Appendix A1: Petrographic Description of Pyroxenite Xenoliths from Quaternary Basanites from Marsabit (Northern Kenya)

General remarks

All investigated pyroxenite xenoliths are hosted by Quaternary basanite scoriae. All samples exhibit rounded shapes and sizes range from 3 to 7 cm diameter. Most xenoliths contain a rind of vesicular lava containing magmatic phenocrysts (clinopyroxene, olivine), as well as xenocrysts and xenolithic fragments. Contacts of xenoliths with these lava rinds are generally sharp and host interaction, if any, is confined to a zone of <1 mm. Occasionally host magma penetrates the xenoliths along cracks.

Xenolith minerals are mostly fresh; only garnet shows kelyphitic rims along grain boundaries and cracks. Olivine from several samples shows partial alteration to iddingsite. Secondary carbonate, clays or zeolite were not observed.

The pyroxenite xenoliths can be subdivided in tow groups: Garnet-bearing websterite (Group V following the classification of Kaeser et al. 2006) and garnet-free clinopyroxenite and websterite (Group VI). Each investigated sample will be described in detail in the following.

Modal abundances of minerals, where indicated, are in vol% and were obtained by classical point counting methods. Given the relatively small size of most samples combined with a very heterogeneous distribution of constituent minerals (e.g., cm-sized garnet crystals, presence of cm-sized orthopyroxenitic domains), modal contents of the major constituents (grt, cpx and opx), however, are subjected to a relative uncertainty of at least 25%,.

The following mineral abbreviations will be used: grt for garnet; opx-I, cpx-I, ol-I, and spl-I for first generation (i.e. porphyroclasts or texturally equilibrated grains) orthopyroxene, clinopyroxene, olivine and spinel, respectively; Suffix 'IIa' indicates neoblasts in porphyroclastic rocks, whereas 'IIb' indicates symplectitic minerals resulting from garnet-breakdown. A subscript 'm' indicates minerals in late-stage metasomatic plagioclase-bearing reaction zones.

Group V – grt websterite

Sample Ke 771/1 is aweakly deformed grt websteritecontaining ~2 vol% opx-I. All cpx-I (2.0-3.5 mm) shows straight opx exsolution lamellae. Grain boundaries are slightly lobed indicating subsolidus grain boundary migration. No olivine or spl-opx-cpx symplectite after garnet was found. Garnet (27 vol%; <1 cm) contains abundant minute rutile needles and euhedral amphibole inclusions.

Brown amphibole is very abundant (>1 vol%) and forms texturally equilibrated interstitial grains. Abundant late-stage reaction zones (plagioclase + cpxm) replace rims of or form domains within cpx-I.

Sample Ke 771/2is a particularly opx-rich (~30 vol%)porphyroclastic grt websterite. Cpx-I porphyroclasts (3-4 mm) are strongly deformed and contain bent opx exsolution lamellae. Opx-I crystals are very large (up to 7 mm) and characterised by abundant kinkbands. Cpx-IIa and opx-IIa neoblasts are smaller (<0.5 mm) and strainfree. Garnet crystals are almost completely transformed to spl-opx-cpx symplectite. Brown amphibole is confined to symplectite.Late-stage reaction zones formed within symplectite (olm+cpxm+plagioclase+sulphide) and along cpx-I and cpx-IIa grain boundaries (cpxm+plagioclase).

Sample Ke 784/1 is aweakly deformed grt websteritewith the lowest degree of deformation of all investigated Group V pyroxenites. Garnet (cm-sized; ~40 vol%) and cpx-I (up to 4.5 mm) are the major constituents. Cpx-I contains straight opx exsolution lamellae. Opx-I (~6 vol%) is texturally identical to cpx-I. The rock fabric is comparable to that of ad-cumulates (e.g., Vernon 2004). Pyroxenes show occasionally weak undulous extinction. Grain boundaries are straight to slightly lobed, indicating sub-solidus grain boundary migration.

No olivine or spl-opx-cpx symplectite after garnet was observed. Brown kaersutitic amphibole is abundant and commonly forms selvages on cpx-I indicating replacement. Accessory minerals include texturally equilibrated interstitial phlogopite and ilmenite, as well as interstitial Fe-Ni-Cu sulphide and its alteration products (Fe-oxides/hydroxides, malachite). Rutile occurs occasionally as inclusions in ilmenite.

Garnet includes dispersed inclusions of large (up to 0.5 mm) euhedral exsolved cpx-I, tiny rutile needles (most abundant, occasionally oriented; 10-100 μm long), as well as euhedral Ti-pargasite and orthopyroxene. Some inclusions are polyphase (rutile + Ti-pargasite + opx).

Sample Ke 1958/2 – This xenolith is the only granular, statically recrystallised grt websterite investigated here. Cpx-I and opx-I form relatively equant, strainfree polygonal grains (~3.5 mm; rarely up to 6 mm) and show abundant 120° triple-junctions. The largest cpx-I grains show weak undulous extinction and preserved few opx lamellae in their core, indicating that this rock underwent a similar sub-solidus evolution (deformation and cooling, respectively) as the deformed pyroxenites, followed by static recrystallisation.

Cpx-I and opx-I contain abundant inclusions. Two inclusion types can be distinguished: large (10-30 μm) randomly dispersed melt (glass) inclusions and smaller (few μm) inclusions aligned along trails. Some of the latter are opaque (sulphide?), while others contain a fluid phase and a vibrating vapour bubble.

Grt is rounded (31 vol%; 5-7 mm), devoid of inclusions and in places rimmed by coarse grained spl-opx-cpx symplectite. Ti-pargasite occurs as ragged relics in fine grained pockets consisting of secondary olivine, plagioclase and presumably devitrified glass. This indicates late-stage amphibole melting such as observed in peridotite xenoliths from the same suite (Kaeser et al. 2007).

Sample Ke 1959/18 is a porphyroclastic grt websterite. Large (up to 1 cm) opx-I porphyroclasts with cpx exsolution lamellae normal to kink bands occur in a ~2 cm wide orthopyroxenitic domain. The latter contain about 10 vol% ol-I porphyroclasts (makes >1 vol% of the whole rock). Cpx-I porphyroclasts (3-4 mm) contain bent opx exsolution lamellae. Cpx-IIa and opx-IIa neoblasts (<1 mm) are polygonal, strainfree and devoid of exsolutions. Amphibole is very common and forms interstitial large (up to 2 mm) texturally equilibrated grains.

Grt crystals (cm-sized) are anhedral and occasionally include euhedral cpx-I. Grt transformation to spl-opx-cpx symplectite ranges from strongly to completely in orthopyroxenitic domains. Symplectites, in turn, become replaced by plagioclase-bearing reaction zones (plm+olm+cpxm+kaersutite+Ti oxides+sulphide + rare silicate glass).

Sample Ke 1960/3 is a porphyroclastic grt websterite with ~7 wt% opx. Cpx-I (with opx exsolution lamellae) and opx-I porphyroclasts (up to 4 mm) show undulous extinction and have ragged and lobed grain boundaries indicating subsolidus grain boundary migration. Cpx-IIa and opx-IIb neoblasts are polygonal and strainfree.

Centimetre-sized rounded grt contains, besides minute rutile needles, particularly large, randomly dispersed euhedral amphibole inclusions. There is only limited grt transformation to spl-opx-cpx symplectite.

Texturally equilibrated interstitial amphibole is common (<1 vol%) whileol-I was not observed. Most cpx-I, -IIa and –IIb grain boundaries are replaced by plagioclase-bearing reaction zones (plm+cpxm).

Sample Ke 1960/4 is a foliated protomylonitic grt websterite characterised by extremely deformed and elongated cpx-I and opx-I porphyroclasts (closely spaced kink bands, bent exsolution lamellae) floating in a fine-grained matrix (0.1-0.5 mm) of cpx-IIa and opx-IIb neoblasts. This xenoliths further contains a cm-wide orthopyroxenitic domain with ~5 wt% ol-I (<1 vol% of the whole rock). Grt in this domain is completely transformed to spl-opx-cpx symplectite.

Grt crystals are ovoid with lobed grain boundaries. Inclusions (rutile needles, euhedral <0.5 mm cpx-I) are concentrated in grt cores. Amphibole exclusively occurs as small-grained neoblasts.

Sample Ke 1960/5 is a slightly foliated porphyroclastic grt websterite. Cpx-I (with opx exsolution lamellae) and opx-I porphyroclasts (up to 4 mm) show undulous extinction and have ragged grain boundaries. Cpx-IIa and opx-IIa neoblasts (<0.5 mm) are strain- and exsolution-free.

Grt (cm-sized) in cpx-rich zones shows only limited transformation to spl-opx-cpx symplectite, whereas almost complete transformation is observed in a ~2 cm wide orthopyroxenitic domain. The latter includes interstitial strongly deformed phlogopite and interstitial ilmenite. Amphibole is rare and occurs together with pyroxene neoblasts or within symplectite.

Sample Ke 1960/6 is a strongly porphyroclastic grt websterite. Cpx-I (with opx exsolution lamellae) and opx-I porphyroclasts (3-6 mm) show undulous extinction and have ragged grain boundaries. Cpx-IIa and opx-IIa neoblasts are very small (0.1-0.5 mm) and strain- and exsolution-free.

Grt crystals are relatively small (~0.5 cm) and partly replaced to spl-opx-cpx symplectite. Amphibole is very abundant (~2 vol%) and forms large (2-3 mm) texturally equilibrated grains, selvages replacing cpx-I and small irregular grains within symplectite.

Sample Ke 1960/8 is a porphyroclastic grt websterite with strikingly apple-green clinopyroxene (in fact the most Cr-rich sample investigated in this study). Opx (~7 vol%) is heterogeneously distributed. Rare ol-I is observed in opx-rich domains. Cpx-I (with opx exsolution lamellae) and opx-I porphyroclasts (3-6 mm) show undulous extinction and have lobed to curvilinear grain boundaries. Cpx-IIa and opx-IIa neoblasts are (<0.5 mm) polygonal and strain- and exsolution-free.

Grt (cm-sized) contains euhedral cpx-I inclusions (≤0.5 mm). Grt transformation to spl-opx-cpx symplectite is strongly texture controlled and restricted to opx-rich parts. Amphibole is rare and only occurs within symplectite.

Sample Ke 1964 is a protomylonitic grt websterite with ~5-10 vol% opx. This xenolith is the most intensely deformed sample from this study. Cpx-I and opx-I porphyroclasts are extremely deformed (kink bands, undulous extinction, bent exsolution lamellae), however, some almost strainfree grains are preserved in pressure shadows adjacent to grt. Cpx-IIa and opx-IIb are very small (0.05-0.25 mm). The smallest grain sizes are concentrated in mm-sized micro-shear zones giving the rock its foliated texture.

Grt crystals are relatively small (0.4-0.7 cm), rounded and partly deformed to very fine-grained spl-opx-cpx symplectite. The latter, in turn, becomes replaced by abundant plagioclase-rich reaction zones (plm + olm + cpxm + Ti oxides). Amphibole is rare and occurs only within symplectite. Primary ol-I was not observed.

Group V – grt websterite

Sample Ke 726 is a coarse-grained (grain sizes of 0.5-1.5 cm)spl-ol clinopyroxeniteand consists of ~60 vol% cpx-I, ~30 vol% ol-I and ~8 vol% spl-I. The rock fabric resembles that of an ad-cumulate, however, ol-I shows abundant kinkbands (Fig. 5.4c), indicating sub-solidus deformation. Cpx-I contains abundant primary Ni-Fe-Cu sulphide inclusions. Grain boundaries of cpx-I are typically replaced by a narrow (<0.1 mm) plagioclase-bearing reaction zone (plm + cpxm). Further, the margin of this xenolith shows some interaction with the host basanite.

Sample Ke 782/1 is a spl-free websterite (~8 vol% opx) exhibiting a metamorphic fabric. It is characterised by large (up to cm-sized) lamellar aggregates of cpx and opx, probably representing former exsolved cpx-I porphyroclast, surrounded by a matrix of equant (~0.3 mm) polygonal cpx-IIa and opx-IIa grains. Cpx-opx aggregates contain carbonate inclusions. In addition, the sample is cross-cut by a layer including small, slightly altered olivine of greenish colour.

Sample Ke 782/2is an olivine-bearing spl-ol clinopyroxenite composed of cpx-I (91 vol%; up to 4 mm) and interstitial ol-I (polygonal; ~0.5 mm), small opx-I and dark-green spl-I (both interstitial; ~0.2 mm). Grain boundaries between cpx-I are typically ragged and tightly lobe-shaped. Cpx-I contains straight opx and spl lamellae, as well as abundant randomly dispersed sulphide and rare carbonate inclusions.

Sample Ke 1973/2 is a typical composite xenolith, similar to those reported by Irving (1980). It consists of undeformed spl-ol clinopyroxenite enclosing a lherzolite 'xenolith' texturally similar to the recrystallised Group II (grt)-spl lherzolites (Kaeser et al. 2006). The pyroxenitic part has a granular texture, composed mainly of equant (2.5-3.0 mm) polygonal cpx-I grains. They are devoid of lamellae but developed (magmatic?) twins. Ol-I occurs as bleb-like inclusions in cpx-I, or as interstitial grains. Spl-I grains exhibit rounded shapes and are of dark-green colour. No orthopyroxene was observed. The contact between pyroxenite and peridotite is marked by a spl-rich layer.

Cited References:

Irving AJ (1980) Petrology and geochemistry of composite ultramafic xenoliths in alkalic basalts and implications for magmatic processes within the mantle. American Journal of Science 280:389-426

Kaeser B, Kalt A, Pettke T (2006) Evolution of the lithopsheric mantle beneath the Marsabit volcanic field (northern Kenya): constraints from textural, P-T and geochemical studies on xenoliths. Journal of Petrology 47(11):2149-2184

Vernon RH (2004) A practical guide to rock microstructures, vol. University Press, Cambridge, p 594