Equilibrium crystallization of massif-type anorthosite residual melts: a case study from the 1.64 Ga Ahvenisto complex, Southeastern Finland

Fe–Ti–P-rich mafic to intermediate rocks (monzodiorites and oxide–apatite–gabbronorites, OAGNs) are found as small intrusions in most AMCG (anorthosite–magnerite–charnokite–granite) suites. The origin of the monzodioritic rocks is still debated, but in many studies, they are presumed to represent residual liquid compositions after fractionation of anorthositic cumulates. In the 1.64 Ga Ahvenisto complex, SE Finland, monzodioritic rocks occur as minor dike-like lenses closely associated with anorthositic rocks. We report new field, petrographic, and geochemical (XRF, ICP-MS, EMPA) data complemented with crystallization modeling (rhyolite-MELTS, MAGFRAC) for the monzodioritic rocks, apatite–oxide–gabbronorite, and olivine-bearing anorthositic rocks of the Ahvenisto complex. The presented evidence suggest that the monzodioritic rocks closely represent melt compositions while the apatite–oxide–gabbronorite and olivine-bearing anorthositic rocks are cumulates. The monzodioritic rocks seem to form a liquid line of descent (LLD) from primitive olivine monzodiorites to more evolved monzodiorites. Petrological modeling suggests that the interpreted LLD closely corresponds to a residual melt trend left after fractional crystallization (FC) and formation of the cumulate anorthositic rocks and minor apatite–oxide–gabbronorite in shallow magma chambers. Consequent equilibrium crystallization (EC) of separate monzodioritic residual magma batches can produce the observed mineral assemblages and the low Mg numbers measured from olivine (Fo25–45) and pyroxenes (En48–63, Mg#cpx 60–69). The monzodioritic rocks and apatite–oxide–gabbronorites show similar petrological and geochemical characteristics to corresponding rock types in other AMCG suites, and the model described in this study could be applicable to them as well.

Lithium-Rich Claystone in the McDermitt Caldera, Nevada, USA: Geologic, Mineralogical, and Geochemical Characteristics and Possible Origin

Lithium deposits in tuffaceous sediments of the McDermitt caldera constitute possibly the world’s largest Li clay resource, yet their characteristics and origin are not established. The 40 × 25 km McDermitt caldera collapsed during the eruption of ~1000 km3 of a 16.4 Ma, zoned peralkaline to metaluminous tuff; minor caldera magmatism ceased by 16.1 Ma. About 200 m of sediments mostly composed of glass from regional pyroclastic eruptions accumulated in the caldera until about 15.7 Ma. Closed hydrologic system diagenesis (CHSD) altered the tuffaceous sediments to a consistent vertical mineral zonation of clay, analcime, K-feldspar, and albite. Entire sedimentary sections in the southern and western parts of the caldera basin have ≥1500 ppm Li. Lithium-rich intervals are dominantly claystone. The most thoroughly studied deposit is a laterally continuous, ~3000 ppm Li zone in the lower sedimentary section that also has high K, Rb, Mo, As, and Sb (and partly Mg and F). Lithium occurs as an illitic clay (tainiolite?). The overlying, upper sedimentary section averages <2000 ppm Li which resides in smectite (hectorite). A transitional zone has variably mixed smectite–illite clay and averages ~2000 ppm Li. An 40Ar/39Ar age of ~14.9 Ma on authigenic K-feldspar in the illite zone is ~1.2 Ma younger than the 16.1 Ma end of magmatism in the caldera, which mitigates against a simple hydrothermal origin. Closed hydrologic system diagenesis was essential to Li mineralization, but Li budget calculations suggest a source of Li in addition to the tuffaceous sediments is required. This additional source could be Li originally in highly enriched magma that entered the diagenetic system through either (1) Li in magma exsolved into a hydrous volatile phase during eruption. The Li-rich volatile phase coated glass shards or was trapped in pumice and was quickly leached by surface or groundwater upon deposition in the caldera. (2) Residual magma immediately following ash-flow eruption and caldera collapse generated Li-rich hydrothermal fluids that mixed with meteoric water in the closed caldera basin, generating a hybrid diagenetic fluid. The hydrothermal fluid and hybrid diagenetic fluids would have existed only during initial basin sedimentation between about 16.4 and 16.1 Ma.

Petrologic Insights into Rift Zone Magmatic Interactions from the 2011 Eruption of Kīlauea Volcano, Hawaiʻi

The high frequency of historical eruptions at Kīlauea Volcano presents an exceptional opportunity to address fundamental questions related to the transport, storage, and interaction of magmas within rift zones. The Nāpau Crater area on Kīlauea’s East Rift Zone (ERZ) experienced nine fissure eruptions within 50 years (1961–2011). Most of the magma intruded during these frequent eruptions remained stored within the rift zone, creating a potential magma mixing depot within the ERZ. The superbly monitored and sampled 2011 eruption (Puʻu ʻŌʻō episode 59) presents an extraordinary opportunity to evaluate magma mixing processes within the ERZ. Whole-rock, glass, and olivine compositions were determined, not only for lava from the 2011 eruption, but also for a new suite of Nāpau Crater area samples from the 1963, 1965, 1968, 1983, and 1997 eruptions, as well as the previously undocumented 1922 eruption. Whole-rock XRF data revealed two geochemically distinct magma batches for episode 59: one less evolved (∼6·6 wt % MgO, 0·46 wt % K2O) than the other (∼6·2 wt % MgO, 0·58 wt % K2O). Episode 59 lava is remarkably aphyric (∼0·1 vol. % phenocrysts), making use of mineralogy to identify parent magma affinities problematic. Linear compositional trends of whole-rock major and trace elements, and reversely zoned olivine crystals indicate episode 59 lavas underwent magma mixing. Least squares regression calculations and plots of major and trace element data, were used to evaluate whether the episode 59 samples are products of mixing summit-derived magma with residual magma from previous Nāpau Crater area eruptions. The regression results and trace element ratios are inconsistent with previously proposed mixing scenarios, but they do support mixing between summit-derived magma and residual magma from the 1983 and 1997 Nāpau Crater area eruptions. These magmas were stored in physically and chemically distinct pods at depths of 1·6–3·0 km prior to mixing with new magma intruded from the summit to produce the episode 59 lava. One pod contained a fractionated equivalent of 1983 lava, and the other a hybrid of compositions similar to 1983 and 1997 lavas. The petrology of episode 59 lava demonstrates that magmas from two previous eruptions (1983 and 1997) were available to mix with magma intruded from the summit region. This study clarifies the pre-eruptive history of the mixed episode 59 lava, and elucidates the evolution of the volcano's magmatic system in a region of frequent eruptions.

Integrating field, textural and geochemical monitoring to track eruption triggers and dynamics: a case-study from Piton de la Fournaise

The 2014 eruption at Piton de La Fournaise (PdF), la Reunion, which occurred after 41 months of quiescence, began with surprisingly little precursory activity, and was one of the smallest so far observed at PdF in terms of duration (less than 2 days) and volume (less than 0.4 Mm3). The pyroclastic material was composed of spiny-opaque, spiny-iridescent, and fluidal scoria along with golden pumice. Density analyses performed on 200 lapilli reveal that the spiny-opaque clasts are the densest (1600 kg/m3) and richest in crystals (54 vol%), and the golden pumices are the lightest (400 kg/m3) and poorest in crystals (14 vol%). The connectivity data indicate that the fluidal and golden (Hawaiian-like) clasts have more isolated vesicles (up to 40 %) than the spiny (Strombolian-like) clasts (0–5 %). These textural variations are linked to primary pre-eruptive magma storage conditions. The golden and fluidal fragments track the hotter portion of the melt, in contrast to the spiny fragments which mirror the cooler portion of the shallow reservoir. Progressive tapping of these distinct portions leads to a decrease in the explosive intensity from early fountaining to Strombolian activity. The geochemical results confirm the absence of new hot input of magma and confirm the involvement of a single, shallow, differentiated magma source, possibly related to residual magma from the November 2009 eruption. We found that the eruption was triggered by water exsolution, favoured by the shallow depth of the reservoir, rather than cooling and chemical evolution of the stored magma.

Petrology of nepheline syenite pegmatites in the Oslo Rift, Norway: Zr and Ti mineral assemblages in miaskitic and agpaitic pegmatites in the Larvik Plutonic Complex

Agpaitic nepheline syenites have complex, Na-Ca-Zr-Ti minerals as the main hosts for zirconium and titanium, rather than zircon and titanite, which are characteristic for miaskitic rocks. The transition from a miaskitic to an agpaitic crystallization regime in silica-undersaturated magma has traditionally been related to increasing peralkalinity of the magma, but halogen and water contents are also important parameters. The Larvik Plutonic Complex (LPC) in the Permian Oslo Rift, Norway consists of intrusions of hypersolvus monzonite (larvikite), nepheline monzonite (lardalite) and nepheline syenite. Pegmatites ranging in composition from miaskitic syenite with or without nepheline to mildly agpaitic nepheline syenite are the latest products of magmatic differentiation in the complex. The pegmatites can be grouped in (at least) four distinct suites from their magmatic Ti and Zr silicate mineral assemblages. Semiquantitative petrogenetic grids for pegmatites in log aNa2SiO5 - log aH2O - log aHF space can be constructed using information on the composition and distribution of minerals in the pegmatites, including the Zr-rich minerals zircon, parakeldyshite, eudialyte, låvenite, wöhlerite, rosenbuschite, hiortdahlite and catapleiite, and the Ti-dominated minerals aenigmatite, zirconolite (polymignite), astrophyllite, lorenzenite, titanite, mosandrite and rinkite. The chemographic analysis indicates that although increasing peralkalinity of the residual magma (given by the activity of the Na2Si2O5 or Nds component) is an important driving force for the miaskitic to agpaitic transition, water, fluoride (HF) and chloride (HCl) activity controls the actual mineral assemblages forming during crystallization of the residual magmas. The most distinctive mineral in the miaskitic pegmatites is zirconolite. At low fluoride activity, parakeldyshite, lorenzenite and wöhlerite are stable in mildly agpaitic systems. High fluorine (or HF) activity favours minerals such as låvenite, hiortdahlite,rosenbuschite and rinkite, and elevated water activity mosandrite and catapleiite. Astrophyllite and aenigmatite are stable over large ranges of Nds activity, at intermediate and low water activities, respectively.

Chemical zoning of muscovite megacrystal from the Brazilian Pegmatite Province

Macroscopically homogenous muscovite plate from the Cruzeiro pegmatite, located in the Eastern Pegmatite Province in Minas Gerais, may show complex distribution patterns of some trace elements. In geochronological and petrological studies, as for example in the distinction of magmatic and post-magmatic mica, the cause of zoning could be taken into consideration. The complex chemical zoning in the studied mica plate can be best explained by growth in an evolving magma followed by alteration due to percolation of hydrothermal fluids. Enrichment of Rb towards the border is interpreted as resulting from the chemical evolution of the residual magma during crystal growth. The depletion in (IV Al+VI Al) as well as the increase in (Fe+Mg) and Si along a fracture could be due to the hydrothermal celadonitic substitution of muscovite. This alteration also caused depletion in the contents of Rb, Ga, Y, Nb, Sn, and Zn and residual concentration of Ti. Elements such as Ga, Y, Nb, Sn, and Zn, rarely considered in the discussion of differentiation or alteration processes in micas, have been shown to be as significant as the alkali-elements.

Geochemical evolution of the Jarawa Younger Granite complex and its related mineralization, northern Nigeria

The Jarawa Younger Granite complex is composed of high silica alkali granites that were emplaced 161 Ma ago. The granites are characterized by high contents of Rb, Li, F, Sn, Nb, W above normal low-Ca granitic rocks and have typical S-type characteristics that are indicative of a substantial component of crustal melt.Mineralization in the complex is associated with the biotite granite which was emplaced as a sheet-like body at relatively shallow depth and occurs as disseminations and as greisen lodes and veins.Chemical studies of the granites have shown that the biotite granite represents a highly fractionated rock that crystallized from a residual magma from which the hornblende-biotite granite had previously crystallized. However the biotite granite is characterized by steep gradients in some minor and trace elements that apparently indicate that liquid-state differentiation and/or volatile complexing processes made significant contributions to their differentiation. Enrichment of Th, Li, Rb, Sn, W and Nb may be more closely linked to roofward migration of F.

Anorthosite and associated plutons, southern Nain complex, Labrador

Plutonic rocks in a southern area of the Nain anorthosite complex consist of leuconorite, anorthosite, granite, and subordinate diorite. An older anorthosite intrusion (CI < 10) has been invaded and broken up by an extensive leuconorite intrusion (CI = 15). Ferrodiorite occurs as small bodies within or adjacent to the leuconorite and is a late stage differentiate of leuconorite, Large granitic plutons of separate crustal origin were emplaced prior to complete solidification of the leuconorite. Where these intrusions invaded leuconorite a marginal hybrid (diorite–granite) zone of heterogeneous matrix developed with chilled pillows of residual magma (mainly ferrodiorite) filter-pressed from the leuconorite. Chilled leuconorite dikes indicate that the parent magma of one leuconorite intrusion was approximately equivalent to its average composition. The older anorthosite body most probably represents a concentration of plagioclase crystals from a different magma; deformation of plagioclase crystals suggest that the mechanism of concentration was filter pressing.