Formation of segregation structures in Hafnarhraun p\u0101hoehoe lobe, SW Iceland: a window into crystal\u2013melt separation in basaltic magma

Paavo Nikkola,O Tapani Rämö,Thorvaldur Thordarson,Pasi Heikkilä

doi:10.1007/s00445-019-1330-9

Paavo Nikkola, O Tapani Rämö + Show 2 more

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https://doi.org/10.1007/s00445-019-1330-9

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Abstract

To gain insights into crystal–melt separation processes during basalt differentiation, we have studied an 8-m-thick pāhoehoe lava lobe from the Hafnarhraun lava flow field in SW Iceland. The lobe has abundant melt segregations, porous cylindrical and sheet-like structures, generally interpreted as separated residual melts of a lava lobe. We divide these melt segregations into three types based on morphology and composition: vesicle cylinders (VC), type 1 horizontal vesicle sheets (HVS1), and type 2 horizontal vesicle sheets (HVS2). Remarkably, the studied VC are not simple residual melts generated by fractional crystallization, but their composition points to removal of plagioclase from the parental lava. HVS1 resemble VC, but have fractionated more olivine (ol) + plagioclase (plg) ± augite and have lost most, if not all, of their olivine phenocrysts. HVS2 are Fe-rich and evolved, corresponding to residual melts after 50–60% fractional crystallization of the lobe. We suggest that the Hafnarhraun VC formed in a two-stage process. Firstly, VC forming residual melt and vapor detached as rising diapirs from ol+plg+melt+vapor mush near the lava base, and later, these VC diapirs accumulated ol phenocrysts and minor plg microphenocrysts in the lava core. HVS1 represent accumulations of VC to the viscous base of the solidifying upper crust of the lobe, and HVS2 formed as evolved vapor-saturated residual melts seeped into voids within the upper crust. Such vapor-aided differentiation, here documented for the Hafnarhraun lava, may also apply to shallow crustal magma storage zones, contributing to the formation of evolved basalts.

Highlights

IntroductionMagmatic differentiation by fractional crystallization is one of the key processes of igneous petrology
To evaluate whether the melt segregations in the Hafnarhraun pāhoehoe lobe are differentiated melts produced by fractional crystallization of the lava lobe, we modeled the fractional crystallization of the lobe and composition of derivate melts along the liquid line of descent (LLD, Fig. 5) with COMAGMAT-5.2.2. (Ariskin 1999; Ariskin et al 2018) software
We have shown that vesicle cylinders (VC) compositions in Hafnarhraun do not correspond to any expected residual melt produced by fractional crystallization of the host lava

Summary

Introduction

Magmatic differentiation by fractional crystallization is one of the key processes of igneous petrology. It is not adequately known how crystals separate from their coexisting melts in various magmatic systems. Various mechanisms of crystal–melt separation have been identified. These include gravitational crystal settling (Darwin 1844), liquid convection (Sparks et al 1984), and filter pressing (Philpotts et al 1996). The physical processes of crystal–melt separation have proven difficult to study (cf Wilson 1993), as the emplacement of differentiated magmatic intrusions is often complex and may involve multiple evolutionary stages (e.g., Gibb and Henderson 1992, 2006; Menand 2011).

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