BH250-112c
Title
BH250-112c
Subject
Spinel Lherzolite Mantle Xenolith, Dish Hill, California
Description
Major Minerals: olivine, orthopyroxene, clinopyroxene
Minor Minerals: spinel, Fe-Ti oxides, minor serpentine alteration
Textures: granular, protogranular, xenolithic, mantle-derived
Overview
BH250-112C is a mantle-derived xenolith enclosed within an alkali basalt erupted at Dish Hill, California. The specimen represents a rare fragment of the upper mantle that was transported rapidly to the Earth's surface by ascending basaltic magma. Mantle xenoliths such as this provide some of the most direct evidence available for the composition, texture, and evolution of the lithospheric mantle beneath western North America.
In hand sample, the xenolith appears as a greenish crystal-rich inclusion sharply enclosed within the darker volcanic host rock. Thin sections reveal a coarse-grained assemblage dominated by olivine and pyroxene that contrasts strongly with the fine-grained basaltic matrix. The mineral assemblage and texture are most consistent with a spinel lherzolite, the most common rock type of the shallow upper mantle.
Geological Setting
Dish Hill is a Quaternary cinder cone located within the eastern Mojave Desert near Ludlow, California. The volcanic center is well known for its abundant mantle xenoliths, which were entrained by rapidly ascending alkali basalt magmas. These eruptions sampled the subcontinental lithospheric mantle and delivered fragments of deep mantle rocks to the surface with minimal modification.
The Dish Hill xenolith suite has been extensively studied because it provides a window into the composition, deformation history, and metasomatic evolution of the mantle beneath the Mojave Desert. BH250-112C represents one of these mantle samples and offers a direct connection between deep Earth processes and surface volcanism.
Mineralogy
The xenolith is composed primarily of olivine accompanied by both orthopyroxene and clinopyroxene. This assemblage is characteristic of lherzolite, a fertile mantle rock capable of producing basaltic magmas through partial melting.
Olivine is the dominant mineral phase and forms large anhedral to subhedral grains throughout the specimen. Orthopyroxene occurs as low-birefringence grains intergrown with olivine, whereas clinopyroxene exhibits slightly higher birefringence and occupies interstitial positions between larger crystals. Small opaque grains are interpreted as spinel-group minerals and occur along grain boundaries and triple junctions.
The coexistence of olivine, orthopyroxene, clinopyroxene, and spinel distinguishes the xenolith from dunite and harzburgite and strongly supports classification as a spinel lherzolite.
Petrography
The thin sections display a coarse-grained granular texture typical of upper mantle peridotites. Individual mineral grains commonly range from several hundred micrometers to more than a millimeter in diameter and form an interlocking mosaic of olivine and pyroxene.
Olivine exhibits bright second-order interference colors, irregular fracturing, and locally undulose extinction. These features record deformation and recrystallization under mantle conditions prior to eruption. Orthopyroxene is recognized by its relatively subdued interference colors and lower birefringence, whereas clinopyroxene displays slightly brighter interference colors and more complex extinction behavior.
Many grain junctions approach 120°, indicating prolonged textural equilibration and recrystallization at high temperatures. Such equilibrium textures are characteristic of mantle rocks that have resided at depth for extended periods. Small opaque grains interpreted as spinel occur between silicate grains and further support a spinel-facies mantle origin.
Evidence for a Mantle Origin
Several petrographic characteristics indicate that BH250-112C originated within the upper mantle:
• Dominance of olivine and pyroxene.
• Presence of both orthopyroxene and clinopyroxene.
• Granular equilibrium texture.
• Triple-junction grain boundaries.
• Absence of volcanic groundmass.
• Sharp contact with the enclosing basalt.
• Presence of spinel consistent with upper mantle pressures.
These features differ fundamentally from those of igneous cumulates or crystal accumulations formed within shallow magma chambers. Instead, they reflect the slow cooling, deformation, and recrystallization expected within the lithospheric mantle.
Interaction with the Host Basalt
Although the xenolith remains remarkably well preserved, its margins record limited interaction with the surrounding basaltic magma. Minor fracturing, localized alteration, and subtle modification of grain boundaries likely developed during transport to the surface.
The preservation of the original mantle texture indicates rapid ascent. Had the xenolith remained within the magma for a prolonged period, extensive melting and recrystallization would have destroyed much of its primary mineralogy and texture. Instead, the basalt acted as a conveyor, transporting the xenolith from mantle depths to the surface while preserving much of its original character.
Interpretation
Based on its mineral assemblage and texture, BH250-112C is interpreted as a spinel lherzolite mantle xenolith derived from the subcontinental lithospheric mantle beneath the Mojave Desert.
Spinel lherzolite is considered a relatively fertile mantle composition and is widely regarded as an important source rock for basaltic magmas. The xenolith therefore provides a direct sample of the mantle reservoir from which many volcanic rocks ultimately originate.
The coarse-grained equilibrium texture, the presence of both pyroxenes, and the occurrence of spinel collectively indicate formation under upper mantle conditions. Subsequent entrainment by alkali basalt magma transported the xenolith rapidly to the surface, preserving a rare record of mantle mineralogy, deformation, and thermal history.
Geological and Educational Significance
Peridotite xenoliths provide a unique opportunity to examine rocks that normally reside tens of kilometers beneath the Earth's surface. BH250-112C allows students to study the mineralogy of the upper mantle directly and serves as an excellent introduction to mantle petrology and basalt genesis.
The specimen is particularly valuable for teaching:
• Mantle composition and structure
• Peridotite classification
• Olivine and pyroxene identification
• Spinel-facies mantle mineralogy
• Partial melting and basalt generation
• Xenolith transport mechanisms
• Mantle deformation and recrystallization
• Relationships between mantle rocks and volcanic magmas
Few teaching specimens provide such a direct connection between deep Earth processes and volcanic activity. BH250-112C is therefore one of the most important mantle petrology samples in the BH250 Collection.
Minor Minerals: spinel, Fe-Ti oxides, minor serpentine alteration
Textures: granular, protogranular, xenolithic, mantle-derived
Overview
BH250-112C is a mantle-derived xenolith enclosed within an alkali basalt erupted at Dish Hill, California. The specimen represents a rare fragment of the upper mantle that was transported rapidly to the Earth's surface by ascending basaltic magma. Mantle xenoliths such as this provide some of the most direct evidence available for the composition, texture, and evolution of the lithospheric mantle beneath western North America.
In hand sample, the xenolith appears as a greenish crystal-rich inclusion sharply enclosed within the darker volcanic host rock. Thin sections reveal a coarse-grained assemblage dominated by olivine and pyroxene that contrasts strongly with the fine-grained basaltic matrix. The mineral assemblage and texture are most consistent with a spinel lherzolite, the most common rock type of the shallow upper mantle.
Geological Setting
Dish Hill is a Quaternary cinder cone located within the eastern Mojave Desert near Ludlow, California. The volcanic center is well known for its abundant mantle xenoliths, which were entrained by rapidly ascending alkali basalt magmas. These eruptions sampled the subcontinental lithospheric mantle and delivered fragments of deep mantle rocks to the surface with minimal modification.
The Dish Hill xenolith suite has been extensively studied because it provides a window into the composition, deformation history, and metasomatic evolution of the mantle beneath the Mojave Desert. BH250-112C represents one of these mantle samples and offers a direct connection between deep Earth processes and surface volcanism.
Mineralogy
The xenolith is composed primarily of olivine accompanied by both orthopyroxene and clinopyroxene. This assemblage is characteristic of lherzolite, a fertile mantle rock capable of producing basaltic magmas through partial melting.
Olivine is the dominant mineral phase and forms large anhedral to subhedral grains throughout the specimen. Orthopyroxene occurs as low-birefringence grains intergrown with olivine, whereas clinopyroxene exhibits slightly higher birefringence and occupies interstitial positions between larger crystals. Small opaque grains are interpreted as spinel-group minerals and occur along grain boundaries and triple junctions.
The coexistence of olivine, orthopyroxene, clinopyroxene, and spinel distinguishes the xenolith from dunite and harzburgite and strongly supports classification as a spinel lherzolite.
Petrography
The thin sections display a coarse-grained granular texture typical of upper mantle peridotites. Individual mineral grains commonly range from several hundred micrometers to more than a millimeter in diameter and form an interlocking mosaic of olivine and pyroxene.
Olivine exhibits bright second-order interference colors, irregular fracturing, and locally undulose extinction. These features record deformation and recrystallization under mantle conditions prior to eruption. Orthopyroxene is recognized by its relatively subdued interference colors and lower birefringence, whereas clinopyroxene displays slightly brighter interference colors and more complex extinction behavior.
Many grain junctions approach 120°, indicating prolonged textural equilibration and recrystallization at high temperatures. Such equilibrium textures are characteristic of mantle rocks that have resided at depth for extended periods. Small opaque grains interpreted as spinel occur between silicate grains and further support a spinel-facies mantle origin.
Evidence for a Mantle Origin
Several petrographic characteristics indicate that BH250-112C originated within the upper mantle:
• Dominance of olivine and pyroxene.
• Presence of both orthopyroxene and clinopyroxene.
• Granular equilibrium texture.
• Triple-junction grain boundaries.
• Absence of volcanic groundmass.
• Sharp contact with the enclosing basalt.
• Presence of spinel consistent with upper mantle pressures.
These features differ fundamentally from those of igneous cumulates or crystal accumulations formed within shallow magma chambers. Instead, they reflect the slow cooling, deformation, and recrystallization expected within the lithospheric mantle.
Interaction with the Host Basalt
Although the xenolith remains remarkably well preserved, its margins record limited interaction with the surrounding basaltic magma. Minor fracturing, localized alteration, and subtle modification of grain boundaries likely developed during transport to the surface.
The preservation of the original mantle texture indicates rapid ascent. Had the xenolith remained within the magma for a prolonged period, extensive melting and recrystallization would have destroyed much of its primary mineralogy and texture. Instead, the basalt acted as a conveyor, transporting the xenolith from mantle depths to the surface while preserving much of its original character.
Interpretation
Based on its mineral assemblage and texture, BH250-112C is interpreted as a spinel lherzolite mantle xenolith derived from the subcontinental lithospheric mantle beneath the Mojave Desert.
Spinel lherzolite is considered a relatively fertile mantle composition and is widely regarded as an important source rock for basaltic magmas. The xenolith therefore provides a direct sample of the mantle reservoir from which many volcanic rocks ultimately originate.
The coarse-grained equilibrium texture, the presence of both pyroxenes, and the occurrence of spinel collectively indicate formation under upper mantle conditions. Subsequent entrainment by alkali basalt magma transported the xenolith rapidly to the surface, preserving a rare record of mantle mineralogy, deformation, and thermal history.
Geological and Educational Significance
Peridotite xenoliths provide a unique opportunity to examine rocks that normally reside tens of kilometers beneath the Earth's surface. BH250-112C allows students to study the mineralogy of the upper mantle directly and serves as an excellent introduction to mantle petrology and basalt genesis.
The specimen is particularly valuable for teaching:
• Mantle composition and structure
• Peridotite classification
• Olivine and pyroxene identification
• Spinel-facies mantle mineralogy
• Partial melting and basalt generation
• Xenolith transport mechanisms
• Mantle deformation and recrystallization
• Relationships between mantle rocks and volcanic magmas
Few teaching specimens provide such a direct connection between deep Earth processes and volcanic activity. BH250-112C is therefore one of the most important mantle petrology samples in the BH250 Collection.
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 112c. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-112c,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/133.