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11.
Gabbroic xenoliths that represent cumulate environments within Mauna Kea Volcano are, in rare examples, penetrated by small-scale
(<7 cm) dikes. We examined four dike/host composite xenoliths to establish how this evidence for magma seemingly injected
into cumulate gabbro fits into the evolution of igneous processes in shield volcano magma reservoirs. Olivine, clinopyroxene,
and plagioclase compositions in both host gabbros and dikes are characteristically tholeiitic and evolved (Fo71–66, cpx-Mg # 79–77, An72–51) with respect to Hawaiian magmatism. Dikes, however, when compared with their host gabbros, have slightly greater abundances
of some incompatible elements and slightly more evolved olivine compositions (e.g., Fo68 vs Fo71). Compared with Mauna Kea lava compositions, both host gabbros and dikes have lower incompatible-element abundances, positive
Eu anomalies, and, notable for dikes, major-element compositions unlike those of lavas (e.g., SiO2<46 wt.%). The small-scale dikes, therefore, also have cumulate characteristics. We interpret them as representing late-stage
liquids (e.g., <5 wt.% MgO, based on <Fo70) "squeezed" from solidifying cumulus piles of evolved (e.g., ∼Fo70) gabbroic assemblages. The compositions of the dikes, however, do not match those of the most evolved liquids expected in
reservoirs because they appear to have lost interstitial liquids (e.g., positive Eu anomalies, low abundances of some trace
elements). Because minerals in the dikes were in equilibrium with highly evolved liquids, conditions for small-scale dike
formation in cumulate environments apparently occur only at the last stages of reservoir magma differentiation and solidification.
Received: 25 February 1997 / Accepted: 14 June 1997 相似文献
12.
Pahoehoe toe dimensions, morphology, and branching relationships were analyzed in flows emplaced during 1972 at Mauna Ulu,
a satellitic shield on the east rift zone of Kilauea Volcano, Hawai'i. In order to characterize regions within flow fields
dominated by networks of pahoehoe toes, measurements of toe length, width, thickness, and orientation were completed for 445
toes at 13 sites. Variations in site characteristics, including slope, substrate, and position in the flow field allow an
evaluation of the effects of such parameters on toe dimensions. Toe surface morphology (ropy or smooth), local flow lobe position
(interior or margin), and connective relationships between toes were documented in the form of detailed toe maps. These maps
show the number of branches connecting a given toe to other toes in its local pahoehoe network and illustrate branching patterns.
Statistical analyses of toe dimensions and comparisons of pahoehoe toe study sites and sub-populations combined with field
observations, evaluation of toe maps, and qualitative examination of toe dimension size distributions show the following:
(a) Although there are significant variations at a given site, toes typically have mean lengths (101 cm) greater than mean
widths (74 cm) and mean widths greater than mean thicknesses (19 cm); sites that have mean widths greater than mean lengths
are those with lower slopes. (b) Where significant site-to-site variations in mean values of a given toe dimension were apparent,
these differences could not be directly related to site characteristics. (c) Ropy toes have significantly larger mean values
of length, width, and number of branches than smooth toes, and toes with three or more branches have greater lengths, widths,
and thicknesses than toes with two or fewer branches, suggesting concentration of flow in these toe types. (d) The skewness
of all size distributions of toe length and width to larger values suggests that toes are transitional to larger sheets and
channels, consistent with field observations; and (e) Two distinct types of branching patterns (called monolayer and centrally
ridged) were observed in preserved pahoehoe flow lobes. The significant variability in measured toe dimensions at Mauna Ulu
suggests that toe dimensions are influenced by numerous locally defined, random factors, and that an approach based on stochastic
methods can be used to model pahoehoe flow emplacement.
Received: 19 January 1998 / Accepted: 24 March 1999 相似文献
13.
This study proposes an efficient new cleaning procedure for measuring in situ cosmogenic 10Be in olivines and pyroxenes. This chemical routine is specially designed to decontaminate the abundant meteoric 10Be from these minerals. The method was tested on mafic minerals from basaltic flows of Mt. Etna volcano and from Hawaiian flows and moraines. A sequential dissolution test shows that 10Be concentrations decrease with the number of cleaning steps until reaching a constant value. This is a necessary condition to demonstrate the efficiency of the method in properly decontaminating samples of meteoric 10Be. Moreover, cross-calibration with cosmogenic 3He measured within the same samples yielded a sea level high-latitude production rate of 4.5±0.4 at g−1 a−1 for cosmogenic 10Be in mafic minerals. This rate is within 1σ uncertainty of empirically or model-derived rates for 10Be on the same targets. Such concordance supports the consistency of the new method. 相似文献