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Zoning and exsolution in alkali feldspars from Laacher See volcano (Western Germany): constraints on temperature history prior to eruption

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Year:
2018
Type of Publication:
Article
Authors:
Rout, Smruti Sourav; Wörner, Gerhard
Journal:
Contributions to Mineralogy and Petrology
Volume:
173
Number:
11
Pages:
95
Month:
October
ISSN:
1432-0967
BibTex:
Abstract:
Compositional zoning and exsolution patterns of alkali feldspars in carbonatite-bearing cognate syenites from the 6.3 km3 (D.R.E) phonolitic Laacher See Tephra (LST) deposit in western Germany (12.9 ka) are reported. These rocks represent the cooler outer portion and crystal-rich products of a cooling magma reservoir at upper crustal levels. Major and trace-element difference between cores and rims in sanidine crystals represent two generations of crystal growth separated by unmixing of a carbonate melt. Trace-element differences measured by LA--ICP--MS are in accordance with silicate--carbonate unmixing. Across the core--rim boundary, we extracted gray-scale profiles from multiple accumulations of back-scattered electron images. Gray scales directly represent K/Na ratios owing to low concentrations of Ba and Sr (<{\thinspace}30 ppm). Diffusion gradients are modeled to solve for temperature using known pre-eruptive U--Th zircon ages (0--20 ky) of each sample (Schmitt et al., J Petrol 51:1053--1085, 2010). Estimated temperatures range from 630 {\textdegree}C to 670 {\textdegree}C. For the exsolution boundaries, a diffusive homogenization model is constrained by the solvus temperature of {\textasciitilde} 712{\_}725 {\textdegree}C and gives short time scales of only 15--50 days. Based on our results, we present a model for the temperature--time history of these rocks. The model also constrains the thermal variation across the cooling crystal-rich carapace of the magma reservoir over 20 ka and suggests a thermal reactivation of cumulates, the cooling carapace, and probably the entire system only a few years prior to the explosive eruption of the remaining molten core of the phonolitic magma reservoir.

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