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Geochemical Behavior of Chromium Tion in Ultramafic Rocks, Then Discuss

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Geochemical Behavior of Chromium Tion in Ultramafic Rocks, Then Discuss RESOURCE GEOLOGY, 47(6), 319•`330, 1997 Geochemical Behavior of Chromium Keiichi SHIRAKI* Abstract: One of the trace elements, chromium, which is incorporated most effectively into the crystallizing minerals with octa- hedral sites from a magma and therefore most sensitive to magmatic fractionation, best indicates the degree of fractionation from the Earth's mantle, and can be used to trace the evolution of the continental crust. Chromium abundances of peridotite constituting the upper mantle are around 3000 ppm,which are slightly lower than those of chondrite. Spinel with Cr as a major constituent, chromite, crystallizes first from most basaltic magmas, and chromite is the only Cr-mineral of economic interest. Chromite orebodies can be divided into layered and podiform types. Chromitite layers in large ultramafic-mafic intrusions, which occupy more than 90 % of world chromite resources, originate from contami- nation of the mafic magma with silicic liquid, because the increase of SiO2 decreases the Cr solubility in basaltic melt. Podiform chromitites, which provide more than half of chromite produced, also form by interaction between basaltic melt and depleted peridotite. Spinels with the highest Cr in terrestrial rocks are found in podiform chromitites, low-Ca boninites and inclusions in diamond. The chromites from low-Ca boninites are higher in Cr/Al than chromites from komatiites with high Cr/Al relative to boninites. The Cr contents of spinels in volcanic rocks tend to increase with increasing SiO2 in primary mag- mas. They also increase with increasing degrees of depletion of basaltic components in the source peridotites. The early crystallized Cr-rich spinels react with the magma to form pyroxenes. Pyroxenes like spinels remove most of the Cr from the magma. In some high-Mg andesites, however, the early formed pyroxenes that coexist with spinels are rather low in Cr, and the Cr contents in the pyroxenes increase with decreasing Mg/Fe in the pyroxenes at the expense of spinels. In primary mafic magmas Cr is reduced relative to mantle rocks by a factor of three to ten through partial melting (1000-300 ppm). However, effects on Cr content of the melting degree are small relative to those of crystal fractionation. Crystal fractionation reduces Cr in the latest felsic differentiates to less than 10 ppm. Chromium contents in clastic sediments are controlled dominantly by the Cr abundances in the source areas, and can be used to estimate the ratio of ultramafic and mafic to felsic rocks in provenance. The Cr abundances of the weathered crust estimated from clastic sediments are 50 to 100 ppm, which are higher than the Cr abundance, 35 ppm, of the exposed Canadian Precambrian shield. Many Archean sediments show elevated Cr concentrations relative to post-Archean sedi- ments, and immature, mafic oceanic terranes have higher Cr abundances than mature, felsic continental crust. The Cr abun- dances of the Earth's surface have decreased with the evolution of the continental crust. tion in ultramafic rocks, then discuss Cr behavior during 1. Introduction igneous differentiation, and finally consider possible Chromium is the seventh most common element in changes of Cr abundances with continental evolution. the Earth's upper mantle, after O, Mg, Si, Fe, Al and Ca. 2. Chromium Abundance in Ultramafic Rocks Chromium abundances in peridotites constituting the upper mantle cluster around 3000 ppm. In the crustal rocks, The Cr abundances in peridotite nodules in alkali basalt however, Cr is a minor or trace element; the average of and kimberlite or in orogenic peridotites, which are con- basalts, 100-600 ppm Cr, depends mainly on the degree sidered to represent the upper, mantle materials, have been of fractionation or melting, and most granites contain only determined by many investigators. MAALƒÓE and AOKI less than 10 ppm Cr (SHIRAKI,1978). Thus Cr is most (1977) compiled 587 available analyses of lherzolitic nod- sensitive to magmatic fractionation and incorporated most ules and showed that continental and oceanic spinel lher- effectively in the early formed minerals with octahedral zolites have the same Cr abundances of 0.44 % Cr2O3 sites. This highly preferential partitioning of Cr into the (3010 ppm Cr), while continental garnet lherzolites have crystallizing minerals from a magma has best been inter- lower Cr2O3 contents of 0.37 % (2530 ppm Cr). On this preted in terms of the crystal field theory; Cr3+ ions have basis they proposed 0.42 % Cr2O3 (2870 ppm Cr) for Cr by far the largest octahedral site preference energy of all abundances, of the primitive mantle. HARTMANN and the common transition metal ions (CURTIS, 1964; BURNS WEDEPOHL(1993) estimated a lower value of 2500 ppm as and FYFE, 1964). In this paper I first review Cr distribu- Cr concentration of the primitive mantle, while JAGOUTZ et al. (1979) and SUN (1982) cited higher values of 3140 Recieved on September 16, 1997, accepted on November 19, and 3000 ppm, respectively, as Cr abundances of the primi- 1997 tive or fertile mantle. *Department of Earth Sciences , Yamaguchi University, The Cr abundances of 2500-3200 ppm in the Earth's Yamaguchi 753, Japan Keywords: Chromium, Geochemistry, Trace element, Cr- primitive mantle are similar to those in the primitive spinel, Pyroxene, Crustal abundance. bulk Moon, 3140 ppm, but lower than those in Cl car- 319 320 Keiichi SHIRAKI RESOURCE GEOLOGY: Table 1 Chromium contents (% Cr2O3) in minerals from some ultramafic rocks . Abbreviations: max, maximum value; Pen, inverted protoenstatite; Ref ., References. 1: ORBERGERet al. (1995); 2: CRAWFORD(1980); 3: MEYER (1975); 4: SOEOLEVet al . (1975); 5: HERVIGet al. (1980); 6: Ross et al. (1954); 7: DICK and BULLEN(1984); 8: OBATAand MORTEN (1987); 9: HARTS et al . (1987); 10: PRESSet al. (1986). bonaceous chondrite, 3810 ppm, and H-group ordinary that chromite was accumulated is economically mined as chondrite, 3660 ppm (O'NEILL, 1991) . Although Cr is Cr-ore, and chromite ore deposits, in most cases, are present in similar abundances in the Earth's mantle and hosted in ultramafic rocks. the Moon, the Cr/Mg ratios of the Earth's mantle and the Chromite orebodies are classed as either podiform type Moon are 0.49 and 0.36-0.40 of the chondritic value , in alpine-type peridotites or layered type in ultramafic respectively. RINGWOODet al. (1990) attribute the Cr cumulates. Podiform ores have accounted for 57.4 % of depletion in the Earth's mantle to its siderophile behav- all chromite produced until 1984, but chromitite layers in ior during formation of the Earth's core at high pressure . the Bushveld and Great Dyke complexes occupy more Chromium is generally considered to be a refractory than 90 % of world chromite resources (STOWE, 1987). element; in basaltic rocks positive correlations between Cr Chromites in podiform chromitites exhibit a wide and Mg are well known, suggesting that the distribution range of composition from high Cr to high Al . The high- coefficientDcyr-liq between crystal and liquidfor Cr is Cr varieties contain occasionally Cr2O3 greater than 60 %; larger than unity. In general, dunite and harzburgite more they are the most Cr-rich terrestrial chromites along with depleted in basaltic components have more Cr than less those in boninites and diamond inclusions (Table 1). Both depleted lherzolite. In a suite of lherzolite-harzburgite high-Cr and high-Al chromitite pods commonly have xenoliths from Austria, however, KURAT et al . (1980) highly depleted dunite rinds that grade outward into less found that Cr contents decrease systematically with depleted harzburgite. ARAI and YURIMOTO(1994) and increasing Mg/Fe. No correlation between Cr and Mg ZHOU et al. (1994) explained the close association of the was also reported from the Ronda peridotite in southern chromitites with dunites and harzburgites in terms of Spain (FREY et al., 1985). The decrease in Cr with interaction between ascending basaltic melts and harzbur- increasing Mg indicates that the bulk distribution coeffi gites or depleted lherzolites in the upper mantle. cient •¬ between peridotite and liquid can be less Chromites in layered chromitites in differentiated than unity under certain conditions . This is indeed the stratiform complexes have a relatively narrow range of case for ultramafic liquids like komatiites where the Cr Cr and Al concentrations; Cr203 contents of chromites contents tend to be independent of the MgO contents . in the Bushveld and the Great Dyke are 38-55 % (HAT- LIANG and ELTHON (1990) showed that a plot of Cr against TON and VON GRUENEWALDT, 1987) and 52-60 % MgO represents a constant value, 2000-3500 ppm Cr , for (WILSON, 1982), respectively. komatutes with>20 % MgO, and gives a nearly horizontal The Cr contents of spinels in igneous rocks depend regression line for some mantle peridotites. mainly on the Cr/Al ratios in the magmas in which they 3. Chromium Distribution in Ultramafic Rocks crystallized. Generally, lherzolite less depleted in basaltic components contains Al-rich spinel with the Table 1 shows Cr distributions among minerals in lowest Cr, and more depleted harzburgite and dunite some types of ultramafic rocks. A spinel, which contains contain chromite with the highest Cr (ARAI, 1987; Cr as a major constituent, crystallizes first from magmas ROEDER,1994). Spinels in abyssal peridotites are low in with basaltic compositions. The Cr-rich spinel, chromite, Cr (Table 1); their Cr#=Cr/(Cr+Al) does not exceed is the only Cr-mineral of economic interest. Chromitite 0.60. In contrast spinels in alpine-type peridotites show 47(6), 1997 Geochemical behavior of chromium 321 a wide variation in Cr2O3 and Cr# (DICK and BULLEN, but its coexisting clinopyroxene has 2.23 % Cr2O3 (SMITH 1984). ARAI (1994) discusses the Fo-Cr# relationships and LEVY,1976). Peridotites often contain emerald-green between peridotite restites and spatially associated Cr-diopsideshaving 0.5-3 % Cr2O3 (e. g., SHIRAKI,1978).
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