Syn-volcanic melt-rock reactions recorded in peridotitic xenoliths from Scania, S Sweden

The fine-grained aggregates usually develop at the margin of orthopyroxene I grains (Fig. 2C,D). The distribution of the minerals and melt in the aggregates are not homogeneous and the modal composition varies according to its thickness: the narrowest (20–30 μm wide) are formed of single, vermicular crystals of clinopyroxene III, glass and minute olivine III, often with enclosed oval grains of orthopyroxene (Fig. 2C); in thicker aggregates (exceeding ~50 μm) clinopyroxene III becomes elongated, perpendicular to the contact with orthopyroxene I, grains of olivine III are oval and glass is present in interstices (Fig. 2D). In the biggest aggregates, orthopyroxene I is lacking (Fig. 2E) and the outermost part of the aggregate is formed of euhedral, olivine III grains (10–50 μm; Fig. 2E), whereas the innermost part is usually formed of radially-arranged grains of clinopyroxene III and olivine III (20–100 μm long); glass is present in interstices in both zones of the aggregates. Moreover, oval, fine (5–20 μm) grains of olivine locally form aggregates up to 100 μm in size embedded in glass (Fig. 2E). In areas closer to the margin of xenolith all the fine-grained structures were noticed, while in the inner parts, only the narrowest type is present.

The altered, hydrous, high-Mg glass-like phase surrounds fine-grained aggregates. It forms pools or veins of varying widths, which are less abundant in areas associated with the narrow aggregates (Fig. 2C). Conversely, in the presence of thicker fine-grained aggregates or between two fine-grained zones, this phase becomes more abundant (Fig. 2D,E).

The spongy textures are consistently in contact with glass or with the altered, hydrous, high-Mg glass-like phase. The thickness of spongy clinopyroxene III rims surrounding clinopyroxene I varies from few to hundreds of μm. Small glass droplets within these spongy rims are from 1 to 5 μm. Clinopyroxene grains exhibiting a fully spongy texture are also present (Fig. 2F). The spongy spinel III rims surrounding spinel I consist either of a 20–100 μm-wide zone of vermicular (2–5 μm thick) spinel III crystals or are formed of a loose network of acicular crystals perpendicular to the margin of spinel I (Fig. 2G).

The composition of type III phases forming fine-grained aggregates varies between samples and thus must be discussed in comparison with the composition of rock-forming minerals from the same sample (Fig. 3).

Figure 3.

The chemical composition of minerals: (A) Mg# vs. Al content in clinopyroxene III and clinopyroxene I; (B) Ca vs. Na content in clinopyroxene III and I; (C) Fo vs. NiO content in olivine III compared to olivine I; (D) Mg# vs. Cr# in spinel III compared to spinel I. Fields for type I minerals are given for comparison with a chemical composition of spongy (type III – SR) and aggregate-forming type III (type III – Agg) minerals (unpublished data by Mikrut et al.). Data presented in the figure are representative for clarity reasons; the full data set is given in the Supplementary Materials.

Clinopyroxene III forming aggregates has Mg#=89.2–93.4 (Fig. 3A; Table S1) and the Al and Na contents are 0.023–0.216 and 0.029–0.059 apfu, respectively (Fig. 3A,B; Table S1). Within a single sample, the Mg# increases, while the Al and Na decrease with the development of the thickness of the aggregate. The Mg# and Al content in our samples are similar to those described by Rehfeldt et al. (2007), while Na content is significantly lower (Fig. 3B). Small grains of orthopyroxene enclosed in clinopyroxene III have Mg#= 90.8–91.9. Olivine III in the intergranular aggregates has Fo=88.6– 91.7, while NiO and Ca are 0.24–0.34 wt.% and 370–2240 ppm, respectively (Fig. 3C, Table S2). The Fo content in olivine I and III does not change significantly, while NiO and CaO contents are elevated in olivine III; exceptions are the olivine III grains occurring at the outer margins of the biggest aggregates which do not show elevated Ca contents. The glass has a composition of trachyte or trachydacite and dacite (Fig. 4; SiO₂= 63.75–68.35 wt. %; Na₂O+K₂O=4.73–8.84 wt.%; Table S3).

Figure 4.

The Total Alkali Silica diagram (after Le Maitre et al., 1989) showing the chemical composition of glass from Scania peridotitic xenoliths from this study in comparison to glass studied by Rehfeldt et al. (2007) and host basanite from Scania (Tappe et al., 2016).

The chemical composition of the altered, hydrous, high Mg glass-like phase is very heterogeneous; Al₂O₃ content varies from 10.7 to 20.4 wt.% and MgO is between 14.7 and 20.7 wt.% (Table S3).

Spongy clinopyroxene III has Mg# from 90.4 to 93.6 (Fig. 3A; Table S1). The Al and Na contents are 0.066–0.271 and 0.024–0.083 apfu, respectively (Fig. 3A,B; Table S1). When compared to clinopyroxene I, spongy clinopyroxene is usually (but not in all cases) lower in Al and Fe (Fig. 3A). The spongy spinel formed of vermicular crystals has a homogenous composition of Cr# = 29.4–32.7 and Mg#=64.2–77.0 (Fig. 3D; Table S4); those values are higher and lower, respectively, than in spinel I in the same sample.

Intergranular aggregates in peridotites from Scania were discussed by Rehfeldt et al. (2007; their “resorbed patches”), who suggested that they were remnants of the breakdown of metasomatic amphibole. However, in the set of xenoliths examined in this study, no hydrous phase (e.g. amphibole) was detected and no textures or minerals characteristic for amphibole decomposition (Shaw, 2009) were stated. Therefore, we claim that the studied intergranular aggregates are not a product of amphibole decomposition.

Glass occurring within the aggregates has trachytic/trachydacitic and dacitic (63.8–68.4 wt.% of SiO₂) composition that differs significantly from the host basanite (42.0–44.5 wt.% of SiO₂; Tappe et al., 2016; Fig. 4) composition precluding the possibility of being a product of its infiltration. Aggregates are typically associated with orthopyroxene I grains and formed along their margins. Only within the thickest aggregates, orthopyroxene I is not present suggesting that the formation of the aggregates may be related to its decomposition. Decomposition of orthopyroxene to olivine, clinopyroxene, and Si-rich glass requires the presence of a Si-undersaturated melt that would trigger the reaction (“solvent” in Shaw & Dingwell, 2008; Shaw et al., 2006). Experimental studies by Shaw and Dingwell (2008) showed that during this reaction a Si-rich melt, similar in composition to the Scania glass, is produced. Furthermore, Kelemen (1990) pointed that during the reaction of peridotite with tholeiitic basalt orthopyroxene dissolves while olivine crystallizes, under nearly isenthalpic conditions. Interaction of such melts with ultramafic wall rock often produces calc-alkaline, Si-rich derivative liquids. Compositions of both experimental and theoretical melts fit well to the composition of glasses in Scania peridotites suggesting that they are products of orthopyroxene dissolution. However, in Scania, remnants of the Siundersaturated melt were not recorded, suggesting that the ratio between orthopyroxene and infiltrating Si-undersaturated melt was <1 and the whole volume of the melt reacted.

The texture and modal composition of the phases participating in the studied intergranular aggregates vary with their thickness (see section 5.1 and Fig. 2C–E). The narrowest aggregates, which envelop the margins of orthopyroxene I consist of clinopyroxene III, olivine III and remnants of orthopyroxene I and represent a product of orthopyroxene decomposition. Textural position and similarities in chemical composition between orthopyroxene enclosed in the fine-grained aggregates and orthopyroxene I (Mg# = 89.4–91.9, Al = 0.106–0.220 apfu; Mikrut et al., 2019) suggest that they constitute the remnants of unreacted orthopyroxene I grain at the initial stage of orthopyroxene I breakdown. Larger aggregates represent a more developed stage of the reaction between Si-undersaturated melt and orthopyroxene. The increase in size and the development of the intergranular structures suggest a prolonged reaction time which ultimately resulted in the complete decomposition of orthopyroxene (Fig. 2E). This interpretation is in agreement with the location of specific types of aggregates – orthopyroxene-free aggregates are predominantly located near the xenolith margins. More than 1 cm away from the contact with the basanite, the aggregates become narrower, but remnants of orthopyroxene I are present. In the central parts of the xenolith (>2.5 cm from contact), the aggregates, which envelop orthopyroxene I grains, are the narrowest and limited to single crystals of olivine III, clinopyroxene III, and remnants of orthopyroxene I grains.

In the thickest, most developed aggregates the outer parts of the aggregates are formed only of olivine III (Fig. 2E). As discussed, the modal composition of the inner part is a result of orthopyroxene I dissolution. Olivine occurring in the outer zones of the aggregates could have been formed in the same process, but it does not show enrichment in Ca, characteristic for olivine III in the aggregates. The formation of those olivine III may result from partial, dissolution of olivine I due to a reaction with Si-undersaturated melt (Shaw & Dingwell, 2008). The lack of Ca enrichment in olivine III suggests an isothermal process (Brey et al., 1990).

Spongy rims are common in xenolithic peridotites (Lu et al., 2015; Pan et al., 2018). Hirose and Kawamoto (1995) and Shaw et al. (2006) described sieve-textured clinopyroxene characterized by a decrease in Na, Al and an increase in Ca content and aligned with the compositional changes induced during the earliest stages of partial melting. Shaw et al. (2006) interpreted the development of the sieve texture as a result of selective removal of more fusible components, leaving a refractory residue. Sieve-textured clinopyroxene and spinel form in the presence of a Si-rich melt, which migrates along grain boundaries and interacts with spinel and clinopyroxene.

The spongy clinopyroxene III form Scania has lower Al₂O₃ and Na₂O but higher CaO and Mg# than the primary clinopyroxene I (Fig. 3A,B) and the spongy spinel III exhibit depletion in Mg and Al content compared to spinel I. Therefore, the spongy domains in Scania peridotites could have been formed due to a reaction with Si-rich melt as described by Shaw et al. (2006).

Fine-grained aggregates and spongy domains are found within the same Scania peridotite xenoliths. Similar structures have been documented worldwide (e.g. in xenoliths from Sal Island; Shaw et al., 2006). Based on petrological observations and geochemical composition, we suggest a mechanism for forming fine-grained aggregates and spongy domains in peridotites from Scania similar to that in xenoliths from Sal Island (Shaw et al., 2006). In this model, the formation of fine-grained aggregates involves the reaction of orthopyroxene with a Si-undersaturated melt, resulting in the production of aggregates composed of olivine, clinopyroxene, and a Si-rich melt. The varying thickness of fine-grained zones, as well as completely reacted grains, indicate differences in the duration of the reaction. The Si-rich melt derived from the breakdown of orthopyroxene migrated along grain boundaries, where it interacted with spinel and clinopyroxene grains, leading to the formation of spongy rims.

The occurrence of a hydrous, high-Mg glass-like phase is associated with fine-grained aggregates. The quantity of this phase varies depending on the width of the fine-grained aggregate zones. The highest concentrations of the hydrous, high-Mg glass-like phase are observed in areas where the fine-grained aggregate zones are the widest or between adjacent fine-grained aggregates. This glass-like phase is highly heterogeneous, with significant variations in Mg and Al content. It is not possible to introduce so high-Mg melt in mantle conditions, so we assume that the process of high Mg glass-like phase formation operated inside the xenolith. We suggest that the hydrous, high-Mg glass-like phase may represent products of in-situ melting of the fine-grained aggregates. The aggregates are formed of unevenly distributed olivine and clinopyroxene grains therefore their melting could result in a Mg-rich melt/mixture with a highly heterogeneous chemical composition. We propose that the local reheating and melting of the fine-grained aggregates could take during their ascent to the surface, under conditions close to the solidus.