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.2014.03.006
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Halogen distribution in continental arc plumbing systems: Sierra Nevada, California
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.
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- Award ID(s):
- 2211243
- PAR ID:
- 10514006
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geological Society of America Abstracts with Programs
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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