Subduction of oceanic plates releases large amounts of chlorine and fluorine into the mantle. These halogens are transported into the crust through hydrous melting, where they may be incorporated into minerals such as biotite, apatite, and amphibole. Halogen concentrations are measured in volcanic or plutonic material, while the concentration of Cl and F released from the subducting slab must be calculated based upon the amount of crystallized material and the partition coefficients of each mineral. As amphibole is the most common halogen bearing igneous mineral, it is commonly studied as a bearer of Cl and F. However, the partition coefficient of F between amphibole and a hydrous melt has not been agreed upon by previous studies. Here we show that F is moderately to highly compatible in amphibole, in agreement with other experiments performed at crustal conditions. As amphibole may be able to incorporate a large amount of F, cryptic amphibole crystallization may raise the Cl/F ratio of residual magma, which will then be transported to the surface bearing this geochemical signature, even with little crystallized amphibole present in erupted material. This provides further evidence for the occurrence of cryptic amphibole crystallization, previously predicted based on REE studies and phase equilibria. A better understanding of the halogen reservoirs present in the crust will allow for more accurate estimates of the amount of Cl and F released by subducting slabs.
more »
« less
Halogen Bearing Amphiboles, Aqueous Fluids, and Melts in Subduction Zones: Insights on Halogen Cycle From Electrical Conductivity
Amphiboles are hydrous minerals that are formed in the oceanic crust via hydrothermal alteration. The partial substitution of halogens for OH−makes amphibole one of the principal hosts of Cl and F in the subducting slab. In this study, we investigated the electrical conductivity of a suite of halogen bearing amphibole minerals at 1.5 GPa up to 1,400 K. The discontinuous electrical behavior indicates dehydration of amphibole at ∼915 K. This is followed by dehydration induced hydrous melting at temperatures above 1,070 K. We find that the released aqueous fluids have an electrical conductivity of ∼0.1 S/m. This high electrical conductivity is likely to explain anomalously high electrical conductivity observed in certain subduction zone settings. This high electrical conductivity of an order of magnitude greater than the electrical conductivity of pure aqueous fluids at similar conditions is likely due to the partitioning of the F and Cl into the aqueous fluids. We also noted that subsequent to the dehydration, secondary phases form due to the breakdown of the primary halogen bearing amphibole. Chemical analyses of these secondary phases indicate that they are repositories of F and Cl. Hence, we infer that upon dehydration of the primary halogen bearing amphibole, first the F and Cl are partitioned into the aqueous fluids and then the halogens are partitioned back to the secondary mineral phases. These secondary minerals are likely to transport the halogen to the deep Earth and may in part explain the halogen concentration observed in ocean island basalt.
more » « less- PAR ID:
- 10450174
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 126
- Issue:
- 3
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Halogens (F, Cl, Br, I) are primary components of volcanic gas emissions and play an essential role in continental arc magmatic environments due to their solubility in fluids that generate metallic ore deposits. Despite their ubiquity, the behavior and budget of halogens in continental arc environments are poorly constrained. We investigated the plutonic and volcanic halogen budgets in intermediate-to-felsic igneous rocks (56–77 wt% SiO2) from the Sierra Nevada (California) - a Mesozoic continental arc where plutonic and volcanic outcrops can be correlated via their geographic, compositional, and geochronologic framework. We measured the halogen concentrations of bulk rock powders and their leachates via ion chromatography (F, Cl) and ICP-MS (Br, I). Halogen concentrations in our rock powders range between 107–727 μg/g F, 13–316 μg/g Cl, 2–323 ng/g Br, and 1–69 ng/g I. In contrast, leachates yielded 3–4 orders of magnitude less Cl and F, one order of magnitude less I, and similar amounts of Br compared to their corresponding bulk rocks. Preliminary data show no significant differences between volcanic and plutonic samples, suggesting that halogen concentrations in these rocks are insensitive to shallow fractionation. Although F and I exhibit no correlation with major element compositions, Cl and Br display negative trends with increasing SiO2 and K2O, and positive trends with increasing Fe2O3T, MnO, MgO, CaO, and TiO2, suggesting mafic minerals as important hosts of structurally bound halogens. Overall, Sierran plutonic rocks display low halogen contents (max. F, Cl = 727, 315 μg/g), consistent with biotite- and apatite-bearing granitoids reported in [1]. This work suggests that halogens do not preferentially enrich in shallow plutonic or volcanic portions of a continental arc system and that mafic mineral phases likely serve as primary reservoirs of these elements in intermediate-to-felsic igneous rocks. These hypotheses will be further investigated in future work through in-situ analysis of halogen concentrations in crystals. [1] Teiber, Marks, Wenzel, Siebel, Altherr & Markl (2014), Chemical Geology, vol. 374–375, pp. 92–109, doi: 10.1016/j.chemgeo.2014.03.006more » « less
-
Abstract A plausible origin of the seismically observed mid-lithospheric discontinuity (MLD) in the subcontinental lithosphere is mantle metasomatism. The metasomatized mantle is likely to stabilize hydrous phases such as amphiboles. The existing electrical conductivity data on amphiboles vary significantly. The electrical conductivity of hornblendite is much higher than that of tremolite. Thus, if hornblendite truly represents the amphibole varieties in MLD regions, then it is likely that amphibole will cause high electrical conductivity anomalies at MLD depths. However, this is inconsistent with the magnetotelluric observations across MLD depths. Hence, to better understand this discrepancy in electrical conductivity data of amphiboles and to evaluate whether MLD could be caused by metasomatism, we determined the electrical conductivity of a natural metasomatized rock sample. The metasomatized rock sample consists of ~87% diopside pyroxene, ~9% sodium-bearing tremolite amphibole, and ~3% albite feldspar. We collected the electrical conductivity data at ~3.0 GPa, i.e., the depth relevant to MLD. We also spanned a temperature range between 400 to 1000 K. We found that the electrical conductivity of this metasomatized rock sample increases with temperature. The temperature dependence of the electrical conductivity exhibits two distinct regimes. At low temperatures <700 K, the electrical conductivity is dominated by the conduction in the solid state. At temperatures >775 K, the conductivity increases, and it is likely to be dominated by the conduction of aqueous fluids due to partial dehydration. The main distinction between the current study and the prior studies on the electrical conductivity of amphiboles or amphibole-bearing rocks is the sodium (Na) content in amphiboles of the assemblage. Moreover, it is likely that the higher Na content in amphiboles leads to higher electrical conductivity. Pargasite and edenite amphiboles are the most common amphibole varieties in the metasomatized mantle, and our study on Na-bearing tremolite is the closest analog of these amphiboles. Comparison of the electrical conductivity results with the magnetotelluric observations constrains the amphibole abundance at MLD depths to <1.5%. Such a low-modal proportion of amphiboles could only reduce the seismic shear wave velocity by 0.4–0.5%, which is significantly lower than the observed velocity reduction of 2–6%. Thus, it might be challenging to explain both seismic and magnetotelluric observations at MLD simultaneously.more » « less
-
The halogens (F, Cl, Br, I) are cycled into the crust via subduction. The presence of F and Cl in arc settings impacts melt viscosity, igneous phase relations, and thermodynamic properties of magma in the pluton-to-volcano system, whereas the systematics of Br and I in melt systems are poorly understood. Mass balance constraints show that more halogens are subducted with the slab than are released during volcanism and passive degassing, suggesting that a halogen sink may exist in the lithosphere. Despite this, the halogen content of the upper continental crust of arc systems and distribution of halogens between plutonic and volcanic arc rocks are poorly quantified. This study presents whole rock halogen (F, Cl, Br, I) concentrations for 22 unaltered, geospatially- and temporally-related Cretaceous granitoid, hypabyssal plutonic, and volcanic rocks from the Sierra Nevada, California. This sampling approach allows direct comparison of plutonic and volcanic counterparts to make inferences about the pluton-volcano relationship. Because F behaves more incompatibly than Cl, Br, and I, late-stage fluid exsolution from melts may concentrate F in plutonic rocks and Cl, Br, and I in volcanic rocks. These whole rock halogen data provide a first-order approximation of the proportion of subducted halogens that are stored in the upper continental crust, and where along the magmatic plumbing path they are stored with important implications for their role in primary igneous processes such as pluton crystallization and volcanism. Ultimately, the results from this work will serve as the preliminary data for a larger study, provide insight into the magnitude of the roles the halogens play during primary igneous processes, and add to the limited halogen data on arc rocks.more » « less
-
Ongoing investigations of halogen element (F, Cl, Br, I) concentrations in rocks and minerals in the Cretaceous Sierra Nevada Batholith, CA, aim to elucidate the spatio-temporal distribution and budget of these important elements in a “typical” continental convergent margin arc. Using a 3.0 kW Axios (Panalytical) wavelength-dispersive X-ray fluorescence spectrometer (XRF) equipped with a Ge 111 crystal to eliminate second order interferences on Cl-Kα lines, the Pomona College XRF lab has undertaken a campaign-style study of Cl in pressed powder samples of metamorphic and metavolcanic rocks in the Sierra. This work complements pyrohydrolysis + ion chromatography (IC) and ICP-MS analyses the research team is undertaking at the University of Texas – Austin. Both labs quadruply wash powders to eliminate Cl contributions from decrepitated fluid inclusions or grain boundary deposits. Intercomparison between the two labs show correlation (r2 = 0.98) between analyses of the same unknown samples, but decreased accuracy of XRF (>30% relative) below 30 µg/g. Despite lower precision, XRF characterization is a relatively rapid and less labor intensive means to identify key Cl variations among rock types and to select samples for full analysis of all four halogen elements by pyrohydrolysis + IC (Cl,F) and ICP-MS (Br,I). Results thus far indicate that Mg- to Al-rich pelites ranging in metamorphic grade from phyllite to migmatite vary widely in Cl: 50–500 µg/g; cordierite-biotite hornfels are typically elevated in Cl (200–400 µg/g) and other lithologies such as skarns and amphibolite are highly varied (50–600 µg/g Cl); a localized study of a high temperature (650–750°C) migmatites surrounding a gabbro-diorite complex shows low and relatively uniform Cl (100 ± 50 µg/g) in the migmatites. This fundings suggests that Cl may have been mobilized into melts during biotite dehydration melting in the migmatites. Metavolcanic rocks vary from 20 to over 2000 µg/g Cl, suggesting post-eruptive exchange with exogenous fluids during hydrothermal alteration and metamorphism. Metavolcanic packages in different pendants, screens and septa show some localized patterns in Cl concentration that are being explored further.more » « less
