An experimental study of amphibole stability in low-pressure granitic magmas and a revised Al-in-hornblende geobarometer

We report new experimental data on the composition of magmatic amphiboles synthesised from a variety of granite (sensu lato) bulk compositions at near-solidus temperatures and pressures of 0.8–10 kbar. The total aluminium content (Al$^\text{tot}$) of the synthetic calcic amphiboles varies systematically with pressure ($\small \textit{P}$), although the relationship is nonlinear at low pressures (<2.5 kbar). At higher pressures, the relationship resembles that of other experimental studies, which suggests of a general relationship between Al$^\text{tot}$ and P that is relatively insensitive to bulk composition. We have developed a new Al-in-hornblende geobarometer that is applicable to granitic rocks with the low-variance mineral assemblage: amphibole + plagioclase (An$_{15–80}$) + biotite + quartz + alkali feldspar + ilmenite/ titanite + magnetite + apatite. Amphibole analyses should be taken from the rims of grains, in contact with plagioclase and in apparent textural equilibrium with the rest of the mineral assemblage at temperatures close to the haplogranite solidus (725 ± 75 °C), as determined from amphibole–plagioclase thermometry. Mean amphibole rim compositions that meet these criteria can then be used to calculate $\small \textit{P}$ (in kbar) from Al$^\text{tot}$ (in atoms per formula unit, apfu) according to the expression: $P$ (kbar) = 0.5 + 0.331(8) × Al$^\text{tot}$ + 0.995(4) × (Al$^\text{tot}$)$^2$ This expression recovers equilibration pressures of our calibrant dataset, comprising both new and published experimental and natural data, to within ±16 % relative uncertainty. An uncertainty of 10 % relative for a typical Al$^\text{tot}$ value of 1.5 apfu translates to an uncertainty in pressure estimate of 0.5 kbar, or 15 % relative. Thus the accuracy of the barometer expression is comparable to the precision with which near-solidus amphibole rim composition can be characterised.BHP Billiton, Royal Society (Wolfson Research Merit Award), California Institute of Technology (Moore Scholarship

Mafic tiers and transient mushes: evidence from Iceland.

It is well established that magmatism is trans-crustal, with melt storage and processing occurring over a range of depths. Development of this conceptual model was based on observations of the products of magmatism at spreading ridges, including Iceland. Petrological barometry and tracking of the solidification process has been used to show that the Icelandic crust is built by crystallization over a range of depths. The available petrological evidence indicates that most of the active rift zones are not underlain by extensive and pervasive crystal mush. Instead, the microanalytical observations from Iceland are consistent with a model where magmatic processing in the lower crust occurs in sills of decimetric vertical thickness. This stacked sills mode of crustal accretion corresponds to that proposed for the oceanic crust on the basis of ophiolite studies. A key feature of these models is that the country rock for the sills is hot but subsolidus. This condition can be met if the porosity in thin crystal mushes at the margins of the sills is occluded by primitive phases, a contention that is consistent with observations from cumulate nodules in Icelandic basalts. The conditions required for the stabilization of trans-crustal mushes may not be present in magmatic systems at spreading ridges. This article is part of the Theo Murphy meeting issue 'Magma reservoir architecture and dynamics'

3D Diffusion of Water in Melt Inclusion-Bearing Olivine Phenocrysts

Publication status: PublishedAbstractOlivine‐hosted melt inclusions are an important archive of pre‐eruptive processes such as magma storage, mixing and subsequent ascent through the crust. However, this record can be modified by post‐entrapment diffusion of H+ through the olivine lattice. Existing studies often use spherical or 1D models to track melt inclusion dehydration that fail to account for complexities in geometry, diffusive anisotropy and sectioning effects. Here we develop a finite element 3D multiphase diffusion model for the dehydration of olivine‐hosted melt inclusions that includes natural crystal geometries and multiple melt inclusions. We use our 3D model to test the reliability of simplified analytical and numerical models (1D and 2D) using magma ascent conditions from the 1977 eruption of Seguam volcano, Alaska. We find that 1D models underestimate melt inclusion water loss, typically by ∼30%, and thus underestimate magma decompression rates, by up to a factor of 5, when compared to the 3D models. An anisotropic analytical solution that we present performs well and recovers decompression rates within a factor of 2, in the situations in which it is valid. 3D models that include multiple melt inclusions show that inclusions can shield each other and reduce the amount of water loss upon ascent. This shielding effect depends on decompression rate, melt inclusion size, and crystallographic direction. Our modeling approach shows that factors such as 3D crystal geometry and melt inclusion configuration can play an important role in constraining accurate decompression rates and recovering water contents in natural magmatic systems.</jats:p

DFENS: Diffusion Chronometry Using Finite Elements and Nested Sampling

In order to reconcile petrological and geophysical observations of magmatic processes in 24 the temporal domain, the uncertainties in diffusion timescales need to be rigorously as25 sessed. Here we present a new diffusion chronometry method: Diffusion chronometry us26 ing Finite Elements and Nested Sampling (DFENS). This method combines a finite el27 ement numerical model with a nested sampling Bayesian inversion, meaning that uncer28 tainties in the parameters contributing to diffusion timescale estimates can be obtained 29 and that observations from multiple elements can be used to better constrain individ30 ual timescales. Uncertainties associated with diffusion timescales can be reduced by ac31 counting for covariance in the uncertainty structure of diffusion parameters rather than 32 assuming that they are independent of each other. We applied the DFENS method to 33 the products of the Skuggafjöll eruption from the Bárðarbunga volcanic system in Ice34 land, which contains zoned macrocrysts of olivine and plagioclase that record a shared 35 magmatic history. Olivine and plagioclase provide consistent pre-eruptive mixing and 36 mush disaggregation timescales of less than 1 year. The DFENS method goes some way 37 towards improving our ability to rigorously address the uncertainties of diffusion timescales, 38 but efforts still need to be made to understand other systematic sources of uncertainty 39 such as crystal morphology, appropriate choice of diffusion coefficients, initial conditions, 40 crystal growth, and the petrological context of diffusion timescales

Deep magma mobilization years before the 2021 CE Fagradalsfjall eruption, Iceland

Abstract The deep roots of volcanic systems play a key role in the priming, initiation, and duration of eruptions. Causative links between initial magmatic unrest at depth and eruption triggering remain poorly constrained. The 2021 CE eruption at Fagradalsfjall in southwestern Iceland, the first deep-sourced eruption on a spreading-ridge system monitored with modern instrumentation, presents an ideal opportunity for comparing geophysical and petrological data sets to explore processes of deep magma mobilization. We used diffusion chronometry to show that deep magmatic unrest in the roots of volcanic systems can precede apparent geophysical eruption precursors by years, suggesting that early phases of magma accumulation and reorganization can occur in the absence of significant increases in shallow seismicity (&amp;lt;7 km depth) or rapid geodetic changes. Closer correlation between geophysical and diffusion age records in the months and days prior to eruption signals the transition from a state of priming to full-scale mobilization in which magma begins to traverse the crust. Our findings provide new insights into the dynamics of near-Moho magma storage and mobilization. Monitoring approaches optimized to detect early phases of magmatic unrest in the lower crust, such as identification and location of deep seismicity, could improve our response to future eruptive crises.</jats:p