ALBERT

Description: Apatite is a superb mineral by which to investigate the nature of fluids that have passed through and altered a rock (metasomatic processes). Its ubiquity allows it to act as a reservoir for P, F, Cl, OH, CO 2 , and the rare earth elements. It is also a powerful thermochronometer and can be chemically altered by aqueous brines (NaCl–KCl–CaCl 2 –H 2 O), pure H 2 O, and aqueous fluids containing CO 2 , HCl, H 2 SO 4 , and/or F. Thus, apatite is the perfect tracker of metasomatic fluids, providing information on the timing and duration of metasomatism, the temperature of the fluids, and the composition of the fluids, all of which can feed back into the history of the host rock itself.

Description: The method of accommodation of solid solution along the OH-Cl binary in calcium phosphate apatites is not fully understood; because of steric constraints in mixtures of OH and Cl anions in the apatite [0,0, z ] anion column, the positions of OH and Cl anions in the pure hydroxylapatite and chlorapatite end-members cannot coexist in the binary anion column. We have undertaken high-precision single-crystal X-ray structure studies of eight synthetic samples along the OH-Cl apatite binary ( R 1 0.0159). We found that for all samples solid solution is attainable in space group P 6 3 / m , but the particular method of solid solution depends on composition. For samples with Cl 〉 OH, three column anion sites (two for Cl, one for OH) provide allowable bond distances with the Ca2 atoms and allow a sequence of column anions that provides sufficient anion-anion distances and also effects reversal of the sense of ordering of the column anions relative to the mirror planes at z = 1/4 and 3/4. In a sample with OH 〉 Cl, three sites exist in the anion column that also provide allowable bond distances to the triangle of Ca2 atoms or its disordered Ca2' equivalent, and afford a sequence of atoms that permits reversal of the anion column and maintenance of P 6 3 / m symmetry. One of those sites is occupied by OH and provides acceptable Ca2-OH distances, and another accommodates Cl with ideal Ca2-Cl distances. A third column anion site is unique among the calcium phosphate apatites. That site, termed the ClOH site, accommodates both OH and Cl. The site has an ideal bond distance for OH to the Ca2 atoms in the Ca2 triangle and also has an ideal bond distance for a Cl occupant to disordered Ca2' atoms; thus, because of the disordering of the Ca2-Ca2' atoms, a single site can accommodate either anion with ideal, but disparate, bond distances to Ca. Finally, in OH-Cl apatites with OH Cl, also crystallizing in space group P 6 3 / m , four anion positions are occupied in the anion column, including the ClOH site that allows occupancy by both OH and Cl. In addition to that site and distinct OH and Cl sites, OH is found to occupy the site within the mirror plane at (0,0,1/4), the site occupied by F in F-bearing apatite. Occupancy of that site is essential to reversing the sense of ordering of the anion column relative to the mirror planes and preserving P 6 3 / m symmetry. Thus, the methods of effecting solid solution along the OH-Cl are composition dependent and complex.

Description: Fluorapatite-monazite-xenotime-allanite mineralogy, petrology, and textures are described for a suite of Kiruna-type apatite-iron oxide ore bodies from the Grängesberg Mining District in the Bergslagen ore province, south central Sweden. Fluorapatite occurs in three main lithological assemblages. These include: (1) the apatite-iron oxide ore bodies, (2) breccias associated with the ore bodies, which contain fragmented fluorapatite crystals, and (3) the variably altered host rocks, which contain sporadic, isolated fluorapatite grains or aggregates that are occasionally associated with magnetite in the silicate mineral matrix. Fluorapatite associated with the ore bodies is often zoned, with the outer rim enriched in Y+REE compared to the inner core. It contains sparse monazite inclusions. In the breccia, fluorapatite is rich in monazite-(Ce) ± xenotime-(Y) inclusions, especially in its cores, along with reworked, larger monazite grains along fluorapatite and other mineral grain rims. In the host rocks, a small subset of the fluorapatite grains contain monazite ± xenotime inclusions, while the large majority are devoid of inclusions. Overall, these monazites are relatively poor in Th and U. Allanite-(Ce) is found as inclusions and crack fillings in the fluorapatite from all three assemblage types as well as in the form of independent grains in the surrounding silicate mineral matrix in the host rocks. The apatite-iron oxide ore bodies are proposed to have an igneous, subvolcanic origin, potentially accompanied by explosive eruptions, which were responsible for the accompanying fluorapatite-rich breccias. Metasomatic alteration of the ore bodies probably began during the later stages of crystallization from residual, magmatically derived HCl- and H 2 SO 4 -bearing fluids present along grain boundaries. This was most likely followed by fluid exchange between the ore and its host rocks, both immediately after emplacement of the apatite-iron oxide body, and during subsequent phases of regional metamorphism and deformation.

Description: We have studied the thermal expansion of 15 synthetic and two natural F-Cl apatite solid solutions through the calculation of unit-cell dimensions at elevated temperatures based on X-ray powder diffraction data collected from room temperature to ~900 °C at 50 to 100 °C intervals. Coefficients of thermal expansion for the a and c unit-cell axes show sensitivity to composition, with α a increasing by about 50% and α c decreasing by a third from chlorapatite to fluorapatite. Despite the relationships observed for a and c , the thermal expansion coefficient for unit-cell volume shows little sensitivity to composition, which can be explained only by a mutually compensating structural adjustment along the latter axes as temperature rises. Results of this study also imply that the thermodynamically ideal volumes of mixing for F-Cl apatite solid solutions observed at ambient conditions continue to at least 900 °C.

Description: Fluids buffered by a plagioclase matrix are experimentally reacted with olivine megacrysts at 800 ºC and 800 MPa (piston-cylinder press, CaF 2 assembly) to form secondary veins of orthopyroxene ± clinopyroxene in the olivine. Fluids utilized were varied in both amount (0–2 wt%) and salinity (0–8 M NaCl). Assuming equilibrium with the plagioclase matrix, they are presumed enriched in Si, Al, Ca, Na, and Cl and are thereby similar in composition to slab-derived fluids. The experiments provide controlled, multi-component analogs of Si-metasomatism in the mantle wedge above subduction zones. The veins are dominated by orthopyroxene with minor clinopyroxene and form complex interconnected networks along fractures in the olivine. The reaction is rate limited by interfacial process of dissolution and precipitation. Porosity is developed throughout the veins and along sub-grain boundaries in the olivine megacrysts. These veins strongly resemble the textures observed in secondary metasomatic orthopyroxene veins widely reported in upper mantle xenoliths within arc magmas. A review of literature data on natural samples and experiments suggests that orthopyroxene ± clinopyroxene veins primarily form between 750–950 ºC and over a large pressure range from 0.8–3.4 GPa. The abundance and composition of these metasomatic veins may vary as a function of pressure, variances in the fluid-rock partition coefficients, and/or by modification of the metasomatic fluid during the reaction.

Description: In the Bafq region, Central Iran, apatite from Kiruna-type iron oxide-apatite deposits is enriched in LREE (up to 2 wt.%), Na, Si, and Cl (up to 15% of the halogen component), but altered apatite is strongly depleted in these elements. Monazite and xenotime inclusions with low Th and U and associated with a distinct pervasive porosity are found in the altered areas of the apatite and are a direct product of fluid-aided coupled dissolution-reprecipitation processes. Monazite-xenotime thermometry indicates that these inclusions formed below 400–300 °C, which agrees well with available fluid inclusion data. Comparison of these iron oxide-apatite deposits with other Kiruna-type iron oxide-apatite deposits suggests that they formed from highly differentiated volcanic magmas which interacted with both magmatic and externally derived fluids shortly after crystallization and during cooling. The concentrations of Sr, Mn, and Y in the apatites ranges from 40–824 ppm, 10–440 ppm, and 66–3700 ppm, respectively, which strongly supports a granitoid source for genesis of the iron ores. An extensive Na-Ca metasomatic alteration zone, with magnetite-apatite mineralization, existence of magmatic bodies grading to breccias, and fluid inclusion data all point to the involvement of both magmatic and hydrothermal processes in the ore formation.

Description: The Varberg–Torpa charnockite–granite association (Varberg, SW Sweden) consists of the magmatic Varberg charnockite (1399 ± 6 Ma) and the Torpa granite (1380 ± 12 Ma). The Torpa granite is both continuous and, based on its whole-rock geochemistry, synmagmatic with the Varberg charnockite. The granite body also contains a number of charnockite inliers. P – T estimation using garnet–clinopyroxene and orthopyroxene–clinopyroxene Fe–Mg exchange thermometry and garnet–orthopyroxene–plagioclase–quartz barometry gives temperatures and pressures (750–850°C; 800–850 MPa) that most probably approximate the P – T conditions during emplacement of the charnockite compared with a lower crystallization temperature (650–700°C) for the granite. The earliest recognized fluid inclusions in both the granite and charnockite consist of H 2 O–CO 2 mixtures (H 2 O volume fraction 0·2–0·7). Fluid inclusions in the charnockite are characterized by high CO 2 densities (up to 1·0 g cm – 3 ; 40–90% bulk CO 2 ), of probable magmatic origin, and are best preserved in garnet, plagioclase, and fluorapatite (in order of decreasing CO 2 densities), and sometimes also in clinopyroxene. Fluid inclusions with the highest CO 2 densities (1·08–1·10 g cm – 3 ) are found in quartz ( T h –31 to –36°C) and may have originated under high P – T conditions during emplacement and cooling of the charnockite. Magmatic fluids in the granite correspond to aqueous–carbonic inclusions with an estimated bulk composition (mol %) of H 2 O 73%, CO 2 25%, NaCl 2%. The salinity of the solutes in the granite (typically 14–20 wt % NaCl-eq.) is generally higher than for the charnockite (0–8 wt % NaCl-eq.). Field, petrographic, mineralogical, geochemical, and fluid inclusion evidence indicates that, compared with the H 2 O-rich granite, the magma responsible for the charnockite had a preponderance of CO 2 over H 2 O, which lowered the H 2 O activity in the melt, stabilizing ortho- and clinopyroxene. This evidence also supports the idea that the granite and charnockite were derived from a common source magma (most probably a fluid-rich basalt at the base of the crust) as a result of fractional crystallization.

Description: The REE enrichment process in fluorapatite and the REE redistribution among fluorapatite, monazite, and allanite were studied in a series of three sets of experimental runs at P-T conditions of 0.5 to 4 GPa and 650 to 900 °C. The first two sets of experimental runs utilized fluorapatite as a P-source, synthetic monazite or allanite as the REE sources, albite, quartz, and NaF-H 2 O or NaCl-H 2 O. The third set of runs was carried out with powdered Ca 3 (PO 4 ) 2 , allanite, quartz, (±Al 2 O 3 ), and a NaF-H 2 O solution. In all runs REE-bearing fluorapatite with up to 28 wt% REE 2 O 3 formed at the expense of monazite or allanite; either as narrow zones at the margin of synthetic fluorapatite in runs 1 and 2 or as discrete grains in run 3. The REE-enrichment of fluorapatite in melt-bearing runs is explained in terms of the high solubility of monazite in the presence of alkali-rich melts together with the high partitioning values for REEs among fluorapatite and alkali-rich melts. The formation of REE-enriched fluorapatite in melt-absent runs implies that the solubility of monazite and the REE-uptake of fluorapatite are similarly high in both alkali-rich melts and fluids and depends foremost on the activity of alkalis in fluids or melts. The results from this study show the importance of fluorapatite as a REE-carrier in rocks whose petrogenesis involved alkali-bearing fluids/melts. In metamorphic rocks, alkali-enriched fluids or melts will likely form under higher-grade conditions, explaining the preferential occurrence of REE-enriched fluorapatite in granulite and eclogite-facies rocks.

Description: 〈span〉〈div〉Abstract〈/div〉The atomic arrangements of eight synthetic samples along the fluorapatite-hydroxylapatite join were examined using X-ray crystallographic techniques; the results of those refinements demonstrate that the incorporation of both F and OH in the apatite anion column, mimicking the human apatite system as modified by fluoridation, is complex. The compositions of the anion columns in the phases ranged from [F〈sub〉0.40〈/sub〉(OH)〈sub〉0.60〈/sub〉] to [F〈sub〉0.67〈/sub〉(OH)〈sub〉0.33〈/sub〉], and the high-precision structure refinements yielded 〈span〉R〈/span〉1 values from 0.0116 to 0.0140. The apatite structure responds to the variable content of the anion columns. Counterintuitively, the OH groups in the anion column move monotonically closer to the mirror planes at 〈span〉z〈/span〉 = ¼, ¾ with increasing F content, despite the decreasing size of the triangle of Ca2 atoms to which the column anions bond and the increasing overbonding of the hydroxyl oxygen. In the structure the F atoms are underbonded and have zero degrees of positional freedom in the (0,0,¼) special position to relieve that underbonding; the bonding deficiency of the anion column is relieved by the overbonding of the O(H) atom in the anion column, overbonding that increases with increasing content of underbonded F in the anion column. Together the underbonded F and the overbonded OH meet the formal bond valence (1.0 v.u.) required by the anion column occupants. The changes in bonding from the individual anion column occupants to the surrounding Ca2 atoms with composition induce bond length changes principally in the irregular Ca2 polyhedron and also affect the 〈span〉a〈/span〉 lattice parameter in the apatites. The bond valence values imparted on the F, OH column anions, when extrapolated to end-member compositions, suggest that different column anion arrangements may exist near the F and OH end-member compositions, as is also seen along the apatite Cl-OH join. These values have implications for the incorporation of fluoride in human teeth during the fluoridation process.〈/span〉

Description: Allanite-fluorapatite reaction coronas around monazite are abundant in metamorphic rocks. We report here special cases where a new generation of "satellite" monazite grains formed within these coronas. Using examples from different P-T regions in the eastern Alps, we examine the origin and the petrological significance of this complex mineralogical association by means of the electron microprobe utilizing Th-U-Pb monazite dating and high-resolution BSE imaging. Satellite monazite grains form when a monazite-bearing rock is metamorphosed in the allanite stability field (partial breakdown of first generation monazite to fluorapatite plus allanite), and is then heated to temperatures that permit a back reaction of fluorapatite plus allanite to secondary satellite monazite grains surrounding the remaining original first generation monazite. Depending on the whole-rock geochemistry satellite monazites can form under upper greenschist- as well as amphibolite-facies conditions. In each of the three examples focused on here, the inherited core monazite was resistant to recrystallization and isotopic resetting, even though in one of the samples the metamorphic temperatures reached 720 °C. This shows that in greenschist- and amphibolite-facies polymetamorphic rocks, individual grains of inherited and newly formed monazite can and often will occur side by side. The original, inherited monazite will preferentially be preserved in low-Ca, high-Al lithologies, where its breakdown to allanite plus fluorapatite is suppressed. Conversely, a medium- or high-Ca, monazite-bearing rock will become particularly fertile for secondary monazite regrowth after passing through a phase of strong retrogression in the allanite stability field. Based on this knowledge, specific sampling strategies for monazite dating campaigns in polymetamorphic basement can be developed.