BH250-234
Title
BH250-234
Subject
Websterite
Description
Major Minerals: orthopyroxene, clinopyroxene, olivine (Fo), serpentine, omphacite
Tonsina near Sheep Mountain lies within a geologically complex region of southern Alaska where fragments of oceanic crust, mantle rocks, and accreted terranes are exposed along major tectonic belts. A websterite from this area is especially significant because it represents an ultramafic rock dominated by both Orthopyroxene and Clinopyroxene, with subordinate Olivine. Such rocks commonly form in the upper mantle or lower crust and are later brought to the surface through tectonic uplift, faulting, and regional accretion.
Websterite is a type of pyroxenite composed mainly of two pyroxenes: orthopyroxene and clinopyroxene. The presence of olivine (Fo-rich, magnesium-rich forsteritic composition) suggests a mantle-derived origin and relatively primitive ultramafic chemistry. In many Alaskan ultramafic bodies, these minerals crystallized from mafic to ultramafic magmas or formed as cumulate layers within deep magma chambers. In other cases, websterites may represent mantle residues or reaction zones where melts interacted with surrounding peridotite.
Ultramafic rocks are commonly classified on ternary diagrams using the relative proportions of olivine (Ol), orthopyroxene (Opx), and clinopyroxene (Cpx). In this scheme, rocks rich in olivine plot toward the peridotite fields, including dunite, harzburgite, lherzolite, and wehrlite. Rocks dominated by pyroxenes with little olivine plot into the pyroxenite fields. Websterite occupies the field where both orthopyroxene and clinopyroxene are abundant, with neither strongly dominant. If orthopyroxene dominates, the rock is classified as orthopyroxenite, whereas clinopyroxene-dominant compositions are clinopyroxenite. Thus, the
This sample would plot near the pyroxene-rich center of the Opx–Cpx side of the ternary diagram, slightly shifted toward olivine if olivine is present in notable amounts.
In thin section, this websterite shows coarse interlocking pyroxene crystals with high relief and two cleavages near 90°. Orthopyroxene commonly displays lower birefringence and straight extinction, whereas clinopyroxene shows higher birefringence and inclined extinction. Many of the pyroxene grains display excellent exsolution lamellae, recording slow cooling and subsolidus re-equilibration at depth. These fine lamellae commonly form as one pyroxene phase unmixes from another during cooling, and they are valuable indicators of the rock’s thermal history.
Olivine commonly appears fractured and partly replaced by Serpentine in mesh textures, indicating later hydration and alteration during uplift and weathering. The presence of Omphacite is particularly noteworthy because it may reflect localized high-pressure metamorphic overprinting or chemical re-equilibration during tectonic transport. Omphacite is more typical of eclogite-facies environments, so its occurrence suggests that this ultramafic body may preserve a more complex metamorphic history than simple igneous crystallization alone.
Tonsina near Sheep Mountain lies within a geologically complex region of southern Alaska where fragments of oceanic crust, mantle rocks, and accreted terranes are exposed along major tectonic belts. A websterite from this area is especially significant because it represents an ultramafic rock dominated by both Orthopyroxene and Clinopyroxene, with subordinate Olivine. Such rocks commonly form in the upper mantle or lower crust and are later brought to the surface through tectonic uplift, faulting, and regional accretion.
Websterite is a type of pyroxenite composed mainly of two pyroxenes: orthopyroxene and clinopyroxene. The presence of olivine (Fo-rich, magnesium-rich forsteritic composition) suggests a mantle-derived origin and relatively primitive ultramafic chemistry. In many Alaskan ultramafic bodies, these minerals crystallized from mafic to ultramafic magmas or formed as cumulate layers within deep magma chambers. In other cases, websterites may represent mantle residues or reaction zones where melts interacted with surrounding peridotite.
Ultramafic rocks are commonly classified on ternary diagrams using the relative proportions of olivine (Ol), orthopyroxene (Opx), and clinopyroxene (Cpx). In this scheme, rocks rich in olivine plot toward the peridotite fields, including dunite, harzburgite, lherzolite, and wehrlite. Rocks dominated by pyroxenes with little olivine plot into the pyroxenite fields. Websterite occupies the field where both orthopyroxene and clinopyroxene are abundant, with neither strongly dominant. If orthopyroxene dominates, the rock is classified as orthopyroxenite, whereas clinopyroxene-dominant compositions are clinopyroxenite. Thus, the
This sample would plot near the pyroxene-rich center of the Opx–Cpx side of the ternary diagram, slightly shifted toward olivine if olivine is present in notable amounts.
In thin section, this websterite shows coarse interlocking pyroxene crystals with high relief and two cleavages near 90°. Orthopyroxene commonly displays lower birefringence and straight extinction, whereas clinopyroxene shows higher birefringence and inclined extinction. Many of the pyroxene grains display excellent exsolution lamellae, recording slow cooling and subsolidus re-equilibration at depth. These fine lamellae commonly form as one pyroxene phase unmixes from another during cooling, and they are valuable indicators of the rock’s thermal history.
Olivine commonly appears fractured and partly replaced by Serpentine in mesh textures, indicating later hydration and alteration during uplift and weathering. The presence of Omphacite is particularly noteworthy because it may reflect localized high-pressure metamorphic overprinting or chemical re-equilibration during tectonic transport. Omphacite is more typical of eclogite-facies environments, so its occurrence suggests that this ultramafic body may preserve a more complex metamorphic history than simple igneous crystallization alone.
Coverage
Tonsina, Alaska, USA
Nearby Geographic Feature: Sheep Mountain
GPS Coordinates: 61.834914, -147.512594
Nearby Geographic Feature: Sheep Mountain
GPS Coordinates: 61.834914, -147.512594
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 234. Housed at Carleton College in Minnesota.
Contributor
Luc Mehl, '00
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-234,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/307.