Trace Element Equilibrium Research Articles

Rapid pre-eruptive mush reorganisation and atmospheric volatile emissions from the 12.9 ka Laacher See eruption, determined using apatite

Magma is commonly thought to be stored as a crystal-rich mush within vertically extensive, crustal storage regions. A key unknown is how to remobilise and erupt such crystal-rich material, and whether the growth of gas bubbles within the mush could promote remobilisation. In order to investigate this, we need improved constraints on the timing of volatile saturation in magmas. The mineral apatite represents a potentially useful record of pre-eruptive magmatic volatiles, but data interpretation is complex because exchange reactions control the volatile partitioning. Model solutions are therefore non-unique. Here, we present a numerical forward modelling program with a sensitivity analysis function, which addresses non-uniqueness by identifying alternative sets of starting parameters that match a target compositional trend through a population of apatite crystals. The model is applied to a new dataset of volatiles in apatite from the 12.9 ka Laacher See eruption, Eifel volcanic region, Germany. The results indicate that the magma was initially strongly volatile-undersaturated and became saturated through progressive crystal fractionation. Apatite crystals are not in volatile or trace element equilibrium with their carrier melts, indicating dispersal of crystals into different chemical environments. Consideration of apatite diffusivities suggests that this reorganisation occurred shortly before eruption. Our modelling results also allow us to constrain directly the amount of pre-eruptive magmatic vapour emitted during the explosive eruption, highlighting the importance of considering the behaviour of halogens during magma storage. Overall, our approach confirms the value of measuring apatite volatile contents and highlights the potential of this method to provide quantitative constraints on magmatic evolution and storage conditions.

Cr-poor and Cr-rich clinopyroxene and garnet megacrysts from southern African Group 1 and Group 2 kimberlites: Clues to megacryst origins and their relationship to kimberlites

Controversies surround the origin of kimberlite megacrysts, including whether and how they are genetically related to their host kimberlites, the relationship between the Cr-poor and Cr-rich suites and the dominant processes responsible for elemental and isotopic variations of megacrysts from a given kimberlite. We present new in-situ major and trace element and Sr isotopic results for clinoyroxene and garnet megacrysts from four southern African kimberlites: Colossus and Orapa (Group 1 kimberlites on the Zimbabwe craton), and Kalkput and Bellsbank (Group 2 kimberlites on the western Kaapvaal craton), that include both Cr-poor and Cr-rich megacryst varieties. Cr-poor megacrysts are present at Colossus, Orapa and Kalkput and the data exhibit tight, well-defined trends on major element diagrams as well as incompatible and rare earth element abundances similar to those previously reported for Cr-poor megacrysts. Cr-rich megacrysts, which are also present at Orapa and are the only variety present at Bellsbank, generally have higher Mg# values, lack well-defined major element trends and show stronger incompatible element enrichments as well as more radiogenic 87Sr/86Sri ratios than Cr-poor megacrysts from the same kimberlite group. Thermobarometry indicates that the Cr-poor megacrysts equilibrated at temperatures of ≈1200 to 1450 °C and pressures of 4.5 to 7.5 GPa. Cr-rich megacrysts, in contrast, extend to temperatures and pressures as low as 700 °C and 3 GPa, respectively. This indicates that, in the studied suites, Cr-poor megacrysts equilibrated at high temperatures in the lower lithosphere (>135 km), whereas Cr-rich megacrysts typically equilibrated at lower temperatures and pressures. Within the Cr-poor megacrysts from Group 1 and Group 2 kimberlites, there is a clear correspondence between kimberlite group, diagnostic incompatible element ratios (e.g., Nb/La) and Sr isotope ratios that parallel the differences noted between whole-rock Group 1 and Group 2 kimberlites. In the case of Cr-poor megacrysts, similar calculated melt compositions in equilibrium with garnet and clinopyroxene from the same kimberlite were obtained using recent high-pressure mineral‑carbonated melt partition coefficients. This suggests formation in conditions close to trace element equilibrium, and is consistent with crystallization from primitive melts with kimberlite-like trace element compositions. In the case of Cr-rich megacrysts, differences in the compositions of melts in equilibrium with clinopyroxene and garnet tend to be larger, and melts in equilibrium with Cr-rich clinopyroxene tend to show significantly greater incompatible element enrichments than those of estimated near-primary kimberlite melts. This could be due to the different behaviour of clinopyroxene and garnet during metasomatic melt-rock interaction, but the apparent disequilibrium between clinopyroxene and garnet could also be due to some of the Cr-rich megacrysts actually being peridotitic xenocrysts. We propose a model for the origin of southern African megacrysts in which carbonated protokimberlite melts formed stockwork-like bodies of variable size in the deep lithosphere (>130 km), which fed networks of melt-filled veins extending into the surrounding and overlying mantle. Crystallization of larger melt bodies resulted in megacryst assemblages dominated by Cr-poor megacrysts, and the incompatible element and isotopic characteristics of these dominantly reflect those of the protokimberlite melt. In contrast, crystallization of smaller melt bodies and their vein networks resulted in megacryst assemblages dominated by Cr-rich megacrysts, which formed as a result of extensive assimilation and metasomatic melt-rock interaction between protokimberlite and peridotite wallrock at low melt/rock ratios, particularly in the middle to shallow lithosphere where pre-existing potassic metasomatic heterogeneities are prevalent. The Cr-rich nature and enrichments in incompatible elements and radiogenic Sr in the Cr-rich megacrysts reflect extensive interaction of their parental magmas with this metasomatized peridotite.

Zircon U–Pb dating of eclogite from the Qiangtang terrane, north-central Tibet: a case of metamorphic zircon with magmatic geochemical features

Zircon is probably the most important mineral used in the dating formation of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks. The origin of zircon, i.e., magmatic or metamorphic, is commonly assessed by its external morphology, internal structure, mineral inclusions, Th/U ratios and trace element composition. In this study, we present an unusual case of metamorphic zircon from the Qiangtang eclogite, north-central Tibet. The zircon grains contain numerous eclogite-facies mineral inclusions, including omphacite, phengite, garnet and rutile; hence, they are clearly of metamorphic origin. However, they display features similar to common magmatic zircon, including euhedral crystal habit, high Th/U ratios and enriched heavy rare earth elements pattern. We suggest that these zircon grains formed from a different reservoir from that for garnet where no trace elements was present and trace element equilibrium between zircon and garnet was achieved. U–Pb dating of zircon gave an age of 232–237 Ma for the eclogite, and that of rutile yielded a slightly younger age of ca. 217 Ma. These ages are consistent with the reported Lu–Hf mineral isochron and phengite Ar–Ar ages. The zircon U–Pb and mineral Lu–Hf isochron ages are interpreted as the time of the peak eclogite-facies metamorphism, whereas the rutile U–Pb and phengite Ar–Ar ages represent the time of exhumation to the middle crust. Thus, the distinction between metamorphic and magmatic zircons cannot be made using only Th/U ratios and heavy REE compositions for HP–UHP metamorphic rocks of oceanic derivation.

Diffusion-induced fractionation of niobium and tantalum during continental crust formation

Differentiation of the Earth into its major spheres – crust, mantle and core – has proceeded dominantly through magmatic processes involving melting and melt separation. Models that describe these differentiation processes are guided by elemental abundances in the different reservoirs. Elements are fractionated between coexisting phases during partial melting, and geochemical models are generally based on the fundamental assumption that trace-element equilibrium is established between the partial melts and the restitic minerals. The element pair niobium and tantalum is key to the distinction of different melting regimes involved in crustal differentiation, but equilibrium partition models have largely failed to reproduce the Nb/Ta patterns observed in nature, posing a long-standing geochemical conundrum. Here we demonstrate that kinetic fractionation of Nb and Ta by diffusion may have produced the low Nb/Ta observed in the continental crust. On the basis of the diffusivities of Nb and Ta in rutile (TiO2) determined experimentally in this study, we conclude that equilibrium cannot be expected for the natural range of grain sizes, temperatures and time scales involved in partial melting of crustal rocks. Instead, the observed fractionation of the geochemical twins, Nb and Ta, in the silicate Earth most likely proceeds by partial – as opposed to complete – equilibration of rutile and melt. Hence, the assumption of bulk equilibrium during partial melting for the processes of crustal differentiation may not be justified, as is demonstrated here for Nb/Ta. The concept presented here is based on kinetic fractionation melting and explains the observed low Nb/Ta ratio of the continental crust.

Large-scale liquid immiscibility at the top of the Bushveld Complex, South Africa

Detailed study of the top 625 m of the Bushveld Complex (South Africa) suggests that the magma developed a siliceous cap generated by liquid immiscibility. The lower ∼300 m of this section, which consists of magnetite-bearing ferrodiorite with several magnetite-rich layers, contains cumulus apatite with relatively low rare earth element (REE) concentrations and no chondrite-normalized Eu anomaly. In contrast, apatite in the overlying 325 m thick portion of the section has three times higher REE concentrations and a negative Eu anomaly, and the cumulate rocks contain cumulus orthoclase and quartz. The contact between the two parts of the section occurs over a limited stratigraphic range (<50 m). Large-scale silicate liquid immiscibility resulting in the physical separation of a dense, Fe-rich magma to form the lower part of the sequence and a buoyant Si-rich magma to form the upper part of the sequence is best able to explain the observations. In addition, the phases in the upper siliceous cap of the Bushveld Complex are in major and trace element equilibrium with portions of the overlying Rooiberg rhyolites and/or Rashoop granophyres, consistent with these rocks having originated as residual liquids after extensive fractional crystallization of the Bushveld magmas.

A Late Oligocene Suprasubduction Setting in the Westernmost Mediterranean Revealed by Intrusive Pyroxenite Dikes in the Ronda Peridotite (Southern Spain)

AbstractContrasting tectonic reconstructions of the westernmost Mediterranean have been proposed to explain the origin of the Alboran marine basin contemporaneously with Cenozoic convergence between the African and European plates. Cr-rich pyroxenites in the Ronda massif record the geochemical processes occurring in the subcontinental mantle of the Alboran domain in the Late Oligocene, thus constraining the geodynamic scenario of Cenozoic extension in the western Mediterranean lithosphere. Clinopyroxene in intrusive Cr-rich websterite dikes crosscutting the Ronda peridotite is strongly depleted in Nb-Ta and enriched in light rare earth elements, as typically observed in arc magmas, and is in trace element equilibrium with Neogene subduction-related lavas from the western and central Mediterranean. Sr-Nd-Pb radiogenic isotopes indicate that the mantle source of the Ronda pyroxenite dikes was contaminated by a subduction component released by detrital sediments likely deposited in passive continental margin...

Selective entrainment of peritectic garnet into S-type granitic magmas: Evidence from Archaean mid-crustal anatectites

Entrainment of restite is commonly invoked to explain both the origin of relatively mafic granites and granodiorites, as well as the chemical connection between granite magmas and their sources. This concept has become linked to models for magma migration out of the source, as restite entrainment is considered to take place when diatexitic sources mobilise en masse. This is at odds with the common occurrence of relatively mafic granites as high level intrusions in the crust or their eruptive equivalents that must have formed from markedly water-undersaturated magmas that ascended through narrow conduits. We investigate pelitic migmatites from the Mkhondo Valley Metamorphic Suite (MVMS) in Swaziland, where a mid-crustal heating event produced metatexitic migmatites with minimal post-anatectic recrystallisation. In these rocks all the garnet is peritectic, having arisen through biotite fluid-absent melting, which produced garnet poikiloblasts characterised by inclusions of melt, quartz and biotite. Leucosomes that represent sites of melt transfer carry similar, smaller (typically < 1 mm), entrained garnet poikiloblasts, which were capable of amalgamating to form larger composite grains. In anatectic structures where melt was present for longer, entrained garnet was extensively recrystallised, via a dissolution-precipitation process, to adopt a more magmatic character. The peritectic garnet in the pelitic source appears to have grown out of equilibrium with feldspar and HREE-rich accessory phases, while the recrystallised garnet in the larger melt-filled structures became progressively better equilibrated with these minerals. Thus, peritectic garnet in the source grew sufficiently rapidly to prevent trace element equilibrium with the bulk-rock composition, and, concurrent rapid magma segregation prevented the development of diatexitic source conditions. The segregated magma consisted of melt, the peritectic assemblage (principally garnet) and the accessory minerals monazite and zircon. These rocks illustrate that mafic granites may arise purely as mixtures of melt and the peritectic assemblage produced by the incongruent melting reaction. Importantly, under the circumstances which produced the MVMS anatectites, peritectic garnet is entrained as < 1 mm poikiloblasts, demonstrating how mafic granitic magmas can migrate out of the source without the source becoming diatexitic.

A Felsic End to Bushveld Differentiation

The similarities in age (c. 2·06 Ga) and isotopic composition of the Bushveld Igneous Complex and the overlying Rooiberg Group felsic volcanic rocks suggest the possibility that the two are cogenetic.To investigate this possibility we derive a robust new estimate of the major and trace element bulk composition of the Bushveld Upper Zone and Upper Main Zone (UUMZ). Using MELTS thermodynamic modeling, we show that this composition fails to reproduce fundamental features of the cumulate sequence, such as the presence of primary orthopyroxene at the base of the magma column.We investigate the possibility that some amount of evolved magma escaped from the magma chamber and derive a new estimate of the UUMZ parent magma composition that reproduces generally the observed cumulate sequence. Our results constrain the amount and composition of the escaped magma and provide evidence of a genetic link between the UUMZ of the Bushveld layered series and the felsic volcanic rocks immediately overlying it. We conclude that between 15 and 25% of the original magma volume was expelled and is manifest either as part of the upper Rooiberg Group lava sequence or the Rashoop Granophyre (distinction is hindered by their almost identical major and trace element compositions). A parent magma in which 15^25% Rooiberg or Rashoop material is added back to the extant cumulate sequence is initially saturated in orthopyroxene and is in major and trace element equilibrium with the phases at the base of the UUMZ.

An alternative model for silica enrichment in the Kaapvaal subcontinental lithospheric mantle

The Mg- and Si-rich nature of the sub-cratonic lithospheric mantle (SCLM) beneath the Kaapvaal Craton indicates extensive melt depletion, followed by a Si-enrichment process. Six highly silica enriched peridotites from Kimberley containing high amounts of orthopyroxene (Opx) or garnet (Grt) that are locally concentrated in clots, were investigated to constrain the timing and nature of the Si-enrichment process. A clinopyroxene-bearing lherzolite containing an Opx-clot was studied to quantify the effects of recent metasomatism on the Si-enriched samples. Minerals from the lherzolite, together with Opx from harzburgites and Opx- and Grt-clots have Hf–Nd isotope ratios at the time of kimberlite eruption, 90 Ma, comparable to group I kimberlites and are close to trace element equilibrium with kimberlitic melts. This implies the xenoliths underwent major interaction with kimberlitic melts close to the time of kimberlite eruption. Harzburgites and mineral clots record equilibration pressures and temperatures of, respectively, between 3.5–4.3 GPa and 930–1060 °C. The garnets in Opx-clots have low Lu/Hf and ε Hf(t) −15, whereas garnets from Grt-clots have high Lu/Hf and ε Hf(t) +10. In contrast, Grt from both Grt- and Opx-clots have low Sm/Nd and ε Nd −10. The whole rock platinum group element (PGE) concentrations are an order of magnitude higher in the Grt-clot than the Opx-clot. Measured 187Os/ 188Os range from 0.1085 to 0.1222. The Grt-clot bearing sample yields Nd–Hf–Os isotope model ages that suggest formation in the Neoproterozoic (∼650 Ma). In contrast, an Opx-clot yields T RD ages of 2.8 Ga, which is interpreted as the time of formation of the host harzburgite. The Opx-clots and host harzburgites have comparable Lu–Hf isotope systematics that imply Opx growth at ∼1.3 Ga and hence their formation is not related to the Grt-clots. Garnets from Opx- and Grt-clots have elevated high-field strength element (HFSE) concentrations, and lack HFSE depletion relative to other trace elements with comparable degrees of incompatibility in the mantle (La/Nb < 0.5). In addition, calculated melts in equilibrium with Grt have strongly fractionated REE (Nd/Yb > 300) and HREE depletion (Yb N < 0.1) suggesting equilibration with a hydrous melt that is more HREE depleted than a kimberlitic melt. Previous models that related Si-enrichment to subduction are inconsistent with the lack of HFSE depletion (La/Nb < 0.5). Therefore the favoured model for Opx- and Grt-clot formation is infiltration of a hydrous melt in a within plate geodynamical environment associated with volcanism in the Mid-proterozoic and Neoproterozoic, respectively. This implies that Si-enrichment of the Kaapvaal SCLM may be a consequence of numerous localised magmatic events rather than a single craton-wide process.

Redistribution of trace elements during prograde metamorphism from lawsonite blueschist to eclogite facies; implications for deep subduction-zone processes

The transfer of fluid and elements from subducting crust to the overlying mantle wedge is a fundamental process affecting arc magmatism and the chemical differentiation of the Earth. While the production of fluid by breakdown of hydrous minerals is well understood, the liberation of trace elements remains generally unconstrained. In this paper, we evaluate the behaviour of trace elements during prograde metamorphism and dehydration using samples of high-pressure, low-temperature metamorphic rocks from New Caledonia. Samples examined include mafic and pelitic rock-types that range in grade from lawsonite blueschist to eclogite facies, and represent typical lithologies of subducting crust. Under lawsonite blueschist facies conditions, the low temperatures of metamorphism inhibit equilibrium partitioning between metamorphic minerals and allow for the persistence of igneous and detrital minerals. Despite this, the most important hosts for trace-elements include lawsonite, (REE, Pb, Sr), titanite (REE, Nb, Ta), allanite (LREE, U, Th), phengite (LILE) and zircon (Zr, Hf). At epidote blueschist to eclogite facies conditions, trace-element equilibrium may be attained and epidote (REE, Sr, Th, U, Pb), garnet (HREE), rutile (Nb, Ta), phengite (LILE) and zircon (Zr, Hf) are the major trace-element hosts. Chlorite, albite, amphibole and omphacite contain very low concentrations of the investigated trace elements. The comparison of mineral trace-element data and bulk-rock data at different metamorphic grades indicates that trace elements are not liberated in significant quantities by prograde metamorphism up to eclogite facies. Combining our mineral trace-element data with established phase equilibria, we show that the trace elements considered are retained by newly-formed major and accessory minerals during mineral breakdown reactions to depths of up to 150 km. In contrast, significant volumes of fluid are released by dehydration reactions. Therefore, there is a decoupling of fluid release and trace element release in subducting slabs. We suggest that the flux of trace elements from the slab is not simply linked to mineral breakdown, but results from complex fluid-rock interactions and fluid-assisted partial melting in the slab.

Petrology and geochemistry of the lower ocean crust formed at the East Pacific Rise and exposed at Hess Deep: A synthesis and new results

The geochemistry and petrology of the lower oceanic crust record information about the compositions of melts extracted from the mantle, how these melts mix and crystallize, and the role of hydrothermal circulation in this portion of the crust. Unfortunately, lower oceanic crust formed at fast spreading ridges is rarely exposed at the seafloor making it difficult to study these processes. At Hess Deep, crust formed at the East Pacific Rise (EPR) is exposed due to the propagation of the Cocos‐Nazca spreading center westward. Here we review our state of knowledge of the petrology of lower crustal material from Hess Deep and document new mineral major and trace element compositions, amphibole‐plagioclase thermometry, and plagioclase crystal size distributions. Samples from the deeper parts of the gabbroic sequence contain clinopyroxene that is close to being in trace element equilibrium with erupted basalts but which can contain primitive (moderate Cr, high Mg#) orthopyroxene and very calcic plagioclase. Because primitive mid‐ocean ridge basalts (MORBs) are not saturated with orthopyroxene or very calcic plagioclase this suggests that melts added to the crust have variable compositions and that some may be in major but not trace element equilibrium with shallow depleted mantle. These apparently conflicting data are most readily explained if some of the melt extracted from the mantle is fully aggregated within the mantle but reacts with the shallow mantle during melt extraction. The occurrence of cumulates with these characteristics suggests that melts added to the crust do not all get mixed with normal MORB in the axial magma chamber (AMC), but rather that some melts partially crystallize in isolation within the lower crust. However, evidence that primitive melts fed the AMC, along with steep fabrics in shallow gabbros (from near the base of the dyke complex), provides support for models in which crystallization within the AMC followed by crystal subsidence is also an important process in lower crustal accretion. More evolved bulk compositions of gabbros from the upper than lower parts of the plutonic section are due to greater amounts of reaction with interstitial melt and not because their parental melt had become highly fractionated through the formation of large volumes of cumulates deeper in the crust. Amphibole‐plagioclase thermometry confirms previous reports that the initial ingress of fluid occurs at high‐temperatures in the shallow gabbros (Tave 713°C) and show that the temperature of amphibole formation was similar in deeper gabbros (Tave 722°C). This thermometry also suggests that fracture and grain boundary permeability for seawater‐derived fluids was open over the same temperature interval.

Formation of Wollastonite by Chemically Reactive Fluid Flow During Contact Metamorphism, Mt. Morrison Pendant, Sierra Nevada, California, USA

Quartz–calcite sandstones experienced the reaction calcite + INTRODUCTION quartz = wollastonite + CO2 during prograde contact metaFlow of chemically reactive fluid may control the minmorphism at P = 1500 bars and T = 560°C. Rocks were in eralogy, stable isotope composition, and trace element equilibrium during reaction with a CO2–H2O fluid with XCO2 = chemistry of rocks during contact metamorphism (e.g. 0·14. The transition from calcite-bearing, wollastonite-free to Labotka et al., 1988; Jamtveit et al., 1992a, 1992b; Nabelek wollastonite-bearing, calcite-free rocks across the wollastonite isograd & Labotka, 1993; Bowman et al., 1994; Roselle et al., is only several millimeters wide. The wollastonite-forming reaction 1999). A number of aspects of metamorphic fluid flow, was driven by infiltration of quartz–calcite sandstone by chemically however, remain the subject of debate, including the reactive H2O-rich fluids, and the distribution of wollastonite directly direction of flow (Labotka et al., 1988; Ferry & Dipple, images the flow paths of reactive fluids during metamorphism. The 1992; Ferry, 1995a; Hanson, 1995a), whether numerical mapped distribution of wollastonite and modeling of an O-isotope models that assume uniform rock properties adequately profile across a lithologic contact indicate that the principal direction predict the geometry of fluid flow in aureoles that contain of flow was layer-parallel, directed upward, with any cross-layer a variety of anisotropic structural features (Hanson, 1992, component of flow <0·1% of the layer-parallel component. Fluid 1995b; Cook et al., 1997), and the degree to which local flow was channeled at a scale of 1–100 m by pre-metamorphic mineral–fluid equilibrium is attained (Lasaga & Rye, dikes, thrust and strike-slip faults, fold hinges, bedding, and 1993; Bolton et al., 1999; Lasaga et al., 2000). These and stratigraphic contacts. Limits on the amount of fluid, based on related questions were addressed in an investigation of minimum and maximum estimates for the displacement of the contact metamorphism in the Mt. Morrison pendant, wollastonite reaction front from the fluid source, are (0·7–1·9) × eastern Sierra Nevada, California. The location is an 10 cm fluid/cm rock. The sharpness of the wollastonite isograd, exceptional site for the study of metamorphic fluid flow the consistency of mineral thermobarometry, the uniform measured for several reasons. First, the Mt. Morrison Sandstone, O–O fractionations between quartz and calcite, and model the focus of this study, is ideal for application of concalculations all argue for a close approach to local mineral–fluid tinuum models of coupled fluid flow and reaction because equilibrium during the wollastonite-forming reaction. it is unusually homogeneous in mineralogy and texture.

Petrogenesis of the crust‐mantle transition zone and the origin of lower crustal wehrlite in the Oman ophiolite

We studied trace element geochemistry and petrology of the crust‐mantle transition zone (MTZ) in the Samail massif of the Oman ophiolite to constrain the location where different primitive magmas mix beneath an oceanic spreading ridge. The MTZ is the deepest location where crystallization took place and thus is an ideal place to determine the compositional diversity of melts leaving the mantle, with various sources and degrees of depletion. We have reached three main conclusions: (1) More than 90% of our samples record equilibration with compositionally indistinguishable parental melts, similar to mid‐ocean ridge basalts (MORB) and the melts that formed the crust in Oman. This suggests that mixing of diverse, polybaric partial melts of mantle peridotite occurred at or below the depth of the MTZ. The presence of distinct heterogeneity in less than 10% of our samples is similar to the nature and frequency of heterogeneity observed in melt inclusions in olivines from MORB. (2) Among the samples recording trace element equilibrium with MORB‐like liquids are wehrlitic rocks, previously suggested to be cumulates from an unusual parental melt on the basis of petrological observation. (3) Systematics of Eu distribution among plagioclase and clinopyroxene in “impregnated peridotites” demonstrate that these minerals did not crystallize from “trapped melt.” As a consequence, it is not possible to use the modal proportion or texture of plagioclase + clinopyroxene impregnations to estimate the instantaneous melt porosity or the shape of melt pores at any time during the formation of these rocks.