Arc Magmatism

The reconstruction of magma pathways at active volcanoes is of paramount importance for the comprehension of their structure and for geohazard assessment. Magma plumbing systems at volcanic arcs may be particularly complicated since the magma rises along fractures that can be consistent with the coeval regional state of stress, the local state of stress, or can form dykes that instead follow pre-existing structures. Magma path orientation can be stable over time or can vary as the consequence of external events like large earthquakes or important modifications in volcano morphology. In order to advance understanding of these issues, we reviewed all available information on the Holocene volcano-tectonics of the Alaska-Aleutian arc and back-arc zones, based on published seismological, interferometric and geological-structural data, geological maps, and official reports. We completed our review with some new measurements of Holocene eruptive fissures, faults, dykes, and morphometric characteristics of pyroclastic cones and volcanic domes aimed at better defining the possible shallow magma paths of the recent-active volcanoes. Finally, we reviewed the possible parameters and models that explain the path configurations. At 32 volcanoes, magma paths strike NW-SE, perpendicular or oblique to the arc but parallel to the regional greatest principal stress. At 20 volcanoes magma paths are parallel to the arc, and 19 volcanoes form rows of coalescent cones that also suggest ascent of magma parallel to the arc. Eight volcanoes display both directions (normal and parallel to the arc), and seismological data indicate that at some volcanoes there has been a rotation of the magma pathway over time. Integration of all data shows that the regional ambient tectonic stress field promotes dyke intrusions normal to the trench. Dykes can also intrude parallel to the trench following stress unclamping from large earthquakes. Trench-parallel dykes and rows of volcanoes can be generated by magma batches that are aligned parallel to the trend of the subduction zone. Once a dyke or a sill is intruded, it locally perturbs the stress field facilitating successive intrusion along a perpendicular direction.

The Longwood Igneous Complex (LIC) is located in Southland, New Zealand on the eastern side of the Carboniferous to Cretaceous, I-type, Median Batholith. Intrusives of the Complex range in age from Permian to Jurassic and show trace element characteristics typical of subduction-related magmas. Gabbro, gabbroic diorite and basaltic dyke rocks show trace and minor element patterns and isotopic compositions indicating that they represent magmas generated in an intra-oceanic subduction system. Radiometric ages decrease across the LIC from 254 Ma in the east to 142 Ma in the west and mineral chemistry and mineral phase relationships indicate emplacement at depths between 15 and 25 km. Thus the petrology and geochemistry of the LIC provides the basis for evaluating the composition of lower–middle crust assembled above a long lived intra-oceanic subduction system and we estimate this to be andesitic and similar to bulk continental crust. Rocks of the LIC range in composition from troctolite and gabbro through diorite to trondhjemite and granite. All of the ultramafic rocks and most of the gabbros have petrographic and geochemical features consistent with a cumulate origin and mineral chemistry shows similarities with arc cumulate sequences from elsewhere. Few of the plutonic rocks making up the LIC have direct analogues among modern intra-oceanic volcanic rocks. The latter are the end products and the former the leftovers from magmatic processes that included fractional crystallisation, crustal assimilation and magma mixing and mingling. Longwood intrusions do not represent magma chambers. They formed as crystal cumulates and mushes left over from the processes that generated magmas erupted at the contemporary volcanic arc. A correlation between decreasing age of emplacement and Sr and Nd isotopic compositions and inheritance in zircons dated by ion probe are indications of crustal recycling. The generation of felsic rocks in the Longwood intra-oceanic arc involved crustal anatexis and, over the 100 million year history of the arc, the crust evolved towards a composition similar to bulk continental crust and average andesite. Dioritic rocks of the LIC contain abundant mafic enclaves, which are argued to represent fragments of mafic magma, derived by fractional crystallisation from basalt, which was intruded into a hot but solid or near solid diorite. Heating and remobilisation of the dioritic host disrupted and disaggregated the intruding mafic magma to form enclaves and zones of intrusion breccia that show every variation from liquid–liquid to liquid– solid mingling and mixing. They were then further modified chemically and mineralogically by diffusion of H 2 O, Na, P, Ba, REE, and, to a lesser extent, Rb. Mafic dykes occur throughout the Complex and a number of these are composite with compositions ranging from dolerite through andesite to dacite. The components of composite dykes do not define unequivocal linear mixing trends and hybridisation processes that took place within them have only localised significance; mingling and hybridisation in the composite dykes do not appear to have controlled geochemical variation among the major intrusive units of the Complex.

Mantle lithologies in orogenic massifs and xenoliths commonly display strikingly different Hf- and Nd-isotope compositions compared to oceanic basalts. While the presence of pyroxenites has long been suggested in the source region of mantle-derived magmas, very few studies have reported their combined Hf\ \Nd isotope compositions.We here report the first Lu\ \Hf data along with Re\Os data and S concentrations on the Cabo Ortegal Complex,where the pyroxenite-rich Herbeira massif has been interpreted as remnants of a delaminated arc root. The pyroxenites, chromitites and their host harzburgites show a wide range of whole-rock 187Re/188Os and 187Os/188Os (0.16–1.44), indicating that Re was strongly mobilized, partly during hydrous retrograde metamorphism but mostly during supergene alteration that preferentially affected low Mg#, low Cu/S pyroxenites. Samples that escaped this disturbance yield an isochron age of 838 ± 42 Ma, interpreted as the formation of Cabo Ortegal pyroxenites. Corresponding values of initial 187Os/188Os (0.111–0.117) are relatively unradiogenic, suggesting limited contributions of slab-derived Os to primitive arc melts such as those parental to these pyroxenites. This interpretation is consistent with radiogenic Os in arc lavas being mostly related to crustal assimilation. Paleoproterozoic to Archean Os model ages confirm that Cabo Ortegal pyroxenites record incipient volcanic arc magmatism on the continental margin of the Western African Craton, as notably documented by zircon U\ \Pb ages of 2.1 and 2.7 Ga. Lu\ \Hf data collected on clinopyroxene and amphibole separates and whole rock samples are characterized by uncorrelated 176Lu/177Hf and 176Hf/177Hf (0.2822–0.2855), decoupled from Nd-isotope compositions. This decoupling is ascribed to diffusional disequilibrium during melt-peridotite interaction, in good agreement with the results of percolation-diffusion models simulating the interaction of an arc melt with an ancient melt-depleted residue. These models notably show that Hf\ \Nd isotopic decoupling such as recorded by Cabo Ortegal pyroxenites and peridotites (ΔƐHf(i) up to +97) is enhanced during melt peridotite interaction by slow diffusional re-equilibration and can be relatively insensitive to chromatographic fractionation. Finally, we discuss the hypothesis that arc-continent interaction may provide preferential conditions for such isotopic decoupling and propose that its ubiquitous recognition in peridotites reflects the recycling of sub-arc mantle domains derived from ancient, reworked SCLM.

To contribute to the understanding of magma evolution in arc settings we investigate the oldest volcanic unit (Kanafià Synthem) of Nisyros volcano, located in the eastern Aegean Sea (Greece). The unit consists of por-phyritic pillow lavas of basaltic andesite composition with trace element signatures that are characteristic of island-arc magmas. Two lava types are distinguished on the basis of geochemistry and the presence or absence of xenoliths, with the xenolith-bearing lavas having distinctly elevated Sr, MREE/HREE and MgO/Fe 2 O 3 compared to the xenolith-free lavas. Xenoliths include relatively rare quartzo-feldspathic fragments that represent continental-type material, and coarse clinopyroxenite xenoliths that consist largely of aluminous and calcic clinopyroxene, and accessory aluminous spinel. Anorthite–diopside reaction selvages preserved around the clinopyroxenite xenoliths demonstrate disequilibrium between the xenoliths and the host magma. The xenolith clinopyroxene is distinctly enriched in most lithophile trace elements compared to clinopyroxene phenocrysts in the host magmas. A notable exception is the Sr concentration, which is similar in both clinopyroxene types. The high Al and low Na contents of the clinopyroxenites preclude a cumulate, deep metamorphic, or mantle origin for these xenoliths. Instead, their composition and mineralogy are diagnostic of skarn rocks formed by magma– carbonate interaction in the mid/upper crust. The Kanafià lavas are interpreted to have undergone crystal fractionation, magma mixing/mingling and crustal assimilation while resident in the upper crust. We show that magma–carbonate reaction and associated skarn formation does not necessarily result in easily recognised modification of the melt composition, with the exception of increasing Sr contents. Carbonate assimilation also releases significant CO 2 , which will likely form a free vapour phase due to the low CO 2 solubility of arc magmas. In the broader context, we stress that the effects of carbonate assimilation by arc magma may be more significant than currently recognised. Carbonate assimilation may modify key trace element ratios, such as Sr/Y, in arc magmas, and will liberate significant CO 2 as vapour, which may influence eruption dynamics, estimates of subduction zone volatile budgets, and deep mantle CO 2 recycling.

Petrological and geochemical investigations were performed on the uniquely distributed Nanzaki basanite
(0.43 Ma) in the northern part of the Izu–Bonin volcanic arc, Japan, to clarify its original magma chemistry,
and to constrain the source mantle and formation process of the magma. The Nanzaki basanite (monogenetic
volcano) is mainly composed of nepheline-bearing basanite lava and scoria. The mineral chemistries are characterized
by high forsterite (Fo) contents of olivines, high Mg# (=Mg/(Mg + Fe)) values of clinopyroxenes, and
low Cr# (=Cr/(Cr + Al)) values of spinels.Whole-rock major element contents have narrow variation ranges
as follows: SiO2 (41.5–44.1%), MgO (10.2–13.1%), CaO (11.9–13.3%), and K2O (0.4–1.9%). Combined with these
mineral and whole rock chemistries, the lowFeO*/MgO (0.81–1.09) values, high Ni and Cr contents, and narrowly
distributed rare earth element (REE) patterns of theNanzaki basanite represent the primary (undifferentiated)
chemical features of the magmas. The incompatible trace element characteristics, especially the high Sr, Ba, and
REE contents and low K, Rb, Zr, Hf, and Ti contents, suggest that the basanite magmas were generated from an
enriched mantle that was affected by metasomatism with carbonatite magma (or carbonate-melt). In addition,
the slight enrichment of Pb, Cs and other alkaline elements in the basanites indicates the close concern of fluids,
and the Sr–Nd isotope characteristics of the basanites (low 87Sr/86Sr and 143Nd/144Nd ratios relative to those of
basaltic rocks in the volcanic front) are consistentwith across arc isotopic variations of the Izu–Bonin volcanic arc.
Themetasomatismof the source mantle by carbonatite (or carbonate-rich)meltwas associated with and potentially
facilitated by the infiltration and interaction of some volatile components (CO2, H2O) from the subducting
slab. Thus, it is presumed that the enriched and metasomatizedmantle parts have been present, ubiquitously in
some regions of the mantle wedge, and that the basaniticmagma, as in the Nanzaki, has been generated in close association with the unique tectonic regime, as in the northernmost part of the Izu–Bonin volcanic arc where
three (or four) plates converged.

The importance of quantifying the amount of H2O dissolved in magmas is obvious. However, pre-eruptive dissolved H2O in magmas is difficult to estimate due to nearly complete degassing of magmas during ascent, eruption, and cooling. Currently, magmatic H2O content estimations are based mostly on two methods: 1) measurements of water dissolved in melt inclusions; 2) plagioclase chemistry ‘hygrometers’ that are thermodyanmically derived models which use composition and/or phase stability as a function of dissolved water content. There is compelling evidence that these methods are only applicable to magmas in the upper-most part of the Earth's crust. Thus, the tools, which are commonly used for magmatic pre-eruptive H2O estimations are not able to provide information about deep/primitive arc magmas, which may contain much larger amounts of water than normally recognized.
  In many ways amphibole may be complementary to plagioclase in its potential use as a hygrometer-type phase. In water-bearing magmas plagioclase crystallization gets suppressed, but amphibole crystallization does not. This leads to amphibole being an early crystallizing phase in deep-crustal hydrous systems, and in particular it crystallizes before plagioclase, and under certain circumstances, coeval with olivine. Primitive amphiboles crystallizing with or possibly even before olivine is a rare but globally ubiquitous assemblege, and has a lot to reveal about just how hydrous are subduction zone magmas when they leave the mantle. The primitive setting for amphibole discussed here is very different from the more typical use of amphibole as a baramoter in evolved acidic rocks. Here we present a survey of published data from plutonic and volcanic rocks that shows magma processing in the lower to middle crust often involves dissolved H2O contents >12 wt%. We also outline how petrologic investigations of primitive igneous amphibole could be carried out to highlight the importance and ubiquity of amphibole as an early crystallizing phase. Amphibole’s lack of a stability field at low pressure and the positive correlation between pressure and H2O solubility are likely major contributing factors to the underestimation of the global significance of H2O-rich primitive melts that are processed through volcanic arc systems. Ongoing and future experiments will help to better interpret the global array of primitive amphibole samples.

Patterns of spatial distribution, and geochemical and isotopic
evolution from subduction-related igneous rocks provide tools
for scaling, balancing and predicting orogenic processes and mechanisms. We discuss patterns from two Andean key arc segments, which developed into fundamentally different types of orogens: (1) A plateau-type orogen with thick crust in the central Andes, and (2) a non-plateau orogen with normal crust in the southern Andes.
Northern Chile (21–26° S) shows a collage of stepwise, eastward migrating arc axes from 200 Ma to the Present. Each arc is characterized by a repeating sequence of magmatic-tectonic events: Magmatism for 30–40 million years with increasing REE fractionation (increasing La/Yb, La/Sm and Sm/Yb ratios); increasing crust-like initial Sr and Nd isotopes; early-stage, back-arc, alkaline magmatism; and late-stage tectonic activity and (mainly) crustal shortening followed by intra-arc strike-slip fault motion, followed by mineralization and magmatic quiescence for 5–12 million years, before the next main-arc evolved up to 100 km further east. Increasing REE fractionation and crust-like Sr and Nd isotopes correlate with crustal thickening by tectonic shortening and magmatic underplating, from 30–35 km (Jurassic) to 45 km (Eocene) to 70 km thick in the modern central Andes. Episodes of magmatic quiescence for 5–12 million years reflect episodes of flat subduction; the repeated nature of these episodes reflects dynamic subduction cycles including flat subduction, slab steepening, and slab breakoff.
Southern Chile (41–46° S) shows stationary arc magmatism from 200 to 50 Ma; followed by trench retreat and arc widening from 50–28 Ma; arc narrowing from 28–8 Ma; and magmatic quiescence from 8–3 Ma. Volcanism from the Pliocene to the Present was concentrated in a narrow volcanic arc. Moderate crustal shortening occurred from 70–55 Ma (mainly back-arc) and ~9–8 Ma (intra-arc). REE fractionation patterns (low and constant La/Yb ratios) are similar to those of Jurassic rocks from northern Chile, consistent with crustal thicknesses of 30–35 km. Initial Sr and Nd isotopes between 200 and 20 Ma evolved from crust-like to mantle-like ratios, with a reversal at 20 Ma to more diffuse and crust-like ratios. This pattern can be related to successive isotopic shielding and/or asthenospheric depletion (200–20 Ma), and increasing crustal assimilation (20 Ma to Recent) due to moderate crustal thickening. Similarly to northern Chile, magmatic quiescence from 8–3 Ma may reflect an episode of flat subduction.
The cyclicity of magmatic, isotopic, and tectonic features in northern Chile suggests that rheologic weakening of the lithosphere plays an important role. Shared magmatic-tectonic features of paleo-arcs in northern Chile and regions of subhorizontal in southern Chile suggests that flat slab episodes may be a typical feature of Andean-type margins.

Dissolved in magma, H2O plays a significant role in generation, evolution, and eruption of arc magmas. Estimating pre-eruptive H2O content is challenged by near surface H2O degassing during ascent and eruption. Currently, the ‘gold-standard’ for determining pre-eruptive volatile contents in magmas is the study of mineral-hosted glassy melt inclusions (MIs). They act as tiny pressure capsules potentially preserving maximum dissolved water contents, while the matrix melt degasses on ascent and gets modified by mixing and differentiation processes. Despite the widespread use of glassy MIs, it has yet to be tested whether they underlie a systematic maximum limit resulting in potentially biasing the inferred magmatic H2O budget in subduction zones. Natural glassy MIs have been found to contain no more than ~8-9 wt.% of dissolved H2O, and the question remains, is this limit representing a natural limit or a preservation limit? Here we explore the limits of mineral hosted glassy MIs as hydrous magma recorders based on an experimental study of quenching water-bearing silicate melts and show that 9 wt.% of dissolved H2O is a physical limit that quenched melt inclusions cannot exceed, while still quenching to a single-phase glass. Our results demonstrate that the maxima of 8-9 wt.% H2O from glassy MIs studies is linked to the ability of quenched glass to incorporate H2O/OH– in its structure, while excess water exsolves as bubbles and/or promotes devitrification through crystallization of quench crystals or hydrous alteration of the glass. Hydrous melts with H2O >9% will not form glassy MIs. As a result glassy MIs are only faithfully recording magmatic pre-eruptive H2O contents in the upper-most part of the Earth’s crust where H2O-solubility is below 9 wt.%. They have no sensitivity to estimate volatile budgets neither in deep/primitive arc magmas nor in midcrustal evolved magmas. Such magmas may contain much larger amounts of water than currently recognized imparting also additional buoyancy on ascent. For dense primitive magmas this may solve a conundrum often found in convergent margins; the fact that such magmas can reach the surface despite a low-density filter in the form of evolved magmas and crust in their path. These results show that we might be drastically underestimating the volatile budgets in subduction zones and they highlight the necessity of using and developing alternative methods for estimating pre-eruptive H2O contents.