Dear Yong-Fei
I'm afraid I have to take issue with your distinction between zeolite facies metamorphism and hydrothermal metamorphism. First, the distinction you make is between burial metamorphism and hydrothermal metamorphism. Zeolite facies metamorphic zones are well known from both settings, although the scale of the zones described by Walker is smaller than that of Coombs' burial zeolite zones. And secondly, both involve hydration to form zeolites from igneous material. Subsequent dehydration is optional. In both settings it is possible for some of the different zone assemblages to grow directly under different conditions, although Coombs showed that the higher grade burial metamorphic zones in which laumontite, adularia and then prehnite and pumpellyite appear are the result of dehydration of lower grade zeolite zones. It is also of course possible to get dehydration of early-formed hydrous assemblages in hydrothermal metamorphism.
Bruce
Bruce Yardley
Emeritus Professor
School of Earth & Environment
University of Leeds
Leeds LS2 9JT
UK
Tel +44 (0)7745 132560
-----Original Message-----
From: Metamorphic Studies Group [mailto:[log in to unmask]] On Behalf Of Yong-Fei?Zheng
Sent: 03 August 2019 01:59
To: [log in to unmask]
Subject: Re: [geo-metamorphism] lower limit of metamorphism
Dear Kent,
The regional metamorphism of basalts at zeolite facies is caused by burial of the supracrustal rocks to shallow depths where the low temperatures of 200-300oC are only determined by the thermal gradient of lithosphere. In contrast, the hydrothermal metamorphism of basalts may take place at a large range of temperatures from as low as 150-200oc to as high as 800-1000oc, depending on the distance from spreading centers. Although both occur at low pressures, the physicochemical mechanism for the two types of metamorphism is significantly different. Zeolite facies metamorphism is realized by dehydration reactions at the low temperatures. In contrast, the hydrothermal metamorphism of basalts is realized by hydration reactions at the variable temperatures. As a consequence, there are a series of differences in mineral paragenesis between the two types of basalt metamorphism.
For the purpose of teaching students, you may introduce them the concept of metamorphic facies series by using a phase diagram, in which not only Alpine, Barrovian and Buchan facies series can be defined clearly by the difference in the thermal gradient of lithosphere but also a few lines for metamorphic reaction and polymorphic transition can be superimposed to show common mineralogical changes.
Best regards
Yong-Fei
> -----原始邮件-----
> 发件人: "Kent Ratajeski" <[log in to unmask]>
> 发送时间: 2019-08-03 01:20:26 (星期六)
> 收件人: [log in to unmask]
> 抄送:
> 主题: lower limit of metamorphism
>
> Hello. I'm brand new to the list. My background is more igneous than metamorphic, but I am teaching an ig/met petrology this semester for the first time in 12 years, so decided to subscribe to this list to get back up to speed.
>
> I would like to pose a question for the community: what defines the lower limit of metamorphism in basalts? Yes, I know about hydrothermal alteration and zeolite facies metamorphism in a general way, but I probably don't really understand on a deeper level what defines the boundary between the two, other than the general idea that metamorphism starts when the original minerals in a protolith begin to participate in chemical reactions, in this case prompted by hydrothermal fluids. Is the boundary sharp or fuzzy? I'm guessing fuzzy. More practically, how would an undergraduate student be able to tell the difference between a hydrothermally-altered basalt and a zeolite-facies metabasalt? Is the presence of amygdaloidal texture (filled with zeolites) sufficient?
>
> Thanks in advance for your thoughts!
>
> - Kent Ratajeski
>
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Dr. Yong-Fei Zheng
Professor of Geochemistry
School of Earth and Space Sciences
University of Science and Technology of China
Hefei 230026, China
Tel & Fax: +86 551 63603554
Email: [log in to unmask]
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