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Abstract Lawsonite is a major host mineral of trace elements (TEs; e.g. REE, Sr, Pb, U, Th) and H2O in various rock types (metabasite, metasediment, metasomatite) over a wide range of depths in subduction zones. Consequently, the composition of lawsonite is a useful archive to track chemical exchanges that occurred during subduction and/or exhumation, as recorded in high-pressure/low-temperature (HP/LT) terranes. This study provides an extensive dataset of major element and TE compositions of lawsonite in HP/LT rocks from two mélanges (Franciscan/USA; Rio San Juan/Dominican Republic), two structurally coherent terranes (Tavşanlı/Turkey; Alpine Corsica/France), and the eclogite blocks of the Pinchi Lake/Canada complex. Bulk major and TE compositions were also determined for lawsonite-bearing host rocks to understand petrogenesis and assess compositional evolution. Most analyzed mélange and coherent-terrane metabasalts have normal mid-ocean ridge/back-arc basin basalt signatures and they preserve compositional evidence supporting interactions with (meta)sediment ± metagabbro/serpentinite (e.g. LILE/LREE enrichments; Ni/Cr enrichments). Most lawsonite grains analyzed are compositionally zoned in transition-metal elements (Fe, Ti, Cr), other TEs (e.g. Sr, Pb), and/or REE, with some grains showing compositional variations that correlate with zoning patterns (e.g. Ti-sector zoning, core-to-rim zoning in Fe, Cr-oscillatory zoning). Our results suggest that compositional variations in lawsonite formed in response to crystallographic control (in Ti-sector zoning), fluid–host rock interactions, modal changes in minerals, and/or element fractionation with coexisting minerals that compete for TEs (e.g. epidote, titanite). The Cr/V and Sr/Pb ratios of lawsonite are useful to track the compositional influence of serpentinite/metagabbro (high Cr/V) and quartz-rich (meta)sediment (low Sr/Pb). Therefore, lawsonite trace and rare earth element compositions effectively record element redistribution driven by metamorphic reactions and fluid–rock interactions that occurred in subduction systems.
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3D zoning of barium in alkali feldspar
Abstract Interpretation of chemical zoning within igneous minerals is critical to many petrologic studies. Zoning in minerals, however, is commonly observed in thin sections or grain mounts, which are random 2D slices of a 3D system. Use of these 2D sections to infer 3D geometries requires a set of assumptions, often not directly tested, introduces several issues, and results in partial loss of zoning information. Computed X-ray microtomography (microCT) offers a way to assess 3D zoning in minerals at high resolution. To observe 3D mineral zoning using microCT, however, requires that zoning is observable as differences in X-ray attenuation. Sanidine, with its affinity for Ba in the crystal lattice, can display large, abrupt variations in Ba that are related to various magma reservoir processes. These changes in Ba also significantly change the X-ray attenuation coefficient of sanidine, allowing for discrete mineral zones to be mapped in 3D using microCT. Here we utilize microCT to show 3D chemical zoning within natural sanidines from a suite of volcanic eruptions throughout the geologic record. We also show that changes in microCT grayscale in sanidine are largely controlled by changes in Ba. Starting with 3D mineral reconstructions, we simulate thin-section making by generating random 2D slices across a mineral zone to show that slicing orientation alone can drastically change the apparent width and slope of composition transitions between different zones. Furthermore, we find that chemical zoning in sanidine can commonly occur in more complex geometries than the commonly interpreted concentric zoning patterns. Together, these findings have important implications for methodologies that rely on the interpretation of chemical zoning within minerals and align with previously published numerical models that show how chemical gradient geometries are affected by random sectioning during common sample preparation methods (e.g., thin sections and round mounts).
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- Award ID(s):
- 1763639
- PAR ID:
- 10453093
- Date Published:
- Journal Name:
- American Mineralogist
- Volume:
- 108
- Issue:
- 2
- ISSN:
- 0003-004X
- Page Range / eLocation ID:
- 297 to 311
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.2138/am-2022-8139
