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

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:

@article{Rout2018, author = "Smruti Sourav Rout and Gerhard W{\"o}rner", 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 ({\&}lt;{\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.", day = "23", doi = "10.1007/s00410-018-1522-x", issn = "1432-0967", journal = "Contributions to Mineralogy and Petrology", month = "Oct", number = "11", pages = "95", title = "{Z}oning and exsolution in alkali feldspars from {L}aacher {S}ee volcano ({W}estern {G}ermany): constraints on temperature history prior to eruption", url = "https://doi.org/10.1007/s00410-018-1522-x", volume = "173", year = "2018", }

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.