Abstract. Processes that move crystals relative to melt, that is crystal fractionation, are of major importance in producing variations that are observed within cogenetic suites of granites. In low‐temperature granite suites, crystal fractionation initially involves the progressive separation of crystals residual from partial melting from that partial melt. Once separation of those crystals, or restite, has been completed, further fractionation may occur through the separation of crystals that had precipitated from the melt, the process known as fractional crystallization. High‐temperature granite magmas are largely or completely molten and elements such as Ca, Mg and Fe, and their associated minor elements, are in that case dissolved in the melt. Such magmas, particularly those that are more potassic and hence contain a higher fraction of low temperature melt, may evolve compositionally through fractional crystallization. Cumulate rocks result, comprising a framework of cumulus minerals with interstitial melt. In this process some of the melt is also displaced to form more felsic rocks. Such cumulate rocks may have distinctive chemical compositions, but that is often not the case. Distinctive features include SiC>2 contents near or below 50 % in rocks that are transitional in the field to more felsic granites, very high Cr and Ni, very low K, P, Ba, Rb and Zr, and anomalous abundances of the anorthite components Ca and Al. These rocks may also have positive Eu anomalies. Cumulate rocks do not necessarily have distinctive textures, at least as such features are understood at this time. Fractional crystallization can also involve the movement of precipitated crystals relative to melt. We refer to rocks as cumulative when formed from the fractions in which the abundance of crystals has increased. The production of cumulative granites typically occurs at more felsic melt compositions than is the case for cumulate granites, and this process may have its greatest significance in the fractional crystallization of the felsic haplogranites. Relative to felsic granites of broadly similar compositions lying on a liquid line of descent, cumulative granites contain more Ca, reflecting the addition from elsewhere of plagioclase crystals with solidus compositions. The abundances of Sr and Ba may be high to very high, and sometimes there are positive Eu anomalies. Cumulative I‐type granites may have low abundances of Y and the heavy REE, while the S‐type granites can be very distinctive with anomalously high abundances of Th and the heavy REE resulting from the concentrating of monazite. Generally, but not always, those who propose fractional crystallization as a mechanism for producing compositional variation within a suite of granites do not state whether the rocks in that particular case are thought to lie on a liquid line of descent or are cumulates/cumulative, although it is generally presumed that they were melts. Our experiences in eastern Australia have shown that the mechanism of fractional crystallization was quantitatively not as important during granite evolution as many workers would expect. However, there are some excellent examples of that process, most notably the Boggy Plain Supersuite. Overall in eastern Australia, varying degrees of separation of restite is a much more common mode of crystal fractionation, and that may also be seen to be the case for some other granite provinces if they are examined with that possibility in mind. 相似文献
Crystallization temperatures (T) and oxygen fugacities (fO2)of kimberlite magma are estimated from oxides included in olivinephenocrysts from the Leslie, Aaron, Grizzly and Torrie kimberlitepipes in the central Slave Province, Canada. Crystallizationtemperatures recorded by olivinechromite pairs at anassumed pressure of 1·0 GPa are 10301170°C± 50°C, with a mean of 相似文献
The Oshurkovo Complex is a plutonic sheeted complex which represents numerous successive magmatic injections into an expanding system of subparallel and subvertical fractures. It comprises a wide range of rock types including alkali monzodiorite, monzonite, plagioclase-bearing and alkali-feldspar syenites, in the proportion of about 70% mafic rocks to 30% syenite. We suggest that the variation within the complex originated mainly by fractional crystallization of a tephrite magma.
The mafic rocks are considered as plutonic equivalents of lamprophyres. They exhibit a high abundance of ternary feldspar and apatite, the latter may attain 7–8 vol.% in monzodiorite. Ternary feldspar is also abundant in the syenites. The entire rock series is characterized by high Ba and Sr concentrations in the bulk rock samples (3000–7000 ppm) and in feldspars (up to 1 wt.%). The mafic magma had amphibole at the liquidus at 1010–1030 °C based on amphibole geothermometer. Temperatures as low as this were due to high H2O and P2O5 contents in the melt (up to 4–6 and 2 wt.%, respectively). Crystallization of the syenitic magmas began at about 850 °C (based on ternary feldspar thermometry). The series was formed at an oxygen fugacity from the NNO to HM buffer, or even higher.
The evolution of the alkali monzodiorite–syenite series by fractional crystallization of a tephritic magma is established on the basis of geological, mineralogical, geochemical and Sm–Nd and Rb–Sr isotope data. The geochemical modeling suggests that fractionation of amphibole with subordinate apatite from the tephrite magma leaves about 73 wt.% of the residual monzonite melt. Further extraction of amphibole and plagioclase with minor apatite and Fe–Ti oxides could bring to formation of a syenite residuum. Rb–Sr isotopic analyses of biotite, apatite and whole-rock samples constrain the minimum age of basic intrusions at ca. 130 Ma and that of cross-cutting granite pegmatites at ca. 120 Ma. Hence the entire evolution took place in an interval of ≤10 My. Initial 87Sr/86Sr ratios for the mafic rocks range from 0.70511 to 0.70514, and for syenites from 0.70525 to 0.70542. Initial Nd (130 Ma) values for mafic rocks vary from −1.9 to −2.4, and for syenites from −2.9 to −3.5. In a Nd(T) vs. (87Sr/86Sr)i diagram, all rock types of the complex fall in the enriched portion of the Mantle Array, suggesting their derivation from a metasomatized mantle source. However, the small but distinguishable difference in Sr and Nd isotopic compositions between mafic rocks and syenites probably resulted from mild (10–20%) crustal contamination during differentiation. Large negative Nb anomalies are interpreted as a characteristic feature of the source region produced by Precambrian fluid metasomatism above a subduction zone rather than by crustal contamination. 相似文献
New analyses of He, Ne, Ar and CO2 trapped in basaltic glasses from the Southeast Indian Ridge (Amsterdam-St. Paul (ASP) region) show that ridge magmas degas by a Rayleigh distillation process. As a result, the absolute and relative noble gas abundances are highly fractionated with 4He/40Ar* ratios as high as 620 compared to a production ratio of ∼3 (where 40Ar* is 40Ar corrected for atmospheric contamination). There is a good correlation between 4He/40Ar* and the MgO content of the basalt, suggesting that the amount of gas lost from a particular magma is related to the degree of crystallization. Fractional crystallization forces oversaturation of CO2 because CO2 is an incompatible element. Therefore, crystallization will increase the fraction of gas lost from the magma. The He-Ar-CO2-MgO-TiO2 compositions of the ASP basalts are modeled as a combined fractional crystallization-fractional degassing process using experimentally determined noble gas and CO2 solubilities and partition coefficients at reasonable magmatic pressures (2-4 kbar). The combined fractional crystallization-degassing model reproduces the basalt compositions well, although it is not possible to rule out depth of eruption as a potential additional control on the extent of degassing. The extent of degassing determines the relative noble gas abundances (4He/40Ar*) and the 40Ar*/CO2 ratio but it cannot account for large (>factor 50) variations in He/CO2, due to the similar solubilities of He and CO2 in basaltic magmas. Instead, variations in CO2/3He (≡C/3He) trapped in the vesicles must reflect similar variations in the primary magma. The controls on C/3He in mid-ocean ridge basalts (MORBs) are not known. There are no obvious correlated variations between C/3He and tracers of mantle heterogeneity (3He/4He, K/Ti etc.), implying that the variations in C/3He are not likely to be a feature of the mantle source to these basalts. Mixing between MORB-like sources and more enriched, high 3He/4He sources occurs on and near the ASP plateau, resulting in variable 3He/4He and K/Ti compositions (and many other tracers). Using 4He/40Ar* to track degassing, we demonstrate that mixing systematics involving He isotopes are determined in large part by the extent of degassing. Relatively undegassed lavas (with low 4He/40Ar*) are characterized by steep 3He/4He-K/Ti mixing curves, with high He/Ti ratios in the enriched magma (relative to He/Ti in the MORB magma). Degassed samples (high 4He/40Ar*) on the other hand have roughly equal He/Ti ratios in both end-members, resulting in linear mixing trajectories involving He isotopes. Some degassing of ASP magmas must occur at depth, prior to magma mixing. As a result of degassing prior to mixing, mixing systematics of oceanic basalts that involve noble gas-lithophile pairs (e.g. 3He/4He vs. 87Sr/86Sr or 40Ar/36Ar vs. 206Pb/204Pb) are unlikely to reflect the noble gas composition of the mantle source to the basalts. Instead, the mixing curve will reflect the extent of gas loss from the magmas, which is in turn buffered by the pressure of combined crystallization-degassing and the initial CO2 content. 相似文献
In this paper, a class of spatio-temporal processes with first-order autoregressive temporal structure and functional spatio-temporal
interaction is introduced. The spatial second-order regularity is allowed to change over time and is characterized in terms
of fractional Sobolev spaces. The associated filtering problem is considered, assuming that observations are defined by spatial
linear functionals of the process of interest, being affected by additive noise. Conditions under which a stable solution
to this problem is obtained are studied. A functional least-squares linear estimate fusion method is derived to calculate
this solution A multiscale finite-dimensional approximation to the problem is obtained from the wavelet-based orthogonal expansions
of the time cross-section spatial processes, which allows the numerical inversion of the linear operator involved. 相似文献
Surface specimens obtained from highly-weathered upland plateau outcrops at three localities in high Arctic Canada have been studied using the scanning electron microscope. Observations are given on factors influencing bedrock surface microfracturing processes. Evidence for the concentration of both salts and organic material in surface cracks which could enhance microfracturing is presented. The importance of lithological parameters including: mineralogy, texture, and structures present in influencing the actual processes of disintegration is reiterated. Under cold, dry high Arctic conditions the combined and apparently inseparable affects of frost action, salt crystallization, and organic activity may have contributed to microfracturing largely responsible for widely scattered examples of highly weathered bedrock terrain. 相似文献