| NSF Org: |
EAR Division Of Earth Sciences |
| Recipient: |
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| Initial Amendment Date: | June 24, 2013 |
| Latest Amendment Date: | April 30, 2015 |
| Award Number: | 1322032 |
| Award Instrument: | Continuing Grant |
| Program Manager: |
Sonia Esperanca
EAR Division Of Earth Sciences GEO Directorate for Geosciences |
| Start Date: | July 1, 2013 |
| End Date: | June 30, 2017 (Estimated) |
| Total Intended Award Amount: | $299,992.00 |
| Total Awarded Amount to Date: | $299,992.00 |
| Funds Obligated to Date: |
FY 2014 = $96,668.00 FY 2015 = $111,319.00 |
| History of Investigator: |
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| Recipient Sponsored Research Office: |
77 MASSACHUSETTS AVE CAMBRIDGE MA US 02139-4301 (617)253-1000 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
77 Mass. Ave., 54-1018 Cambridge MA US 02139-4307 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | Petrology and Geochemistry |
| Primary Program Source: |
01001415DB NSF RESEARCH & RELATED ACTIVIT 01001516DB NSF RESEARCH & RELATED ACTIVIT |
| Program Reference Code(s): | |
| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
Understanding the origin of the highly diverse nature of felsic magmas, erupted as rhyolite or emplaced as large upper-crustal batholiths, is crucial to the investigation of the mechanisms that generate the silica-enriched continental crust. General consensus exists that strongly so-called peraluminous granites are derived, at least to some significant extent, from muscovite- and biotite-dehydration melting of metasedimentary protholiths. The origin of so-called meta-aluminous granites, however, is less certain.
This project aims to investigate the hydrous liquid line of descent (LLD) of an alkaline magma and the relative roles of hydrous fractional crystallization versus partial melting in the formation of meta-aluminous alkaline granites through a field-based study. While alkaline granites are volumetrically less significant than calk-alkaline granites, their petrogenesis is of particular importance as they are often associated with economically significant mineralization. We will study a newly discovered exposures in the Dariv arc complex (Altai region of western Mongolia). The project will investigate a layered sequence ranging from phologopite-dominated ultramafic/mafic to intermediate (quartz-) monzonite and (quartz-) syenites rocks is spatially associated with large alkali-feldspar granite bodies. A similarly complete alkaline plutonic sequence has, to our knowledge, never been described in the geological literature before. In particular, the ultramafic rock assemblages exposed in Mongolia are currently only described as xenoliths from alkaline basaltic magmas. The complete exposures from ultramafic to felsic rocks provide an unrivaled opportunity to investigate crystal fractionation as a possible mechanism of alkali-feldspar granite formation.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
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This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
This study focused on a newly discovered exposures in the Dariv arc complex (Altai region of western Mongolia). The project investigated a layered sequence ranging from phologopite-dominated ultramafic/mafic to intermediate (quartz-) monzonite and (quartz-) syenites rocks is spatially associated with large alkali-feldspar granite bodies. The complete exposures from ultramafic to felsic rocks provided an unrivaled opportunity to investigate crystal fractionation as a possible mechanism of alkali-feldspar granite formation.
The result of this project demonstrates that the rock successions exposed in Dariv are formed by fractional crystallization of a hydrous liquid line of descent. We developed a quantitative model that demonstrates that the different rock suites are related to a common parental magma by the process of magmatic differentiation.
The presence of a complete alkaline magmatic differentiation sequence allowed us to constrain the time scales of magmatic differentiation. Crucial is the first appearance of magmatic zircon crystals. While zircon crystallizes in calk-alkaline melts only in intermediate to evolved composition, the Zr concentration is significantly higher in alkaline melts and hence zircon crystallizes from basaltic melts and can be separated from ultramafic cumulates that crystallized from these melts. Our results demonstrate the Dariv igneous suite formed in less than ~590kyr.
Finally, we compared our rock sequence in Dariv to other large igneous intrusion like the Bushveld to understand the role of different magmatic differentiation trends can have on the oxygen isotopic evolution. Oxygen isotopes are often used as a traces to understand how magma composition and from that continental crust formed through Earth history. However, the role of magmatic differentiation of oxygen isotope in the melt was poorly constrained, yet crucial to interpret the global dataset of oxygen isotope measure e.g. in zircon crystals.
Using natural mineral and rock composition from the Dariv and Bushveld complexes we could develop a quantitative model of oxygen evolution in hydrous and anhydrous melts. We can show that magmatic differentiation can significantly change the oxygen isotopic composition of the magma depending on the differentiation sequence.
Our broader impact included support for a female graduate student that now is faculty member at Caltech. We also supported undergraduate students from Mongolia to accompany us in the field and their visit at MIT.
Last Modified: 09/28/2017
Modified by: Oliver Jagoutz
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