Halogen Enrichment Research Articles

Halogen enrichment on the continental surface: a perspective from loess

Halogen enrichment on the continental surface: a perspective from loess

Heavy halogen compositions of peridotite massifs in the Ivrea-Verbano Zone and implications for strong modification of mantle rocks

The lithospheric mantle affected by metasomatism is an important reservoir for halogens and often preserves significant heterogeneity in halogen abundances. However, the halogen compositions of the non-metasomatized mantle rocks with depleted geochemical features have not been well studied, impeding a comprehensive understanding of the halogen distribution in the mantle. The Balmuccia (BM) and Baldissero (BD) peridotite massifs in the Ivrea-Verbano Zone, Italian Alps, represent fragments of the sub-continental lithospheric mantle (SCLM) that are fresh and devoid of subduction-related mantle metasomatism. Here we examine the heavy halogen contents of well-characterized spinel-facies mantle peridotites (n = 20) and pyroxenites (n = 16) from the BM and BD massifs. The results indicate significant variation in whole rock halogen contents of peridotites and pyroxenites, with chlorine (Cl) ranging from <30 to 244 μg/g, bromine (Br) from <0.10 to 0.94 μg/g, and iodine (I) from <0.015 to 0.12 μg/g. Some peridotites and pyroxenites show rather low halogen contents (below the detection limit), consistent with their depleted features. However, a large number of samples display halogen contents far higher than the depleted MORB mantle (i.e., Cl: 3–10 μg/g) and peridotite xenoliths from the strongly metasomatized mantle, although ∼30 % of the halogens were leached out by ultrapure water or dilute nitric acid. The data suggest variable halogen enrichment in many mantle rocks. The halogen contents of different types of mantle rocks do not vary with major elements and incompatible trace elements (e.g., Nb), indicating that halogen compositions are not controlled by magmatic processes such as melting and melt-peridotite reaction. Instead, secondary fluid inclusions are abundant and mainly distributed as trails within olivine and pyroxene in both peridotites and pyroxenites, which were likely trapped during the exhumation of these mantle rocks. Their elevated halogen/Nb ratios compared with MORB mantle and strong correlations with Ba/Nb support the effect of fluids. Micro-Raman analyses identify hydrous serpentines within fluid inclusions. Moreover, mass balance calculation indicates that halogen enrichment is mainly caused by secondary fluids (> 90 %). Together, our findings highlight that the fluid percolation in the mantle rocks during exhumation can intensely modify the halogens, to a similar degree as mantle metasomatism, suggesting the sensitivity of halogens to secondary modification.

Halogen enrichment in the North American lithospheric mantle from the dehydration of the Farallon plate

A greater quantity of halogens (F, Cl, Br, and I) appear to be subducted globally than are erupted at magmatic arcs, requiring either an increase in the mantle halogen budget through time or an additional output pathway or reservoir. The sub-continental lithospheric mantle (SCLM) is one such reservoir. SCLM can be enriched in volatile elements after metasomatism by fluids or melts from subducting oceanic lithosphere. We analyzed the bulk halogen content (F, Cl, Br, and I) of 28 variably metasomatized spinel and garnet peridotite xenoliths from the Navajo Volcanic Field (NVF; Central Colorado Plateau) to constrain the effects of the subducting Farallon plate-derived metasomatism on the halogen content of the western North American SCLM. We also analyzed 33 anhydrous spinel peridotites from various localities on and around the Colorado Plateau, as well as xenoliths from Oahu, Hawaii, and Eifel, Germany to compare them to the modally hydrated NVF xenoliths. Cl, Br, and I are enriched in NVF xenoliths relative to the depleted mantle, correlating with olivine oxygen isotope values and indices of metasomatism. F is less enriched, indicating preferential retention in the slab residue. Br/Cl and I/Cl are similar to partially dehydrated serpentinite, indicating that the source of these fluids is consistent with serpentinized oceanic lithosphere of the Farallon plate. Anhydrous xenoliths from the southwestern United States have more depleted mantle-like Cl and F contents but have highly enriched Br contents similar to the NVF xenoliths. Similar to NVF xenoliths, anhydrous xenolith Br/Cl and I/Cl suggest a Farallon derived serpentinite source of halogen enrichment. These results indicate that the Farallon slab sequestered halogens in the North American SCLM of the central and south Colorado Plateau and the southern Rio Grande Rift. Therefore, halogen sequestration in the SCLM may be a critical component in the global halogen cycle.

Halogen Enrichment of Siberian Traps Magmas During Interaction With Evaporites

Volatile emissions to the atmosphere associated with the Siberian Traps eruptions at the Permian-Triassic boundary were sourced from the outgassing of primary magmas and the sedimentary host rocks into which they were intruded. Halogens in volcanic gases may have played an important role in environmental degradation and in stratospheric ozone destruction. Here we investigate how halogens behave during the interaction between salts and basalt magma emplaced as sills and erupted as lava. We present whole-rock, trace, and halogen concentrations for a suite of samples from three locations in the Siberian Traps Large Igneous Province, including basalt lavas erupted, and dolerites intruded into both organic-bearing shales and evaporites. Dolerites are enriched in Cl, Br, and I; their enrichment in Cl is similar to MORB and OIB that have been inferred to have assimilated seawater. The dolerites exhibit halogen compositional systematics, which extend towards both evaporites and crustal brines. Furthermore, all analyzed samples show enrichment in Rb/Nb; with the dolerites also showing enrichment in Cl/K similar to MORB and OIB that have been inferred to have assimilated seawater. We infer that samples from all three locations have assimilated fluids derived from evaporites, which are components of crustal sedimentary rocks. We show that up to 89% of the chlorine in the dolerites may have been assimilated as a consequence of the contact metamorphism of evaporites. We show, by thermal modeling, that halogen transfer may occur via assimilation of a brine phase derived from heating evaporites. Halogen assimilation from subcropping evaporites may be pervasive in the Siberian Traps Large Igneous Province and is expected to have enhanced emissions of Cl and Br into the atmosphere from both intrusive and extrusive magmatism.

Using magmatic biotite chemistry to differentiate barren and mineralized Silurian–Devonian granitoids of New Brunswick, Canada

The geochemistry of biotite crystals from thirty fertile and barren Silurian–Devonian granitoids of New Brunswick, Canada, was studied in situ using electron microprobe and laser ablation inductively coupled plasma-mass spectrometry to investigate the suitability of biotite geochemistry as a diagnostic fertility index among these intrusions. The Fe2+/(Fe2+ + Mg2+) ratio of biotite varies as a function of intrusion metal affinity, increasing from Cu–Mo-related (mean of 0.56 ± 0.12), to Mo-related (mean of 0.69 ± 0.06) to Sn–W-related (mean of 0.77 ± 0.16), with barren granitoids lying between Cu–Mo and Mo types (mean of 0.66 ± 0.06). The results show a distinctive geochemical contrast between mineralized and barren samples. Compatible elements (Ti, Mg, Co, Ni, V, Cr, Ba, and Sr) decrease from barren to Cu–Mo, Mo, and Sn–W granitoids, whereas incompatible elements (Mn, Zn, Sn, W, Rb, Cs, and Li) show the opposite trend. These two trends might indicate higher degree of fractionation indicated by biotite chemistry in Sn–W-related granites. Furthermore, barren intrusions have the lowest water content (1–3 wt.% H2O), whereas Sn–W and Cu–Mo-related intrusions have between 3 and 6 wt.% H2O. Mo-bearing intrusions have a limited range of H2O contents (4–4.5 wt.%). A high degree of halogen enrichment related to degree of fractional crystallization results in enrichment of incompatible elements in the magmas associated with Sn–W mineralization and is reflected by the geochemical characteristics of biotite from these systems. New metallogenic classifications are introduced using ternary V–Na–Li (ppm) and Sn + W (ppm) versus Ga (ppm) to differentiate barren and mineralized granitic systems in New Brunswick.

Sea-ice-related halogen enrichment at Law Dome, coastal East Antarctica

Abstract. The Law Dome site is ideal for the evaluation of sea ice proxies due to its location near to the Antarctic coast, regular and high accumulation throughout the year, an absence of surface melting or remobilization, and minimal multiyear sea ice. We present records of bromine and iodine concentrations and their enrichment beyond seawater compositions and compare these to satellite observations of first-year sea ice area in the 90–130° E sector of the Wilkes coast. Our findings support the results of previous studies of sea ice variability from Law Dome, indicating that Wilkes coast sea ice area is currently at its lowest level since the start of the 20th century. From the Law Dome DSS1213 firn core, 26 years of monthly deposition data indicate that the period of peak bromine enrichment is during austral spring–summer, from November to February. Results from a traverse along the lee (western) side of Law Dome show low levels of sodium and bromine deposition, with the greatest fluxes in the vicinity of the Law Dome summit. Finally, multidecadal variability in iodine enrichment appears well correlated to bromine enrichment, suggesting a common source of variability that may be related to the Interdecadal Pacific Oscillation (IPO).

Seawater cycled throughout Earth’s mantle in partially serpentinized lithosphere

The extent to which water and halogens in Earth's mantle have primordial origins, or are dominated by seawater-derived components introduced by subduction is debated. About 90% of non-radiogenic xenon in the Earth's mantle has a subducted atmospheric origin, but the degree to which atmospheric gases and other seawater components are coupled during subduction is unclear. Here we present the concentrations of water and halogens in samples of magmatic glasses collected from mid-ocean ridges and ocean islands globally. We show that water and halogen enrichment is unexpectedly associated with trace element signatures characteristic of dehydrated oceanic crust, and that the most incompatible halogens have relatively uniform abundance ratios that are different from primitive mantle values. Taken together, these results imply that Earth's mantle is highly processed and that most of its water and halogens were introduced by the subduction of serpentinized lithospheric mantle associated with dehydrated oceanic crust.

Halogen and Cl isotopic systematics in Martian phosphates: Implications for the Cl cycle and surface halogen reservoirs on Mars

The Cl isotopic compositions and halogen (Cl, F, Br, and I) abundances in phosphates from eight Martian meteorites, spanning most rock types and ages currently available, have been measured in situ by Secondary Ion Mass Spectrometry (SIMS). Likewise, the distribution of halogens has been documented by x-ray mapping. Halogen concentrations range over several orders of magnitude up to some of the largest concentrations yet measured in Martian samples or on the Martian surface, and the inter-element ratios are highly variable. Similarly, Cl isotope compositions exhibit a larger range than all pristine terrestrial igneous rocks. Phosphates in ancient (>4 Ga) meteorites (orthopyroxenite ALH 84001 and breccia NWA 7533) have positive δ37Cl anomalies (+1.1 to +2.5‰). These samples also exhibit explicit whole rock and grain scale evidence for hydrothermal or aqueous activity. In contrast, the phosphates in the younger basaltic Shergottite meteorites (<600 Ma) have negative δ37Cl anomalies (−0.2 to −5.6‰). Phosphates with the largest negative δ37Cl anomalies display zonation in which the rims of the grains are enriched in all halogens and have significantly more negative δ37Cl anomalies suggestive of interaction with the surface of Mars during the latest stages of basalt crystallization. The phosphates with no textural, major element, or halogen enrichment evidence for mixing with this surface reservoir have an average δ37Cl of −0.6‰, supporting a similar initial Cl isotope composition for Mars, the Earth, and the Moon. Oxidation and reduction of chlorine are the only processes known to strongly fractionate Cl isotopes, both positively and negatively, and perchlorate has been detected in weight percent concentrations on the Martian surface. The age range and obvious mixing history of the phosphates studied here suggest perchlorate formation and halogen cycling via brines, which have been documented on the Martian surface, has been active throughout Martian history.

Tracking halogens through the subduction cycle

Research Article| December 01, 2012 Tracking halogens through the subduction cycle Mark A. Kendrick; Mark A. Kendrick 1School of Earth Sciences, University of Melbourne, Victoria 3010, Australia Search for other works by this author on: GSW Google Scholar Jon D. Woodhead; Jon D. Woodhead 1School of Earth Sciences, University of Melbourne, Victoria 3010, Australia Search for other works by this author on: GSW Google Scholar Vadim S. Kamenetsky Vadim S. Kamenetsky 2ARC Centre of Excellence in Ore Deposits and Institute for Marine and Antarctic Studies, University of Tasmania, Tasmania, Australia Search for other works by this author on: GSW Google Scholar Geology (2012) 40 (12): 1075–1078. https://doi.org/10.1130/G33265.1 Article history received: 03 Feb 2012 rev-recd: 08 May 2012 accepted: 27 May 2012 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Mark A. Kendrick, Jon D. Woodhead, Vadim S. Kamenetsky; Tracking halogens through the subduction cycle. Geology 2012;; 40 (12): 1075–1078. doi: https://doi.org/10.1130/G33265.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The flux of halogens into the Earth’s mantle at subduction zones is a critical yet poorly constrained parameter in the geochemical evolution of the planet. Here we report the first ever combined high-precision measurements of chlorine, bromine, and iodine for backarc basin basalt (BABB) and ocean island basalt (OIB) glasses. The measurements were undertaken in order to evaluate the depth and extent of the halogen subduction cycle by comparing: (1) melts formed in the Manus Basin (Papua New Guinea) proximal to a modern subduction zone, and (2) melts formed from enriched mantle (EM) reservoirs that have been linked to ancient subduction recycling [EM1 and EM2 sampled by the Pitcairn and Society seamounts (central Pacific Ocean), respectively]. As expected from previous studies, the BABBs are strongly enriched in chlorine relative to other trace elements and mid-oceanic ridge basalts (MORB); however, the combined Br/Cl and I/Cl data provide additional insights. The BABBs have I/Cl weight ratios of up to 5.3 × 10−4, that are up to five times higher than typical MORB; and the BABBs with the highest I/Cl have Br/Cl ratios of 2–3 × 10−3, that are lower than typical MORB, and significantly lower than either iodine-rich sediments or seawater-derived sedimentary pore fluids. The final breakdown of iodine-rich serpentine is considered the most likely source of the halogen enrichment in the BABB, suggesting that subduction of serpentinized peridotites enables transport of strongly incompatible, fluid-mobile, volatile elements, like iodine, beyond zones of arc-magma generation. The Pitcairn and Society melts exhibit a remarkable correlation between K/Cl and 87Sr/86Sr. The K/Cl ratios vary from MORB-like values of ∼15 to maxima of ∼40 in the isotopically most enriched EM end members. The trend reflects the lower subduction efficiency of halogens compared to K and other lithophile elements. Melts formed from EM and MORB mantle reservoirs have very similar Br/Cl and I/Cl weight ratios of 3.6 ± 0.8 × 10−3 and 85 ± 42 × 10−6 (2σ) respectively, that could indicate that subducted volatiles have been mixed throughout the mantle. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Compositional variability of terrestrial mantle apatites, thermodynamic modeling of apatite volatile contents, and the halogen and water budgets of planetary mantles

Apatites in ultramafic xenoliths from Hawaii, Kamchatka, Nunivak Island, Lunar Crater volcanic field and the Navajo volcanic field have variable volatile (F, Cl, OH) compositions that correlate with their petrogenesis. Apatites in metasomatized lherzolite/harzburgite xenoliths are relatively Cl-rich whereas apatites in igneous pyroxenite and amphibolite xenoliths are relatively F-rich. Our results support the generalization of O'Reilly and Griffin (2000) that metasomatic apatites tend to be Cl-rich and magmatic apatites tend to be F-rich. Mantle metasomatism may largely be the consequence of devolatilization of Cl-rich fluids from subducted lithosphere. However, the occurrence of metasomatic xenoliths far removed from subduction zones (e.g., Bullenmerri) as well as the geochemistry of Cl in some plume basalts and diamonds indicates that not all metasomatism is subduction-triggered. Thermodynamic modeling of the influence on apatite compositions of variable F, Cl, H2O, and P abundances, as well as temperature, is consistent with the idea that terrestrial mantle fluids are H2O-rich, and that halogen enrichment in terrestrial apatites need not represent large absolute halogen abundances in the fluids. The models indicate that the occurrence of volatile-poor calcium phosphate (merrillite/whitlockite) in the Martian and terrestrial mantles is facilitated by halogen- and phosphorus-rich and water-poor environments — the rarity of this assemblage in the terrestrial mantle compared to Mars is consistent with the relatively wet and halogen- and phosphorus-poor nature of the terrestrial mantle compared to that of Mars.

Fate of the chlorine and fluorine in a sub-bituminous coal during pyrolysis and gasification

The fate of the chlorine and fluorine present in a sub-bituminous coal from Indonesia during pyrolysis and gasification has been studied with fixed and entrained bed reactors. The rate profile for HCl evolved in the temperature programmed pyrolysis exhibits the main and shoulder peaks at 480 and 600 °C, respectively. Model experiments and subsequent Cl 2p XPS measurements show that HCl reacts with metal impurities and carbon active sites at 500 °C to be retained as inorganic and organic chlorine forms, from which HCl evolves again at elevated temperatures. It is suggested that the HCl observed in the coal pyrolysis may originate from the above-mentioned chlorine functionalities formed by secondary reactions involving the nascent char. In the CO 2 gasification of the 900 °C char at 1000 °C and 2.5 MPa, any measurable amounts of HCl and HF could not be detected even at a high conversion of 75 wt% (daf), suggesting the accumulation of these halogens in the residual char. When the coal is injected into an O 2-blown, entrained bed gasifier at 1200–1400 °C under 2.6 MPa, the partial oxidation proceeds to a larger extent at a higher O 2/coal ratio, whereas the chlorine and fluorine are enriched in the remaining char, and the extent of the enrichment at the latter stage of gasification is larger with the fluorine. The XPS measurements of the chars reveal the presence of the broad F 1 s peak, which can cover a wide range of binding energies attributable to inorganic and organic fluorine. The halogen enrichment during gasification is discussed in terms of secondary reactions of HCl and HF with char.

Halogen retention, organohalogens, and the role of organic matter decomposition on halogen enrichment in two Chilean peat bogs.

Natural formation of organohalogen compounds can be shown to occur in all natural environments. Peat bogs, which are built up exclusively of organic matter and cover approximately 3% of the total continental world area, are potentially significant reservoirs for organohalogen formation. Up to now, fluxes and retention rates of halogens and organohalogen formation in peat bogs were mostly unquantified. In our study, we investigated the retention of atmospheric derived halogens and the natural formation of organohalogens by differential halogen analysis in two peat bogs in southernmost Chile. Atmospheric wet deposition rates of chlorine, bromine, and iodine range between 600 and 36000, 6 and 160, and 1 and 3 mg m(-2) yr(-1), respectively. Mean annual net accumulation rates of these halogens in peat are calculated to be 12-72 mg of Cl m(-2), 1.7-12 mg of Br m(-2), and 0.4-1.2 mg of l m(-2). Retention rates are similarly high for iodine (36-46%) and bromine (7.5-50%), and substantially lower for chlorine (0.2-2%). To evaluate influences of peat decomposition processes on halogen enrichment, halogen concentrations were compared to carbon/nitrogen ratios (C/N). Our results indicate that up to 95% of chlorine, 91% of bromine, and 81% of iodine in peat exist in an organically bound form. The results also indicate that the concentrations of halogens, especially of bromine and iodine, in peat are largely determined by peat decomposition processes and that halogens are not conservative in bogs.

Geochemical and petrographical characteristics of a domed bog, Nova Scotia: a modern analogue for temperate coal deposits

A freshwater domed bog in Nova Scotia, Canada, was investigated for its petrographic, mineralogical and geochemical characteristics. Three cores were taken from the lagg zone, margin and centre, to examine different stages of evolution in a fen to domed bog sequence. Based on this study, a model was developed, which shows the relationship between factors controlling peat geochemistry and petrography, all of them being dependent on trophic conditions. The bog may represent a modern analogue for temperate coal deposits, and thus can be used to help understand spatial variability within coal beds. Rheotrophic conditions in the early stages of mire development result in the formation of high-ash, highly humified Carex-dominated peat, while the evolution toward ombrotrophic conditions is reflected in a progressive replacement by low-ash and well-preserved Sphagnum-dominated peat. The associated decrease in pH reflects a decrease of groundwater influence, which is also supported by the mineralogical assemblage and petrographic data (decrease in reflectance and change in maceral composition). Except for Cl, Br, I and Se, which are associated with the organic fraction, variations in elemental composition can be explained in terms of mineral matter abundance, i.e. decrease upward and with increasing distance from the margin. Sea spray is the most probable origin for halogen enrichment in the peat.

Extreme halogen abundances in tin-rich magma of the Taylor Creek Rhyolite, New Mexico

Crystal-bearing glass inclusions are hosted by quartz phenocrysts in eruptive units of the tin-mineralized Taylor Creek Rhyolite, New Mexico. Inclusions from four units were refused at 2 to 4 kbars and temperatures > or = 890 degrees C and analyzed for major and minor elements by electron microprobe and for H, Li, Be, B, Rb, St, Y, Nb, Mo, Sn, Cs, Ce, Th, and U by ion microprobe. Average glass inclusion compositions are chemically representative of Taylor Creek melt, and comparison of inclusion compositions with their whole-rock equivalents suggests that Li, F, Cl, Ca, and H 2 O degassed from cooling lava and/or magma after inclusion entrapment.Taylor Creek magma was strongly and variably enriched in F and Cl prior to eruption; average F concentrations of melt apparently ranged from 0.15 to 3.9 wt percent. Eruptive units containing the most F-enriched glass inclusions are characterized by comparatively high average molar (Na 2 O/Na 2 O + K 2 O) ratios, low Li concentrations, and relatively low phenocryst contents which are compatible with high preeruptive concentrations of F in magma. Average C1 concentrations were more uniform, ranging from 0.23 to 0.37 wt percent. High concentrations of F and Cl are in accord with the style and extent of alteration in vapor-phase altered parts of Taylor Creek Rhyolite and are consistent with alkali chloride-rich fluid inclusions in vapor-phase minerals of altered rhyolites. Preeruptive enrichments in other volatiles were not as significant; B in melt was <100 ppm and the apparent H 2 O content of melt was <2 wt percent.Geochemical studies of other tin-topaz rhyolites and molybdenite-bearing, Climax-type, porphyritic granites imply that their magmatic F concentrations were highly variable and that some fractions of magma were strongly enriched in halogens. We suggest that such high levels of halogen enrichment may be generally representative of other highly evolved granitic magmas genetically associated with lithophile mineralization.

Halogen contamination in Antarctic H5 and H6 chondrites and relation to sites of recovery

The distribution of the elements F, Cl, Br and I was analysed in layers removed stepwise from Antarctic H5 and H6 chondrites. All meteorites show higher concentrations of these elements on their surfaces than in their interiors. The degree of halogen enrichment cannot be correlated with the degree of visual weathering and is proportional to the time the meteorites were residing on the surface of the Antarctic ice. During this period, aerosols, salts and gaseous components are deposited on the surfaces of the meteorites and diffuse into their interior. The observed contamination level of the meteorites is influenced by leaching properties and diffusion behaviour of the individual halogens. The major contamination source for F, Cl and probably Br is air-borne seaspray; for I it is the gaseous compound methyl iodide (CH 3I) produced by biological processes in the sea. Methyl iodide and its oxidation products formed in the Antarctic atmosphere (e.g., I 2) can be transported over longer distances to the interior of Antarctica than air-borne seaspray. Therefore, the ratio of the halogen contamination is related to the collection site of the meteorites. All meteorites that were found in the Antarctic interior are contaminated to a lesser degree by F, Cl and Br relative to I than those found near the coast. The measured enrichment ratio of I/F is a function of the distance between the collection site and the open sea, and increases from the Allan Hills to the Thiel Mountains. By revealing the degree of contamination of a meteorite it is possible to determine its maximum surface residence time on the Antarctic ice.

Geochemistry of biotites from mineralized and barren intrusive systems

Ninety-two biotite separates from 35 different igneous systems in western North America and the Caribbean region have been analyzed for Cl, F, H 2 O, Cu, Zn, Fe, and Mg. Biotites in fresh magmatic samples from igneous systems genetically related to significant mineralization (dominantly porphyry copper type) form a population with average chlorine and fluorine contents greater than and average copper content less than the average of the remaining barren intrusive systems (at the 90 percent confidence level). However, the magnitude of this halogen enrichment is too small to permit its use as a routine exploration tool. There are no systematic differences in Cl:F, F:H 2 O or other multicomponent ratios of biotite compositions that can be used to distinguish between mineralized and barren host intrusions with a high degree of confidence. Although the average halogen content of biotites from both mineralized and barren potassium-rich intrusions is higher than that for potassium-poor intrusions, biotites from mineralized potassium-rich intrusions are slightly enriched in halogens with respect to their barren counterparts. Magmatic biotites from the Boulder batholith do not exhibit systematic spatial variations in halogen abundance related to known areas of mineralization, although biotites from altered rocks around Butte exhibit chlorine depletion. The copper content of biotites from the same samples indicates the location of mineralization at Butte more reliably, however.