https://bereket-haileab.geology.sites.carleton.edu/files/original/da98f6b873d85c5ec3a25358375f1225.jpeg df8533def48341d8a24862a55165e7b2 Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Igneous Rocks Subject The topic of the resource This collection covers both volcanic and intrusive rocks, including rhyolites, granites, diorites, andesites, basalts, gabbros, diabases, pumices, obsidians, syenites, phonolites, peridotites, carbonatites, dacites, trachyte, dunites, xenoliths, ignimbrites, and pegmatites. Creator An entity primarily responsible for making the resource Bereket Haileab Source A related resource from which the described resource is derived Housed in Anderson Hall, Carleton College. Format The file format, physical medium, or dimensions of the resource Thin sections, thin section scans, hand samples, hand sample images, photomicrographs, chemical analyses (major elements, trace elements and REE analyses for selected samples). Thin section Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource BH250-15 Subject The topic of the resource Andesite, true andesite Description An account of the resource Major Minerals: amphibole, plagioclase <br />Minor Mineral: glass <br />Texture: amphibole dehydration, trachytic flow<br /><br /> <p>BH250-15b is an andesite from Mt. Shasta, California. When plotted on the TAS (Total Alkali–Silica) diagram, it falls within the dacite field, indicating a slightly more evolved composition than typical andesite. Twinned amphiboles and plagioclase feldspar are common. Its mineralogy includes phenocrysts of hornblende and plagioclase feldspar. The matrix consists of fine-grained feldspar and volcanic glass. Both phenocryst types exhibit zoning, suggesting changes in magmatic conditions during crystallization. The hornblende phenocrysts display dehydration reaction rims, with the extent of alteration ranging from thin margins to rims so extensive that they have nearly consumed the amphibole entirely.<br /><br /></p> <p><span style="color: black;">In this sample, you can observe<span class="apple-converted-space">&nbsp;</span><strong data-start="120" data-end="146"><span style="font-weight: normal;">hornblende dehydration</span></strong><b>,</b> a key mineral reaction that records changing magma conditions during ascent. Hornblende is a hydrous amphibole, stable only at relatively high pressure and in water-rich magmas. As magma rises, pressure decreases and dissolved water exsolves, destabilizing hornblende. It then breaks down into minerals stable under drier, lower-pressure conditions, typically forming pyroxene, plagioclase, and iron oxides such as magnetite, while releasing water vapor.<o:p></o:p></span></p> <p style="caret-color: #000000; font-variant-caps: normal; orphans: auto; text-align: start; widows: auto; -webkit-text-stroke-width: 0px; word-spacing: 0px;" data-start="604" data-end="1212"><span style="color: black;">This process commonly produces<span class="apple-converted-space">&nbsp;</span><strong data-start="635" data-end="652"><span style="font-weight: normal;">reaction rims</span></strong><span class="apple-converted-space">&nbsp;</span>around hornblende crystals. These dark, opaque “opacite rims” consist of very fine-grained oxides and pyroxenes and indicate that the dehydration reaction began but did not go to completion, usually due to relatively rapid magma ascent. In contrast, complete alteration occurs when hornblende is entirely replaced by these secondary minerals, preserving only the original crystal shape as a<span class="apple-converted-space">&nbsp;</span><strong data-start="1044" data-end="1059"><span style="font-weight: normal;">pseudomorph</span></strong>. In some cases, the breakdown products may also include minor glass or extremely fine-grained intergrowths, reflecting rapid quenching near the surface.<o:p></o:p></span></p> <p style="caret-color: #000000; font-variant-caps: normal; orphans: auto; text-align: start; widows: auto; -webkit-text-stroke-width: 0px; word-spacing: 0px;" data-start="1214" data-end="1671"><span style="color: black;">The thickness and development of these rims provide important clues about magma dynamics. Thin rims suggest rapid ascent and limited time for reaction, whereas thick rims or complete replacement indicate slower ascent, longer residence at shallow depth, or more extensive degassing prior to eruption. These textures are therefore considered classic<span class="apple-converted-space">&nbsp;</span><strong data-start="1563" data-end="1590"><span style="font-weight: normal;">disequilibrium features</span></strong>, showing that minerals did not have time to fully adjust to changing conditions.<o:p></o:p></span></p> <p style="caret-color: #000000; font-variant-caps: normal; orphans: auto; text-align: start; widows: auto; -webkit-text-stroke-width: 0px; word-spacing: 0px;" data-start="1673" data-end="2187"><strong data-start="1673" data-end="1821"><span style="color: black; font-weight: normal;">Compared to BH250-15b and BH250-120, where most of the hornblende is completely dehydrated and replaced, this sample preserves earlier stages of the reaction.</span></strong><span class="apple-converted-space"><span style="color: black;">&nbsp;</span></span><span style="color: black;">The presence of partial reaction rims here suggests that magma ascent was faster or that the crystals experienced less time under low-pressure, degassing conditions. In contrast, the more complete dehydration observed in BH250-120 indicates longer residence at shallow depths or more extensive pre-eruptive degassing, allowing the reaction to proceed to completion.<o:p></o:p></span></p> <p style="caret-color: #000000; font-variant-caps: normal; orphans: auto; text-align: start; widows: auto; -webkit-text-stroke-width: 0px; word-spacing: 0px;" data-start="2189" data-end="2640" data-is-last-node="" data-is-only-node=""><span style="color: black;">Overall, hornblende dehydration textures are powerful indicators of magma evolution. Because hornblende stability is strongly tied to water content, its breakdown provides direct evidence of<span class="apple-converted-space">&nbsp;</span><strong data-start="2380" data-end="2424"><span style="font-weight: normal;">pre-eruptive degassing and decompression</span></strong><span class="apple-converted-space">&nbsp;</span>in volcanic systems. These features are especially common in intermediate magmas such as andesite and dacite and are widely used to interpret ascent rates, storage conditions, and eruption dynamics in volcanic arcs.</span></p> <br /><br />Optics: pleochroism in amphibole, Carlsbad twining Source A related resource from which the described resource is derived From the rock collection of Bereket Haileab. Sample BH250-15. Housed at Carleton College in Minnesota. Type The nature or genre of the resource Thin section Relation A related resource <iframe width="100%" height="600" style="border: 0;" src="https://carleton.maps.arcgis.com/apps/instant/media/index.html?appid=87cb287111644d20a3e727d83a795ebd" frameborder="0" allowfullscreen="allowfullscreen">iFrames are not supported on this page.</iframe> <a href="&lt;iframe src=&quot;https://carleton.maps.arcgis.com/apps/instant/media/index.html?appid=87cb287111644d20a3e727d83a795ebd&quot; width=&quot;400&quot; height=&quot;600&quot; frameborder=&quot;0&quot; style=&quot;border:0&quot; allowfullscreen&gt;iFrames are not supported on this page.&lt;/iframe&gt;"></a><br />View on ArcGIS Online <a href="https://carleton.maps.arcgis.com/home/item.html?id=1556b48fc03540b8ada472f0d3898607">here</a><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1PH1WZBRbYyGwWIIVKjTXjty3EuuyPYZv/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/15C51AErjn3uuvHFr43n4LlT_sE05W3TY/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1liRBBlmDfx2Kz1O2zH_6RdDuKVrktIGP/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1lECGx-VS8E82aLsboB-AlvPzttCZvYIV/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1wRtiI5ouH9z13twFZvMcKVCOGC4onQ_G/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1akJjToYhi_k7g7O05-WDpl7yyvpin-Rv/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1O1wubeYi99yo0HLadOHFCF5X0B8hqCme/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1691cxuMO9uSKM-OkM45r7EA0t7f8j2Ad/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1g3MpeQE7MgLR5Ao86WOhdWySOmXA0pFn/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/15YS5CGEvCiUWUzhNzO4Hv9A0PCEa6Lw9/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1olveWFKtwgPe4XEhcmEwVZvMoA_cGk_N/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1QGSnmx5dbGrd-sDD9tnz5kdZB11Qqzw7/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/12KSdFWvzb1j86AQDcCylAtr4zILSriR_/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1KAWsU-CLwiFFVNvaohn9IQ5ePgGwqT5P/preview" allow="autoplay"></iframe> Creator An entity primarily responsible for making the resource Bereket Haileab Contributor An entity responsible for making contributions to the resource Bereket Haileab andesite dacite Dehydration reaction in amphiboles Dehydration Rim felsic Hornblende igneous intermediate Mt Shasta phenocryst Plagioclase TagsAmphibole Alteration TAS diagram Total Alkali–Silica diagram volcanic Volcanic glass Zoning