BH250-250a
Title
BH250-250a
Subject
Pillow Basalt
Description
Pillow Basalt sample showing a chilled margin, vesicles, vesicle pipes and Fe-Mn oxide coatings.
BH250-250a specimen was collected in 2018 aboard the Nathaniel B. Palmer during a cruise to the South Atlantic Ocean, targeting the Rio Grande Rise. The sample was generously provided by Peter C. Davidson, whose contributions and scholarship is gratefully acknowledge. According to Peter’s personal communication, the exact coordinates of collection are 28.1133° S, 31.1755° W, at a feature named the Gienah Gurab Escarpment along the northern edge of the Rio Grande Rise in the Atlantic Ocean. The sample was recovered from a depth between 4300 and 3400 meters.
BH250-250a is a fragment of a pillow basalt with striking characteristics. It contains numerous vesicles, including excellent examples of pipe vesicles, which indicate the basalt originally erupted in much shallower water than where it presently resides. Many of these vesicles are now partially filled with secondary clay minerals, giving the rock an amygdaloidal texture. Particularly notable is the sharp color transition from the reddish, oxidized, quenched outer rim (top view of the pillow basalt) to the greyer, more coarsely crystalline interior.
This pattern reflects variations in cooling rate, oxidation, and alteration. A thin (~1 mm) Mn-oxide crust, visible along the sides and top of the specimen, records subsequent seafloor chemical processes. High-resolution photographs document all sides of the sample, highlighting the diversity in vesicle sizes, orientations, and the remarkable preservation of pipe vesicles. Reference: Davidson, P. C., Koppers, A. A. P., Class, C., Sager, W. W., & Heaton, D. (2025). 40Ar/39Ar dating reveals over 30 million years of plume–ridge interaction formed the Rio Grande Rise. Communications Earth & Environment, 6, 588. https://www.nature.com/articles/s43247-025-02572-y Thanks to Peter C. Davidson not only for generously sharing this specimen, but also for providing the detailed notes and context that make this catalog entry possible. Kein Dolge assisted with data collection.
Chemical analysis: A quick chemical analysis was carried out on specimen BH250-250a using a Bruker portable handheld XRF. Measurements were taken from the bottom, the center, the chilled margin, and the top of the sample to compare chemical variations across the pillow basalt. The results show that the bottom, center, and chilled margin are consistent with a basaltic composition, with SiO₂ contents ranging from ~50–55 wt. %. The chilled margin (49.79 wt. % SiO₂) and the center (54.72 wt. % SiO₂) also contain moderate Al₂O₃ (14–18 wt. %), Fe (10–11 wt. %), MgO (2–5 wt. %), and Ca (3–5 wt. %), typical of basaltic glass and crystalline interiors. These values suggest the rock preserves its primary igneous composition. In contrast, the top of the pillow basalt exhibits strikingly different chemistry. It contains much lower SiO₂ (14.40 wt. %) and Al₂O₃ (3.08 wt. %), but is enriched in Fe (18.08 wt. %) and Mn (15.25 wt. %), along with elevated Cl (3.55 wt. %). These features indicate that the top is not primary basalt but rather a surface enriched in iron- and manganese-oxide precipitates.
Such oxide coatings commonly develop on submarine basaltic surfaces, where hydrothermal or seawater interaction promotes the deposition of Fe–Mn crusts. Taken together, the data highlight the basaltic core of BH250-250aa while also documenting the secondary precipitation of Fe- and Mn-rich oxides on its outer surface, providing a clear chemical distinction between primary igneous composition and later seafloor alteration.
BH250-250a specimen was collected in 2018 aboard the Nathaniel B. Palmer during a cruise to the South Atlantic Ocean, targeting the Rio Grande Rise. The sample was generously provided by Peter C. Davidson, whose contributions and scholarship is gratefully acknowledge. According to Peter’s personal communication, the exact coordinates of collection are 28.1133° S, 31.1755° W, at a feature named the Gienah Gurab Escarpment along the northern edge of the Rio Grande Rise in the Atlantic Ocean. The sample was recovered from a depth between 4300 and 3400 meters.
BH250-250a is a fragment of a pillow basalt with striking characteristics. It contains numerous vesicles, including excellent examples of pipe vesicles, which indicate the basalt originally erupted in much shallower water than where it presently resides. Many of these vesicles are now partially filled with secondary clay minerals, giving the rock an amygdaloidal texture. Particularly notable is the sharp color transition from the reddish, oxidized, quenched outer rim (top view of the pillow basalt) to the greyer, more coarsely crystalline interior.
This pattern reflects variations in cooling rate, oxidation, and alteration. A thin (~1 mm) Mn-oxide crust, visible along the sides and top of the specimen, records subsequent seafloor chemical processes. High-resolution photographs document all sides of the sample, highlighting the diversity in vesicle sizes, orientations, and the remarkable preservation of pipe vesicles. Reference: Davidson, P. C., Koppers, A. A. P., Class, C., Sager, W. W., & Heaton, D. (2025). 40Ar/39Ar dating reveals over 30 million years of plume–ridge interaction formed the Rio Grande Rise. Communications Earth & Environment, 6, 588. https://www.nature.com/articles/s43247-025-02572-y Thanks to Peter C. Davidson not only for generously sharing this specimen, but also for providing the detailed notes and context that make this catalog entry possible. Kein Dolge assisted with data collection.
Chemical analysis: A quick chemical analysis was carried out on specimen BH250-250a using a Bruker portable handheld XRF. Measurements were taken from the bottom, the center, the chilled margin, and the top of the sample to compare chemical variations across the pillow basalt. The results show that the bottom, center, and chilled margin are consistent with a basaltic composition, with SiO₂ contents ranging from ~50–55 wt. %. The chilled margin (49.79 wt. % SiO₂) and the center (54.72 wt. % SiO₂) also contain moderate Al₂O₃ (14–18 wt. %), Fe (10–11 wt. %), MgO (2–5 wt. %), and Ca (3–5 wt. %), typical of basaltic glass and crystalline interiors. These values suggest the rock preserves its primary igneous composition. In contrast, the top of the pillow basalt exhibits strikingly different chemistry. It contains much lower SiO₂ (14.40 wt. %) and Al₂O₃ (3.08 wt. %), but is enriched in Fe (18.08 wt. %) and Mn (15.25 wt. %), along with elevated Cl (3.55 wt. %). These features indicate that the top is not primary basalt but rather a surface enriched in iron- and manganese-oxide precipitates.
Such oxide coatings commonly develop on submarine basaltic surfaces, where hydrothermal or seawater interaction promotes the deposition of Fe–Mn crusts. Taken together, the data highlight the basaltic core of BH250-250aa while also documenting the secondary precipitation of Fe- and Mn-rich oxides on its outer surface, providing a clear chemical distinction between primary igneous composition and later seafloor alteration.
Coverage
Rio Grande Rise, South Atlantic Ocean
GPS coordinate: 28.1133° S, 31.1755° W
GPS coordinate: 28.1133° S, 31.1755° W
Date
2018
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. BH250-250a. Housed at Carleton College in Minnesota.
Publisher
Bereket Haileab
Contributor
Bereket Haileab
Peter C Davidson
Peter C Davidson
Type
Hand Sample
Relation
Collection
Citation
Bereket Haileab, “BH250-250a,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/371.