Chemical heterogeneities reveal early rapid cooling of Apollo Troctolite 76535

The evolution of the lunar interior is constrained by samples of the magnesian suite of rocks returned by the Apollo missions. Reconciling the paradoxical geochemical features of this suite constitutes a feasibility test of lunar differentiation models. Here we present the results of a microanalytical examination of the archetypal specimen, troctolite 76535, previously thought to have cooled slowly from a large magma body. We report a degree of intra-crystalline compositional heterogeneity (phosphorus in olivine and sodium in plagioclase) fundamentally inconsistent with prolonged residence at high temperature. Diffusion chronometry shows these heterogeneities could not have survived magmatic temperatures for >~20 My, i.e., far less than the previous estimated cooling duration of >100 My. Quantitative modeling provides a constraint on the thermal history of the lower lunar crust, and the textural evidence of dissolution and reprecipitation in olivine grains supports reactive melt infiltration as the mechanism by which the magnesian suite formed.

Crystal Forensics of Historical Lava Flows from Mt Ngauruhoe

<p>Mt Ngauruhoe is a 900 m high andesitic cone constructed over the last 2500 yr, and is the youngest cone of the Tongariro Massif. It was previously one of the most continuously active volcanoes in New Zealand, with ash eruptions having occurred every few years since written records for the volcano began in 1839. However, it has now been more than 30 yr since the last eruption. Eruptions in 1870, 1949, 1954 and 1974-1975 were accompanied by lava and block-and-ash flows. Detailed sampling of these historical lava and block-and-ash flows was conducted, including sampling from seven different lava flows erupted over the period June-September 1954 to investigate changes in magma geochemistry and crystal populations over short timescales, and to enable observed changes to be related back to known eruption dates. Mineral major and trace element chemistry highlights the importance of mixing between distinct basaltic and dacitic melts to generate the basaltic andesite whole rock compositions erupted. The basaltic end member can be identified from the presence of olivine crystals with Mg# 75-87, clinopyroxene cores with Mg# 82-92, and plagioclase cores of An80-90. The dacitic melt is identified by SiO2-rich clinopyroxene melt inclusions, clinopyroxene zoning with Mg# 68-76 and plagioclase rims of An60-70. Textural evidence from complex mineral zoning and large variability in the widths of reaction rims on olivine crystals suggests that mafic recharge of the more evolved system is frequent, and modelling of Fe-Mg inter-diffusion applied to the outermost rims of the clinopyroxene crystal population indicates that such recharge events have occurred weeks to months or even shorter prior to each of the historical eruptions, and thus likely trigger the eruptions.</p>

Crystal Forensics of Historical Lava Flows from Mt Ngauruhoe

<p>Mt Ngauruhoe is a 900 m high andesitic cone constructed over the last 2500 yr, and is the youngest cone of the Tongariro Massif. It was previously one of the most continuously active volcanoes in New Zealand, with ash eruptions having occurred every few years since written records for the volcano began in 1839. However, it has now been more than 30 yr since the last eruption. Eruptions in 1870, 1949, 1954 and 1974-1975 were accompanied by lava and block-and-ash flows. Detailed sampling of these historical lava and block-and-ash flows was conducted, including sampling from seven different lava flows erupted over the period June-September 1954 to investigate changes in magma geochemistry and crystal populations over short timescales, and to enable observed changes to be related back to known eruption dates. Mineral major and trace element chemistry highlights the importance of mixing between distinct basaltic and dacitic melts to generate the basaltic andesite whole rock compositions erupted. The basaltic end member can be identified from the presence of olivine crystals with Mg# 75-87, clinopyroxene cores with Mg# 82-92, and plagioclase cores of An80-90. The dacitic melt is identified by SiO2-rich clinopyroxene melt inclusions, clinopyroxene zoning with Mg# 68-76 and plagioclase rims of An60-70. Textural evidence from complex mineral zoning and large variability in the widths of reaction rims on olivine crystals suggests that mafic recharge of the more evolved system is frequent, and modelling of Fe-Mg inter-diffusion applied to the outermost rims of the clinopyroxene crystal population indicates that such recharge events have occurred weeks to months or even shorter prior to each of the historical eruptions, and thus likely trigger the eruptions.</p>

Rapid eruptive transitions from low to high intensity explosions and effusive activity: insights from textural analysis of a small-volume trachytic eruption, Ascension Island, South Atlantic

Proximal deposits of small-volume trachytic eruptions are an under-studied record of eruption dynamics despite being common across a range of settings. The 59 ± 4 ka Echo Canyon deposits, Ascension Island, resulted from a small-volume explosive-effusive trachytic eruption. Variations in juvenile clast texture reveal changes in ascent dynamics and transitions in eruption style. Five dominant textural types are identified within the pumice lapilli population. Early Strombolian-Vulcanian eruption phases are typified by macro- and micro-vesicular equant clast types. Sheared clasts are most abundant at the eruption peak, transitioning to dense clasts in later phases due to shear-induced coalescence, outgassing and vesicle collapse. Melt densification and outgassing via tuffisite veins increased plume density, contributing to partial column collapse and the explosive-effusive transition. Bulk vesicularity distributions indicate a shift in dominant fragmentation mechanism during the eruption, from early-stage bubble interference and rupture to late-stage transient fragmentation, with a transient peak of Plinian activity. Dome and lava groundmass crystallinities of up to 70% indicate near-complete degassing during effusive phases, followed by shallow over pressurisation and a final less explosive phase. We provide textural evidence for high-intensity explosive phases and rapid transitions in eruptive style during small-volume trachytic eruptions and consider the impact of trachytic melt compositions on underlying dynamics of these short-lived, explosive events. This analysis demonstrates the value of detailed stratigraphy in understanding critical changes in eruption dynamics and the timescales over which they may occur which is of particular value in anticipating future eruptions of this type.

Compositional Variation of Amphiboles During Magma Mixing: A Case Study of Huangyangshan A-Type Granite in Kalamaili Metallogenic Belt, East Junggar, China

The Huangyangshan A-type granitic pluton, distributed along the thrust fault in the Kalamaili region of East Junggar, Xinjiang, China, consists of alkaline granite containing abundant dioritic enclaves that formed via magma mixing. Both the host granite and the enclaves contain sodic amphiboles. The textural evidence indicates that amphiboles crystallized as a magmatic phase in both units. We determined major and trace element contents of amphiboles from both units to investigate the compositional variation of the amphiboles during the magma mixing process. The results show that cations of W- and C-site are influenced by chemical compositions of the magma whereas cations of A-, B- and T-site and Al3+ are controlled by crystal structure. Therefore, the variations of W- and C-site cations can reflect magma evolution. The core and rim of the amphiboles show similar trace element patterns, which also suggests that the amphiboles are late-stage phases. Furthermore, the amphibole-only thermometers yield reasonable estimates that are consistent with petrographic evidence. However, thermometers based on partition coefficients and all the currently available amphibole-based barometers that rely on Al contents or DAl cannot be applied to Fe-rich and Al-poor amphiboles.

Conduit processes of the Sf. Ana (TGS) sub-Plinian-Vulcanian eruption sequence of the Ciomadul volcano (SE Carpathians)

&lt;p&gt;The Sf. Ana crater is the young volcanic crater of the dacitic Ciomadul volcano located at the SE end of the C&amp;#259;limani-Gurghiu-Harghita volcanic chain in the Eastern Carpathians. The crater was formed at ~60-30 kyr-s ago probably by several eruptions. The Sf. Ana also called as TGS eruption sequence was the main event that shaped the crater to the present form. The eruption produced fall and PDC deposits, but it is unclear what caused the change in the eruption style. The stratigraphically controlled analyses of the Mohos Layered Pyrolcalstic Sequence (MLPS) provide deep insight into the evolution of the eruption. Assuming that juvenile clast density is primarily controlled by the magma vesiculation within the conduit, the processes close to the fragmentation level can be studied. The vesicularity, vesicle texture, microlite texture, and glass H2O content of the juvenile pyroclasts were studied to reveal the conduit processes. The juvenile clasts show textural evidence for different stages of the vesiculation from bubble nucleation to collapse indicating degassing and outgassing processes in the conduit. The increase of the juvenile clast density upward in the MLPS and the sharp increase of the dense clasts in the PDC deposits indicate the effect of magma column heterogeneity on the eruption style. The conduit heterogeneity was induced by the effective outgassing of the slowly ascending magma portion due to the evolution of vesicle textures together with localized shearing. The eruption column collapse was preceded by a vent failure event which caused densification in the conduit. Banded pumices suggest that the observed conduit heterogeneity was small scale.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;The study is supported by the PD130214 project National Research, Development, and Innovation Fund of Hungary.&lt;/p&gt;

Oxidation state and metasomatism of the lithospheric mantle beneath the Rae Craton, Canada: strong gradients reflect craton formation and evolution

We present the first oxidation state measurements for the subcontinental lithospheric mantle (SCLM) beneath the Rae craton, northern Canada, one of the largest components of the Canadian shield. In combination with major and trace element compositions for garnet and clinopyroxene, we assess the relationship between oxidation state and metasomatic overprinting. The sample suite comprises peridotite xenoliths from the central part (Pelly Bay) and the craton margin (Somerset Island) providing insights into lateral and vertical variations in lithospheric character. Our suite contains spinel, garnet-spinel and garnet peridotites, with most samples originating from 100 to 140 km depth. Within this narrow depth range we observe strong chemical gradients, including variations in oxygen fugacity (ƒO2) of over 4 log units. Both Pelly Bay and Somerset Island peridotites reveal a change in metasomatic type with depth. Observed geochemical systematics and textural evidence support the notion that Rae SCLM developed through amalgamation of different local domains, establishing chemical gradients from the start. These gradients were subsequently modified by migrating melts that drove further development of different types of metasomatic overprinting and variable oxidation at a range of length scales. This oxidation already apparent at ~ 100 km depth could have locally destabilised any pre-existing diamond or graphite.

Formation mechanisms of macroscopic globules in andesitic glasses from the Izu–Bonin–Mariana forearc (IODP Expedition 352)

The Izu–Bonin–Mariana volcanic arc is situated at a convergent plate margin where subduction initiation triggered the formation of MORB-like forearc basalts as a result of decompression melting and near-trench spreading. International Ocean Discovery Program (IODP) Expedition 352 recovered samples within the forearc basalt stratigraphy that contained unusual macroscopic globular textures hosted in andesitic glass (Unit 6, Hole 1440B). It is unclear how these andesites, which are unique in a stratigraphic sequence dominated by forearc basalts, and the globular textures therein may have formed. Here, we present detailed textural evidence, major and trace element analysis, as well as B and Sr isotope compositions, to investigate the genesis of these globular andesites. Samples consist of $$\hbox {K}_2\hbox {O}$$ K 2 O -rich basaltic globules set in a glassy groundmass of andesitic composition. Between these two textural domains a likely hydrated interface of devitrified glass occurs, which, based on textural evidence, seems to be genetically linked to the formation of the globules. The andesitic groundmass is Cl rich (ca. $$3000\, \mu \hbox {g/g}$$ 3000 μ g/g ), whereas globules and the interface are Cl poor (ca. $$300\, \mu \hbox {g/g}$$ 300 μ g/g ). Concentrations of fluid-mobile trace elements also appear to be fractionated in that globules and show enrichments in B, K, Rb, Cs, and Tl, but not in Ba and W relative to the andesitic groundmass, whereas the interface shows depletions in the latter, but is enriched in the former. Interestingly, globules and andesitic groundmass have identical Sr isotopic composition within analytical uncertainty ($$^{87}\hbox {Sr}/^{86}\hbox {Sr}$$ 87 Sr / 86 Sr of $$0.70580 \pm 10$$ 0.70580 ± 10 ), indicating that they likely formed from the same source. However, globules show high $$\delta ^{11}$$ δ 11 B (ca. + 7$$\permille$$ ‱ ), whereas their host andesites are isotopically lighter (ca. – 1 $$\permille$$ ‱ ), potentially indicating that whatever process led to their formation either introduced heavier B isotopes to the globules, or induced stable isotope fractionation of B between globules and their groundmass. Based on the bulk of the textural information and geochemical data obtained from these samples, we conclude that these andesites likely formed as a result of the assimilation of shallowly altered oceanic crust (AOC) during forearc basaltic magmatism. Assimilation likely introduced radiogenic Sr, as well as heavier B isotopes to comparatively unradiogenic and low $$\delta ^{11}\hbox {B}$$ δ 11 B forearc basalt parental magmas (average $$^{87}\hbox {Sr}/^{86}\hbox {Sr}$$ 87 Sr / 86 Sr of 0.703284). Moreover, the globular textures are consistent with their formation being the result of fluid-melt immiscibility that was potentially induced by the rapid release of water from assimilated AOC whose escape likely formed the interface. If the globular textures present in these samples are indeed the result of fluid-melt immiscibility, then this process led to significant trace element and stable isotope fractionation. The textures and chemical compositions of the globules highlight the need for future experimental studies aimed at investigating the exsolution process with respect to potential trace element and isotopic fractionation in arc magmas that have perhaps not been previously considered.

Silicate-replacive high sulfidation massive sulfide orebodies in a porphyry Cu-Au system: Bor, Serbia

The Cu-Au deposit of Bor (Serbia) represents a continuum of mineralization styles, from porphyry-style ore occurring in quartz-magnetite-chalcopyrite veins and chalcopyrite disseminations to high-sulfidation epithermal Cu-Au ores in pyrite-chalcopyrite and anhydrite-sulfide veins. Decisive for the great economic importance of Bor is the presence of exceptionally rich high-sulfidation massive sulfide orebodies, composed of pyrite + covellite + chalcocite/digenite and minor anhydrite and enargite. They form irregular bodies measuring 0.5–10 million tons of ore grading up to 7% Cu, hosted by andesites and surrounded by intense argillic alteration. This study focuses on a small but rich underground orebody mined out recently, where limited drillcore is preserved for quantitative geochemical study. This paper documents the vein relationships within the deep porphyry-style orebody of Borska Reka, the transitional porphyry-epithermal veins, and the overlying and laterally surrounding epithermal massive sulfides of the Bor deposit. Geological observations indicate that the formation of massive sulfide orebodies concludes the ore formation. Mass balance calculations, recast into geologically realistic bulk fluid-rock reactions, confirm textural evidence that near-isovolumetric replacement of andesite host rocks is the dominant formation mechanism of massive sulfide orebodies at Bor, whereby all lithophile elements including Si are dissolved and only Ti stays relatively immobile. While net volume changes for individual mineralization styles within the massive sulfide orebody vary from − 16% volume loss to + 127% volume gain, overall volume change for the whole massive sulfide orebody was probably slightly negative. Brecciation is important only as means of creating channelways for reactive fluid that turns the andesite protolith into massive sulfide, whereas net breccia infill occurred only locally.