Contamination and Melt Aggregation Processes in Continental £Ood Basalts: Constraints from Melt Inclusions in Oligocene Basalts from Yemen

Earthand Planetary Science Letters 202 (2002) 577^594 www.elsevier.com/locate/epsl

Contamination and melt aggregation processes in continental £ood basalts: constraints from melt inclusions in Oligocene basalts from Yemen

Adam J.R. Kent a;, Joel A. Baker a, Michael Wiedenbeck b

a Danish Lithosphere Centre, Òster Voldgade 10, KBH 1350-K Copenhagen, Denmark b GeoForschungsZentrum Potsdam, Telegrafenberg, D 14473 Potsdam, Germany

Received 13 February 2002; received in revised form 1 May 2002; accepted 5 July 2002

Abstract

Melt inclusions from Oligocene continental flood basalts (CFB) erupted in Yemen provide unique insight into the timing and nature of the processes that lead to crustal contamination and melt aggregation in CFB magmas. Large variations in trace element indices that are sensitive to the degree and composition of assimilated crustal material (e.g. K2O = 0.20^1.94 wt%, Ba = 13^543 ppm, K/Nb = 128^1603, Ba/Th= 9^303) are evident in many inclusions, even where these derive from the same host lava, and reflect the complexity of the processes that lead to contamination within individual CFB melting and melt-transport systems. The compositions of melt inclusions relate to differences in the degree of contamination, but in addition require that there is substantial heterogeneity in the composition of the contaminant material itself. Many inclusions also appear to contain more primitive melts than typical Yemen CFB lava compositions, and as such would be highly sensitive to addition of crustal materials. Overall melt inclusions provide a markedly better record of the diversity of melt compositions present within given CFB magma systems than the bulk compositions of erupted lavas. Highly contaminated melts (with K/Nb and Ba/Nb up to 957 and 22, respectively) trapped within fosterite-rich olivines (Fo8590) require very high rates of assimilation relative to crystal fractionation, with Ma/Mc values (the mass ratio of assimilated to crystallized material)E1. Suchrapid assimilation may reflect decoupling of heatand mass transfer at the margins of larger magma chambers, within feeder dyke complexes, or at other sites where primitive magma is juxtaposed against wall-rocks that are already heated to temperatures near, or above, their solidus. In addition, relatively little assimilation appears to have occurred after crystallization of the phases that host melt inclusions, consistent with a thermal link between assimilation and phenocryst formation. Melt inclusions also show trace element variations related to mantle source compositions and mantle melting processes. Two inclusions with unusually Sr-richand rare earthelement-poor compositions are similar to thoserecognized from Mauna Loa, Hawaii, and may be related to melting of recycled gabbroic material within the upwelling Afar plume. In addition, many melt inclusions with crustally contaminated compositions also show large variations in trace element ratios that are essentially insensitive to crustal contamination (e.g. Zr/Y = 2^10) but are fractionated during progressive partial melting within an upwelling mantle column. The presence of both mantle and crustal-derived trace element signatures

* Corresponding author. Present address: Department of Geosciences, Oregon State University, Corvallis, OR 97331-5506, USA. E-mail address: [email protected] (A.J.R. Kent).

EPSL 6342 11-9-02 578 A.J.R. Kent et al. / Earth and Planetary Science Letters 202 (2002) 577^594 in the same inclusions demonstrates that melt transport systems in Yemen CFB were capable of transporting compositionally distinct melt batches, without complete mixing, through the asthenospheric and lithospheric mantle until final mixing and aggregation (and contamination) within crustal magma reservoirs. Thus, regardless of contamination, the ultimate compositions of many CFB lavas may be determined by magma mixing within the crust, rather than representing primitive compositions derived directly from the mantle. ß 2002 Elsevier Science B.V. All rights reserved.

Keywords: Yemen; £ood basalts; melts; inclusions; magma contamination

1. Introduction contaminant and a constant mass ratio of assimilated to crystallized material (Ma/Mc). However, Continental £ood basalt (CFB) provinces and other work, based on £uid dynamic investigations related rocks are widely distributed throughout [2], thermodynamically constrained models where the geological record [1^4]. They represent extra- the amount and/or rate of assimilation is con- ordinarily large volumes of mantle-derived melt strained by the amount of heat available from erupted over very short periods of time, and com- the fractionating (and/or recharged) magma [19^ prise an important mode of mass transfer from 22], and geochemical and petrological studies the mantle to the crust. The formation of CFB [15,16,23] suggest that contamination may be far provinces have implications for models of conti- more complex. The rate of assimilation is a func- nental growth, rifting and dispersal [1,3,4] and tion of the pressure^temperature history of the may also have played a role in biotic extinction magma, the temperature of crustal wall-rocks, events [5^7]. and the £uid dynamic regime under which magma Geochemical studies of CFB lavas provide im- movement occurs, as well as bothliquidus and portant constraints on the nature of the mantle solidus temperatures of the melt and assimilant sources which contribute to magmatism and the [2,19^22]. Contaminated melt compositions may array of processes which lead to generation and also re£ect variations in primary magmas, pro- eruption of suchvoluminous mantle melts. How- gressive melting of crustal wall-rocks, the rate ever, one consistent and widespread complication and mode of magma recharge and changes in dis- is the extent to which these magmas have inter- tribution coe⁄cients withchangingpressure, tem- acted with continental crust through which they perature and composition. Given this complexity, pass en route to the surface [2,8^16]. Continental it is highly likely that a wide range of melt com- crust generally has a lower solidus temperature positions exist within individual crustal-scale CFB than basalt, is highly enriched in incompatible magma plumbing systems, and examination of elements, and typically has stable and radiogenic this diversity might provide valuable insight into isotopic signatures that di¡er from mantle-derived the nature of CFB magmatism. However, it is melts [17]. Addition of even small amounts of also unlikely that this variation will be fully ex- continental material may have a profound in£u- pressed in the compositions of CFB lavas, as ence on the trace element and isotopic composi- high-level fractionation and mixing of diverse tion of mantle-derived melts, complicating and melt batches prior to eruption may drastically obscuring more subtle mantle-derived composi- limit the range of compositions shown by erupted tional relationships. As such, understanding the lavas [24^34]. mechanisms of crustal contamination is a crucial To examine CFB contamination processes in aspect of the study of CFB lavas [2]. more detail, we have analyzed the chemical com- Assimilation and fractional crystallization positions of melt inclusions and phenocrysts in a (AFC) processes in basaltic systems are often well-documented suite of CFB lavas from Yemen modeled using the equations of DePaolo [18], us- [8,9,35]. Melt inclusions provide a unique means ing bulk end-member compositions for basalt and to probe the range of melt compositions present

EPSL 6342 11-9-02 A.J.R. Kent et al. / Earth and Planetary Science Letters 202 (2002) 577^594 579 within individual basaltic plumbing systems as tion’ to refer to changes in bulk magma compo- they often sample melts prior to the extensive sition caused by subsequent loss of crystallized magma mixing and homogenization processes material). We also note that, in contrast to bulk that precede eruption [24^34]. Inclusion composi- lava analyses, the compositions of melt inclusions tions, particularly incompatible trace element and host phenocrysts will be sensitive to crystal abundances and ratios, allow us to assess varia- fractionation regardless of whether or not crystal- tions in both the extent of assimilation and the lized material is subsequently removed from the composition of the contaminant material within melt. magma systems. The composition of the host To our knowledge, this is the ¢rst study of melt phenocryst may also be used to monitor the de- inclusions in primitive crustally contaminated gree of fractionation at the time of trapping, pro- CFB lavas, and our primary aim is to investigate viding a novel means to examine the relative rates whether melt inclusions provide useful insight into of assimilation and crystal fractionation (note the processes of crustal contamination in CFB. that we use the term ‘crystal fractionation’ to re- However, the melt inclusions we have studied fer to chemical changes within a melt driven by also record chemical variations related to mantle sequestration of chemical components into crys- source and melting processes and show that inclu- talline phases, and the term ‘fractional crystalliza- sions can also be used to examine primary melt variations in CFB, even in highly contaminated suites. Moreover, the presence of both contamination and melting-driven trace element variations in individual inclusions provides a unique insight into the relative timing and location of

6 Fig. 1. (A) Map of the Red Sea region (after [35]) showing distribution of Cenozoic volcanic rocks and locations of the 87 86 samples used for this study. (B) Sr/ Sri vs. Ba/Nb for Ye- men Oligocene CFB lavas. The three lavas analyzed for this study are also labeled. For this ¢gure and elsewhere in this study we have calculated mixing relations based on two crustal compositions (also see Background Data Set1, [49]): (i) the average Pan-African granulite xenolith composition from Saudi Arabia reported by McGuire et al. [38] for Pan-Afri- can granulite lower crust; and (ii) and the estimated average upper crustal composition [17,50] for felsic upper crust. Although these two crustal compositions share many chemical features withtheprojected compositions of contaminants in Yemen lavas, it should be noted that they are only in- tended as a guide to interpreting lava and melt inclusion compositions. In reality the compositions of crustal contaminants may vary substantially, and in some cases show compositional features intermediate between these two compositions. Although the Sr isotope composition of these crustal components is not well known (and is probably highly variable) we have used existing studies of Arabian crustal rocks as a guide [13,38]. Mixing proportions shown represent weight percent of contaminant material added. (C) Primitive mantle-normalized [43] compositions of average upper crust and Pan-African granulite contaminant compositions; lava sample 281 is also shown for reference.

1 http://www.elsevier.com/locate/epsl

EPSL 6342 11-9-02 580 A.J.R. Kent et al. / Earth and Planetary Science Letters 202 (2002) 577^594 melt aggregation, crystal fractionation and contamination processes within the mantle and crust. E (Gubara Q 00 P

2. Samples and geological background granulite N, 44‡44

Oligocene volcanic rocks in Yemen (Fig. 1A) Q 31 P are part of the youngest known CFB province Ba/ThContaminant associated withthenascent development of an N

oceanic basin. The province formed at the Afro- ^ 13‡53 Arabian triple junction withapproximately 350 000 km3 of basalt lavas and associated rhyo- lite pyroclastic rocks erupted in Yemen, Ethiopia, MNY 361 Eritrea and Djibouti [36,37]. In Yemen the bulk of preserved £ood volcanism occurred in the Oli- gocene between 26 and 31 Ma [35]. Magmatism is considered to be related to the arrival of anoma- lously hot mantle material (the Afar plume) be- neatheastern northAfrica V30 Ma ago, which may have coincided with the onset of continental rifting in the Gulf of Aden and eastern Africa. ), 2200 m above sea level;

The chemical and isotopic compositions of Ye- [9] O olivine K/Nb Ba/Nb (La/Sm)

men Oligocene CFB lavas vary considerably, re- 18 N £ecting variations in primary magmas, crystal fractionation histories and degree of assimilation of crustal materials of di¡ering age and composi- i N tion [8,9]. In contaminated lavas, variations are O particularly evident in radiogenic isotope compo- i Pb

sitions and incompatible trace element abundan- 204 ces. Oxygen isotope systematics of phenocryst Pb/ 206

phases from contaminated Yemen CFB also E (lower series, Sana’a section Q

clearly show that acquisition of enriched isotopic 48 P i and trace element signatures is a crustal phenom- Sr 86

enon, and not related to interaction between melts Sr/ N, 44‡05 and enriched lithospheric material [8]. Lavas from Q 50 eastern Yemen are typically contaminated with P a87 late Archean felsic crust, resulting in enriched Sr represents mole fraction. 206 204 X 6859 0.703765 0.7039 18.94 0.7049 18.22 5.6 18.21 3.8 5.12 ^ 5.9 ^ 137 323 5.4 15.2 1.5 491 2.0 17.6 52 2.1 170 uncontaminated Pan-African 121 felsic Archean and Nd isotopic compositions, low Pb/ Pb ^ 15‡24 ratios, and enrichments in large ion lithophile el- 282

ement (LILE) contents, withdistinctively high ), where Fe and

K/Nb. In contrast, many contaminated lavas X + from western Yemen have radiogenic isotope 281 Mg X compositions that are closer to those of unconta- Sana’a (NW Yemen) Sana’a (NW Yemen) Gubara(SE Yemen) /( and J.A. Baker, unpublished data. minated Yemen basalts, but show strong enrich- Mg X [8,9]

ment of Ba relative to K, Rb, Thand U. This b contaminant is considered to be primitive lower

crustal ma¢c granulite of Pan-African age b b Sample locations: Mg = 100 281 282 Data from MNY 361 Table 1 Whole-rock chemical and isotopic compositionsSample of lava samples used for this study Location Mg# a b (V700 Ma), similar to that observed in lower Village), 2063 m above sea level.

EPSL 6342 11-9-02 A.J.R. Kent et al. / Earth and Planetary Science Letters 202 (2002) 577^594 581 crustal xenolithsuites from Yemen and elsewhere 6 10 Wm). All inclusions we report data for are in Arabia [8,38]. Although there is considerable v 30 Wm in diameter. diversity in the composition of contaminated Our homogenization procedure provides no in- lavas and the exact compositions of the crustal formation regarding the trapping temperatures of end-members are not well constrained, assimila- individual melt inclusions, but does provide an tion of various proportions of these (or similar) e¡ective means of homogenizing large numbers crustal components can explain many of the iso- of inclusions [25,29,31]. Although bulk heating topic and compositional characteristics of con- at a single temperature may not bring all inclu- taminated lavas [8,9] (e.g. Fig. 1B,C). sions to equilibrium with their host phase, we We have examined melt inclusions from olivine note that for olivine-hosted inclusions the average and clinopyroxene phenocrysts in three Oligocene calculated KD values for FeO^MgO exchange (V30 Ma) ma¢c lavas from Yemen. These lavas [KD = (FeO/MgOolivine)/(0.9FeO/MgOmelt)] for in- have whole-rock chemical and isotopic composi- clusions from eachsample [ 281, 0.33 0.07 (1c); tions (Table 1 and Fig. 1B) that show they repre- 282, 0.37 0.07; MNY 361, 0.28 0.06] are all sent a range in boththedegree of contamination within 1 S.D. of the accepted value for mantle and the bulk composition of assimilated material: melts of 0.30 [39]. Further, as addition or removal 281 is an apparently uncontaminated near-pri- of the host mineral component will only a¡ect mary basalt (Mg# 68), MNY 361 is a highly ma¢c incompatible element concentrations through (Mg# 65) lava withhigh 87Sr/86Sr considered to changes in closure (generally 6 10%), incompat- be contaminated by Archean felsic crust, and 282 ible element ratios will remain una¡ected. Incom- is a slightly more fractionated basalt (Mg# 59) plete inclusion^host equilibration thus does not withlower 87Sr/86Sr that is considered to have preclude substantial geochemical information been contaminated by Pan-African granulite being derived from inclusion compositions. Natu- [8,9]. Lava sample 281 directly overlies 282 in ral processes suchas di¡usive re-equilibration be- the same stratigraphic section. All lavas examined tween host and inclusion may also substantially contain abundant olivine phenocrysts, and 281 alter FeO and MgO contents of melt inclusions and MNY 361 also contain clinopyroxene. [40,41].

2.1. Melt inclusions 3. Analytical methods Olivine and clinopyroxene separates were pre- pared by standard heavy mineral separation tech- Melt inclusions and phenocryst phases were an- niques and puri¢ed by handpicking. In order to alyzed for major element compositions using a produce a homogenous glass for analysis, olivine Jeol 8000 Superprobe at the University of Copen- and clinopyroxene separates were heated to hagen, Denmark. Analyses were performed using 1200‡C for 15 min in a gas-mixing furnace at the a combination of energy-dispersive (for SiO2, University of California, Davis, CA, USA, at an Al2O3, CaO, K2O, MgO, FeOT and TiO2) and oxygen fugacity two log units below the magne- wavelength-dispersive analysis (MnO and Na2O). tite^fayalite bu¡er, and were quenched in water. Analyses were made using an accelerating voltage Homogenized and quenched melt inclusions of 15 kV and witha 15 nA electron beam defo- consist of clear glass and occasional small shrink- cused to 10 Wm in diameter. Major element com- age bubbles. Inclusions are round to oval in ap- positions of unknowns were calculated by refer- pearance in botholivine and clinopyroxene and ence to mineral and oxide standards. From repeat have generally smooth walls. Small spinel crystals, analyses of standard and unknown glasses the an- interpreted to have been trapped alongside melt in alytical reproducibility is estimated at 2% inclusions (rather than crystallized in situ from (2 S.D.) for Si, 4% for Al, Mg, Fe and Ca, 6% trapped melt) are also occasionally apparent. for Ti and Na, 10% for K, and 50% for Cr and Melt inclusions sizes vary from V200 to Mn.

EPSL 6342 11-9-02 582 A.J.R. Kent et al. / Earth and Planetary Science Letters 202 (2002) 577^594

Trace element contents of melt inclusions were tively low SiO2 as well as generally lower Na2O analyzed using a modi¢ed Cameca 3f ion and higher CaO and CaO/Al2O3. Clinopyroxene- microprobe at Lawrence Livermore National hosted inclusions from MNY 361 generally have Laboratory, CA, USA, and a Cameca 6f ion higher SiO2 than olivine-hosted inclusions. Host microprobe at GeoForschungsZentrum (GFZ), olivine and clinopyroxene crystals also vary in Potsdam, Germany, following procedures detailed composition, particularly in Mg# (Fig. 2E; Table elsewhere [25]. Eachanalysis consisted of 10 2), which ranges from 74.4^89.8 (olivine) and sequential scans of the following masses 30Si, 76.7^85.7 (clinopyroxene). Typically the host phe- 42Ca, 86Sr, 89Y, 90Zr, 93Nb, 138Ba, 139La, 140Ce, nocrysts are either normally zoned or homogene- 141Pr, 145Nd, 147Sm, 232Thand 238U (analyses ous, and reverse zoning was not documented in at GFZ did not include U and Th). Trace element any crystals. concentrations were calculated withreference Trace element abundances in melt inclusions to sensitivity factors measured from repeat are also highly variable. Incompatible element analysis of NBS 612 and 610 glass using 42Ca abundances vary by factors of 5^10 for moder- as a normalizing isotope. Repeat analyses of ately incompatible elements and by factors of BCR-1 and BCR-2 glass standards were used to 20^40 for the most incompatible elements. Incom- monitor accuracy and precision throughout the patible trace element abundances are also typi- analytical session (see Background Data Set1). Es- cally at the low end, or below the range evident timated analytical uncertainties are 9 6% for in typical Yemen lava compositions, particularly Sr, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm; 10% for non-LILE incompatible elements suchas Nb, for Thand 12.5% for U. All errors reported Zr and rare earthelements (REE) thatare less herein are given at the level of 2 S.D., unless in£uenced by crustal contamination (Figs. 2 and otherwise noted. 3). This suggests that many inclusions contain melts that are less fractionated than the majority of lava compositions. Melt inclusions from 281 4. Results have among the lowest and least-variable incompatible element abundances. However, two inclu- Major and trace element analyses of 10 repre- sions (281-2-6, 281-3-2) have distinctively low in- sentative melt inclusions are given in Table 2.Full compatible element abundances but show large data for all 64 inclusions analyzed (56 olivine positive Sr anomalies on mantle-normalized dia- hosted, eight clinopyroxene hosted) are provided grams (Sr/Sr* = 3.3 and 5.3; Fig. 3). Olivine- in the Background Data Set1. hosted melt inclusions from 282 and MNY 361 Key features of the major element compositions have the highest incompatible element abundan- of melt inclusions are summarized in Fig. 2B^E ces and show the most variation in LILE (Ba, K, where we have plotted selected oxides against U; Fig. 3). Inclusion 282-1-10b also has distinc- SiO2. Analyzed melt inclusions show substantial tively low overall elemental abundances and high variations in major element compositions, with Sr (Sr/Sr* = 5). Clinopyroxene-hosted inclusions variations of factors s 2^3 for most oxides, and from MNY 361 show more pronounced K enrich- by factors of 10^15 for K2O and Na2O. SiO2 ment than olivine-hosted inclusions from the same contents also vary from 42 to 54 wt%. However, lava. Ratios of LILE to other incompatible ele- there are also compositional consistencies between ments are also highly variable (Figs. 4^6). For our inclusions from the same host lava. Melt inclu- complete data set, the Ba/Th, K/Nb, Ba/Nb and sions from 281 have typically lower and less var- Ba/La ratios in melt inclusions vary by factors of iable Al2O3,K2O, Na2O and FeOT and lower 34, 14, 77 and 52, respectively. Other incompat- K2O/TiO2 ; whereas those from 282 tend to lower ible element ratios are less variable [e.g. La/Nb, MgO, CaO and CaO/Al2O3 and higher and more Zr/Y, Ce/Y and (La/Sm)N all vary by factors of variable Al2O3,K2O, Na2O and K2O/TiO2. Oliv- V5], although ratios of REE to Sr vary by fac- ine-hosted inclusions from MNY 361 have distinc- tors of V20.

EPSL 6342 11-9-02 A.J.R. Kent et al. / Earth and Planetary Science Letters 202 (2002) 577^594 583

Fig. 2. (A^E) SiO2 vs. Ba/Th, K2O, Al2O3, FeOT and Mg# of the host phenocrysts [Mg# = 100XMg/(XFe+XMg), where X denotes mole fraction] and (F^H) Zr vs. MgO, K2O, Nb and Ba contents. Symbols are as per legend, and the range of whole-rock Ye- men CFB compositions is shown as a gray ¢eld. Unless shown otherwise all concentrations are in wt%. Note di¡erent scales for K2O in (A) and (E).

5. Discussion canic rocks [25^34] and re£ect chemical fractiona- tions caused by a wide range of igneous processes, 5.1. Relationship between melt inclusions and their including mantle melting, mantle source heteroge- host lavas neity, mantle^melt re-equilibration, melt mixing and assimilation. It is considered that the large One of the most notable features of melt inclu- compositional ranges evident in inclusions re£ects sions from the three Yemen CFB samples we have the fact that they trap and preserve composition- examined are their highly variable chemical com- ally diverse melt batches that are present within positions (Figs. 2^6; Table 2), even for inclusions the magma storage and transport systems where in phenocrysts from the same lava. Such large phenocrysts form [26,27,29^31]. These melts then compositional variations appear to be a near-uni- aggregate and mix to form the more homogenous versal feature of melt inclusion suites from vol- compositions that are ultimately erupted as lavas.

EPSL 6342 11-9-02 584 A.J.R. Kent et al. / Earth and Planetary Science Letters 202 (2002) 577^594

Our data are consistent withthis general model tion and crustal contamination in lavas and melt as, for eachlava sample examined, theaverage inclusions. melt inclusion major and trace element composition (after correction for di¡erences in fractiona- 5.2. E¡ects of crustal contamination on melt tion) corresponds closely to the bulk lava compo- inclusion compositions sition (i.e. the average melt inclusion composition for eachsample is within1 S.D. of composition of Melt inclusions in Yemen CFB show large var- the host lava, and at the 95^99% con¢dence level, iations in trace element concentrations and ratios average melt inclusion compositions are indistin- that are sensitive to the addition of continental guishable from their host lava). This suggests that crustal material (e.g. K, Ba, Th, U, K/Nb, Ba/ the same melts sampled by inclusions also mixed Nb, Ba/Th; Figs. 2^5), and many inclusions to form their host lavas (and that melts within have LILE contents and Ba/Nb, K/Nb and Ba/ inclusions were analyzed in approximately the Th ratios that are considerably greater than typ- same proportions as they contributed). Impor- ically observed in uncontaminated basaltic melts tantly, this also implies that a genetic relationship [42^44]. Although mantle melting processes can exists between inclusions and host lava composi- in£uence LILE systematics, these e¡ects are often tions, and thus constraints on petrogenetic pro- relatively subtle. Highly fractionated Ba/Nb, cesses based on whole-rock data (i.e. isotopic K/Nb and Ba/Thratios are generally restricted compositions) will apply to the melts trapped to very low degrees of partial melting (Fig. 6)or within inclusions (and vice versa). As we outline to where LILE-bearing phases are present in the below, this observation also provides a useful con- mantle source. However, addition of continental straint on the relationship between crystal forma- crust, even in small quantities, can substantially

Fig. 3. Primitive mantle-normalized diagrams for olivine, (A^C) and clinopyroxene-hosted (D) melt inclusions from this study. Also shown as a gray ¢eld in each plot is the range of Yemen CFB lava compositions (data from [9] and J.A. Baker, unpublished data) as well as the host lava composition (thick black dashed line) for plots (A^C). Normalizing values are taken from [43]. Plot D shows clinopyroxene-hosted inclusions from both samples 281 (solid lines) and MNY 361 (dashed lines). High-Sr inclusions in A and C are labeled.

EPSL 6342 11-9-02 Table 2 Major (wt%) and trace element (Wg/g) compositions of representative melt inclusions from Yemen CFB Inclusion 361- 361- 361- 361- 361- 281- 281- 281- 281- 281- 282- 282- 282- 282- 282- 282- 1-15 2-6 4-1 4-13 4-12d 2-6 3-2 1-2 3-4 3-5a 1-10b 1-25 3-4d 3-4e 3-5 3-6 Hosta ol ol ol ol cpx ol ol ol ol cpx ol ol ol ol ol ol

SiO2 47.64 47.69 46.57 44.60 51.78 51.81 52.50 49.86 51.43 51.30 48.46 49.37 51.56 49.34 46.88 49.15 TiO2 2.04 1.88 1.87 2.03 2.43 2.75 2.51 3.46 3.71 1.84 0.74 1.50 2.49 2.42 4.29 3.16 Al2O3 13.98 14.27 12.90 12.72 10.99 11.49 12.55 10.64 12.32 11.80 19.99 14.79 15.87 15.71 15.80 14.92 Cr2O3 0.07 0.02 0.14 0.15 1.02 0.01 0.01 0.05 0.01 0.10 0.01 0.24 0.01 0.01 0.01 0.01 577^594 (2002) 202 Letters Science Planetary and Earth / al. et Kent A.J.R. b FeOT 7.93 7.44 13.01 14.32 12.63 8.49 7.50 11.27 5.90 10.13 11.72 12.55 9.96 10.92 9.39 11.17 MnO 0.16 0.01 0.15 0.01 0.12 0.01 0.01 0.01 0.16 0.18 0.27 0.35 0.27 0.23 0.48 0.29 MgO 8.44 8.92 9.07 7.78 11.45 9.96 8.91 9.83 8.17 9.31 6.92 6.48 5.63 7.05 8.26 7.55 CaO 14.97 14.25 12.76 15.35 14.20 12.99 12.88 12.56 15.17 12.71 9.49 9.08 8.59 9.11 10.30 9.05 Na2O 2.94 3.17 1.49 1.08 3.10 2.05 2.68 1.55 2.10 1.67 2.05 3.45 3.40 3.63 2.85 3.07 K2O 1.04 0.89 1.06 0.33 1.81 0.28 0.23 0.26 0.78 0.35 0.32 1.08 1.88 1.56 0.68 1.09 Total 99.23 98.63 99.08 98.43 100.54 99.84 99.84 99.50 99.80 99.45 100.03 98.92 99.70 99.98 98.94 99.48 Mg#c 67.82 70.37 57.99 51.85 64.22 69.91 70.17 63.34 73.29 64.53 53.89 50.55 52.83 56.10 63.52 57.24 d KD 0.26 0.27 0.23 0.16 0.31 0.38 0.42 0.30 0.37 0.34 0.31 0.28 0.28 0.32 0.44 0.42

PL64 11-9-02 6342 EPSL e Mg#host 89.2 89.8 85.9 86.9 85.1 85.9 84.9 85.0 88.1 84.1 78.9 78.4 79.8 79.9 79.9 76.3 Sr 505 257 439 312 342 715 869 293 427 175 546 459 344 263 467 635 Y 15.3 13.3 12.0 17.3 17.2 24.1 25.5 23.1 26.5 14.4 6.1 23.4 22.9 27.9 35.8 43.2 Zr 110 72 121 110 95 83 56 143 221 65 39 173 123 137 190 310 Nb 17.0 7.7 18.3 12.0 12.2 6.2 4.5 16.5 25.5 5.4 3.4 17.7 9.8 18.7 18.0 31.0 Ba 280 112 406 100 163 57 54 72 142 47 90 263 544 298 223 459 La 15.9 7.7 23.1 13.7 14.8 7.4 4.9 12.7 21.9 6.8 4.7 18.7 13.0 22.3 16.6 33.7 Ce 35.6 19.2 41.8 33.0 33.6 17.7 12.9 29.6 53.1 16.3 10.7 42.5 29.2 39.5 41.9 76.5 Pr 4.7 2.9 4.6 4.6 5.6 2.6 2.0 4.5 7.3 2.3 1.5 5.7 4.1 4.7 6.6 10.3 Nd 20.8 13.2 17.2 20.2 20.3 14.5 10.6 20.3 31.4 11.0 6.3 25.5 19.9 22.6 33.2 44.6 Sm 4.1 3.3 3.5 4.8 5.5 5.0 3.7 4.7 6.9 3.0 1.6 5.9 5.1 6.3 7.8 9.5 Th1.65 0.46 3.07 1.37 2.50 na 0.35 1.16 1.84 0.48 0.30 1.39 3.43 4.85 1.13 2.26 U 0.37 0.11 0.62 0.35 1.23 na 0.09 0.39 0.67 0.22 0.10 0.68 1.90 1.83 0.37 0.71 K/Nb 509 957 481 229 1233 372 431 128 254 533 772 508 1604 693 316 294 Ba/Nb 16.4 14.4 22.2 8.3 15.4 9.3 11.8 4.3 5.6 8.7 26.4 14.8 55.7 15.9 12.4 14.8 Ba/Th170 1.16 132 73 108 153 62 77 98 303 190 159 61 197 203 Zr/Y 7.18 5.38 10.04 6.34 6.78 3.45 2.19 6.19 8.32 4.55 6.30 7.40 5.37 4.92 5.29 7.17 Ce/Y 2.36 1.44 3.48 1.91 1.95 0.73 0.51 1.28 2.00 1.13 1.75 1.82 1.28 1.42 1.17 1.77 (Sr/Sr*)f 1.43 1.16 1.38 0.91 0.90 3.28 5.33 0.86 0.79 0.97 5.02 1.06 1.06 0.71 0.88 0.83 a Host phenocryst: ol ^ olivine, cpx ^ clinopyroxene. b All Fe measured reported as FeOT. c Melt inclusion Mg# [ = 100XMg/(XMg+0.9XFe), where X denotes mole fraction]. d KD = (FeO/MgOhost)/(0.9FeO/MgOinclusion). e Host phenocryst [Mg# = 100XMg/(XMg+XFe), where X denotes mol fraction]. f k W (Sr/Sr*) = SrN/( PrNm NdN), where subscript ‘N’ refers to primitive mantle normalized values (normalizing factors from [43]). 585 586 A.J.R. Kent et al. / Earth and Planetary Science Letters 202 (2002) 577^594 fractionate LILE systematics in basaltic melts. also tend to magnify the e¡ect of contaminant Variations in LILE systematics in melt inclusions heterogeneity on relative trace element abundan- are also much greater (and extend to higher de- ces, and thus may be one part of the explanation grees of LILE enrichment) in melt inclusions from for suchvariable compositions in melt inclusions. the two contaminated lavas we have studied com- Addition of relatively small amounts of contami- pared to those from the uncontaminated lava 281. nant to highly depleted primitive melts is also From this we suggest that the large LILE var- consistent with the observation that major ele- iations in melt inclusions, particularly those from ment systematics of melt inclusions (apart from the two contaminated lava samples, primarily re- K2O) show little indication of the addition of £ect assimilation of crustal material during stor- crustal material beyond those variations that can age and transport of basaltic magma within the be plausibly attributed to melting and crystal frac- continental crust. The similarity between average tionation (implying that contamination has not melt inclusion compositions and host lavas also substantially altered the major element contents supports this interpretation (as the host lavas of melt inclusions). also have distinctive contamination-derived iso- We also stress that heterogeneity in the assimi- topic signatures; Table 1), as does the observation lated component strongly in£uenced the composi- that elements not substantially enriched in conti- tions of contaminated melt inclusions. This is well nental crust (e.g. Zr, Nb and Y) show much more illustrated by the plots of Ba/Th vs. K/Nb shown systematic variations in inclusions than LILE. in Fig. 4. On this ¢gure, where we plot two in- Large variations in LILE in melt inclusions are compatible element ratios, the trajectories pre- apparent on a number of spatial scales: between dicted by assimilation, calculated using either inclusions from di¡erent samples, between inclu- AFC [18], energy-constrained AFC (where the en- sions from the same sample, and even between ergy required for assimilation is balanced by that inclusions trapped within the same olivine grain released by the fractionating magma [19^22]), or (Table 2). This implies that an equivalent (or even bulk mixing models, are similar (although the ac- greater) range of melt compositions co-exist inti- tual mixing proportions may be substantially dif- mately within the magma transport and storage ferent), and depend primarily on the composition systems through which CFB erupt (and in which of the assimilant and basaltic end-members. phenocrysts form). Our data therefore provide Moreover, changes in the latter (within reasonable dramatic and direct evidence for the small-scale limits for primitive CFB melts) will again largely complexity of contamination processes in CFB a¡ect the mixing proportions. In contrast, melt and other crustally hosted ma¢c magma systems. inclusions from contaminated lavas do not fall In detail, the compositions of contaminated on a relatively small number of simple linear or melts trapped within inclusions probably re£ect hyperbolic trends, as would be expected if a lim- a number of factors, suchas: (i) thecomposition ited number of contaminants were involved, and of the assimilant(s), (ii) the amount of material require a range of contaminants compositions ^ assimilated by a given melt, and (iii) the compo- even for inclusions from the same host lava (and sition of the primary basaltic melt itself (which is thus that trap melts present within the same mag- itself a function of the initial composition and ma transport system). We also note that, although subsequent crystal fractionation history). The the mixing lines shown in Fig. 4 suggests that up generally low abundances of non-LILE trace ele- to 20^30 wt% of crustal contaminant may be re- ments suchas Zr, Y, Nb and REE in Yemen melt quired to produce the compositions of contami- inclusions (e.g. Figs. 2 and 3) indicate that many nated melt inclusions, this is probably an overes- melt inclusions contain relatively primitive melts timate, given both: (i) that the degree of (compared to typical Yemen CFB whole rock contamination in inclusions does not appear to compositions). Suchmelts would be highlysus- relate to major element composition (e.g. SiO2), ceptible to modi¢cation during crustal contamina- which might be expected to be the case for addition. Low incompatible element abundances will tion of large amounts of continental crust (Fig. 2);

EPSL 6342 11-9-02 A.J.R. Kent et al. / Earth and Planetary Science Letters 202 (2002) 577^594 587 and (ii) based on Zr and Nb abundances, many inclusions appear to represent melts withlow primary incompatible element abundances (and thus would be highly sensitive to addition of crustal materials) than the uncontaminated melt composition (from lava 281) used to calculate mixing trajectories. The contaminant heterogeneity that we have identi¢ed re£ects the heterogeneity of the continental crust traversed by melt transport systems. Progressive partial melting of crustal material ad- jacent to basaltic magma reservoirs could also fractionate contaminant compositions, and thus contribute to the apparent diversity of contaminants [23,45]. However, the overall contribution of partial melts appears to be relatively minor given the overall similarity between contaminated melt inclusion compositions and the expected granulite and felsic crust contaminants, which suggests that substantial fractionation of bulk crustal compositions (which might be expected during the formation of partial melts) has not occurred during assimilation. Furthermore, strong positive correlations between OHfi and ONdi values of contaminated Yemen basalts (J.A. Baker, unpublished data) imply that, at least for bulk lava compositions, assimilation of partial melts from crustal materials is not a widespread process (as Hf is dominantly hosted in refractory zircon which would not contribute to partial melts, whereas Nd has a number of less-refractory hosts). Regardless, the diversity of contaminated melt inclusion compositions clearly shows that a broad range of contaminants were present within the individual melt transport and storage systems, and argues against broad-scale mixing and/or homogenization of contaminant material prior to incorporation within basaltic melts. Our data also suggest that, despite the dramatic e¡ect of contamination on most melt inclusion compositions in contaminated lavas, small vol- Fig. 4. K/Nb vs. Ba/Thfor melt inclusions from (A) 281, umes of relatively uncontaminated melts may (B) 282, and (C) MNY 361. Symbols are the same as Fig. 2. Curves represent bulk mixing between lava 281 and the aver- also exist within most magma plumbing systems. age upper continental crust and Pan-African granulite com- For example, a small number of inclusions from positions (see the Background Data Set1). Mixing proportions samples 282 and MNY 361 (three out of 46) have represent weight percent of contaminant material added. In- K/Nb and Ba/Nb ratios distinctly lower than oth- clusions from olivine grain 282-3-4 are joined by a gray er inclusions and that lie within the range of typ- dashed line. ical uncontaminated basalt compositions (Figs. 4

EPSL 6342 11-9-02 588 A.J.R. Kent et al. / Earth and Planetary Science Letters 202 (2002) 577^594 and 5). Conversely we also note that one inclusion for details) as a starting point, we have calculated (281-3-5a) from lava 281 has K/Nb s 500 (as do the trajectories that represent assimilation of Pan- the high-Sr inclusions from 281), which suggests a African granulite and average upper continental small proportion of contaminated melt may also crust material during progressive olivine fraction- contribute to apparently uncontaminated lava ation at Ma/Mc ratios of 1 and 10. The composi- compositions. tions of the majority of melt inclusions from contaminated lavas are consistent with Ma/McE1, 5.3. Constraints on Ma/Mc ratios during and some suggest Ma/Mc = 10. Suchhigh Ma/Mc contamination ratios are signi¢cantly in excess of the maximum

The compositions of contaminated melt inclusions and host phenocrysts also constrain the relative rates at which assimilation and crystal fractionation occur in primitive CFB magmas, using the trace element composition of inclusions to monitor assimilation, and the Mg# of their host minerals to monitor fractionation. This approach requires that melt inclusions were trapped from melts in chemical equilibrium with their enclosing phenocrysts, however the reasonable average KD values that we calculate for FeO^MgO exchange between olivine^melt inclusion pairs (Table 2) suggests this condition generally holds. In Fig. 5 we compare the Ba/Nb and K/Nb ratios of melt inclusions from Yemen CFB with the Mg# of the olivine and clinopyroxene host phenocrysts. Melt inclusion compositions from contaminated lava samples (282 and MNY 361) show large variations in Ba/Nb and K/Nb at a given host Mg#, suggesting a range of di¡erent contaminated melt compositions were undergoing crystal fractionation, and again underlines the complexity of individual melt transport systems. In particular, there is no clear relationship between the apparent degree of contamination of Fig. 5. (A) K/Nb and (B) Ba/Nb vs. host phenocryst Mg# for melt inclusions from Yemen lavas. Symbols are the same melt inclusions and the Mg# of the host phase, as Fig. 2. Also shown are calculated trends for coupled AFC as might be expected from a simple AFC model of olivine by lava sample 281 (calculated to be in FeO^MgO (involving a single magma batchbeing progres- equilibrium withMg# = 92 olivine by incremental addition of sively contaminated by a single contaminant), olivine in equilibrium with the bulk composition until the re- where the most contaminated compositions would calculated bulk composition is in equilibrium withMg# = 92 olivine and using olivine^melt KD½MgOFeO = 0.30; resulting in be expected to occur in the most fractionated host a composition withMgO = 20.5 wt%, FeO = 10.7 wt%, phases [18], or in other models for crustal contam- SiO2 = 46.6 wt%, and incompatible element concentrations ination of CFB where the most primitive melts are that are 18% lower than the original composition). AFC expected to have the most contaminated compo- trends are calculated for boththeaverage upper continental crust and Pan-African granulite crustal compositions (see sitions [2,15,16]. 1 Background Data Set ) and Ma/Mc = 1 and 10. Mixing pro- Using the composition of the uncontaminated portions marked along trajectories represent progressive crys- lava sample 281 (recalculated to be in equilibrium tallization of 10 wt% olivine. Inclusions from olivine grain witholivine withMg# = 92 ^ see ¢gure caption 282-3-4 are joined by a gray dashed line.

EPSL 6342 11-9-02 A.J.R. Kent et al. / Earth and Planetary Science Letters 202 (2002) 577^594 589 values normally estimated for this parameter (gen- withassimilated components (as thermaldi¡usion erally V1, but up to V2^3 for energy-balanced within melts is much faster than mass transfer AFC simulations [19^21]). [23,46]). This situation could also re£ect recharge Although re-equilibration of relatively evolved of hot ma¢c melts into locations where previous host phenocrysts (containing highly contaminated magma batches have already heated the wall- melt inclusions) withprimitive melts could result rocks, suchas theboundaries of a large magma in highly contaminated melts trapped within ap- chamber or in closely spaced feeder dykes [23]. parently primitive host phases, there is little petro- Rapid heat transfer could also be aided by turbu- graphic evidence for this (i.e. resorbed and/or re- lent £ow in primitive magmas [2]. In turn, cooling verse-zoned phenocrysts), and the similarity and/or chemical changes induced by interaction between average melt inclusion and host lava withcontaminant melts may also promote crys- compositions limits the amount of the contribu- tallization and melt inclusion formation [19].For- tion of ‘exotic’ melts that did not contribute to mation of melt inclusions within enriched bound- the ¢nal lava composition. ary layers at the margins of magma chambers has Assimilation of a highly enriched partial melt also been described in silicic volcanic systems [46]. material, or selective assimilation of less-refrac- We also note that, in contrast to studies based tory phases at reasonable Ma/Mc ratios could on bulk lava compositions, the high apparent Ma/ also potentially result in apparently highly con- Mc ratios implied by melt inclusion and pheno- taminated melts trapped in primitive olivine crys- cryst compositions must re£ect genuinely low tals. For example, the highest K/Nb, Ba/Nb, K overall rates of crystal fractionation (although and Ba contents in melt inclusions could be pro- the presence of melt inclusions shows that some duced at Ma/Mc = 1 by addition of an assimilant crystallization was occurring at this time), regard- with V6% K2O, 1200 ppm Ba and 10 ppm Nb to less of whether phenocrysts are subsequently re- a primitive basaltic melt with0.12 wt% K 2O, moved from the crystallizing melt. 70 ppm Ba and 5^10 ppm Nb. This process is Melt inclusions from sample 282 are hosted in hard to constrain further, as the potential compo- more fractionated olivines, and the majority of sitions of partial melts are highly variable [23,45], inclusions from this sample (with the exception although (as noted above) the overall contribu- of those in grain 282-3-4) tend to have more ho- tion of highly enriched and fractionated partial mogenous compositions compared to inclusions melts to contaminated Yemen CFB appears mi- from MNY 361 (e.g. Figs. 4 and 5). This is con- nor. sistent witha longer period of fractionation and Our preferred explanation of the high apparent mixing prior to trapping of these inclusions pro- Ma/Mc values required to model melt inclusion ducing a more homogenous set of inclusions, and phenocryst compositions is that inclusions although many inclusion compositions still sug- represent melts are trapped within highly contam- gest Ma/Mc ratiosE1. There is also dramatic evi- inated zones, probably located at the margins of dence for localized assimilation accompanied by magma chambers and conduits, where hot and minimal crystal fractionation. Five inclusions primitive basaltic magma lies in close proximity hosted in a single olivine crystal (282-3-4) have to wall-rock material. In order for assimilation the most LILE-rich compositions recorded in to occur without substantial crystal fractionation this study (Fig. 3; Table 2), and K/Nb ratios (as we observe), the temperature of the wall-rock are higher than any known Oligocene Yemen has to be close to, or above its solidus prior to CFB composition (e.g. Fig. 4). These inclusions contamination, otherwise a period of cooling and also de¢ne a clear trend in the plot of K/Nb vs. fractionation withlittle or no assimilation occurs Ba/Th( Fig. 4), suggesting that, unlike the major- until the wall-rock starts melting [20,21]. This may ity of other inclusions, they may contain melts occur at the boundaries of large magma chambers formed during progressive assimilation of a single, where thermal di¡usion provides heat from a highly enriched crustal contaminant. However, muchlarger volume of magma thancan mix the tremendous range of melt inclusion composi-

EPSL 6342 11-9-02 590 A.J.R. Kent et al. / Earth and Planetary Science Letters 202 (2002) 577^594 tions is accompanied by only a small change in the composition of the host olivine, which exhibits slight normal zoning from Fo79:9 to Fo78:7 (although there is a broad spatial association between the most contaminated inclusion located at the grain margin and the least contaminated located within in the interior of the crystal), suggesting limited crystal fractionation during inclusion entrapment. Thus, even in more fractionated systems, it still seems that very rapid localized assimilation, accompanied by minimal fractionation, can occur. This may accompany sudden changes in the magma transport system, and in this regard we note that the apparent composition of the assimilant in grain 282-3-4 is markedly dif- ferent from that in the majority of other inclusions in sample 282 and may represent contamination by more potassic Pan-African upper crust material (e.g. Fig. 4).

5.4. Location and timing of contamination and melt aggregation

Our data also place constraints on the location and relative timing of melt aggregation, contamination and fractionation in CFB magma transport systems (Fig. 7). Firstly, we note that the similarity between the average melt inclusion and host lava compositions in both our contaminated lava samples implies that relatively little assimilation (within the melts contributing to this lava) occurred after trapping of melt inclusions. If this was not the case then we would ex- pect that the bulk lava composition would be signi¢cantly more contaminated than the average inclusion composition [26]). This suggests that crystal growthand assimilation were intimately Fig. 6. Zr/Y vs. Ba/Nb for melt inclusions from (A) sample linked processes; either because assimilation re- 281, (B) sample 282, (C) sample MNY 361. Symbols are the quired the heat released by crystallization same as Fig. 2. Also shown are trends for the compositions (although we have argued above that the heat of aggregated modal fractional melts of spinel and garnet required to melt the wall-rocks was probably de- lherzolite (using the modal composition for garnet and spinel rived from additional sources from that released lherzolite from [32] and partition coe⁄cients from [34] and [48]). Proportions marked represent melt fraction in percent. solely by the contaminated magma batches them- Black dashed lines represent mixing between lava 281 with selves) or because cooling induced by the addition the average upper continental crust and Pan-African granu- of assimilants stimulated phenocryst growth. The lite compositions detailed in the Background Data Set1. Mix- relative dearthof uncontaminated compositions ing proportions represent weight percent assimilant added. In in melt inclusions from contaminated lavas also order to preserve a useful scale we have not plotted inclusion 282-3-4d (Ba/Nb = 55) in (B). supports this idea, showing that little olivine crys-

EPSL 6342 11-9-02 A.J.R. Kent et al. / Earth and Planetary Science Letters 202 (2002) 577^594 591 tallization occurred prior to the assimilation of continental crust material. The presence of mantle-derived trace element variations in contaminated melt inclusions also places important and novel constrains on the location and timing of melt aggregation. As shown in Fig. 6, melt inclusions show a large range of Zr/Y ratios (V2^10). This ratio is relatively insensitive to crustal assimilation, but can be fractionated during progressive melting of mantle peridotite (Fig. 6). Thus the large range of Zr/Y (and other ratios that are insensitive to contamination suchas Ce/Y, Zr/Nb) in melt inclusions suggest that melts produced within the mantle column did not fully aggregate and mix within the mantle prior to melt inclusion trapping (had this occurred then all melt inclusions would be expected to have similar Zr/Y ratios). Further- more, elevated and variable Ba/Nb and/or K/Nb Fig. 7. Schematic sketch of a CFB melting and melt trans- in the same inclusions also indicate that trapped port system showing the major ¢ndings of this work (not to melts did not completely aggregate and mix until scale). after they entered, and were contaminated by, the continental crust. Despite boththesubstantial thickness of the Yemen crust (V40 km) and the and thus mixing could also occur within the density di¡erence between mantle and crust, rela- uppermost mantle. tively little mixing of di¡erent mantle melts appears to have occurred during upwelling or via 5.5. High-Sr inclusions ponding of melts at the base of the crust. Large variations in Zr/Y also characterize melt inclu- Although this study concentrates on melt ag- sions from the more fractionated lava sample gregation and melt^crust interaction processes, 282 (Table 2; Fig. 6) and suggest that mantle- we also note that three melt inclusions (281-2-6, derived magmas may also remain as discrete and 281-3-2, and 282-1-10b) have anomalous compo- unmixed melt batches (that assimilate di¡erent sitions, withdistinctively low overall incompatible amounts of crustal materials) through more pro- trace element abundances ( 6 10Uprimitive man- tracted episodes of crystal fractionation within the tle) and large positive Sr anomalies (Fig. 3). crust. Although the high Sr in these inclusions could Regardless of contamination, the compositions be the result of contamination of mantle-derived of Yemen CFB lavas (as well as those from other melts by plagioclase-richmaterial, only inclusion CFB provinces) appear to be largely the result of 282-1-10b has the elevated Al2O3 and low CaO/ crustal mixing processes, rather than re£ecting Al2O3 expected from suchcontamination (and primary mantle melt compositions. This ¢nding even for this inclusion the major element and may be important for interpreting other aspects low rare earthelement abundances are di⁄cult of the geochemistry of CFB lavas. Analogous to produce by plagioclase assimilation alone). conclusions have been made for MORB (i.e. man- The two Sr-rich inclusions from sample 281 tle-derived melt batches primarily mix within (281-2-6 and 281-3-2) are hosted in Fo-rich oliv- magma chambers within the ocean crust [29,30]), ines (Fo85:9 and Fo84:9) from an uncontaminated although these are based on the observation that host lava, and have major element compositions diverse melt inclusions are trapped in plagioclase, that are similar to other inclusions from this sam-

EPSL 6342 11-9-02 592 A.J.R. Kent et al. / Earth and Planetary Science Letters 202 (2002) 577^594 ple, and that are inconsistent with plagioclase as- the processes that lead to crustal contamination similation (e.g. Al2O3 V12 wt% and CaO/Al2O3 within CFB magma chambers and melt transport V0.9; Table 2). The low overall LILE contents of systems. Our main conclusions are: these inclusions also suggest minimal crustal contamination (Table 2), and although both inclu- 1. Melt inclusions sample the same melts that ul- sions have K/Nb ratios s 300, this is a function timately amalgamate and mix to produce their of unusually low Nb contents. An alternative in- host lavas, and signi¢cant assimilation did not terpretation is that Sr-rich inclusions represent occur after melt inclusion trapping. In contam- low-volume melts derived from the mantle source inated lavas there also appears to have been of Yemen CFB volcanism. In particular there are little olivine crystallization prior to contamina- marked similarities between high-Sr inclusions tion. from Yemen and high-Sr melt inclusions from 2. Trace element abundances in inclusions are Mauna Loa, Hawaii that have been attributed consistent withvariations in thedegree of as- to melting of recycled oceanic crust within the similation and the precontamination composi- Hawaiian mantle plume [47]. Bothsets of inclu- tions of primitive melts, but also require sub- sions show enrichment in Sr over REE, low over- stantial heterogeneity within contaminant all REE abundances, and depletion of Thand Nb materials themselves. withrespect to Ba. Sobolev et al. [47] suggested 3. Highly contaminated melt inclusions hosted in that the high-Sr inclusions from Mauna Loa rep- fosterite-rich olivine show that very rapid as- resent recycled gabbroic material, and that decou- similation, withlittle concomitant crystal frac- pling of major element and trace element signa- tionation (withapparent Ma/Mc ratios that tures of gabbroic material occurred during range up to V10) may occur within crustal subduction and slab dispersion within the mantle. melt plumbing systems ^ particularly at the Sucha conclusion for our Yemen inclusions is initial stages of fractionation. This probably more equivocal, given the limited number of Sr- re£ects decoupling of mass and heat transfer rich inclusions that have been identi¢ed, although processes and/or magma recharge at the mar- the compositions of some uncontaminated Yemen gins of large magma chambers, in magma con- basalts are consistent withthepresence of re- duits, or at other locations where hot primitive cycled oceanic crustal material within the Afar magmas came into contact withpreheated plume [9]. If the high-Sr inclusions do represent wall-rocks. recycled gabbroic material then these melts are 4. Trace element signatures derived from mantle probably not a major component within the melting processes preserved in crustally con- Afar plume, as only three inclusions out of taminated melt inclusions demonstrate that V120 inclusions analyzed from Yemen CFB discrete melt batches from the mantle did not (Table 2; A.J.R. Kent, unpublished data) have completely mix prior to entering the continen- the Sr-rich ‘exotic’ composition. tal crust (and being contaminated). This is consistent withlimited mixing during upwelling and/or ponding of melts at the base of the 6. Conclusions continental crust. 5. Inclusions with unusual high-Sr compositions Melt inclusions and phenocryst compositions in may re£ect low-volume melts derived from re- Oligocene CFB from Yemen provide a novel cycled oceanic gabbros within the mantle method for examining crustal contamination pro- plume source.[BW] cesses in basaltic magma systems that traverse the continental crust in greater detail than is possible from the study of whole-rock lava compositions Acknowledgements alone. In particular, melt inclusions provide dramatic evidence for the small-scale complexity of Ian Hutcheon and Doug Phinney provided as-

EPSL 6342 11-9-02 A.J.R. Kent et al. / Earth and Planetary Science Letters 202 (2002) 577^594 593 sistance with SIMS analysis and Chip Lesher and [11] A.D. Brandon, P.R. Hooper, G.G. Goles, R.S.J. Lam- Greg Baxter helped with homogenizing melt in- bert, Evaluating crustal contamination in continental basalts: the isotopic composition of the Picture Gorge basalt clusions. Berit Wendel and Jrn Rnsbo provided of the Columbia River Basalt Group, Contrib. Mineral. assistance withelectron microprobe analyses. Re- Petrol. 114 (1993) 452^464. views by Jon Davidson, Chris Hawkesworth, [12] R.W. Carlson, J. Lugmair, J.D. Macdougall, Columbia Sally Gibson and Frank Spera and additional dis- River Volcanism: the question of mantle heterogeneity cussions withFrank Spera, David Peate and Lara or crustal contamination, Geochim. Cosmochim. Acta 45 (1981) 2483^2499. Heister substantially improved the quality of this [13] J.P. Davidson, I.R. Wilson, Evolution of an alkali basalt- manuscript. 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