BH250-80b
Title
BH250-80b
Subject
Serpentinized Peridotite
Description
Major Mineral: serpentine
Minor Mineral: olivine altered to serpentine
BH250-86b is a sample collected from the Upper Peninsula of Michigan, specifically from the tip of Presque Isle. It represents part of an ultramafic body commonly known as the Presque Isle Peridotite. This body is unconformably overlain by the Jacobsville Sandstone, a widespread Proterozoic sedimentary unit prominently exposed along the southern shoreline of Lake Superior.
Degree of Alteration:
The Presque Isle peridotite has undergone extensive serpentinization, a hydrothermal alteration process in which primary minerals like olivine and pyroxene have been transformed into serpentine minerals, predominantly antigorite and lizardite. The resulting rock is a magnetite-rich serpentinite, characterized by a bluish-green color and weak magnetism due to the presence of fine-grained magnetite.
Petrographic analysis of thin sections confirms that the original plagioclase lherzolite has been altered to serpentinite. Secondary minerals such as chlorite and calcite are present, indicating ongoing interaction between the rock and circulating fluids during and after the serpentinization process.
Conjugate Fractures:
The outcrop at this site exhibits well-defined conjugate fracture sets, which are clearly visible in the field and documented in photographs. These fractures play a significant role in both structural deformation and chemical alteration. Fluid movement through these fractures is evident from mineral-filled veins and alteration halos surrounding them.
Conjugate fractures occur in pairs, typically intersecting at high angles—often between 60° and 70°. In the Presque Isle peridotite, they are found cutting across massive serpentinite and other altered ultramafic zones. These fractures often act as conduits for fluid flow and are commonly filled with secondary minerals such as calcite, serpentine, or chlorite.
Their formation is attributed to a combination of tectonic stress and hydration-induced volume expansion. As olivine and pyroxene transform into serpentine minerals during serpentinization, the rock undergoes a net volume increase—typically up to 30–40%. This expansion generates internal pressure, which contributes to the development of these brittle fracture networks, facilitating further alteration and mineral deposition.
Minor Mineral: olivine altered to serpentine
BH250-86b is a sample collected from the Upper Peninsula of Michigan, specifically from the tip of Presque Isle. It represents part of an ultramafic body commonly known as the Presque Isle Peridotite. This body is unconformably overlain by the Jacobsville Sandstone, a widespread Proterozoic sedimentary unit prominently exposed along the southern shoreline of Lake Superior.
Degree of Alteration:
The Presque Isle peridotite has undergone extensive serpentinization, a hydrothermal alteration process in which primary minerals like olivine and pyroxene have been transformed into serpentine minerals, predominantly antigorite and lizardite. The resulting rock is a magnetite-rich serpentinite, characterized by a bluish-green color and weak magnetism due to the presence of fine-grained magnetite.
Petrographic analysis of thin sections confirms that the original plagioclase lherzolite has been altered to serpentinite. Secondary minerals such as chlorite and calcite are present, indicating ongoing interaction between the rock and circulating fluids during and after the serpentinization process.
Conjugate Fractures:
The outcrop at this site exhibits well-defined conjugate fracture sets, which are clearly visible in the field and documented in photographs. These fractures play a significant role in both structural deformation and chemical alteration. Fluid movement through these fractures is evident from mineral-filled veins and alteration halos surrounding them.
Conjugate fractures occur in pairs, typically intersecting at high angles—often between 60° and 70°. In the Presque Isle peridotite, they are found cutting across massive serpentinite and other altered ultramafic zones. These fractures often act as conduits for fluid flow and are commonly filled with secondary minerals such as calcite, serpentine, or chlorite.
Their formation is attributed to a combination of tectonic stress and hydration-induced volume expansion. As olivine and pyroxene transform into serpentine minerals during serpentinization, the rock undergoes a net volume increase—typically up to 30–40%. This expansion generates internal pressure, which contributes to the development of these brittle fracture networks, facilitating further alteration and mineral deposition.
Coverage
Location: Upper Peninsula, Michigan, USA
Presque Isle
46°35'35.95"N
87°22'48.63"W
Presque Isle
46°35'35.95"N
87°22'48.63"W
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-80b. Housed at Carleton College in Minnesota.
Contributor
Bereket Haileab
Type
Thin section and hand sample
Relation
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Collection
Citation
Bereket Haileab, “BH250-80b,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/348.