The effect of crystallization time on plagioclase grain shape in dolerites

Holness, Marian B.

doi:10.1007/s00410-014-1076-5

Cited by 49 publications

(103 citation statements)

References 53 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…At first sight, this demonstrates that plagioclase in the sills examined by Holness (2014) grew in situ in inwardspropagating solidification fronts, rather than as a suspended crystal load in a vigorously convecting magma. However, the symmetrical variation of AR observed in sills thinner than ~40 m is not apparent in either the 130 m-thick Portal Peak sill, nor in the 189 m-thick Koffiefontein sill, in which the minimum value of AR is displaced upwards from the mid-point of the sill (Holness 2014). This is consistent with the accumulation on the floor of relatively non-equant plagioclase grown at the sill roof, suggestive of convective redistribution of material.…”

Section: Microstructural Evidence For Convection In Other Sillsmentioning

confidence: 99%

Crystal settling and convection in the Shiant Isles Main Sill

Holness

Farr

Neufeld

2017

Contrib Mineral Petrol

Self Cite

View full text Add to dashboard Cite

clusters of olivine are present at the roof, forming a foreshortened mirror image of the coarsening-upwards component of the floor accumulation. The coarsening-upwards sequence records the growth of olivine crystals while in suspension in a convecting magma, and their aggregation into clusters, followed by settling over a prolonged period (with limited trapping at the roof). As olivine was progressively lost from the convecting magma, crystal accumulation on the (contemporaneous) floor of the PCU was increasingly dominated by plagioclase, most likely forming clusters and aggregates with augite and olivine, both of which form large poikilitic grains in the crinanite. While the PCU is unusual in being underlain by an earlier, still hot, intrusion that would have enhanced any driving force for convection, we conclude from comparison with microstructures in other sills that convection is likely in tabular bodies >100 m thickness.

show abstract

Section: Microstructural Evidence For Convection In Other Sillsmentioning

confidence: 99%

Crystal settling and convection in the Shiant Isles Main Sill

Holness

Farr

Neufeld

2017

Contrib Mineral Petrol

Self Cite

View full text Add to dashboard Cite

show abstract

“…Further validation of our 2-20 days disaggregation timescale estimate, and hence our choice of plagioclase growth rates, is provided by the elevated water content of primitive olivinehosted inclusions in magmatic tephra samples: the extent of diffusive over-hydration observed in these inclusions requires that entrained macrocrysts spent a minimum of 2.5-19.1 days in the evolved Laki carrier liquid prior to eruption (Hartley et al 2015). Moreover, the high mean aspect ratios of plagioclase microcrysts (length/width > 4) are indicative of crystallization timescales in the order of days to tens of days, whereas the low mean aspect ratios (length/width = 2-3) of large plagioclase macrocrysts are consistent with much longer crystallization timescales of years to hundreds of years, assuming that crystallization was continuous (Holness 2014). As noted by Hartley et al (2016), a 2-20 days timescale is not only much shorter than the total duration of the eruption (245 days), but is also comparable in length with the intervals between eruptive episodes (1-28 days; Thordarson and Self 1993).…”

Section: Timescales Of Mush Disaggregationmentioning

confidence: 51%

“…Plagioclase morphology varies systematically as a function of grain size: macrocrysts (A 0.5 > 0.2 mm, which is equivalent to L > 0.5 mm) are equant and have mean aspect ratios of 2-3, whereas microcrysts (A 0.5 < 0.2 mm, which is equivalent to L < 0.5 mm) are elongate and usually have aspect ratios >4. Given that the shape of plagioclase grains reflects their crystallization histories (Lofgren 1974;Higgins 1996;Holness 2014), these size-morphology relationships confirm the presence of multiple plagioclase populations (cf. Neave et al 2013).…”

Section: Plagioclase Size-morphology Relationshipsmentioning

confidence: 68%

Continuous mush disaggregation during the long-lasting Laki fissure eruption, Iceland

Neave

Buisman

Maclennan

2017

American Mineralogist

View full text Add to dashboard Cite

Plagioclase textures were investigated in the products of the voluminous 1783-1784 CE Laki eruption from the Eastern Volcanic Zone (EVZ) of Iceland to establish whether mush disaggregation occurred solely at the onset of the eight-month eruption or throughout its whole duration. Phase proportions and plagioclase size distributions were determined using standard optical and manual techniques as well as automated approaches based on Quantitative Evaluation of Minerals by SCANing electron microscopy (QEMSCAN). Based on optical microscopy and the explicit combination of textural and compositional information in QEMSCAN images, plagioclase crystals were divided into two populations: small (<0.5 mm long), high-aspect ratio (length/width >4) microcrysts with low-anorthite (0.5 mm long), low-aspect ratio (length/width = 2-3) macrocrysts with high-anorthite (An 84 -An 92 ) cores. Small microcrysts grew from their carrier liquid during the final phase of pre-eruptive crystallization while large macrocrysts, which are out of geochemical equilibrium with their carrier liquids, were entrained from crystal mushes. Changes in phase proportions and plagioclase size distributions between eruptive episodes demonstrate that macrocryst entrainment efficiency varied substantially during the eruption; material erupted in later episodes contain proportionally more mush-derived material. Using stereologically corrected plagioclase size distributions, we estimate that the pre-eruptive residence times of microcrysts in the Laki carrier liquid were probably of the order of 2-20 days. Because microcryst crystallization was concurrent with macrocryst rim growth, these day-to-week residence times also indicate that macrocryst entrainment occurred on much shorter timescales than the eruption's eight-month duration. In line with constraints from independent geochronometers, macrocryst entrainment and mush disaggregation thus appears to have continued throughout the eruption. Magmas were assembled on an episode by episode basis, and the volume of eruptible magma in the plumbing system at any given time was probably closer to 1-2 km 3 than the final erupted volume of 15.1 km 3 .

show abstract

“…Under the conditions of interface-controlled growth that pertain to most geological environments, plagioclase forms variably elongated tabular facetted grains, with elongation commonly along the c axis, and tablets flattened parallel to the b axis. In basaltic rocks in which the plagioclase is randomly oriented (i.e., those in which the grains have not been rearranged by the action of magmatic currents or by gravitational instability and slumping of a crystal pile) the average grain size and shape carries key information about the cooling and crystallization history [12]. Currently, these data are invariably extracted from 2D sections through the rock.…”

Section: Introductionmentioning

confidence: 99%

Reconstructing grain-shape statistics from electron back-scatter diffraction microscopy

Farr

Vukmanovic

Holness

et al. 2018

Phys. Rev. Materials

View full text Add to dashboard Cite

Reconstructing the three-dimensional (3D) size and shape distribution of randomly oriented grains using only images of cross sections remains an important challenge. Even for ellipsoids, a solution is only possible when they are solids of revolution, and may still be numerically unstable. Here we show that crystallographic orientation data, for example from electron back-scatter diffraction (EBSD), provides enough additional information to obtain moments of the 3D grain distribution, provided grain shapes can be assumed to align with crystal axes. We show that this moment method can give an average 3D grain size and shape (with error estimate) which is rigorous for ellipsoids and a good approximation for cuboidal grains, indicating that it may be a useful technique for polycrystalline materials in general. High throughput image analysis and EBSD now make the necessary sample sizes practical. We illustrate by applying the method to a basaltic rock specimen.

show abstract

The effect of crystallization time on plagioclase grain shape in dolerites

Cited by 49 publications

References 53 publications

Crystal settling and convection in the Shiant Isles Main Sill

Crystal settling and convection in the Shiant Isles Main Sill

Continuous mush disaggregation during the long-lasting Laki fissure eruption, Iceland

Reconstructing grain-shape statistics from electron back-scatter diffraction microscopy

Contact Info

Product

Resources

About