C. K. Shearer

Are discontinuous chondrite-normalized REE patterns in pegmatitic granite systems the results of monazite fractionation?

Abstract All 14 stable rare earth elements (REEs) in biotite-muscovite granite and tourmaline-rich granite of the Harney Peak Granite, Black Hills, South Dakota, USA, have been analyzed using inductively coupled plasma mass spectrometry. Chrondrite-normalized REE patterns of the tourmaline-rich granites are discontinuous between Nd and Sm. The discontinuity was modeled successfully by the fractional crystallization of monazite from a biotite-muscovite granite initial composition. This explanation may also apply to the development of such discontinuous patterns in other highly evolved rocks.

Mineralogical and chemical evolution of a rare-element granite-pegmatite system: Harney Peak Granite, Black Hills, South Dakota

Abstract The Harney Peak Granite (1.7 b.y.) in the Black Hills, South Dakota, is a well-exposed granite complex surrounded by a rare-element pegmatite field (barren to Nb-, Ta-, Be, Li-enriched pegmatites). It consists of a multitude of large and small sills and dikes, which exhibit great variation in texture, mineralogy and geochemistry. This granite is moderately to strongly peraluminous with the following mineralogy: plagioclase (An0–An21) + potassium feldspar (Or70–96) + quartz + muscovite ± apatite ± biotite ± garnet ± tourmaline. The granitic intrusions in the interior of the complex have similar K Rb ratios (> 190), whereas this ratio decreases and is more variable for intrusions which are structurally higher or along the perimeter of the complex. Substitutions of (Fe, Mn)Mg−1 in the ferromagnesian minerals, NaCa−1 in plagioclase and RbK−1 in muscovite and potassium feldspar increase in the perimeter granites and vary systematically with K Rb . These more evolved intrusions are commonly enriched in incompatible elements such as Nb, Li, Cs, Be, and B and depleted in Ba, Ca, and Sr relative to the interior, primitive granites. Biotite-bearing assemblages are common in the interior granites but are replaced by tourmaline-bearing granites in the more evolved intrusions. A series of discontinuous reactions may explain this assemblage transition. Observations and trace element modeling suggest that: (1) within individual units volatile transfer mechanisms have resulted in mineral and chemical segregation; (2) 75–80% fractional crystallization of a primitive biotite-muscovite granite was the dominant mechanism in producing the more evolved tourmaline-bearing granites; and (3) extreme fractional crystallization aided by high volatile activity produced the associated rare-element pegmatites.

Chemistry of potassium feldspars from three zoned pegmatites, Black Hills, South Dakota: Implications concerning pegmatite evolution

Abstract An initial phase of an extensive geochemical study of pegmatites from the Black Hills, South Dakota, indicates potassium feldspar composition is useful in interpreting petrogenetic relationships among pegmatites and among pegmatite zones within a single pegmatite. The K Rb and Rb Sr ratios and Li and Cs contents of the feldspars within each zoned pegmatite, to a first approximation, are consistent with the simple fractional crystallization of the potassium feldspar from a silicate melt from the wall zone to the core of the pegmatites. Some trace element characteristics (i.e. Cs) have been modified by subsolidus reequilibration of the feldspars with late-stage residual fluid. K Rb ratios of the potassium feldspar appear to be diagnostic of the pegmatite mineral assemblage. The relationship between K Rb and mineralogy is as follows: Harney Peak Granite (barren pegmatites) > 180; Li-Fe-Mn phosphate-bearing pegmatites = 90−50; spodumene-bearing pegmatites = 60−40; pollucitebearing pegmatites K Rb ratios suggest that the pegmatites studied are genetically related by fractional crystallization to each other and the Harney Peak Granite, overlapping Rb Sr ratios and the general increase in Sr and Ba with decreasing K Rb indicate the genetic relationship is much more complex and may also be dependent upon slight variations in source (chemistry and mineralogy) material composition and degrees of partial melting.

An ion microprobe study of the intra-crystalline behavior of REE and selected trace elements in pyroxene from mare basalts with different cooling and crystallization histories

Abstract To study the effects of crystallization sequence and rate on trace element zoning characteristics of pyroxenes, we used combined electron microprobe-ion microprobe techniques on four nearly isochemical Apollo 12 and 15 pigeonite basalts with different cooling rates and crystallization histories. Major and minor element zoning characteristics are nearly identical to those reported in the literature. All the pyroxenes have similar chondrite-normalized REE patterns: negative Eu anomalies, positive slopes as defined by Yb Ce , and slopes of REE patterns from Ce to Sm much steeper than from Gd to Yb. The characteristics of the REE patterns of pigeonite and augite can be rationalized in terms of REE size and charge and M2 site characteristics. Numerous REE and other trace element (V, Sr, Zr, Na, Sc, Cr) zoning characteristics can be correlated with quadrilateral composition. The total REE abundances increase with increasing Ca in the M2 site and indicate that the distribution coefficients of REE between pyroxene and melt vary systematically with the Wo component. The total REE abundances also increase with decreasing Mg (Mg + Fe) and increasing Al prior to plagioclase crystallization. The Eu anomaly in pyroxene is not substantially affected by plagioclase crystallization because Eu3+ is effectively partitioned into pyroxene relative to Eu2+ (( Eu 2+ Eu 3+ ) = .03 in pigeonite) and the plagioclase effect is diluted by the low Eu distribution coefficient for the more abundant pyroxene. The Cr and V components behave similarly in that Cr and V decrease with increasing Ti Al in basalt sample 15058 and decrease with crystallization and at similar Ti Al . Na and Sc increase in abundance from pigeonite to augite with similar Ti Al in both 15058 and 15499. In 15058, Na decreases coherently with increasing Ti Al , whereas Sc shows a limited variability. Sr and Zr in 15058 are somewhat more enriched in the augite with low Ti Al relative to pigeonite. The augite does not become enriched in Zr or Sr until the pyroxene has high Ti Al and low Mg (Mg + Fe) . In 15499, Sr and Zr show slight enrichment in augite relative to pigeonite. These trace element zoning characteristics in pyroxene and the partitioning of trace elements between pyroxene and the melt are intimately related to the interplay among the efficiency of the crystallization process, the kinetics at the crystal-melt interface, the kinetics of plagioclase nucleation and the characteristics of the crystal chemical substitutions within both the pyroxene and the associated crystallizing phases (i.e. plagioclase).

Inter- and intra-crystal REE variations in apatite from the Bob Ingersoll pegmatite, Black Hills, South Dakota

Abstract Concentrations of rare earth elements (REE) have been measured on a suite of apatite crystals from an internally zoned granitic pegmatite enriched in Li, B, Be, F, Nb, Ta, Sn and U with a Cameca IMS 3f ion microprobe using energy filtering. An apatite specimen from the Tin Mountain pegmatite, analyzed previously by isotope dilution, was used as a standard. The chondrite-normalized pattern determined with the ion microprobe closely matches the pattern determined by isotope dilution, with maxima at Sm and Dy, and minima at Nd and Er. Apatite samples from the Bob Ingersoll pegmatite show a large range of REE patterns and concentrations. In one case, apatite crystals within millimeters show differences in REE concentrations and pattern shapes, including a switch from positive to negative Eu anomalies. Samples from several mineral assemblages show patchy individual crystal zoning with respect to the REE that is not mirrored by major element zoning. This indicates disequilibrium conditions on a small scale, consistent with rapid growth from a melt/fluid system that was either of locally heterogeneous melt structure, or in which the melt structure fluctuated rapidly as volatile rich minerals such as tourmaline crystallized. These effects may be coupled with non-ideal partitioning of REE in a heterogeneous mixture of melt, aqueous fluid and crystals. REE concentrations in apatite samples from the different pegmatite zones indicate a large variation in outer zones (10–500X chondrite), high concentrations (100–1000X chondrite) near the pegmatite core, and very low concentration in the core (2–20X chondrite). Patterns are flat to slightly inclined (Ce/Yb: 1 to 5), and most samples have positive Eu anomalies. The magnitude of positive Eu anomalies decreases with inward position in the pegmatite, possibly indicating a progressive increase in ƒ O 2 , and a sharp increase may be indicated by systematic Ce depletion in apatite from the pegmatite core. REE-specific volatile complexes may contribute to variations, including unusual kinks, observed in REE patterns of apatite from mineral assemblages in upper parts of the pegmatite.

Fractionation trends in mica and tourmaline as indicators of pegmatite internal evolution: Bob Ingersoll pegmatite, Black Hills, South Dakota☆

Abstract Pegmatitic micas from the Bob Ingersoll No. 1 Dike, a large, internally zoned, rare element pegmatite body, have been analyzed for trends of compositional variation that indicate the sequence of crystallization and record the progressive compositional differentiation of the pegmatite. The dike consists of six primary zones and one replacement assemblage. 2M 1 , muscovite occurs in the border and wall zones and outer parts of the first intermediate zone. Intermediate-Li micas of mixed polytype, but with a dominant 2M 1 component, occur in inner parts of the pegmatite, including the core and areas of contact between intermediate zones and the core. 2M 2 lepidolite occurs as a replacement of K-feldspar along margins between the first intermediate zone and core. Fractionation trends of the micas include increasing Li substitution, decreasing Rb Cs and increasing Ta (Nb + Ta) in a general sequence from the outer wall zone to the pegmatite core, but with considerable overlap among intermediate zones. When compared to coexisting tourmaline, overlapping compositional trends of the interior zones suggest overlap in timing of crystallization and interzonal geochemical communication. Border zone mineral compositions do not fit major variation trends due to the combined effects of wallrock contamination, replacement of early microcline by lithian mica during differentiation of the pegmatite, and possibly rapid, disequilibrium crystallization. Strong zoning of mica and tourmaline crystals reflects disequilibrium conditions as crystalline pegmatite reacted with compositionally evolving fluids. Lithian mica and lepidolite that replaced early-formed K-feldspar may have been generated by fluids that were in equilibrium with a residual silicate melt in the pegmatite core. Aqueous fluids exsolved from silicate melt appear to have leaked from the pegmatite throughout much of its crystallization history, and the late stage Li, Rb, and Cs-enriched fluids contributed to the exomorphic halo surrounding the pegmatite.

Ion microprobe studies of trace elements in Apollo 14 volcanic glass beads - Comparisons to Apollo 14 mare basalts and petrogenesis of picritic magmas

Abstract The major element geochemistry of picritic lunar glass beads indicates that they represent primary basaltic liquid compositions and, as such, provide unique information concerning the origin of mare basalts and characteristics of the lunar interior. This study used ion microprobe techniques for trace element analysis of individual glass beads representing seven compositionally distinct types of picritic glass beads from the Apollo 14 site [high-Ti glasses (17–11% TiO2): Red/Black, Orange; intermediate-Ti glasses (5–4% TiO2): Yellow; low-Ti glasses (2.8% TiO2): LAP (low alkali, picritic; Papike et al., 1989); very low-Ti glasses ( Ba Sr ) > 1 ] of these glasses are different from low-Ti mare basalts at other sites but are similar to crystalline basalts at the Apollo 14 site. The intermediate to high-Ti picritic glasses exhibit a flat to slightly positive LREE slope and a negative HREE slope. The Orange glass has a higher total REE abundance and a slightly larger negative Eu anomaly than the Red/Black and Yellow glasses. Relative to the Apollo 17 high-Ti glasses, Apollo 14 high-Ti glasses are enriched in REE, LREE/HREE, Y, V, Zr, Sr, Ba, and Ba Sr and are similar in alkali elements (Li, Rb), Co, and Sc. Trace element modeling, within the context of liquid lines of descent and major element characteristics, indicates that the picritic glass beads at the A-14 site are not related by low pressure fractional crystallization to each other or to crystalline basalts at the Apollo 14 or other landing sites. A possible exception is the relationship between LAP and basalts of the high-Al basalt suite. The wide range of primary magma compositions and the lack of petrogenetic linkage (via crystal fractionation) to crystalline basalts indicates that either a wide compositional range of evolved mare basalts has not yet been sampled or a unique mechanism is selectively tapping these picritic magmas directly from their mantle source region. The wide range of major and trace element characteristics of the volcanic glass beads is consistent with derivation from mineralogically distinct sources which consist of varying proportions of olivine + orthopyroxene ± clinopyroxene ± ilmenite ± plagioclase ± KREEP component. The evolved KREEP component may have been incorporated into these primary picritic magmas by either assimilation-fractional crystallization-type processes (AFC) or by hybridization of the mantle source. The former appears less likely due to the general systematic increase in incompatible elements relative to Ti concentration and the apparent lack of crystallization that is required in AFC-type models. The hybridization processes may be the result of “sinker” mechanisms as proposed by Ringwood and Kesson (1976), a manifestation of original mantle inhomogeneities, or a magma ocean stirred by large impacts.

Pegmatite/wallrock interactions, Black Hills, South Dakota: Progressive boron metasomatism adjacent to the Tip Top pegmatite

Abstract Interaction between country rock and fluids derived from the Tip Top pegmatite has resulted in a series of boron enriched assemblages. Between unaltered quartz-mica schist to the pegmatite contact is a succession of four mineral assemblages: 1. (1) Quartz-Biotite-Potassium Feldspar assemblage (Q-B-K), which consists essentially of the original metamorphic silicate assemblage plus anomalously high amounts of modal tourmaline 2. (2) Quartz-Biotite-Tourmaline assemblage (Q-B-T) 3. (3) Tourmaline-Quartz-Muscovite assemblage (T-Q-M) 4. (4) Tourmaline-Quartz assemblage (T-Q). Alkali elements (Cs, Rb, K, Li), SiO 2 , and Ba show a decrease from the Q-B-K assemblage to the T-Q assemblage. A1 2 O 3 , Ga, B, total Fe and Zn increase moderately from the Q-B-K assemblage to the T-Q assemblage. The mineral chemistries also change considerably. The Mg/(Mg + Fe 2+ ) ratios in biotites range from 0.54 to 0.50 in samples from the Q-B-K assemblage to 0.39 in the (Q-B-T) assemblage. The range in tourmaline end-member components from the Q-B-K assemblage to the T-Q assemblage is as follows: Q-B-K: Dravite .63 Schorl .23 Elbaite .05 Buergerite .09 T-Q: Dravite .23 Schorl .37 Elbaite .17 Buergerite .23 . Observed variations in mineral assemblage and whole rock chemistry within the alteration zone appear to a first approximation to be a function of μB 2 O 3 (boron metasomatism) and μK 2 O (alkali leaching). The breakdown of feldspar and biotite may be approximated by reactions: 2HCl + 2(K, Na)AlSi 3 O 8 /ai 2(K, Na)Cl + Al 2 SiO 5 + 5SiO 2 + H 2 O and 2 Annite + SiO 2 + 5Al 2 SiO 5 + 2NaCl + 6H 3 BO 3 /ai 2 Tourmaline + 2KCl + 7H 2 O. The alteration zone may represent either a single episode (B-, Cs-, Li-, Rb-enriched fluid) or multiple episodes (B, Zn, Mn fluid and Cs, Li, Rb fluid) of pegmatite fluid-schist interactions. In both situations, B in the aqueous fluid from the pegmatite reacts with the schist breaking down sheet silicate “traps” for Cs, Rb, Li, and K and forming tourmaline-rich assemblages.

Is plagioclase removal responsible for the negative Eu anomaly in the source regions of mare basalts

The nearly ubiquitous presence of a negative Eu anomaly in the mare basalts has been suggested to indicate prior separation and flotation of plagioclase from the basalt source region during its crystallization from a lunar magma ocean (LMO). Are there any mare basalts derived from a mantle source which did not experience prior plagioclase separation? Crystal chemical rationale for REE substitution in pyroxene suggests that the combination of REE size and charge, M2 site characteristics of pyroxene, fO2, magma chemistry, and temperature may account for the negative Eu anomaly in the source region of some types of primitive, low TiO2 mare basalts. This origin for the negative Eu anomaly does not preclude the possibility of the LMO as many mare basalts still require prior plagioclase crystallization and separation and/or hybridization involving a KREEP component.

Chemical migration by contact metamorphism between pegmatite and country rocks: natural analogs for radionuclide migration

Comparison of trace element signatures of country rocks as a function of distance from the contact with two pegmatites, Tin Mountain and Etta, in the Black Hills of South Dakota, suggests that some elements such as K, Li, Rb, Cs, As, Sb, Zn and Pb, have migrated to distances of 4 to 40 meters during contact metamorphism. The relative degree of migration varies depending on the element. On the other hand, there is virtually no migration of rare earth elements (REE), Al, Sc, Cr, Hf, U, and Th. Biotite and muscovite are effective trace element traps for Li, Rb, and Cs. Biotite has a greater affinity for Rb, Cs and Li than muscovite. 9 references, 5 figures, 1 table.