Bernard Charlier

Large-scale silicate liquid immiscibility during differentiation of tholeiitic basalt to granite and the origin of the Daly gap

The dearth of intermediate magmatic compositions at the Earths surface, referred to as the Daly gap, remains a major issue in igneous petrology. The initially favored explanation invoking silicate liquid immiscibility during evolution of basalt to rhyolite has lost support because of the absence of any firm geological evidence for separation of Fe- and Si-rich liquids in igneous rocks. This work presents a record of large-scale magmatic differentiation due to immiscibility in the tholeiitic Sept Iles intrusion (Canada), one of the largest layered plutonic bodies on Earth. Gabbroic cumulate rocks from the Critical Zone of this intrusion show a bimodal distribution in density and P2O5 content, despite identical major element chemistry of the principal magmatic phases. Immiscibility is supported by the presence of contrasting Fe-rich and Si-rich melt inclusions trapped in cumulus apatite. Phase diagrams and well-documented occurrences of small-scale immiscibility confirm that liquid-liquid unmixing and the separation of Fe-rich and Si-rich liquids may contribute significantly to the Daly gap along the tholeiitic liquid line of descent.

Efficiency of compaction and compositional convection during mafic crystal mush solidification: the Sept Iles layered intrusion, Canada

Adcumulate formation in mafic layered intrusions is attributed either to gravity-driven compaction, which expels the intercumulus melt out of the crystal matrix, or to compositional convection, which maintains the intercumulus liquid at a constant composition through liquid exchange with the main magma body. These processes are length-scale and time-scale dependent, and application of experimentally derived theoretical formulations to magma chambers is not straightforward. New data from the Sept Iles layered intrusion are presented and constrain the relative efficiency of these processes during solidification of the mafic crystal mush. Troctolites with meso- to ortho-cumulate texture are stratigraphically followed by Fe–Ti oxide-bearing gabbros with adcumulate texture. Calculations of intercumulus liquid fractions based on whole-rock P, Zr, V and Cr contents and detailed plagioclase compositional profiles show that both compaction and compositional convection operate, but their efficiency changes with liquid differentiation. Before saturation of Fe–Ti oxides in the intercumulus liquid, convection is not active due to the stable liquid density distribution within the crystal mush. At this stage, compaction and minor intercumulus liquid crystallization reduce the porosity to 30%. The velocity of liquid expulsion is then too slow compared with the rate of crystal accumulation. Compositional convection starts at Fe–Ti oxide-saturation in the pore melt due to its decreasing density. This process occurs together with crystallization of the intercumulus melt until the residual porosity is less than 10%. Compositional convection is evidenced by external plagioclase rims buffered at An61 owing to continuous exchange between the intercumulus melt and the main liquid body. The change from a channel flow regime that dominates in troctolites to a porous flow regime in gabbros results from the increasing efficiency of compaction with differentiation due to higher density contrast between the cumulus crystal matrix and the equilibrium melts and to the bottom-up decreasing rate of crystal accumulation in the magma chamber.

Magma chamber-scale liquid immiscibility in the Siberian Traps represented by melt pools in native iron

Magma unmixing (i.e., separation of a homogeneous silicate melt into two or more liquids) is responsible for sudden changes in the evolution of common melts, element fractionation, and potential formation of orthomagmatic ore deposits. Although immiscible phases are a common phenomenon in the mesostasis of many tholeiitic basalts, evidence of unmixing in intrusive rocks is more difficult to record because of the transient nature of immiscibility during decompression, cooling, and crystallization. In this paper, we document a clear case of liquid immiscibility in an intrusive body of tholeiitic gabbro in the Siberian large igneous province, using textures and compositions of millimeter-sized silicate melt pools in native iron. The native iron crystallized from a metallic iron liquid, which originated as disseminated globules during reduction of the basaltic magma upon interaction with coal-bearing sedimentary rocks in the Siberian craton. The silicate melts entrapped and armored by the native iron are composed of two types of globules that represent the aluminosilicate (60–77 wt% SiO2) and silica-poor, Fe-Ti-Ca-P–rich (in wt%: SiO2, 15–46; FeO, 15–22; TiO2, 2–7; CaO, 11–27; P2O5, 5–30) conjugate liquids. Different proportions and the correlated compositions of these globules in individual melt pools suggest a continuously evolving environment of magmatic immiscibility during magma cooling. These natural immiscible melts correspond extremely well to the conjugate liquids experimentally produced in common basaltic compositions at <1025 °C. Our results show that immiscibility can occur at large scale in magma chambers and can be instrumental in generating felsic magmas and Fe-Ti-Ca-P–rich melts in the continental igneous provinces.

Igneous Layering in Basaltic Magma Chambers

Layering is a common feature in mafic and ultramafic layered intrusions and generally consists of a succession of layers characterized by contrasted mineral modes and/or mineral textures, including grain size and orientation and, locally, changing mineral compositions. The morphology of the layers is commonly planar, but more complicated shapes are observed in some layered intrusions. Layering displays various characteristics in terms of layer thickness, homogeneity, lateral continuity, stratigraphic cyclicity, and the sharpness of their contacts with surrounding layers. It also often has similarities with sedimentary structures such as cross-bedding, trough structures or layer termination. It is now accepted that basaltic magma chambers mostly crystallize in situ in slightly undercooled boundary layers formed at the margins of the chamber. As a consequence, most known existing layering cannot be ascribed to a simple crystal settling process. Based on detailed field relationships, geochemical analyses as well as theoretical and experimental studies, other potential mechanisms have been proposed in the literature to explain the formation of layered igneous rocks. In this study, we review important mechanisms for the formation of layering, which we classify into dynamic and non-dynamic layer-forming processes.

Silicate Liquid Immiscibility in Layered Intrusions

More and more evidence for the development of silicate liquid immiscibility during cooling of magmas in layered intrusions have been presented. Here, we review some theoretical principles with a focus on the separation of two silicate melts, i.e. silica-rich vs. iron-rich. We discuss the role of melt structure and present phase equilibria relevant to stable and metastable immiscibility. The understanding of immiscibility in magmas has strongly benefited from recent progress in experimental approaches. Kinetics studies evidence the importance of nucleation barriers in producing unmixing , coarsening and potential separation of equilibrium melts. Improvement of analytical tools has also enabled detailed study of major and trace element partitioning. The study of immiscible emulsion in volcanic rocks also brings important information on the evolution of plutonic systems and on the potential formation of compositional gap along liquid lines of descent. We then present the most recent evidence for immiscibility in some major layered intrusions, i.e. the Skaergaard, Sept Iles, intrusions of the Emeishan province, and the Bushveld complex. Paired melts are identified as contrasted melt inclusions trapped in apatite and their segregation can be responsible for the formation of Fe–Ti–P-rich rocks. We finally discuss more broadly the potential effect of immiscibility in interstitial melt and the implications on the evolution of the crystal mush.

The thickness of the mushy layer on the floor of the Skaergaard magma chamber at apatite saturation

Natural Environment Research Council Royal Society of London Carlsberg Foundation Danish Council of Independent Research Danish National Research Foundation

Uncommon behavior of plagioclase and the ancient lunar crust

Calcic plagioclase, the dominant mineral of the anorthositic lunar crust, fails to show the Na enrichment during cooling that is typical of magmatic plagioclase. We show that this enigmatic behavior may arise during fractionation of highly calcic plagioclase at depths greater than ~70 km in the lunar magma ocean because of the development of a negative azeotropic configuration at high anorthite contents that impedes and may even reverse the standard plagioclase albite enrichment with dropping temperature. This result supports a high-pressure origin of this plagioclase consistent with the lunar magma ocean model. It also provides a new mechanism for forming lunar lithologies with sodic plagioclase from a highly Na-depleted Moon through gravitational settling of spinel and refines the compositional characteristics of the late stage residual liquids of the lunar magma ocean.

Ti Substitution Mechanisms in Phlogopites from the Suwalki Massif-type Anorthosite, NE Poland

Intercumulus titanian phlogopite occurs in leuco- and gabbro-noritic cumulates from the Suwalki anorthosite massif, NE Poland. The degree of Ti enrichment in the micas ranges from 2.59 to 9.41 wt.% TiO2. The chemical composition is highly variable for several other elements: Al 2O3 (13.07-16.75 wt.%), K2O (7.90-10.16 wt.%), FeOtot (6.92-16.69 wt.%), Fe2O3 (0.82-2.95 wt.%), and MgO (9.86-19.54 wt.%), with a Mg/(Fe + Mg) ratio ranging from 0.47 to 0.83. Substitution mechanisms for Ti are proposed, which suggest the presence of exchange vectors involving octahedral and tetrahedral cations. In samples characterized by low Ti contents (0.147-0.239 Ti a.p.f.u.), the Ti incorporation mechanism is: [6]Ti4+ + [6]□ = 2( [6]Mg2+, [6]Fe2+, [6]Mn2+), where [6]□ corresponds to a vacancy in octahedral coordination (Ti-vacancy). In the two groups with intermediate (0.164-0.326 Ti a.p.f.u.) and high Ti contents (0.477-0.532 Ti a.p.f.u.), the Ti substitution mechanism corresponds to the reaction: [6]Ti4+ + 2([4]A13+, [4]Fe3+) = ([6]Mg2+, [6]Fe2+, [6]Mn2+) + 2 [4]Si4+ (Ti-Tschermak). The Mossbauer spectral investigation shows that 0.046-0.167 a.p.f.u. Fe3+ occur on the octahedral sites of the structure. The substitution mechanism responsible for the incorporation of Fe3+ in phlogopites from Suwalki is 3( [6]Mg2+, [6]Fe2+) = 2( [6]Al3+, [6]Fe3+) + [6](M3+-vacancy). The use of the Ti content of phlogopite as geothermometer reveals crystallization temperatures from 729 ± 15 to 874 ± 15 °C for the phlogopites.

Immiscible hydrous Fe–Ca–P melt and the origin of iron oxide-apatite ore deposits

The origin of iron oxide-apatite deposits is controversial. Silicate liquid immiscibility and separation of an iron-rich melt has been invoked, but Fe–Ca–P-rich and Si-poor melts similar in composition to the ore have never been observed in natural or synthetic magmatic systems. Here we report experiments on intermediate magmas that develop liquid immiscibility at 100 MPa, 1000–1040 °C, and oxygen fugacity conditions (fO2) of ∆FMQ = 0.5–3.3 (FMQ = fayalite-magnetite-quartz equilibrium). Some of the immiscible melts are highly enriched in iron and phosphorous ± calcium, and strongly depleted in silicon (<5 wt.% SiO2). These Si-poor melts are in equilibrium with a rhyolitic conjugate and are produced under oxidized conditions (~FMQ + 3.3), high water activity (aH2O ≥ 0.7), and in fluorine-bearing systems (1 wt.%). Our results show that increasing aH2O and fO2 enlarges the two-liquid field thus allowing the Fe–Ca–P melt to separate easily from host silicic magma and produce iron oxide-apatite ores.The origin of iron oxide-apatite deposits remains enigmatic and controversial. Here, the authors perform experiments on intermediate magmas and show that increasing aH2O and fO2 enlarges the two-liquid field thus allowing the Fe–Ca–P melt to separate easily from host silicic magma and produce iron oxide-apatite ores.

Stable and metastable silicate liquid immiscibility in ferrobasalts

Abstract The onset of immiscibility in ferrobasaltic systems has been the subject of much research recently. The compositional space of the two-liquid field and the maximum temperature of the binodal surface have been investigated experimentally, but results from static and centrifuge experiments are controversial. In the article by Hou and Veksler (2015, May-June issue) entitled “Experimental confirmation of high-temperature silicate liquid immiscibility in multicomponent ferrobasaltic systems,” the authors present experimental evidence for immiscibility between silica- and iron-rich melts at 1150-1200 °C, which are significantly higher to previous studies (ca. 1000-1025 °C). These results have important implications for potential largescale differentiation of magmas by liquid unmixing and for the formation of both Fe-Ti-P-rich melts and rhyolites.