employ is in some way distinct from those of other groups because of Confucianism and unique ‘Asian values’. This interpretation is found severely wanting. Entrepreneurs are seen to be but one type of migrant from China and Hong Kong, and there are significant differences within Chinese migrant groups. We need to move away from vague cultural explanations to consider the context in which migrant groups find themselves before a satisfactory analytical framework for ethnic entrepreneurship can be constructed. The case of entrepreneurs moving to Canada is taken to illustrate recent policy changes and the impact of entrepreneurs on destination economies.     

The importance of double diffusion to the settling of suspended material

The vertical fingering motions which can occur when warm, salty water overlies colder, fresher water are important to the vertical flux of salt in the ocean. An analogous and not uncommon situation is that where turbid water overlies clearer and denser water. For the case of an originally sharp horizontal interface between the two water layers (associated, as an example with an interflow due to a warm, muddy river entering a stratified lake), the resulting downward sediment flux is compared with that occurring by way of gravitational settling. Rules are proposed to ascertain when the fingering effect could be important to sedimentation. For example, it seems that fingering could be of consequence in the situation where the sediment has a diameter of 2 μm and the upper layer is about 1°C warmer than the (clear) lower layer, when the upper-layer concentration is 1 ppm. The effect of fingering on the residence time of a gravitationally stable turbid layer is calculated. Such layers may be more short-lived than commonly thought.     

The Origin and Significance of Garnet Phenocrysts and Garnet-Bearing Xenoliths in Miocene Calc-alkaline Volcanics from Northland, New Zealand

Xenoliths, considered to be of igneous origin and consistingof hornblendegarnetplagioclase clinopyroxene, occur in associationwith high-pressure phenocrysts in early Miocene high-silicaandesites and dacites, Northland, New Zealand. Microstructuresof these xenoliths range from coarse, even-grained sub-ophitictypes to others with coarse glomerocrysts set in a finer-grainedmesostasis. The xenoliths are commonly flow-banded and are arguedto represent direct crystallization products and crystal aggregationsfrom the calc-alkaline host or related magmas at depth. Manygarnets within these high-pressure aggregates and also discretegarnet phenocrysts are rimmed by medium—coarse-grained,interlocking hornblendeplagioclase, representing partial adjustmentto an assemblage stable at shallower levels. The garnets aretypically pyrope—almandine with 17–28 mol.% grossularand show normal, reverse and oscillatory zoning; the associatedamphibole is pargasite trending to hornblende in phenocrystrims and reaction rims. Metamorphic xenoliths with plagioclase-hornblende-quartzassemblages are also found in the rocks and are characterizedby fine-grained granoblastic mosaic microstructures with well-developedfoliation defined by preferred orientation of elongate grainsand a mineral layering. These metamorphic xenoliths are interpretedas fragments of lower-crustal country rocks accidentally incorporatedinto rising andesitic magma. Application of established experimental high-pressure phasediagrams for andesites indicates crystallization of these assemblagesat depths corresponding to 10–20-kb pressure, and appropriategeothermometers indicates the following temperatures for equilibrationof assemblages at a nominal pressure of 12 kb: garnet-augite980C; garnet-augite-hornblende 920–1020 C. Geobarometryon a single garnet—orthopyroxene-bearing xenolith indicatesa pressure of 10–12 kb for a likely temperature rangeof 950–1000C. Thus the xenoliths point to the generationof host andesite-dacite magmas at suberustal depths of 35–45km, from fractional crystallization of more mafic mantlederivedmagmas, and demonstrate that relatively silicic calc-alkalinemagmas may evolve in the mantle. The rarity of evidence forsuch a process may be linked with the obduction-related tectonicevents operative in Northland just before the magmatic episode,and to the unusually high water content in the magma.     

The spectra of singular values in a regional model

Large portions of the spectra of singular values are determined for both moist and dry versions of a tangent linear, regional model for 4 different synoptic cases. Norms considered include the usual energy norm and versions of a norm measuring only the energy in some set of rotational mode perturbations. At most, only a few percent of the singular vectors possible with any of the norms are growing ones. Inclusion of moist physics in the tangent linear model greatly affects the leading singular vectors but does not increase the number of growing singular vectors much. Most singular vectors are damping ones, and therefore random perturbations drawn from a white‐noise distribution will likely damp during the 24‐h forecast periods considered. Only a few singular vectors are required to explain a significant portion of the final‐time variance of such perturbations, however, because the leading singular values are so large compared with the rest. The truncated rotational mode norm is shown to be very useful for investigating these properties.