Crystals Detail Magma Production and Crust Formation at an Active Back-Arc Submarine Volcano

Magmatic reservoirs feed active volcanoes and contribute to Earth’s crust formation. Among the processes operating in these systems, those controlling the early chemical evolution stages of magma are the most difficult to identify. This paper reports the first multi-mineral crystal archive that offers insights on the magmatic dynamics governing the production of basalt to andesite lavas at an oceanic back-arc spreading centre. As a result of these dynamics, a mush-dominated transcrustal system developed in the last 0.7 million years. The micro-scale crystal view identifies physicochemical magmatic environments that pass unobserved using whole-rock chemistry alone. It also supplies an interpretative formation model for plumbing system components, functional for the interpretation of geophysical data.

Open-to closed-system processes exercise control on the production of magmas erupted at volcanoes, resulting in the construction of complex plumbing systems. Information retrieved from the composition and zoning patterns of crystals carried in erupted lavas significantly contribute to identify the variety of complementary magmatic processes producing the feeding magmas 1 , and provide evidence for the presence of vertically extended magmatic plumbing systems beneath active volcanoes 2 . An igneous crystal viewpoint makes it increasingly apparent that the interplay of these mechanisms passes unobserved under the whole-rock geochemical protocol 2 .
Here we focus on the crystal records preserved in a set of lava samples recovered from the submarine Marsili Volcano (MV), Southern Tyrrhenian Sea, to shed light on the subcrustal magmatic mechanisms that have generated the entire sample suite. The MV 3 is a large (c. 70 km x 20 km) NNW-SSE elongated seamount that rises from 4000 m to 500 m depth along the active spreading centre of the Tyrrhenian Sea (Fig. 1). The Tyrrhenian represents an example of the seafloor spreading stage in a continental back-arc setting 4 , formed following continental crust rifting behind the Aeolian Volcanic Arc and Calabrian suture zone during subduction and retreat of the Ionian plate 5 . Magmatic activity of the MV took place in the last 0.7 million years, producing primarily basaltic and basaltic andesite lavas; volumetrically minor andesite lavas were recovered at the summit cone of the volcano 6 ( Fig. 1). The major element variations (Fig. 2a) and the trace element and isotopic compositions exhibited by the MV lava suite 6 suggest that the magma evolution history was controlled by closed-system fractionation of olivine, clinopyroxene, and plagioclase, as basaltic mantle-derived melts are injected into the MV crust cool and differentiate. This crystallization history also recalls the results of the petrological experiments on cooling hydrous basaltic melts simulated in closed-system conditions 7 (Fig. 2a). Instead, the recent detailed studies of olivine, clinopyroxene and plagioclase crystals carried in the MV lavas [8][9][10][11] have retrieved information on a wide range of opensystem magmatic mechanisms that govern magma evolution within a mush dominated system, where small volumes of melt-rich pockets are spatially and temporally arranged (Fig. 2b).
We combine the diverse records supplied by the MV crystal cargoes to demonstrate that magma genesis is not simply related to cooling following liquidus conditions (Fig. 2a). We show that the petrogenetic indicators supplied by the crystal cargoes capture the more complete spectrum of the physicochemical environments within the transcrustal system ( Fig. 2b) feeding cogenetic basic and andesitic magmas.

Results
Information on the igneous lithologies formed by the MV magmatic activity have been retrieved from both free crystals and those within aggregates found in the erupted lavas [8][9][10][11] . The mineral assemblage mainly consists of olivine, clinopyroxene, plagioclase and magnetite; rare amphibole-orthopyroxene-plagioclase glomerocrysts were found only in one of the MV basaltic andesite lavas. The glomerocrysts exhibit a variety of mineralogy and textures, from monomineralic (made only by either clinopyroxene or plagioclase crystals, arranged with a locked texture) to polymineralic clusters (made of the mineralogical assemblage described above, with open to locked structures). Overall, the crystal archive is made of minerals crystallizing from the carrier lava with the addition of accidental cogenetic components (antecrysts) of the transcrustal system, extracted by the carrier lava during migration to the surface.
Mineral-melt geothermobarometry calculations coupled with petrological simulations 8,9,11 constrain the pre-eruptive P-T crystallization conditions during the 0.7 Ma of the MV activity (Fig. 2b). These studies document a crustal lithostratigraphy made of maficultramafic lithologies in the deep crust while gabbroic, at places plagioclase-rich, lithologies dominate at the mid-to upper-crustal levels. A similar crustal structure has been seismically detected along back-arc spreading centres 12 , where crustal lithostratigraphy is defined by applying thermodynamic models to the available whole-rock datasets. The MV crystal archive offers the opportunity to investigate the broad range of processes that play a role in the generation of the back-arc crustal lithostratigraphy.
The MV crystals document a magma solidification history controlled by a crystallization sequence made of olivine-clinopyroxene-plagioclase, similar to the one exposed in exhumed crustal sections of volcanic arcs (e.g. by ref. 13 ) and obtained by the crystallization experiments on arc melt compositions (e.g. by ref. 7,14 ).
In this contribution we interpret the origin of the crystals using the following differentiation indices: forsterite (Fo = 100 x Mg/[Mg + Fe] in mol%) versus Ca (ppm) for olivine (Fig. 3a); Mg# (100 x Mg/[Mg + Fe 2+ ]) versus TiO2 (wt%) for clinopyroxene (Fig. 3b); anorthite (An = 100 x Ca/[Ca + Na + K] in mol%) versus Mg (ppm) for plagioclase (Fig. 3c). These mineral differentiation indices combine major and minor elements easily accessible with electron microprobe (EMP) and which have different degrees of compatibility during the magmatic crystallization process (e.g. by ref. 13,[15][16][17][18] ). Furthermore, we show that plagioclase and clinopyroxene zoning provide more information about magmatic history than olivine zoning. This is due to the slow post-crystallization diffusion re-equilibration rate of An in plagioclase and Mg# in clinopyroxene that allows to preserve the original magmatic An and Mg# zoning, while Fo in olivine generally does not retain this information due to fast diffusional-re-equilibration of Fe 2+ and Mg between olivine and host melt (e.g. by ref. 19 ).

Discussion
The pre-eruptive P-T estimates obtained for the MV lavas ( Fig. 2b) combined with the information supplied by the record preserved in their crystal cargoes (Fig. 3) constrain several magmatic environments that governed the chemical evolution of the MV lava suite (Fig. 2a).
The first refers to the interconnected mush and melt-rich environments (Fig. 2b) formed by melt migration mechanisms sourcing basalt to andesite lavas (Fig. 2a). The linear Mg#-Ti trend defined by the primitive clinopyroxene crystals (Mg# > 0.80) found in the deep sourced basic lavas (D5 basalt; D1 and D4 basaltic andesites) (Fig. 3b) documents the mafic-ultramafic mush formed by the crystallization stage of hydrous, high-Mg basaltic mantle melts injected at the base of the MV crust (Fig. 2b). Whether this deep crustal mush environment formed by subsidence of cumulates or by in-situ crystallization is of little consequence for this stage. It is noteworthy that the clinopyroxene found in the exposed crustal mush roots of volcanic arcs 13 and in lavas sourced from deep crustal mush environments 20,21 exhibit similar Mg#-Ti systematics (Fig. 3b).
The D2 basaltic andesite lava sample carries a clinopyroxene cargo that documents crystal growth stage in a melt-rich horizon, formed by melts progressively extracted and collected above the top of the deep crustal mush environment (Fig. 2b). Indeed, the D2 cargo defines a Mg#-Ti trend where the high Mg# part overlaps with the most evolved compositions of the D5 clinopyroxene (Fig. 3b), thus suggesting that these primitive crystals formed in melts within interconnected magmatic environments. At lower Mg# values, the D2 clinopyroxenes split into Ti-depleted and Ti-enriched compositions, pointing towards orthopyroxene and amphibole antecrysts, respectively (Fig. 3b). These antecrysts originate from a mush lithology formed by crystallization of earlier, more evolved hydrous melts stored at the D2 pre-eruptive zone. Some of these antecrysts exhibit a thin clinopyroxene overgrowth rim of evolved composition (i.e., with low Mg# value), associated to a large range in Ti, documenting the chemical variability of melt in this environment 11 . Clinopyroxene compositions from other amphibole-bearing arc rocks 22-24 follow the same forked trend (Fig. 3b), showing that this Mg#-TiO2 clinopyroxene signature identifies the fractionation of amphibole with precursory clinopyroxene in the evolution of arc magmas elsewhere.
The heterogeneity in the textural and compositional features found in the D2 plagioclase cargo indicates that magma feeding the carrier lava, once extracted from the deep crustal mush environment, migrated through a crustal column consisting of pockets of melt distributed within a crystal-mush framework (Fig. 2b). Indeed, the plagioclase cargo spans the widest and most continuous range of An-Mg compositions, from the An-rich plagioclases found in amphibole-bearing glomerocrysts to the An-poor plagioclases found in monomineralic glomerocrysts (Fig. 3c). It is noteworthy that both have a well-packed texture, characteristic of crystal-rich mush 9,11 . A wide An-Mg compositional range was also found in the plagioclase cargo of the D6 sample (Fig. 3c), a basaltic lava sourced from a mid-crustal storage zone (Fig. 2b). Clinopyroxene crystals are rare in the D6 lava, documenting a further crystallization stage within this environment.
The basalt and basaltic andesite lavas belonging to this magmatic environment contain a diversity of olivine crystals, compatible with an evolutionary path of the magma by melt migration in the envisaged interconnected mush and melt-rich regions 10 . Overall, these olivine cargoes record as much Ca-Fo variation as the regional data set from the nearby Aeolian Volcanic Arc 25 (Fig. 3a), but are not able to identify the magmatic processes with the same detail as clinopyroxene and plagioclase.
The mush-melt lens architecture of the MV plumbing system must have survived for sufficient time to sustain the pooling at the base of the volcano edifice of evolved melts (andesitic or more silica-rich) extracted by compaction and percolation from deeper crustal levels (Fig. 2b). Our suggestion is supported by the consideration that the crystal-poor andesite lavas experienced shallow pre-eruptive conditions (Fig. 2b) and carry clinopyroxene with a composition that records growth in melts which had already undergone fractional crystallization at the deep MV crust (Fig. 3b). A recent study 26 speculates that extraction of andesitic melts from a hydrous basic mush system is favoured at a crystallinity window of 50-70 vol% and we suggest that a similar scenario sourced the evolved MV melts. As noted by Trua et al. 9 , the occurrence in most of the MV basic lavas of monomineralic plagioclase glomerocrysts with An content lower than those found in plagioclase from andesites (Fig. 3c) provides evidence that andesite lavas are not the most evolved products of the MV magma evolution history.
The second magmatic environment is documented by basaltic magmas rapidly migrating through an already compacted mush portion of the crustal plumbing system. This is recorded by crystals found in high-Mg basalts (Fig. 2a) erupted at the northern sector of the MV (i.e., MRS 1, 2, 3, 4; Fig. 1). This magmatic setting allows the eruption of basaltic magmas almost free of clinopyroxene, with primitive crystals exhibiting a wide range in minor and trace elements (e.g., Ca in Fo-rich olivine; Mg in An-rich plagioclase) (Fig. 3a  and c). Olivine and plagioclase crystals strongly zoned in minor elements but buffered at primitive compositions (i.e., high Fo and An contents) have been found in basaltic lavas elsewhere 15,16,27 and document crystal growth immediately preceding eruption of the carrier magma. These basaltic lavas carry plagioclase cargoes with An-Mg systematics (Fig. 3c) revealing that the carrier magma briefly stalled in shallow melt lenses before eruption 9 (Fig. 2b).
A third magmatic environment is that in which hybrid basis lavas formed by magma mixing. It is documented by the crystal cargoes from the D16 basaltic and the D11 and D19 basaltic andesite lavas, all erupted at the off-axis, southeastern flank of the MV (Fig. 1) and showing a narrow whole-rock composition (Fig. 2a). Indeed, the large diversity of olivine, plagioclase and clinopyroxene crystals found in these lavas (Fig. 3) supports an ascent history of the carrier magma punctuated by stalling at shallow crustal levels (Fig.  2b). Here, mixing with resident basic magma bodies occurred, favoured by the similar viscosity of the involved basic magmas, thus the hybrid nature of the resulting lavas passes unobserved in their bulk rock chemistry (Fig. 2b).

Conclusion
The micro-scale view supplied by the MV crystal archive contrasts markedly with the simple closed-system fractional crystallization scenario suggested by the bulk-rock chemistry of the carrier lavas, demonstrating that cogenetic basalt-andesite suites may form by a great variety of open-system magma and crustal accretion dynamics. This is the first multi-mineral archive that preserves a spectrum of petrogenetic indicators which reconstruct magma emplacement and basalt to andesite evolution scenarios along a 12 km thick oceanic transcrustal system. These petrogenetic indicators can be used for the interpretation of crystal archives in basaltic-andesite lava suites globally, increasing the ability for decoding the incomplete or cryptic record preserved in crystals. Furthermore, the ability of crystals to identify the distribution of melt rich bodies supplies an interpretative crustal formation model that can be functional for the interpretation of geophysical data at back-arc basins.
Recognising similar scenarios at basaltic-andesite lava suites elsewhere has strong implications for a realistic assessment of magma-crust forming processes.

Data and methods
The mineral and bulk-rock chemical data are from literature (ref. [8][9][10][11]. The MV mineral chemical dataset have been collected using the EMP facility (i.e., CAMECA SX50) at the Padova (Italy) branch of the Istituto di Geoscienze e Georisorse of the Italian National Research Council. For a detailed description of analytical conditions refer to the original papers (ref. [8][9][10][11].   7 , reproduced experimentally following shallow (P < 1 GPa) crystallization paths from moderately hydrous basaltic melts: olivine-only (black arrow); olivine-clinopyroxene-plagioclase-amphibole (black dashed lines). The Marsili lava samples are colour-coded for the specific magmatic scenario experienced by their crystal cargo (see legend). The grey fields and arrows labelled Ol, Cpx, Pl and Am refer to compositional data of MV olivine, clinopyroxene, plagioclase, and amphibole from literature [8][9][10][11] . (b) P-T diagram with the pre-eruptive storage conditions of the Marsili lava samples (modified after 11 ). The lithostratigraphy inferred for the feeding transcrustal mushdominated system is schematically illustrated on the right of the diagram. The arrows refer to magma ascent paths, coloured for the varying magmatic scenarios as in the legend in (a). Wet basalt and wet andesite liquidi are from ref. 28 ; amphibole (Am) crystallization conditions for the MV lavas (grey field) and Am-out curve are from ref. 8,11 . The white field marks compositional variations in deep mush-sourced clinopyroxene from arc rocks: basic lavas from Stromboli (Aeolian Volcanic Arc) 21 and Ruapehu (New Zealand) 20 ; exhumed island arc crustal section 13 . Grey field contains clinopyroxene literature data from others amphibole-bearing arc-rocks [22][23][24] . (c) Mg (ppm) versus anorthite (mol%) in plagioclase cargoes from basalt to andesite lavas. Yellow field contains plagioclase literature data for mid-ocean ridge basalts 16,26 .