Xenocryst assimilation and formation of peritectic crystals during magma contamination: An experimental study

International audienceContamination of magmas by country rocks may contribute xenoliths and xenocrysts to the magma, but also melt and peritectic crystals that form through incongruent melting or dissolution of the original contaminants. Identifying contaminant-derived peritectic crystals and former melt components in igneous rocks is particularly challenging, but also particularly important, because their assimilation significantly affects melt composition and magma temperature. To facilitate the identification of peritectic crystals in igneous rocks, the aim of this study was to experimentally control partial assimilation of xenocrysts and examine the formation, textures, and composition of resulting peritectic crystals. Our experiments mimic contaminant melting and contamination of a partially crystallized basaltic andesite by melanorite- and monzodiorite-derived xenocrysts and micro-xenoliths. Micro-xenoliths and xenocrysts partially survive assimilation, and yet peritectic crystals form ~ 1/3 of all solid contaminants. Anhydrous xenocrysts either develop laterally continuous, subhedral to euhedral, and inclusion-poor overgrowths, or progressively decompose. Hydrous and partially altered xenocrysts decompose to peritectic crystals. The peritectic crystals form clusters of subhedral to euhedral, randomly-oriented olivine, clinopyroxene, olivine–plagioclase, and olivine–plagioclase–clinopyroxene that texturally resemble primary magmatic crystals. We propose that natural peritectic crystals with short residence times form clusters of one or more minerals with textures as those of our experiments, and that peritectic crystals with longer residence times likely anneal to subhedral or euhedral single crystals or coarse-grained mineral clusters. They hold crucial evidence for largely assimilated country-rock components and estimates of open-system magma evolution, but the longer their magma residence time the more easily they are overlooked

Textures of Peritectic Crystals as Guides to Reactive Minerals in Magmatic Systems: New Insights from Melting Experiments

International audiencePeritectic crystals in igneous rocks may be derived from either the source or country rocks, or may have formed by reactive assimilation of source-inherited solids, primary magmatic minerals during self- or magma mixing, or country-rock xenoliths or xenocrysts. Identifying such peritectic crystals is important for constraining the components and textures of igneous rocks and the underlying processes of magmatic evolution. In this study we demonstrate that peritectic olivine formed in melting experiments crystallizes as clusters of euhedral to subhedral crystals. Olivine replacing orthopyroxene, amphibole, and phlogopite forms crystal clusters with distinct crystal to melt ratios, 2D surface area, grain boundary segmentation, and inclusion relations. In our experiments the textures of peritectic crystals are primarily controlled by the stability temperature and decomposition rate of reactive minerals. High-temperature minerals such as orthopyroxene slowly decompose to form high-density clusters of large crystals with long grain boundary segments. The SiO2-rich peritectic melt produced favours formation of melt inclusions. Low-temperature minerals such as amphibole and phlogopite rapidly decompose to form low-density clusters of small crystals with short grain boundary segments. The relatively SiO2-poor peritectic melt produced results in the formation of fewer melt inclusions. Host melt composition has a minor effect on the textures of peritectic olivine formed in the melting experiments of this study and previous contamination experiments, but affects the assemblages of the peritectic crystal clusters. Cluster density and 2D surface area of peritectic olivine tend to decrease, whereas grain boundary segment length increases with increasing experimental temperature and H2O content. Using textural criteria that distinguish olivine formed after different minerals in our melting experiments, we hypothesize that two olivine populations from a basaltic-andesitic lava flow of the Tatara-San Pedro volcanic complex, Chile, may be peritectic crystals formed after amphibole and orthopyroxene. Both amphibole and orthopyroxene are common in xenoliths preserved in some Tatara-San Pedro lava flows. One notable difference between the experimental and natural olivine crystals is that the natural olivine crystals have 2D surface areas and 2D grain boundary segments up to ∼1000 and ∼100 times larger, respectively, than those produced in our experiments. We propose that this size difference is primarily controlled by comparatively slow heating and decomposition of reactive crystals and textural coarsening of peritectic crystals during prolonged magma residence in the natural system

The Relation Between the Surface Brightness and the Diameter for Galactic Supernova Remnants

In this work, we have constructed a relation between the surface brightness ($\Sigma$) and diameter (D) of Galactic C- and S-type supernova remnants (SNRs). In order to calibrate the $\Sigma$-D dependence, we have carefully examined some intrinsic (e.g. explosion energy) and extrinsic (e.g. density of the ambient medium) properties of the remnants and, taking into account also the distance values given in the literature, we have adopted distances for some of the SNRs which have relatively more reliable distance values. These calibrator SNRs are all C- and S-type SNRs, i.e. F-type SNRs (and S-type SNR Cas A which has an exceptionally high surface brightness) are excluded. The Sigma-D relation has 2 slopes with a turning point at D=36.5 pc: $\Sigma$(at 1 GHz)=8.4$^{+19.5}_{-6.3}$$\times10^{-12}$ D$^{{-5.99}^{+0.38}_{-0.33}}$ Wm$^{-2}$Hz$^{-1}$ster$^{-1}$ (for $\Sigma$$\le3.7\times10^{-21}$ Wm$^{-2}$Hz$^{-1}$ster$^{-1}$ and D$\ge$36.5 pc) and $\Sigma$(at 1 GHz)=2.7$^{+2.1}_{-1.4}$$\times$ 10$^{-17}$ D$^{{-2.47}^{+0.20}_{-0.16}}$ Wm$^{-2}$Hz$^{-1}$ster$^{-1}$ (for $\Sigma$$>3.7\times10^{-21}$ Wm$^{-2}$Hz$^{-1}$ster$^{-1}$ and D$<$36.5 pc). We discussed the theoretical basis for the $\Sigma$-D dependence and particularly the reasons for the change in slope of the relation were stated. Added to this, we have shown the dependence between the radio luminosity and the diameter which seems to have a slope close to zero up to about D=36.5 pc. We have also adopted distance and diameter values for all of the observed Galactic SNRs by examining all the available distance values presented in the literature together with the distances found from our $\Sigma$-D relation.Comment: 45 pages, 2 figures, accepted for publication in Astronomical and Astrophysical Transaction

Initial spin periods of neutron stars in supernova remnants

We present estimates of initial spin periods, $P_0$, for radio pulsars associated with supernova remnants. By using the published data on 30 objects, we were able to derive a reliable estimate for the initial spin period, assuming standard magneto-dipole spin-down (braking index n=3), in many cases. Our set of estimates is still not sufficient to infer the exact shape of the initial period distribution. However, we show that a gaussian distribution with mean and deviation $\sim 0.1$ s is consistent with our results, while flat, wide distributions and very narrow ones are disfavored.Comment: 28 pages, subm. to Ap&S