633 research outputs found
Bubble formation and decrepitation control the CO2 content of olivine-hosted melt inclusions
Author Posting. Ā© American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 18 (2017): 597ā616, doi:10.1002/2016GC006633.The CO2 contents of olivine-hosted melt inclusions have previously been used to constrain the depth of magma chambers in basaltic systems. However, the vast majority of inclusions have CO2 contents which imply entrapment pressures that are significantly lower than those obtained from independent petrological barometers. Furthermore, a global database of melt inclusion compositions from low inline image settings, indicates that the distribution of saturation pressures varies surprisingly little between mid-ocean ridges, ocean islands, and continental rift zones. 95% of the inclusions in the database have saturation pressures of 200 MPa or less, indicating that melt inclusion CO2 does not generally provide an accurate estimate of magma chamber depths. A model of the P-V-T-X evolution of olivine-hosted melt inclusions was developed so that the properties of the inclusion system could be tracked as the hosts follow a model P-T path. The models indicate that the principal control on the saturation of CO2 in the inclusion and the formation of vapor bubbles is the effect of postentrapment crystallization on the major element composition of the inclusions and how this translates into variation in CO2 solubility. The pressure difference between external melt and the inclusion is likely to be sufficiently high to cause decrepitation of inclusions in most settings. Decrepitation can account for the apparent mismatch between CO2-based barometry and other petrological barometers, and can also account for the observed global distribution of saturation pressures. Only when substantial postentrapment crystallization occurs can reconstructed inclusion compositions provide an accurate estimate of magma chamber depth.Natural Environment Research Council Grant Number: (NE/I012508/1)2017-08-1
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Clinopyroxene Dissolution Records Rapid Magma Ascent
Magma ascent rates control volcanic eruption styles. However, the rates at which basaltic magmas ascend through the crust remain highly uncertain. Although recent studies have successfully exploited records of decompression driven degassing to estimate the rates at which H2O-rich basalts ascend, such approaches cannot readily be applied to primitive and H2O-poor basalts that erupt in ocean island and mid-ocean ridge settings. Here we present magma ascent rates obtained by modelling the dissolution of clinopyroxene crystals in a wehrlitic nodule from the primitive Borgarhraun lava flow in North Iceland. High-Al2O3 clinopyroxene core compositions are consistent with crystallisation near the Moho (_800 MPa), whereas low-Al2O3 clinopyroxene rims and inclusion compositions are consistent with crystallisation at or near the surface. We interpret low-Al2O3 rims and inclusions as the crystallised remnants of boundary layers formed by the dissolution of high-Al2O3 clinopyroxene during magma ascent. By combining characteristic rim dissolution lengths of 50ā100 m with published experimental calibrations of clinopyroxene dissolution behaviour, we estimate that the Borgarhraun magma most likely decompressed and ascended at rates of 3.0ā15 kPa.s-1 and 0.11ā0.53 m.s-1 respectively. These rates are slightly faster than published estimates obtained by modelling the diffusive re-equilibration of olivine crystals, suggesting that the Borgarhraun magma either accelerated upwards or that it stalled briefly at depth prior to final ascent. Comparisons with other basaltic eruptions indicate that the H2O-poor magma that fed the dominantly effusive Borgarhraun eruption ascended at a similar rate to some H2O-rich magmas that have fed explosive eruptions in arc settings. Thus, magma ascent rates do not appear to correlate simply with magma H2O contents. Overall, our findings confirm that primitive and H2O-poor basalts can traverse the crust within days, and may erupt with little precursory warning of magma ascent
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Acknowledgements The authors would like to acknowledge helpful discussions on this topic held with various colleagues in connection with a variety of projects. Funding No funding was received for this work.Peer reviewedPublisher PD
Rate of Melt Ascent beneath Iceland from the Magmatic Response to Deglaciation
Observations of the time lag between the last deglaciation and a surge in
volcanic activity in Iceland constrain the average melt ascent velocity to be
. Although existing theoretical work has explained why
the surge in eruption rates increased - fold from the steady-state rates
during the last deglaciation, they cannot account for large variations of Rare
Earth Element (REE) concentrations in the Icelandic lavas. Lavas erupted during
the last deglaciation are depleted in REEs by up to ; whereas, existing
models, which assume instantaneous melt transport, can only produce at most
depletion. Here, we develop a numerical model with finite melt ascent
velocity and show that the variations of REEs are strongly dependent on the
melt ascent velocity. When the average melt ascent velocity is
, the variation of calculated by our model is
comparable to that of the observations. In contrast, when the melt ascent
velocity is or above, the model variation of
becomes significantly lower than observed, which explains why
previous models with instantaneous melt transport did not reproduce the large
variations. We provide the first model that takes account of the diachronous
response of volcanism to deglaciation. We show by comparing our model
calculations of the relative volumes of different eruption types (subglacial,
finiglacial and postglacial) and the timing of the bursts in volcanic eruptions
with the observations across different volcanic zones that the Icelandic
average melt ascent velocity during the last deglaciation is likely to be
Quantifying lithological variability in the mantle
We present a method that can be used to estimate the amount of recycled material present in the source region of mid-ocean ridge basalts by combining three key constraints: (1) the melting behaviour of the lithologies identified to be present in a mantle source, (2) the overall volume of melt production, and (3) the proportion of melt production attributable to melting of each lithology. These constraints are unified in a three-lithology melting model containing lherzolite, pyroxenite and harzburgite, representative products of mantle differentiation, to quantify their abundance in igneous source regions.
As a case study we apply this method to Iceland, a location with sufficient geochemical and geophysical data to meet the required observational constraints. We find that to generate the 20 km of igneous crustal thickness at Iceland's coasts, with 30Ā±10%30Ā±10% of the crust produced from melting a pyroxenitic lithology, requires an excess mantle potential temperature (ĪTp) of ā©¾130āĀ°C (View the MathML sourceTpā©¾1460Ā°C) and a source consisting of at least 5% recycled basalt. Therefore, the mantle beneath Iceland requires a significant excess temperature to match geophysical and geochemical observations: lithological variation alone cannot account for the high crustal thickness. Determining a unique source solution is only possible if mantle potential temperature is known precisely and independently, otherwise a family of possible lithology mixtures is obtained across the range of viable ĪTp. For Iceland this uncertainty in ĪTp means that the mantle could be >20% harzburgitic if View the MathML sourceĪTp>150Ā°C (View the MathML sourceTp>1480Ā°C).
The consequences of lithological heterogeneity for plume dynamics in various geological contexts are also explored through thermodynamic modelling of the densities of lherzolite, basalt, and harzburgite mixtures in the mantle. All lithology solutions for Iceland are buoyant in the shallow mantle at the ĪTp for which they are valid, however only lithology mixtures incorporating a significant harzburgite component are able to reproduce recent estimates of the Iceland plume's volume flux. Using the literature estimates of the amount of recycled basalt in the sources of Hawaiian and Siberian volcanism, we found that they are negatively buoyant in the upper mantle, even at the extremes of their expected ĪTp. One solution to this problem is that low density refractory harzburgite is a more ubiquitous component in mantle plumes than previously acknowledged
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Microstructural constraints on magmatic mushes under KÄ«lauea Volcano, Hawai'i.
Distorted olivines of enigmatic origin are ubiquitous in erupted products from a wide range of volcanic systems (e.g., Hawai'i, Iceland, Andes). Investigation of these features at KÄ«lauea Volcano, Hawai'i, using an integrative crystallographic and chemical approach places quantitative constraints on mush pile thicknesses. Electron backscatter diffraction (EBSD) reveals that the microstructural features of distorted olivines, whose chemical composition is distinct from undistorted olivines, are remarkably similar to olivines within deformed mantle peridotites, but inconsistent with an origin from dendritic growth. This, alongside the spatial distribution of distorted grains and the absence of adcumulate textures, suggests that olivines were deformed within melt-rich mush piles accumulating within the summit reservoir. Quantitative analysis of subgrain geometry reveals that olivines experienced differential stresses of ā¼3-12āMPa, consistent with their storage in mush piles with thicknesses of a few hundred metres. Overall, our microstructural analysis of erupted crystals provides novel insights into mush-rich magmatic systems
A Statistical description of concurrent mixing and crystallisation during MORB diļ¬erentiation: Implications for trace element enrichment
The pattern of trace element enrichment and variability found in diļ¬erentiated suites of basalts is a sim-
ple observable, which nonetheless records a wealth of information on processes occurring from the mantle
to crustal magma chambers. The incompatible element contents of some mid-ocean ridge basalt (MORB)
sample suites show progressive enrichment beyond the predictions of simple models of fractional crystalli-
sation of a single primary melt. Explanations for this over-enrichment have focused on the diļ¬erentiation
processes in crustal magma chambers. In this paper we consider an additional mechanism, and focus instead
on the deviation from simple fractionation trends that is possible by mixing of diverse mantle-derived melts
supplied to magma chambers. A primary observation motivating this strategy is that there is signiļ¬cant
chemical diversity in primitive high MgO basalts, which single liquid parent models cannot match. Models
were developed to simulate the compositional eļ¬ects of concurrent mixing and crystallisation (CMC): diverse
parental melts were allowed to mix, with a likelihood that is proportional to the extent of fractional crys-
tallisation. Using a simple statistical model to explore the eļ¬ects of concurrent mixing and crystallisation on
apparent liquid lines of descent, we show how signiļ¬cant departure from Rayleigh fractionation is possible
as a function of the diversity of trace elements in the incoming melts, their primary MgO, and the relative
proportion of enriched to depleted melts.
The model was used to make predictions of gradients of trace element enrichment in log[trace element]ā
MgO space. These predictions were compared with observations from a compilation of global MORB and
provide a test of the applicability of CMC to natural systems. We ļ¬nd that by considering the trace element
variability of primitive MORB, its MgO content and degree of enrichment, CMC accurately predicts the
pattern of trace element over-enrichment seen in global MORB. Importantly, this model shows that the
relationship between over-enrichment and incompatibility can derive from mantle processes: the fact that
during mantle melting maximum variability is generated in those elements with the smallest bulk K d .
Magma chamber processes are therefore ļ¬ltering the signal of mantle-derived chemical diversity to produce
trace element over-enrichment during diļ¬erentiation.
Finally, we interrogate the global MORB dataset for evidence that trace element over-enrichment varies
as a function of melt supply. There is no correlation between over-enrichment and melt supply in the
global dataset. Trace element over-enrichment occurs at slow-spreading ridges where extensive steady-state
axial magma chambers, the most likely environment for repeated episodes of replenishment, tapping and
crystallisation, are very rarely detected. This supports a model whereby trace element over-enrichment is
an inevitable consequence of chemically heterogeneous melts delivered from the mantle, a process that may
operate across all rates of melt supply
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