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Towards a Petrologically Constrained Thermal Model of Mid-Ocean Ridges.

Record Type:	Language materials, manuscript : Monograph/item
Title/Author:	Towards a Petrologically Constrained Thermal Model of Mid-Ocean Ridges./
Author:	Scott, Jameson Lee.
Description:	1 online resource (241 pages)
Notes:	Source: Dissertation Abstracts International, Volume: 79-05(E), Section: B.
Contained By:	Dissertation Abstracts International79-05B(E).
Subject:	Geology. -
Online resource:	click for full text (PQDT)
ISBN:	9780355442816

Towards a Petrologically Constrained Thermal Model of Mid-Ocean Ridges.
Scott, Jameson Lee.

Towards a Petrologically Constrained Thermal Model of Mid-Ocean Ridges. - 1 online resource (241 pages)

Source: Dissertation Abstracts International, Volume: 79-05(E), Section: B.

Thesis (Ph.D.)

Includes bibliographical references

Plate spreading at mid-ocean ridges (MOR) is responsible for the creation of most of the crust on earth. The ridge system is very complex and many questions remain unresolved. Among these are the controls on the architecture of magma plumbing systems beneath mid-ocean ridges of different spreading rates and in proximity to transform faults. Previous studies have called into question the hypothesis that a decrease in magma flux and increase in conductive cooling along transforms faults promotes higher pressures of partial crystallization, and that this also explains the higher partial pressures of crystallization inferred for magmas erupted along slow spreading ridges compared to magmas erupted along faster spreading ridges. To test these hypothesis, I undertook a detailed analysis of pressures of partial crystallization (PPC) for magmas erupted along the slow spreading Reykjanes Ridge (RR), indeterminate spreading Juan de Fuca Ridge (JdF), 3 transforms along the fast to intermediate spreading East Pacific Rise (Blanco, Clipperton, and Siqueiros), and 5 transforms along the slow spreading Mid Atlantic Ridge (Oceanographer, Famous Transform A & B, Kane, and 15°20'N). PPC were calculated from the compositions of glasses (quenched liquids) lying along the P (and T) dependent olivine, plagioclase, and augite cotectic using the method described by Kelley and Barton (2008). Published analyses of MOR basalt glasses sampled from the ridges and transforms were used as input data. Samples with anomalous chemical compositions and samples that yielded pressures associated with unrealistically large uncertainties were filtered out of the database. The calculated pressures for the remaining 459 samples for the RR, 564 samples for the JdF, and 1056 samples for the transforms were used to calculate the depths of partial crystallization and to identify the likely location of magma chambers. The RR results indicate that the pressure of partial crystallization decreases from 102 +/- 37 MPa at the Charlie Gibbs Fracture Zone to 12 +/- 14 MPa at 56°N, then increases to 357 +/- 68 MPa as Iceland is approached. Five magma lenses were identified at depths of 3.6 +/- 1.15 km, 0.2 +/- .48 km, 2.2 +/- 1km, 6.7 +/- 1km, and 5.1 +/- 0.8km. The magma lens at 2.2 +/- 1 km agrees very well with seismically imaged sill at 2.5 km. The JDF results indicate that the pressure of partial crystallization decreases from 207 +/- 90 MPa near the Blanco fracture zone to 107 +/- 54 MPa along the Cleft segment of the ridge to the north. Calculated pressures remain approximately constant at 87 +/-73 MPa along ridge segments to the north of Cleft. One magma lens was identified at depths of 2.90 +/- 0.9 km which is in good agreement with a nearby seismically imaged magma lens at 2.5 km depth. The pressures of partial crystallization for the transforms ranged from 0 to 520 MPa with most samples returning pressures of less than 300 MPa. Pressures of < 300 MPa are within error of the pressure range associated with partial crystallization within oceanic crust with a thickness of ~6 km. Except for the Blanco, pressures of partial crystallization do not increase as transforms are approached. These observations contrast with those of previous workers, who reported anomalously high pressures (up to 1000 MPa) for many samples erupted near both Atlantic and Pacific Transforms. The average depth of magma chambers along the slower spreading Atlantic MOR is only slightly higher than that along the intermediate and fast spreading Pacific MOR. Moreover, the average depth of partial crystallization along the RR increases with increasing crustal thickness that is thought to reflect increasing magma flux towards the Iceland hotspot. These results suggest that the relationship between magma chamber depth and magma flux maybe more complex that previously assumed in the literature. The results obtained for samples from virtually every locality also suggest partial crystallization in the crust beneath these lenses, and therefore the results support the many sill or crystal mush models for accretion of oceanic crust for both slow and fast spreading ridges. I conclude that lower magma flux along slow spreading ridges or the higher rates of cooling along transform does not have a major effect on the onset of partial crystallization along the MOR. Use of the method described by Herzberg (2004) yields slightly lower pressures for most locations, but differences between pressures calculated with both methods are within the uncertainties of the calculations and does not change the conclusions of this study.

Electronic reproduction.
Ann Arbor, Mich. :
ProQuest,
2018

Mode of access: World Wide Web

ISBN: 9780355442816Subjects--Topical Terms:

670379
Geology.
Index Terms--Genre/Form:

554714
Electronic books.

Towards a Petrologically Constrained Thermal Model of Mid-Ocean Ridges.
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245 1 0 $a Towards a Petrologically Constrained Thermal Model of Mid-Ocean Ridges.
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500 $a Source: Dissertation Abstracts International, Volume: 79-05(E), Section: B.
500 $a Adviser: Michael Barton.
502 $a Thesis (Ph.D.) $c The Ohio State University $d 2017.
504 $a Includes bibliographical references
520 $a Plate spreading at mid-ocean ridges (MOR) is responsible for the creation of most of the crust on earth. The ridge system is very complex and many questions remain unresolved. Among these are the controls on the architecture of magma plumbing systems beneath mid-ocean ridges of different spreading rates and in proximity to transform faults. Previous studies have called into question the hypothesis that a decrease in magma flux and increase in conductive cooling along transforms faults promotes higher pressures of partial crystallization, and that this also explains the higher partial pressures of crystallization inferred for magmas erupted along slow spreading ridges compared to magmas erupted along faster spreading ridges. To test these hypothesis, I undertook a detailed analysis of pressures of partial crystallization (PPC) for magmas erupted along the slow spreading Reykjanes Ridge (RR), indeterminate spreading Juan de Fuca Ridge (JdF), 3 transforms along the fast to intermediate spreading East Pacific Rise (Blanco, Clipperton, and Siqueiros), and 5 transforms along the slow spreading Mid Atlantic Ridge (Oceanographer, Famous Transform A & B, Kane, and 15°20'N). PPC were calculated from the compositions of glasses (quenched liquids) lying along the P (and T) dependent olivine, plagioclase, and augite cotectic using the method described by Kelley and Barton (2008). Published analyses of MOR basalt glasses sampled from the ridges and transforms were used as input data. Samples with anomalous chemical compositions and samples that yielded pressures associated with unrealistically large uncertainties were filtered out of the database. The calculated pressures for the remaining 459 samples for the RR, 564 samples for the JdF, and 1056 samples for the transforms were used to calculate the depths of partial crystallization and to identify the likely location of magma chambers. The RR results indicate that the pressure of partial crystallization decreases from 102 +/- 37 MPa at the Charlie Gibbs Fracture Zone to 12 +/- 14 MPa at 56°N, then increases to 357 +/- 68 MPa as Iceland is approached. Five magma lenses were identified at depths of 3.6 +/- 1.15 km, 0.2 +/- .48 km, 2.2 +/- 1km, 6.7 +/- 1km, and 5.1 +/- 0.8km. The magma lens at 2.2 +/- 1 km agrees very well with seismically imaged sill at 2.5 km. The JDF results indicate that the pressure of partial crystallization decreases from 207 +/- 90 MPa near the Blanco fracture zone to 107 +/- 54 MPa along the Cleft segment of the ridge to the north. Calculated pressures remain approximately constant at 87 +/-73 MPa along ridge segments to the north of Cleft. One magma lens was identified at depths of 2.90 +/- 0.9 km which is in good agreement with a nearby seismically imaged magma lens at 2.5 km depth. The pressures of partial crystallization for the transforms ranged from 0 to 520 MPa with most samples returning pressures of less than 300 MPa. Pressures of < 300 MPa are within error of the pressure range associated with partial crystallization within oceanic crust with a thickness of ~6 km. Except for the Blanco, pressures of partial crystallization do not increase as transforms are approached. These observations contrast with those of previous workers, who reported anomalously high pressures (up to 1000 MPa) for many samples erupted near both Atlantic and Pacific Transforms. The average depth of magma chambers along the slower spreading Atlantic MOR is only slightly higher than that along the intermediate and fast spreading Pacific MOR. Moreover, the average depth of partial crystallization along the RR increases with increasing crustal thickness that is thought to reflect increasing magma flux towards the Iceland hotspot. These results suggest that the relationship between magma chamber depth and magma flux maybe more complex that previously assumed in the literature. The results obtained for samples from virtually every locality also suggest partial crystallization in the crust beneath these lenses, and therefore the results support the many sill or crystal mush models for accretion of oceanic crust for both slow and fast spreading ridges. I conclude that lower magma flux along slow spreading ridges or the higher rates of cooling along transform does not have a major effect on the onset of partial crystallization along the MOR. Use of the method described by Herzberg (2004) yields slightly lower pressures for most locations, but differences between pressures calculated with both methods are within the uncertainties of the calculations and does not change the conclusions of this study.
533 $a Electronic reproduction. $b Ann Arbor, Mich. : $c ProQuest, $d 2018
538 $a Mode of access: World Wide Web
650 4 $a Geology. $3 670379
650 4 $a Petrology. $3 783490
655 7 $a Electronic books. $2 local $3 554714
690 $a 0372
690 $a 0584
710 2 $a ProQuest Information and Learning Co. $3 1178819
710 2 $a The Ohio State University. $b Earth Sciences. $3 1183560
773 0 $t Dissertation Abstracts International $g 79-05B(E).
856 4 0 $u http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=10702704 $z click for full text (PQDT)