Textures of Basalts and Gabbros

Introduction

Basalts and gabbros are volcanic rocks that are common to the Earth, Moon, Mars, many large asteroids, and probably to Mercury and Venus. From the Apollo Missions and meteorites that we have received from the Moon, Mars, and asteroids, we have learned that the general characteristics of all basaltic samples are very similar and for the most part we can apply the textural characteristics of Earth rocks to those from extraterrestrial sources. With some limitations, we can also constrain the physical and chemical conditions under which these extraterrestrial rocks crystallized.

Textures

Texture refers to the degree of crystallinity, grain size, and fabric (geometrical relationships) among the constituents of a rock. Textural features are probably the most important aspect of an igneous rock because they are a necessary aid in understanding the conditions under which igneous rocks crystallized (e.g., cooling and nucleation rates and order of crystallization) that in turn depend on initial composition, temperature, pressure, gaseous contents and the viscosity of the magma.

Degree of crystallinity – Rocks composed entirely of crystals are called holocrystalline; those composed entirely of glass are holohyaline; rocks that contain both crystals and glass are hypocrystalline.

Grain size - Overall, there is a distinction between the grain size of rocks that have crystallized at depth and are medium to coarse grained (e. g., gabbros) and those that crystallized at shallow depth as dikes or were effusive are finer grained (e. g., basalts). Grain sizes of terrestrial and meteorite igneous rocks are defined as follows:

The cut-off grain size between basalt and gabbro for both sets of rocks is 2 mm.

Rock fabric – Fabric is the shape and mutual relationships among rock constituents:

1. Euhedral, idiomorphic, or automorphic refer to grains that are bounded by crystal faces
2. Subhedral or hypidiomorphic are grains that are bounded by some crystal faces
3. Anhedral, allotrimorphic, or xenomorphic grains are devoid of crystal faces

Terms

Essential textural terms (those that may be encounter in reading meteorite classifications and research papers):

• Amygdules – vesicles filled with low temperature or alteration minerals.
• Cumulate – igneous rocks formed by the accumulation of early-formed minerals by the action of gravity. Orthocumulates form when the intercumulus liquid is compositionally different from earlier formed cumulate crystals and precipitates different intercumulus minerals. Adcumulates form when the intercumulus liquid is similar in composition to cumulate crystals and results in additional growth of cumulus grains of similar composition. Adcumulates are essentially monomineralic, although the growth boundary between the original cumulate grain and later adcumulus growth is recognizable. Those rocks that are intermediate between orthocumulates and adcumulates are called mesocumulates.
  
  

  Cumulate texture – An olivine gabbro with cumulate olivine, plagioclase, pyroxene, and magnetite. Plagioclase and magnetite are strongly oriented. Base width = 8 mm. Skaergaard Layered Intrusion (stratiform structure), Lower Zone C. Image © T. E. Bunch, 2007.

  

  Orthocumulate texture – Harzburgite with cumulate olivine and chromite and intercumulus plagioclase. Stillwater Layered Intrusion. Width = 8 mm. Image © T. E. Bunch, 2007.

• Granular – most of the grains are equant.
• Granoblastic – typical of non-foliated metamorphic rocks without porphyroblasts. Grains have sutured boundaries, are approximately equidimensional, and meet at ~120° triple junctions. Characteristic of recrystallization.
  
  

  Recrystallization texture (granoblastic) - Mantle xenolith composed of polygonal grains of olivine and pyroxene that recrystallized under deformational pressures at depth. Similar polygonal textures are shown by metachondrites and “primitive achondrites” that have recrystallized from precursor chondrites. Width = 13 mm. Image © T. E. Bunch, 2007.

• Hyaloophitic – angular interstices between feldspars filled with glass.
• Hypidiomorphic or hypautomorphic granular – the most common granular texture in which a mixture of euhedral, subhedral, and anhedral grains are present.
• Intergranular – angular interstices between feldspars occupied by pyroxene granules (very small grains).
• Intersertal – interstices filled with a mixture of glass and some pyroxene.
• Matrix, groundmass, or mesostasis – fine-grained or glassy medium in which large grains are set.
• Ophitic texture – is one where random plagioclase laths are enclosed by pyroxene or olivine. If plagioclase is larger and encloses the ferromagnesian minerals, then the texture is subophitic and the laths typically impinge on one another to form sharp angles. Note that the change from intergranular through subophitic to ophitic textures in basaltic rocks results from slower cooling and slower nucleation rates. This textural sequence is typically found at the margins toward the center in diabasic or doleritic rocks (basaltic dikes) or from the chilled surface to depth of basaltic flows. If the cooling rate is very fast, the interstitial material between plagioclase laths may be quenched to glass to form an intersertal texture.
  
  

  Ophitic texture – An olivine gabbro that contains cumulate olivine and plagioclase in a matrix of intercumulate pyroxene and magnetite (black) that crystallized from trapped intercumulate liquid. Base width = 17 mm. From the Skaergaard layered intrusion (stratiform structure). Lower Zone A. Image © T. E. Bunch, 2007.

  

  Subophitic texture – Poikilitic pyroxene (oikocrysts) grew between large plagioclase laths and enclosed smaller plagioclase. Base width = 17 mm. Skaergaard Intrusion, Upper Border Group α. Image © T. E. Bunch, 2007.

• Pilotaxitic – rocks in which feldspar microlites (tiny grains) are arranged in a flow oriented, felty matrix.
• Porphyritic or phyric – refers to a rock that contains grains of distinctly different sizes. Large crystals that are surrounded by finer-grained matrix are referred to as phenocrysts. If the matrix or groundmass is glassy, then the rock has a vitrophyric texture. Phenocrysts that cluster and grow together form a glomeroporphyritic texture.
  
  

  Olivine phyric basalt – Phenocrysts of olivine and a glomerocryst of smaller olivine (purple) and plagioclase set in an intergranular matrix of plagioclase and granular pyroxene. Width = 6 mm. Maui, Hawaii. Image © T. E. Bunch, 2007.

  
  

  Porphyritic or phyric texture – Olivine phenocrysts set in a fine-grained, intergranular matrix. Round clear structures are gas vesicles. Olivine basalt from the DPSP (Deep Sea Drilling Program) Leg 34. Base width = 8 mm. Image © T. E. Bunch, 2007.

• Poikilitic texture – refers to small, typically euhedral crystals (chadacrysts), that are enclosed (included) within a much larger mineral of different composition. Unlike the porphyritic texture, the large crystals known as oikocrysts, are devoid of crystal faces. Chadacryst also refers to a grain that is foreign to the rest of the rock a.k.a. xenocryst.
  
  

  Poikilitic texture. Orthopyroxene oikocryst that encloses rounded chadacrysts of olivine. Image height = 17 mm. Image © T. E. Bunch, 2007.

• Spherulitic – radial aggregates of acicular or fibrous crystals.
• Stratiform or layered igneous rocks – Mafic to ultramafic rocks in stratiform intrusions are formed by crystal settling in much the same way that sedimentary rocks form. The resulting textures and structures reflect sedimentary processes controlled by gravity, density, viscosity, grain shapes and sizes, and magmatic currents.
• Variolitic (varioles) – divergent plagioclase laths with interstitial glass or intergrown with pyroxene granules.
  
  

  Multi-textured basalt - Plumose to variolitic texture on the right hand side intermingled with an intersertal texture to the left. Width = 5 mm. DSDP Leg 16. Image © T. E. Bunch, 2007.

• Vesicles – cavities from expanded gases (vesicular texture).