Origin of internal flow structures in columnar-jointed basalt from Hrepphólar, Iceland: I. Textural and geochemical characterization

Abstract

Basalt columns from Hrepphólar (Iceland) show distinct internal structures produced by alternating brighter and darker bands through the column, locally exhibiting viscous fingering features. Here, we present geochemical and petrographic data retrieved from analyses of major and trace elements and mineral chemistry from a cross section of a single basaltic column. This is combined with petrographic descriptions and data on crystal size distributions of plagioclase. We use our data from Hrepphólar to test four existing models that have been proposed to explain banded structures inside columns: (1) deuteric alteration, (2) doublediffusive convection, (3) constitutional supercooling, and (4) crystallization-induced melt migration. We find that the internal structures at Hrepphólar represent primary magmatic features, because approximately 20 % of the observed structures crosscut the column-bounding fracture for each meter along the main axis of the column. These features must thus have been formed before the column-delimiting crack advanced. Major and trace element analyses show small but significant variations across the column and strong correlation between oxides like FeO and TiO 2 , as well as K 2 O and P 2 O 5 . The geochemical variations correlate with the presence of darker/brighter bands visible on a polished surface and can be explained by a variation in the modal proportions of the main phenocryst phases (specifically variable plagioclase and titanomagnetite content). This banding enhances the internal structures apparent in the polished cross section from columnar joints at Hrepphólar. The measured variations in major and trace element geochemistry, as well as mineral chemistry, are too small to distinguish between the proposed band-forming models. Plagioclase crystal size distributions, however, display a systematic change across the column that is consistent with late-stage migration of melt inside the column (i.e., the crystallization induced melt migration hypothesis). The central part of the columns have plagioclases indicative of slow cooling and these are also more steeply oriented (i.e., subparallel to the column axis) compared with plagioclases present in the more rapidly cooled edges. This redistribution of melt within individual columns may significantly affect the cooling rate of columnar-jointed lava flows and intrusions.

Internal flow structures in columnar jointed basalt from Hrepphólar, Iceland: II. Magnetic anisotropy and rock magnetic properties

“…The specimens show a Curie temperature of 200 °C or below, which indicates that the bulk ferromagnetic (s.l.) mineral is titanomagnetite with a composition The inferred composition, which is based on the Curie temperature, is supported by the compositional data for grains of titanomagnetite obtained with the electron microprobe (Bosshard et al 2012). Production of stoichiometric magnetite, which is evident from the increase in susceptibility during cooling, arises either at the expense of titanomagnetite (i.e., producing exsolution end members of Tirich and Ti-poor oxides), or through the creation of magnetite from alteration of other minerals present in the basalt (e.g., olivine).…”

“…Rock magnetic experiments are performed to evaluate differences in composition and grain-size of the ferrimagnetic minerals, which occur in zoned areas of the basalt plate. The characterization and interpretation of mineral texture, petrography, and geochemistry from the same sample is described in Bosshard et al (2012).…”

Scale selection in columnar jointing: Insights from experiments on cooling stearic acid and numerical simulations

“…Though thermal contraction is widely believed to be the mechanism governing the column formation process, other models where fingering in the solidifying lava governs the column scale, such as large‐scale constitutional supercooling [ Guy , ; Gilman , ] or double‐diffusive convection [ Kantha , ], have been proposed. These models were not found consistent with a small‐scale study of a single basalt column [ Bosshard et al , ] and do not fully explain the striking similarity between columnar jointing in basalt and corn starch.…”

Development of columnar jointing in albite rhyolite in a rapidly cooling volcanic environment (Rupnica, Papuk Geopark, Croatia)

“…Pollard and Aydin, ). Beside the mechanism of thermal contraction that was generally advocated for the development of the phenomenon (Goehring and Morris, ; Bahr et al ., ; Milazzo et al ., ), recent studies imply significant influence of solidification fingering and/or large‐scale constitutional supercooling (Guy and Le Coze, ; Gilman, ; Guy, ), chemical differences and melt‐migration (Mattsson et al ., ; Bosshard et al ., ; Hetényi et al ., ) on the formation of columnar jointing.…”