S. A. Morse

Cation diffusion in plagioclase feldspar.

Steep compositional gradients in igneous plagioclase feldspar from slowly cooled intrusive bodies imply a maximum value of the intracrystalline diffusion coefficient for NaSi [unknown] CaAl exchange, Dmax ∼10-20 centimeters squared per second for temperatures in the range 1250� to 1000�C. Millimeter-sized grains cannot be homogenized in all geologic time; hence reactive equilibrium crystallization of plagioclase from the melt does not occur in dry systems.

Kiglapait Geochemistry IV: The Major Elements

Abstract The summed bulk composition of the Kiglapait intrusion is SiO 2 47.46 (31). TiO 2 0.81 (20), Al 2 O 3 19.46 (48), Fe 2 O 3 1.91 (46), FeO 9.36 (36), MnO 0.15 (01). MgO 8.04 (21), CaO 9.27 (46). Na 2 O 3.13 (05), K 2 O 0.22 (01), P 2 O 5 , 0.09 (01), and BaO 0.02 (01). total 99.98 wt% (estimated standard deviations of the last two decimals in parentheses). The CIPW norm is ap 0.34, mt 2.78, il 1.52, or 1.11, ab 26.27, an 38.64, ( fsp 66.47), di 5.38, hy 2.45, ol 21.58. Molar X An is 0.58; molar x fo is 0.65. The variation curves for oxides in calculated liquids are explained in detail by the petrographic variations shown in earlier papers of this series. The Kiglapait FeO + MnO:MgO:Alkalies (FMA) trend reaches higher iron concentrations than the Skaergaard liquid and constitutes a new limiting case of the Fenner trend of fractional crystallization, implying low oxygen fugacity and silica activity. The end-stage Kiglapait ferrosyenite lies on the FeO + MnO-Alkalies sideline, being essentially devoid of MgO. Except in K and K Rb. the Kiglapait composition is the same within experimental error as the chilled margin of the nearby Hettasch intrusion (BERG, 1980.), but slight systematic differences show that the Hettasch composition is slightly more fractionated or represents a smaller fraction of melting at the source. Both are low-augite troctolite compositions, and they require troctolitic parent liquids. Such liquids are related to common augite-rich basalts by addition of spinel, and they imply exhaustion of clinopyroxene in the source. Except for their low augite and high Al 2 O 3 , the Kiglapait and Hettasch compositions resemble modern MORB and suggest a depleted source.

Sapphirine reactions in deep-seated granulites near Wilson Lake, Central Labrador, Canada

Abstract Sapphirine and magnetite are closely associated in elongate pods in the granulites of Wilson Lake, central Labrador, where quartz, hypersthene, sillimanite, kyanite, corundum, and spinel also occur in textural equilibrium with sapphirine. Evidence exists for at least five retrograde reactions involving sapphirine, in the absence of cordierite. The metamorphic conditions to be inferred for these rocks are extreme, based on the experimental work of Hensen and Green (1970) . Conditions in excess of 11 kbar, 1100°C, and possibly as high as 13 kbar, 1150°C are suggested by the experimental work.

Origin of strongly reversed rims on plagioclase in cumulates

Abstract Narrow reversed rims on plagioclase are ubiquitous in troctolites and olivine gabbros of the Kiglapait intrusion and may be a common feature of all such cumulates. The rims occur at plag/plag, plag/ol, and less strongly at plag/aug grain boundaries. They are optically obvious at ΔAn and can reach ΔAn = 32 mol.% or more. In parallel, K/Na drops sharply. Although ubiquitous from sample to sample, the reversed rims are only locally present at grain boundaries even for the same pair of crystals in contact; they are prominent in linear networks suggesting the last trace of intercumulus liquid. A subsolidus origin is ruled out by the absence of reactants at plag/plag and plag/ol boundaries and by the local rather than pervasive development of rims. The rims are required to grow from intercumulus liquid, in which the partition of An component between crystals and liquid increases with the trapped augite component of the liquid. Calculations from published experimental data show that ΔAn > 30 can easily be achieved by such a process. It is also probable that the trapped liquid is part of an An-rich boundary layer generated by solute rejection during adcumulus growth. The ability of the rims to sustain steep K/Na gradients despite a long subsolidus cooling history proves that the K Na exchange rate is vanishingly small over a geologic time scale in An-rich feldspar, suggesting that at low K content the potassium is site-bound to the tetrahedral Al/Si distribution. Reversed rims therefore provide important information on diffusion limits as well as on the late-stage solidification history of plagioclase-rich cumulates. Moreover, they demonstrate that plagioclase geothermometry cannot be divorced from effects of liquid composition and structure as monitored, for example, by augite content.

Crystal Identification with the Polarizing Microscope

List of symbols. Preface. Efficient use of this book. Crystal identification and optical principles. Equipment and preparation of materials. The immersion method. Isotropic crystal identification. Phase relationships and interference. Anisotropic crystals. Uniaxial crystal optics. Uniaxial interference figures. Identification of uniaxial crystals. Biaxial crystal optics. Biaxial interference figures. Identification of biaxial crystals. Crystallographic relations of biaxial crystals. Optic orientation in stereo. Special orientation methods. Use of the dispersion method. Crystal identification practicum Appendix A: optical properties of common rock-forming minerals. Appendix B: identification of fibrous asbestos References. Index.

Kiglapait Geochemistry III: Potassium and Rubidium

Abstract The regular geometry and completeness of the Kiglapait intrusion permit its bulk composition to be obtained by summation, and the composition of successive liquids to be obtained by subtraction. The summations for K and Rb give 1806 and 1.08 ppm, yielding R  frsol | K / Rb = 1670 for the intrusion, taken as equal to the parent magma. R increases slightly from this initial value to 2000 at the end of crystallization where MgO approaches zero in the rocks. K and Rb are therefore closely coherent and their distribution coefficients can differ only by a small amount in the Kiglapait system. Apparent feldspar/liquid distribution coefficients (D F/L ) can be estimated from detailed plots of feldspar and liquid compositions against F L . The Kiglapait data imply that these coefficients are linear 1:1 functions of plagioclase composition within experimental error, having values given by D K F / L = 1.42− X An D Rb F / L = 1.13− X An with minimum values of 0.75 and 0.49, respectively. The ratio R F R L lies in the range of 1.53± 0.03 for the plagioclase composition range X An = 0.34 to 0.67 showing that high-R rocks such as anorthosite crystallized from high- R liquids. The apparent feldspar distribution coefficients are much closer to 1.0 than common literature values. They can be reduced by assuming that the cumulate pile was continuously recharged by the circulating magma until an advanced stage of differentiation was reached, and assuming that alkalies were exchanged to the feldspars from the magma. When such an ‘aquifer recharge’ model is calibrated using olivine-liquid equilibria as a time marker for the liquid, the inferred minimum equilibrium values of the distribution coefficients are D K F L = 0.42, D Rb F L = 0.25 at the base of the intrusion. Their variation is given by D K F L = 1.66−1.88X An , D Rb F L = 1.17−1.41X An , The equilibrium values are considered to be appropriate for deducing liquid compositions in plutonic bodies where alkali exchange can be shown or inferred to have been inhibited, such as in small intrusions. The apparent values are considered to be appropriate, even though they may be artificial, for large intrusions similar to the Kiglapait. The bulk K and Rb concentrations in the Kiglapait intrusion are consistent with a plagioclase-rich abyssal tholeiite magma. Clinopyroxene and olivine fractionation in the mantle may contribute to the production of such high- R magmas.

Kiglapait geochemistry V: Strontium

Abstract The Kiglapait intrusion contains 330 ppm Sr and has Sr Ca = 5 × 10 −3 and Rb Sr = 3 × 10 −3 , as determined by summation over the Layered Group of the intrusion. Wholerocks in the Lower Zone contain 403 F L 0.141 ppm Sr, where F L is the fraction of liquid remaining; Sr drops to 180 ppm at the peak of augite production ( F L = 0.11) and rises to a maximum of 430 ppm in the Upper Zone before decreasing to 172 ppm at the end of crystallization. Feldspars in the Lower Zone contain 532 F L 0.090 ppm Sr, increasing to 680 ppm in the Upper Zone before decreasing to 310 ppm at the end. Clinopyroxenes contain 15 to 30 ppm Sr and have a mineral-melt distribution coefficient D = 0.06 except near the top of the intrusion where D = 0.10. The calculated feldspar-liquid distribution coefficient has an average value near 1.75 but shows four distinct trends when plotted against X An of feldspar. The first two of these are strongly correlated with the modal augite content of the liquid, on average by the relation D = 1.4 + 0.02 Aug L . The third (decreasing) trend is due to co-crystallization of apatite, and the fourth (increasing) trend can best be attributed to a triclinic-monoclinic symmetry change in the feldspar at An 26 , 1030°C. The compound feldspar-liquid distribution coefficient K D for Sr Ca bears out these deductions in detail and yields ΔG r for the Sr-Ca exchange ranging from nearly zero at the base of the Lower Zone to −26 k J /gramatom at the end of crystallization. The compound feldspar-liquid distribution coefficient K D for Rb Sr varies from 0.3 in the Lower Zone to 1.1 at the end of crystallization. The ratio Ca F Ca L is about 1.45 for troctolitic liquids containing 5% augite, for which K D (Sr-Ca) = 1.0 and D Ca = D Sr . For common basaltic liquids containing 20% augite, the Kiglapait data predict solSr F Sr L = 1.8 , as commonly found elsewhere. The strong dependence of D sr on augite content of the liquid illuminates the role of liquid composition and structure in determining the feldspar-liquid distribution coefficient. Conversely, a discontinuous change in the trend of D Sr when apatite arrives shows that the effect is due to apatite crystallization itself, not to the continuous variation of the liquid as it becomes enriched in apatite component.

Marginal rocks resembling the estimated bulk composition of the Kiglapait Intrusion

Abstract The southern margin of the Kiglapait Intrusion, Labrador, contains fine-grained rocks 5–6 m above a contact with anorthosite of the Nain Complex, and a laminated unit 3 m from the contact Three fine-grained samples and one of the laminated unit have a mean composition closely resembling that previously calculated by summation of the exposed Layered Group. Analyzed values agree closely with the earlier estimates for alkalies, Sr, and Y, thus they support the mineral/liquid partition coefficients previously calculated for these elements and for the rare earth elements. Based on new calculations, the value for ferric oxide is revised to 1.04% from its original value of 1.91%. The new samples are depleted in the incompatible elements titanium and phosphorus, suggesting that they have lost some residual liquid by sidewall fractional crystallization and downward motion of rejected solute. The marginal rocks provide no evidence for repeated injection of magma during Lower Zone time.

Kiglapait geochemistry VII: Yttrium and the rare earth elements

Abstract Based on 51 wholerock analyses by XRF and summation over the layered group, the Kiglapait Intrusion contains 4.7−1.6+1.2 ppm Y, which resides principally in augite and apatite. Using liquid compositions calculated by summation, the partition coefficient DAUG/LY is 0.95 ± 0.12 from 84 to 97 PCS (percent solidified) and 1.5 ± 0.4 above 97 PCS. For feldspar, the most likely value for D is 0.028 ± 0.02 (N = 6). REE analyses for 13 whole rocks were interpreted with the aid of yttrium models to yield trends for wholerocks and liquids vs PCS. Summations over the rocks of the layered group gave La = 2.5, Ce = 5.8, Nd = 3.9, Sm = 1.0, Eu = 0.8, Tb = 0.17, Yb = 0.37, and Lu = 0.06 ppm, with 2 s.d. errors near ± 30%. All these elements are highly incompatible until the arrival of augite, which affects chiefly the HREE, and apatite, which affects all (but more strongly, the LREE). The net result is that after apatite arrival at 94 PCS, the liquid compositions are nearly constant, hence DWR/LREE ≈ 1.0. These results are compatible with the mineralogy of the intrusion and the estimated partition coefficients for feldspar, olivine, augite, apatite, and Fe-Ti oxide minerals. For pre-apatite liquids, DFSP/LREE vary regularly with the normative di content of the liquid and change by an order of magnitude, hence the bulk liquid composition must be considered in any attempt to invert the compositions of feldspars to parent liquids. The Eu anomaly at first decreases in Kiglapait liquids due to plagioclase fractionation, but then increases due to removal of augite and apatite with negative Eu anomalies. The features dominantly responsible for Eu partitioning are liquid structure and, for monoclinic ternary feldspars, crystal structure. The former is best monitored by the augite or diopside content of the liquid and the latter, by the K content of the feldspar. The chondrite-normalized REE pattern for the intrusion has LaN = 7.4, LuN = 1.6, (Ce/Yb)N = 3.6, and Eu/Eu* = 2.4, indicating its feldspar-rich nature. The chilled margin of the nearby Hettasch Intrusion has a similar but more evolved pattern, corresponding roughly to the Kiglapait liquid at 70 PCS. As with other data, those for the REE suggest source differences for the two intrusions rather than a relationship due to fractionation.

Plagioclase lamellae in hypersthene, Tikkoatokhakh Bay, Labrador

Abstract Abundant lamellae of plagioclase are present in the (100) planes of hypersthene megacrysts in andesine anorthosite along Tikkoatokhakh Bay, northwest of Nain, Labrador. Spongy intergrowths of plagioclase in hypersthene also occur. Plagioclase lamellae have mean compositions ranging from An 43 to An 92 , with extreme compositions from An 39 to An 97 ; the calcic compositions are the more abundant. Such lamellae are always accompanied in the hypersthene by grains or lamellar segments of magnetite, and rarely by lamellae of olivine, augite, magnetite, or ilmenite. Some calcic plagioclase lamellae contain antiperthitic spindles of orthoclase. The host rocks of the hypersthene megacrysts are layered leuconorites and anorthosites with mean plagioclase compositions ranging from An 41 to An 55 . The plagioclase lamellae in hypersthene are characteristically much more calcic than the host-rock plagioclase. There is little doubt that the lamellae exsolved from a pyroxene host, dominantly by a coupled redox reaction which generated magnetite, thereby releasing silica to combine with the Ca-Tschermak and jadeite components of the precursor pyroxene. Rapid growth of megacrysts may account for their aluminous nature.