Investigations of mineral textures and zoning as evidence for open system processes during magmatic evolution have always been a centerpiece of petrological studies and have provided some of the best evidence for magma mixing and crustal contamination for many decades (e.g., Milch 1905; Kuno 1936; Eichelberger 1975; Sato 1975; Anderson 1976). In fact, evidence from mineral studies was instrumental in the acceptance of magma mixing as important petrological process ever since it was initially proposed by Bunsen (1851). In recent years, mineral studies are invigorated by the development of high-precision, high-resolution analytical instruments and techniques through which textural information and compositional data are combined (e.g., Jerram and Davidson 2007). As minerals respond texturally and compositionally to changing magmatic environments, they preserve in their crystal growth stratigraphy a wealth of information regarding their past history of magmatic processes and compositions (cf. Ginibre et al. 2007). On the other hand, magmatic liquids (melts) are snapshots of current magmatic states, and provide less direct evidence of the processes responsible for their evolution. In addition, liquids may crystallize thereby destroying the direct evidence they provide. With an appropriate set of observations and measurements, we can correlate textures with mineral compositions and thus produce a richer composite picture of magmatic evolution than compositional data of minerals alone. This contribution reviews the zoning and textures of minerals frequently encountered in volcanic rocks (many of which apply equally to plutonic rocks) and the interpretations ascribed to these features in terms of open system magmatic processes.

First, I will address the analytical tools that are used to acquire textural and compositional data, followed by a brief review of open system processes and a summary of the relevant textures. From that point on, I will address some mineral specific aspects followed by a discussion …

Mineral Textures and Zoning as Evidence for Open System Processes

Apatite in the mantle: implications for metasomatic processes and high heat production in Phanerozoic mantle

2·1 my) includesIts particularly simple elemental and isotopic geochemistry suggests, early and waning stages with alkalic activity bracketing a steady-however, a rather homogeneous mantle source and makes this volcano state stage of tholeiitic activity. We estimate that Re´union volcanismideally suited for petrogenetic studies. We report mineralogical results from the impingement on the base of lithosphere of a solitarydescriptions, major element, trace element and Sr isotope compositions wave of hotspot material with a radius of 100–130 km and aof four volcanic sequences spanning the 530 ka of the known velocity of 5–9 cm/yr.lifetime of the volcano. The lavas change with time from mildlyalkalic to mildly tholeiitic. This change is due to crystal fractionationbecoming shallower with time as olivine replaces clinopyroxeneas the liquidus phase at lower pressure. Fractionation of anolivine–clinopyroxene assemblage at lithospheric–mantle pressure

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The Geochemical Regimes of Piton de la Fournaise Volcano (Reunion) During the Last 530 000 Years

All subaerial lavas at Mauna Kea Volcano, Hawaii, belong to the postshield stage of volcano construction. This stage formed as the magma supply rate from the mantle decreased. It can be divided into two substages: basaltic (∼240–70 ka) and hawaiitic (∼66–4 ka). The basaltic substage (Hamakua Volcanics) contains a diverse array of lava types including picrites, ankaramites, alkalic and tholeiitic basalt, and high Fe-Ti basalt. In contrast, the hawaiitic substage (Laupahoehoe Volcanics) contains only evolved alkalic lavas, hawaiite, and mugearite; basalts are absent. Sr and Nd isotopic ratios for lavas from the two substages are similar, but there is a distinct compositional gap between the substages. Lavas of the hawaiitic substage can not be related to the older basalts by shallow pressure fractionation, but they may be related to these basalts by fractionation at moderate pressures of a clinopyroxene-dominated assemblage. We conclude that the petrogenetic processes forming the postshield lavas at Mauna Kea and other Hawaiian volcanoes reflect movement of the volcano away from the hotspot. Specifically, we postulate the following sequence of events for postshield volcanism at Mauna Kea: (1) As the magma supply rate from the mantle decreased, major changes in volcanic plumbing occurred. The shallow magma chamber present during shield construction cooled and crystallized, and the fractures enabling magma ascent to the magma chamber closed. (2) Therefore subsequent basaltic magma ascending from the mantle stagnated within the lower crust, or perhaps at the crust-mantle boundary. Eruptions of basaltic magma ceased. (3) Continued volcanism was inhibited until basaltic magma in the lower crust cooled sufficiently to create relatively low-density, residual hawaiitic melts. Minor assimilation of MORB-related wall rocks, reflected by a trend toward lower 206Pb/204Pb in evolved postshield lavas, may have occurred at this time. A compositional gap developed because magma ascent was not possible until a low-density hawaiitic melt could escape from a largely crystalline mush. Eruption of this melt created aphyric hawaiite and mugearite lavas which incorporated cumulate gabbro, wherlite, and dunite xenoliths during ascent.

Evolution of Mauna Kea Volcano, Hawaii: Petrologic and geochemical constraints on postshield volcanism

Post-collisional transition from calc-alkaline to alkaline volcanism during the Neogene in Oranie (Algeria): magmatic expression of a slab breakoff

The Quaternary foidites and basanites of the West Eifel (Germany) contain optically and chemically heterogeneous clinopyroxenes, some of which occur as discrete zones within individual crystals: Most clinopyroxene phenocrysts are made up of a core and a normally zoned comagmatic titanaugite mantle Most cores are greenish pleochroic and moderately resorbed (fassaitic augite) Some are pale green and strongly resorbed (acmitic augite) Cores of Al-augite composition and of Cr-diopside derived from peridotite xenoliths are rare The fassaitic augites are similar in trace element distribution pattern to the titanaugites, but are more enriched in incompatible elements The acmitic augites, in contrast, are clearly different in their trace element composition and are enriched in Na, Mn, Fe and depleted in Al, Ti, Sr, Zr A model for polybaric magma evolution in the West Eifel is proposed: Primitive alkali basaltic magma rises through the upper mantle precipitating Al-augite en route It stagnates and differentiates near the crust/mantle boundary crystallizing Fe-rich fassaitic augites The magma differentiated at high pressure is subsequently mixed with new pulses of primitive magma from which the rims of pyroxene are crystallized Sporadic alkali pyroxenite xenoliths are interpreted to represent cumulates of cognate phases formed within the crust and not metasomatized upper mantle material (Lloyd and Bailey 1975)

Antje Duda

Papers

Polybaric differentiation of alkali basaltic magmas: evidence from green-core clinopyroxenes (Eifel, FRG)