Publication Date:
2022-05-25
Description:
Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Contributions to Mineralogy and Petrology 172 (2017): 51, doi:10.1007/s00410-017-1368-7.
Description:
The fluorine (F) and chlorine (Cl) contents of arc magmas have been used to track the
composition of subducted components, and the F and Cl contents of MORB have been used to
estimate the halogen content of depleted MORB mantle (DMM). Yet, the F and Cl budget of the
Earth's upper mantle, and their distribution in peridotite minerals, remains to be constrained.
Here we developed a method to measure low concentrations of halogens (≥ 0.4 μg/g F and ≥ 0.3
μg/g Cl) in minerals by secondary ion mass spectroscopy. We present a comprehensive study of
F and Cl in natural olivine, orthopyroxene, clinopyroxene, and amphibole in seventeen samples
from different tectonic settings. We support the hypothesis that F in olivine is controlled by melt
polymerization, and that F in pyroxene is controlled by their Na and Al contents, with some
effect of melt polymerization. We infer that Cl compatibility ranks as follows: amphibole 〉
clinopyroxene 〉 olivine ~ orthopyroxene, while F compatibility ranks as follows: amphibole 〉
clinopyroxene 〉 orthopyroxene ≥ olivine, depending on the tectonic context. In addition, we
show that F, Cl, Be and B are correlated in pyroxenes and amphibole. F and Cl variations suggest
that interaction with slab melts and fluids can significantly alter the halogen content of mantle
minerals. In particular, F in oceanic peridotites is mostly hosted in pyroxenes, and proportionally
increases in olivine in subduction-related peridotites. The mantle wedge is likely enriched in F
compared to un-metasomatized mantle, while Cl is always low (〈 1 μg/g) in all tectonic settings
studied here. The bulk anhydrous peridotite mantle contains 1.4–31 μg/g F and 0.14–0.38 μg/g
Cl. The bulk F content of oceanic-like peridotites (2.1–9.4 μg/g) is lower than DMM estimates,
consistent with F-rich eclogite in the source of MORB. Furthermore, the bulk Cl budget of all
anhydrous peridotites studied here is lower than previous DMM estimates. Our results indicate
that nearly all MORB may be somewhat contaminated by seawater-rich material and that the Cl
content of DMM could be overestimated. With this study, we demonstrate that the halogen
contents of natural peridotite minerals are a unique tool to understand the cycling of halogens,
from ridge settings to subduction zones.
Description:
This research was supported by grant NSF EAR-P&G 1524311 and DOEI award 18563 to VLR. Urann was supported by
the Stanley W. Watson Student Fellowship Fund based at WHOI.
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
Permalink
