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Melting of the Shallow Upper Mantle: A New Perspective 总被引：4，自引：3，他引：4
Detailed examination of liquidus phase relationships in binaryand ternary joins of the CFMAS +Cr system has permitted a rigorousdetermination of the dry melting path of an initially fertilespinel peridotite composition resembling Bulk Silicate Earthor MORB-pyrolite. It is demonstrated that it is impossible tomodel mantle melting accurately using only one set of ratiosof phases entering the melt; this implies that the melting processis primarily controlled by solid solution rather than eutecticbehaviour. The proportions of phases entering a melt dependon whether a phase reacts and/or disappears from a system, andon the choice of the initial and final peridotite compositions.Four discrete domains in the melting regime of upper-mantleperidotites are distinguished, each characterized by differentphase melting coefficients, relating to the melting of: (1)lherzolites, (2) clinopyroxene-bearing harzburgites (i.e., free-clinopyroxene),(3) clinopyroxene-saturated harzburgites (i.e., clinopyroxenein solid solution in orthopyroxene), and (4) clinopyroxene-freeharzburgites (i.e., no clinopyroxene). The proposed non-linearfashion in which mantle lithologies melt explains the inadequacyof all previous models to reproduce the observed compositionsof upper-mantle peridotite melting residues. It is suggestedthat: (1) olivine and orthopyroxene will melt cotectically;(2) clinopyroxene and spinel will lose most of their aluminouscomponent after {small tilde}8% melting within the first 4 kb({smalltilde} 12 km) of ascent from the dry solidus; and that (3) clinopyroxenewill disappear completely from a MORB-pyrolite mantle after{small tilde}42% melting. Although such a number is significantlyhigher than that dictated by the position of the clinopyroxene-outcurves from peridotite isobaric equilibrium melting experiments({small tilde}22%), it is emphasized that the latter are a grossoversimplification of the natural melting process and are notequivalent to melting during adiabatic upwelling. It is concludedthat the commonly postulated disappearance of clinopyroxenefrom fertile peridotite compositions at {small tilde}22% meltingis greatly in error if melting in an adiabatically rising mantleis considered, thus providing an explanation for many unsuccessfulattempts by various authors to model the behaviour of transitionelements in sub-oceanic and supra-subduction-zone mantle andderivative magmas. 相似文献
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Parameterization of melting phenomena in the upper mantle hasprimarily focused on two basic themes, namely the physical andchemical processes that govern partial melting. Parameterizationof physical processes mainly refers to establishing relationshipsbetween parameters such as the temperature, pressure, matrixand melt flow geometry, lithospheric stretching, and volumeof magma. By contrast, parameterization of chemical processeslargely implies unravelling the relationships between type anddegree of melting, and source and melt composition. Few attemptshave been made, however, to interrelate the two processes. Thepresent work is an effort to provide a link between physicaland chemical parameters associated with mantle melting and toallow in-depth modelling of partial melting processes in upwellingasthenosphere in a rigorous yet simplified manner. Several correlationsamong the most important physical parameters (e.g., equilibrationand extrusion temperature and pressure of magma, melt fractionand thickness, stretching factor, etc.) are explored. On thisbasis, a model for the compositional stratification of the lithosphereis proposed, and its bearing on the nature of intra-oceanicarc magmatism is emphasized. Trends of melting residues in termsof modal olivine and clinopyroxene are calculated for a widerange of possible potential temperatures that may be appliedto xenolith or abyssal peridotite suites to constrain furthertheir original depth of upwelling. Dry solidus equations fordepleted peridotite compositions are also derived that may beused to infer the effects of volatiles on the melting of refractorysupra-subduction zone mantle. The sensitivity of certain elementsto temperature variations during melting in a column of ascendingmantle is highlighted using Ni as an example, and the dangersof using single-value distribution coefficients to predict concentrationsof transition metals in magmas are emphasized. MORB-normalizedmulti-element profiles calculated using a variety of sources,mantle potential temperatures, and stretching factors are presented,and the differences between instantaneous and pooled melts arediscussed. A technique to calculate mineral proportions duringtransformation of garnet lherzolite to spinel lherzolite, togetherwith estimates of the modal composition of fertile spinel andgarnet lherzolite are included. Selected trace-element abundancesin various sources [bulk silicate Earth, depleted MORB (mid-oceanridge basalt) mantle, N-MORB) and distribution coefficientsfor common rock-forming minerals are also tabulated. 相似文献
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