Cl In Apatite Research Articles

An inversion-based self-calibration for SIMS measurements: Application to H, F, and Cl in apatite

Measurements of volatile abundances in igneous apatites can provide information regarding the abundances and evolution of volatiles in magmas, with applications to terrestrial volcanism and planetary evolution. But measurement of volatiles in apatite (especially H) is challenging, and the most commonly utilized method- the electron probe-is often inadequate in precision and accuracy. Secondary ion mass spectrometry (SIMS) measurements can produce accurate and precise measurements of H and other volatiles in many materials including apatite. SIMS standardization generally makes use of empirical linear transfer functions that relate measured ion ratios (i.e., of an element of interest normalized to a reference element) to independently known concentrations or abundance ratios. However, this approach is often limited by the lack of compositionally diverse, well-characterized, homogeneous standards. In general, SIMS calibrations are developed for minor and trace elements, and any two calibrations are treated as independent of one another. However, in crystalline materials, additional stoichiometric constraints may apply. In the case of apatite, the sum of concentrations of abundant volatile elements (H, Cl, and F) must closely approach 100% occupancy of their collective structural site (recognizing the possibility of trace substitutions or vacancies and other elemental species). Here we propose and document the efficacy of a method for standardizing SIMS analyses of abundant volatiles in apatites that takes advantage of this stoichiometric constraint. The principle advantage of this method is that it requires no independently known homogeneous reference standards, and under specific conditions it is effectively self-standardizing. We define a system of independent linear equations relating measured ion ratios (OH - /P - , Cl - /P - , F - /P - ) and unknown calibration slopes (i.e., the empirical transfer functions described above). Given sufficient range in the concentrations of the different elements among apatites measured in a single analytical session, solving this system of equations allows for the calibration slope for each element to be determined without standards, using only blank-corrected ion ratios. In the case that a data set of this kind lacks sufficient range in measured compositions of one or more of the relevant ion ratios, one can employ measurements of additional apatites of a variety of compositions to increase the statistical range and make the inversion more accurate and precise. These additional non-standard apatites need only be wide-ranging in composition. They need not be homogenous nor have known H, F, or Cl concentrations. Tests utilizing synthetic data and data generated in the laboratory indicate that this method should yield satisfactory results for a wide range of compositions provided apatites have “halogen” sites dominated by OH, F, and Cl. The inversion method is able to reproduce conventional calibration curves to within <2.5%, a level of accuracy comparable to-or even better than-the uncertainty of the conventional calibration, and one that includes both error in the inversion method as well as any true error in the independently determined values of the standards. Uncertainties in the inversion calibrations range from 0.1-1.7% (2σ), typically an order of magnitude smaller than the uncertainties in conventional calibrations (~4-5% for H 2 O, 1-19% for F and Cl). However, potential systematic errors stem from the model assumption of 100% occupancy of this site by the measured elements. To a first order, each percent of deviation from 100% occupancy of this site by the measured elements results in a systematic error of 1% (relative) in all species. Carbonate-rich apatites appear to violate the assumptions of the model, and should not be used for calibration purposes-though their volatile contents can be accurately and precisely determined by an independent inversion calibration that omits the high-C data. Use of this method simplifies analysis of H, F, and Cl in apatites by SIMS, and may also be amenable to other stoichiometrically limited substitution groups, including P+As+S+Si in low-C apatite, and Zr+Hf+U+Th in non-metamict zircon.

Partitioning behavior of chlorine and fluorine in the system apatite-silicate melt-fluid

The partitioning behavior of Cl among apatite, mafic silicate melt, and aqueous fluid and of F between apatite and melt have been determined in experiments conducted at 1066 to 1150 °C and 199–205 MPa. The value of D Cl apatite/melt (wt. fraction of Cl in apatite/Cl in melt) ≈0.8 for silicate melt containing less than ∼3.8 wt.% Cl. At higher melt Cl contents, small increases in melt Cl concentration are accompanied by large increases in apatite Cl concentration, forcing D Cl apatite/melt to increase as well. Melt containing less than 3.8% Cl coexists with water-rich vapor; that containing more Cl coexists with saline fluid, the salinity of which increases rapidly with small increases in melt Cl content, analogous to the dependency of apatite composition on melt Cl content. This behavior is due to the fact that the solubility of Cl in silicate melt depends strongly on the composition of the melt, particularly its Mg, Ca, Fe, and Si contents. Once the melt becomes “saturated” in Cl, additional Cl must be accommodated by coexisting fluid, apatite, or other phases rather than the melt itself. Because Cl solubility depends on composition, the Cl concentration at which D Cl apatite/melt and D Cl fluid/melt begin to increase also depends on composition. The experiments reveal that D F apatite/melt ≈3.4. In contrast to Cl, the concentration of F in silicate melt is only weakly dependent on composition (mainly on melt Ca contents), so D F apatite/melt is constant for a wide range of composition. The experimental data demonstrate that the fluids present in the waning stages of the solidification of the Stillwater and Bushveld complexes were highly saline. The Cl-rich apatite in these bodies crystallized from interstitial melt with high Cl/(F + OH) ratio. The latter was generated by the combined processes of fractional crystallization and dehydration by its reaction with the relatively large mass of initially anhydrous pyroxene through which it percolated.

Identification of multiple detrital sources for Otway Supergroup sedimentary rocks: Implications for basin models and chronostratigraphic correlations

Correlation of apatite chlorine content (wt%) with apatite fission track age (Ma) from Lower Cretaceous Otway Supergroup sediments at present‐day low temperatures, allows identification of two characteristic detrital source regions. Apatites from eroded Palaeozoic basement terrains yield low Cl content (generally < 0.5 wt%) and fission track ages significantly older than the stratigraphic age. By contrast, apatites from contemporaneous volcanogenic detritus record a characteristic spread of chlorine content (0–2.5 wt% Cl, many > 0.5 wt%) and syndepositional fission track ages. Where post‐depositional thermal annealing (> 70°C) has significantly reduced the fission track age, provenance information is preserved in the apatite Cl composition alone. In the Otway Supergroup, evidence for contemporaneous volcanism was found in both the Eumeralla Formation (Albian‐Aptian), and Crayfish Group (Aptian‐Berriasian) in samples located towards the central rift, where less sandy facies dominate. Results suggest that Crayfish Group sediments deposited along the northern margin of the basin were predominantly derived from eroding basement material, while the section located towards the central rift contains a greater proportion of volcanogenic detritus. Evidence from this study suggests that volcanogenic detritus was a distal sediment source throughout the entire early rift phase, prior to the main influx of arc‐related volcanogenic material during deposition of the Eumeralla Formation. As diagenesis of volcanogenic sediments significantly reduces porosity and permeability of the sandstones, reservoir quality and petroleum potential may be significantly reduced in the Crayfish Group in deeper parts of the basin where a greater proportion of volcanogenic detritus is suggested. The results presented here provide important information regarding Lower Cretaceous Otway Basin stratigraphy and clearly indicate that this methodology may have wider application.

An evaluation of models of apatite compositional variability using apatite from the Middle Banded series of the Stillwater Complex, Montana

Apatite is a relatively common accessory mineral in the olivine-bearing zones (OB-III and OB-IV) of the Middle Banded series of the Stillwater complex, occurring interstitial to cumulus grains, as monomineralic inclusions in cumulus grains, and in polymineralic clusters which may or may not be included in the cumulus grains. Eighty-nine of 185 samples examined were found to contain some apatite. The F-Cl-OH content of the apatite show distinct stratigraphic variations. XClAp (mole fraction Cl in apatite) of interstitial apatite increases upsection in both OB-III and OB-IV with average values ranging from 0.15 to 0.85 in OB-III and from 0.03 to 0.60 in OB-IV. XFAp varies inversely with XClApwith average values ranging from 0.00 to 0.70, while XOHAp remains relatively constant near 0.40 or decreases slightly with height. These variations are remarkable given that no appreciable stratigraphic variations in either the major or trace element compositions of any of the cumulus minerals are found in the 800 m of section that comprise OB-III and OB-IV. Within-sample variation of XClAp for samples containing Cl-rich apatite is substantially larger (up to 0.65 XClAp) than for samples with more F-rich apatite (XClAp varies by approximately 0.15). Although interstitial apatite is found throughout OB-III and OB-IV, apatite occurring as monomineralic inclusion in cumulus grains or in polymineralic clusters is almost exclusively found in samples with Cl-rich apatite. The data are best explained by a model involving the degassing of a Cl-rich volatile phase from the crystallizing interstitial liquid. The up-section migration of this fluid resulted in the crystallization of F-rich apatite in the lower portion and progressive Cl-enrichment in the apatite with height. The presence of hornblende-bearing dikes, veins and pegmatoids at the level of maximum Cl-enrichment is consistent with a fluid migration model.

Compositional Influences on Fission Track Annealing in Apatite and Improvements in Routine Application of AFTA&amp;reg;: ABSTRACT

Use of Apatite Fission Track Analysis (AFTA{reg_sign}) to reconstruct thermal histories in sedimentary basins relies critically on detailed knowledge of fission track annealing kinetics. The influence of apatite Cl content on annealing rates has long been recognized. Initial quantitative information on annealing rates came from a monocompositional apatite with {approximately}0.45 wt% Cl. Subsequent work showed that annealing rates decrease with Cl content, so that pure-fluorapatite are totally annealed at temperatures 10 to 20{degrees}C less than those containing 0.5 wt% Cl. We have recently completed a major investigation of compositional influences on fission track annealing, using laboratory experiments and data from geological conditions using apatites from sedimentary sequences currently at their maximum burial depth-in which AFTA parameters are dominated by the present-day temperatures. Incorporation of VR data from the same sequences has assisted in confirming the simple nature of the thermal history in each well. This study has included full investigation of compositional influences and has resulted in kinetic models for annealing which allow quantitatively for the influence of Cl content. This work has emphasized the importance of quantitatively allowing for compositional influences on fission track annealing in apatite. In routine application of AFTA, Cl contents are now determined in everymore » grain analyzed, with data grouped into 0.1 wt% Cl divisions and interpreted using the kinetic models described above. Paleotemperatures and timing determined using this approach have improved precision and provide a well-calibrated scale with which other paleotemperature dictators can be compared.« less

Crystallization and alteration of quartz monzonite, Iron Springs mining district, Utah; relation to associated iron deposits

The Iron Springs mining district of southwestern Utah contains the largest iron deposits in the western United States. These are related to emplacement, at minimum depths of 1 to 2.3 km, of three Miocene laccoliths of porphyritic quartz monzonite. When emplaced, the magma was nearly half crystallized, on the verge of brittle behavior, and contained phenocrysts of plagioclase, amphibole, biotite, and clinopyroxene. Granophyric dikes in quartz monzonite formed from residual liquid, but miarolitic cavities in these dikes are lined with alpha quartz, indicating that deposition continued from a volatile-rich phase below the solidus. Pervasive high-temperature alteration led to: replacement of augite by diopside; oxidation of magnetite and ilmenite; replacement of amphibole (completely) and biotite (partially) by aggregates of diopside, alkali feldspars, smectite-chlorite, magnetite, ilmenite, and calcite; albitization of groundmass alkali feldspar; exchange of Mg for Fe and of F for OH in biotite; and exchange of F and OH for Cl in apatite. This alteration produced whole-rock enrichment in Fe (super 3+) , Na, and H 2 O, and losses in Fe (super 2+) , Ca, and K. Total Fe was effectively unchanged. A chilled, relatively impermeable margin, surrounding the pervasively altered interior of each laccolith, was mechanically bonded to wall rock more strongly than to the magmatic interior, and deformed with the wall rock during subsequent intrusion and structural readjustment. In this peripheral shell, clinopyroxene, amphibole, and biotite were less altered. Where the roof of each laccolith had a radius of curvature of less than 2 km, a heterogeneous jointed zone formed below the peripheral shell. In this zone, pervasively altered quartz monzonite was fractured by renewed intrusion or by structural readjustment of roof rocks. Along the resulting joints, quartz monzonite was altered to bleached zones (selvages) during a second stage of alteration. Within these selvages, complete decomposition of biotite that had survived pervasive alteration was accompanied by further growth of diopside, albitization of feldspars, and dissolution of some apatite. Unlike early pervasive alteration, selvage alteration was fracture controlled and caused strong whole-rock depletion in light REE, total Fe, Mg, P, Rb, Y, Ba, and H 2 O (super -) , but caused enrichment in Na. Magnetite-apatite veins commonly formed along the joint. Adjacent to zones of selvage joints, strata-bound orebodies of magnetite, hematite, and apatite formed in limestone around the three laccoliths. Iron-bearing fluid was derived by subsolidus alteration from quartz monzonite; there is no evidence supporting immiscible separation of iron-oxide-rich liquid from silicate magma. Mass balance calculations indicate that the Fe, P, S, Al, and Mg in the orebodies could have been entirely supplied by selvage alteration of 40 km 3 of quartz monzonite, in agreement with the exposed and confidently inferred volume of altered intrusive rock. A fourth pluton, lacking magnetite veins, orebodies, and selvages, was emplaced closer to the surface and against sandstone and share rather than limestone. Iron liberated from hydrous mafic phenocrysts in this body was oxidized in place, forming disseminated hematite.

Fluorine chlorine in biotite from Basin and Range plutons

The concentrations of fluorine and chlorine in biotite, apatite, hornblende, sphene, and chlorite from plutons of intermediate composition in the Basin and Range province have been determined by microprobe methods. Maximum and minimum concentrations in weight percent are 0.4 to 3.8 F and 0.0 to 0.26 Cl in biotite and 0.7 to 3.6 F and 0.0 to 0.8 Cl in apatite. Apatite is generally richer in Cl and F than associated biotite.Chlorine and fluorine concentrations differ significantly between individual plutons, but no differences which might prove useful for exploration could be recognized between mineralized and nonmineralized plutons. Hydrothermal biotite and apatite have equilibrated with saline, hydrothermal fluid and may contain either more or less chlorine and fluorine than magmatic minerals which reflect the relative abundance of HCl, HF, and H 2 O in the magma. The highest Cl content in biotites occurs in plutons containing pyroxene, whether mineralized or not.